nuclear reactor[1], also known asatomic reactororreactor, is able to maintain controllable self-sustainingChain fission reactionTo implementnuclear energyDevices used.Nuclear reactor through reasonable layoutnuclear fuelSo that the self-sustaining chain nuclear fission process can occur in the neutron source without additional neutron source.Strictly speaking, the term reactor should cover fission reactorFusion reactorFission fusion hybrid reactor, but generally only refers to fission reactor.
Manhattan Project During this period, the first human nuclear reactor was built by a famous Italian American physicistEnrico Fermi The team led by him was built at the University of Chicago in December 1942, named Chicago Pile-1.[2-7]The reactor adopts uranium fission chain reaction, which has opened the era of atomic energy for mankind, and thus the University of Chicago has become the "birthplace of atomic energy" for mankind.[2-7]
On November 5, 2018, the first nuclear reactor project for scientific research in Saudi Arabia was started.[8]
As early as 1929,CocroftWe successfully realized the transformation of atomic nucleus by using protons.However, the nuclear reaction caused by protons consumes a lot of energy, so there is very little chance for protons to hit the target nucleus.[1]
In 1938, GermanOtto Hahn And Hugh Tross succeeded in colliding neutrons with uranium atoms.This experiment is of great significance. It not only makes the uranium atom split simply, but also reduces the total mass after fission, and releases energy at the same time.Especially important is that when uranium atom is fissionFission fragmentIn addition, 2 to 3 neutrons are emitted, which can cause the fission of the next uranium atom, thus causing a chain reaction.
The designer of the first nuclear reactor: Fermi
In January 1939, uranium was induced by neutronsNuclear fissionThe news ofFermiIn his ears, he had fled toColumbia UniversityFermi is indeed a genius scientist. As soon as he heard the news, he immediately visualized the possibility of an atomic reactor and began to work hard for its realization.Fermi organized a research team to conduct a thorough study on the problem of building an atomic reactor.Fermi, together with his assistants, often spent all night doing theoretical calculations and thinking about the shape design of the reactor,
Sometimes I have to solve the problem of purchasing graphite materials in person.
December 2, 1942Manhattan Project period,Enrico Fermi The research team ofUniversity of ChicagoIn front of a huge graphite reactor below the Stagg Field Stadium.[2-7]Then Fermi sent a signal, and then from the huge reactor composed of 7 tons of uranium fuel buried between graphite,control rodIt was pulled out slowly. As the counter made a clicking sound, when the control rod rose to a certain extent, the sound of the counter became loud, which indicated thatchain reactionHere we go.[2-7]This is the first time that mankind has released and controlled atomic energy. This reactor is named“Chicago One”（Chicago Pile-1）。[2-7]
1954Former Soviet UnionBuild the first building in the worldAtomic power stationEnriched uranium is used as fuelGraphite water cooled reactor, the electrical output power is 5000 kW.In 1956, Britain was also builtAtomic power plant。The development of atomic power plants has not been smooth. Many people are worried and afraid about the radioactive pollution of nuclear power plants, so there has been an anti nuclear movement.In fact, under strict scientific management, atomic energy is a safe energy source.The level of radioactivity around atomic power stations is actually not much different from that of the natural background.
In March 1979, the Three Mile Island Atomic Energy Power Station in the United States suffered an unprecedented failure in the history of atomic energy development due to operating errors and equipment failuresSerious accident。However, as the reactor shutdown system, emergency cooling system, containment and other safety measures played a role, the radioactive escape was minimal, and people and the environment were not affected, which fully shows that the development of modern science and technology can ensure the safe use of atomic energy.
On November 9, 2021, French President Makron said in his speech that France would restart its nuclear reactor construction plan and continue to develop renewable energy.[20]
theoretical research
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Research history
2 billion years ago[9]More than a dozen natural nuclear reactors in Africa's Okrobando region were mysteriously started, stably outputting energy, and safely operated for hundreds of thousands of years.Why didn't they destroy themselves in the explosion?Who ensured the safe operation of these nuclear reactions?Are they really like the rumors in the world, evidence of extraterrestrial visits, or masterpieces of the previous generation of civilization?Through the detailed analysis of the relics, the truth of ancient nuclear reactors is increasingly exposed to us.
Uranium ore
In May 1972, a worker at a nuclear fuel processing plant in France noticed a strange phenomenon.At that time, he was conducting routine analysis on a piece of uranium ore, which was mined from a seemingly ordinary uranium mine.Like all natural uranium ores, this ore contains three types of uranium isotopes - in other words, the uranium element exists in three different forms, and their atomic weights are different: the most abundant is uranium 238;The rarest is uranium 234;The isotope that people covet and can maintain the chain reaction is uranium 235.In almost all places on the earth, even on the moon or in meteorites, the atomic number of uranium 235 isotope always accounts for 0.720% of the total uranium element.However, in these samples taken from AfricaGabonOf the ore samples, the content of uranium 235 is only 0.717%!Although the difference is so subtle, it has aroused the vigilance of French scientists, and something strange must have happened.Further analysis shows that uranium 235 in a part of ore mined from the mine is seriously short of weight: about 200 kg has disappeared - enough to make six atomic bombs.
KurodaIt is believed that the first condition for self-sustaining fission reaction to occur is that the size of uranium ore veins must exceed the average distance of fission induced neutrons passing through the ore, that is, about 0.67 meters.This condition can ensure that neutrons released by fission nuclei can be absorbed by other uranium nuclei before escaping from the ore vein.
The second necessary condition is that uranium 235 must be abundant enough.Today, even the uranium ore vein with the largest reserves and highest concentration cannot become a nuclear reactor, because the concentration of uranium 235 is too low, even less than 1%.However, this isotope is radioactive, and its decay rate is about 6 times faster than that of uranium 238. Therefore, in the long past, the proportion of this more easily decayed isotope must be much higher.For example, when the Orkro uranium ore vein was formed 2 billion years ago, the proportion of uranium 235 was close to 3%, which is roughly equivalent to the concentration of artificially purified enriched uranium fuel used in most nuclear power plants.
The third important factor is that there must be some kind of neutron“Moderator”ModeratorMovement speedSo that these neutrons can be more handy in inducing uranium nuclear fission.Finally, there should be no large amount of boron, lithium or other "toxins" in the ore vein. These elements will absorb neutrons, so anynuclear fission reaction cease abruptly.
Finally, the researchers in Oklo and the adjacentOkloAmong the uranium deposits in Bando area, 16 separate regions have been identified. Two billion years ago, the real environment there was strikingly similar to the general situation described by Kuroda.Although these areas were all identified decades ago, the details of the operation process of ancient nuclear reactors were not completely revealed until my colleagues and I.
Hydrogen provides evidence
The hydrogen element produced by the splitting of heavy elements provides conclusive evidence that self-sustaining nuclear fission reactions did occur in the Orkro uranium mine 2 billion years ago, and lasted for hundreds of thousands of years.
Shortly after Oklo's uranium anomaly was discovered, physicists determined that the natural fission reaction led to the depletion of uranium 235.When a heavy nucleus is split in two, new lighter elements will be produced.Finding these elements is tantamount to finding conclusive evidence of nuclear fission.Facts have proved that the content of these fission products is so high that there can be no other explanation except nuclear chain reaction.This chain reaction is like 1942Enrico Fermi (Enrico Fermi) and his colleagues made the famous demonstration (when they built the world's first controllable nuclear fission chain reactor), and the reaction was maintained by their own power, just 2 billion years earlier.
Shortly after such an astonishing discovery was announced, physicists around the world began to study the evidence of these natural nuclear reactorsLibrevilleAt a special meeting of, they shared their research results on the "Oklo phenomenon".The next year, George A. Cowan, who represented the United States at the meeting, was a famous AmericanSanta Fe Institute He wrote an article for Scientific American (see the article "Natural Nuclear Fission Reactor" written by George A. Cowen on July 1976), in which he explained the scientists' guess on the operating principle of these ancient nuclear reactors.
For example, Cowen describedPlutonium 239The formation process of uranium 238, which is more abundant, captures some neutrons released by the fission of uranium 235 and converts them into uranium 239, which then releases two electrons and converts them into plutonium 239.stayOrkro uranium mineMore than two tons of plutonium 239 have been produced.However, this isotope almost disappeared later (mainly through natural radioactive decayhalf life24000 years), some plutonium has also experienced fission itself, which is proved by its unique fission products.The rich content of these light elements led scientists to speculate that the fission reaction must have lasted for hundreds of thousands of years.According to the amount of uranium 235 consumed, they calculated the total energy released by the reactor, which is roughly equivalent to the energy consumed by a 15 million kilowatt machine operating for a whole year;Combined with some other evidence, the average output power of the reactor can be calculated: no more than 100 kilowatts, enough to maintain the operation of dozens of ovens.
A dozen natural reactors work spontaneously and maintain a moderate power output, which has been operating for several hundred thousand years, which is really amazing.Why didn't these veins explode and self destruct immediately after the nuclear chain reaction started?What mechanism enables them to have the necessary self-regulation ability?Are these reactors stable or intermittent?Since the first discovery of the Oklo phenomenon, these questions have not been answered for a long time.In fact, the last question puzzled people for 30 years, until IUniversity of WashingtonThe mystery was gradually revealed after colleagues at the St. Louis campus detected a piece of ore from this mysterious African uranium mine.
Inert gas reveals the mystery
In the Orkro reactor relic, the composition ratio of xenon isotope is abnormal.To find out the source of this anomaly, we can uncover the mystery of the operation of ancient nuclear reactors.
One of the reactor relics in Ocro was studied, focusing on the analysis of xenon gas.Xenon is a heavyinert gas(inert gas) can be stored by minerals for billions of years.There are 9 kinds of xenonstable isotope , produced by different nuclear reaction processes, with different contents.As an inert gas, it is difficult to form chemical bonds with other elements, so it is easy to purify them for isotope analysis.Xenon is so rare that scientists can use it to detect and trace nuclear reactions, or even to study nuclear reactions in primitive meteorites that occurred before the formation of the solar system.
XenonicIsotopic compositionA mass spectrometer is needed, which can separate different atoms according to different atomic weights.I was lucky to be able to use an extremely accurate xenon mass spectrometerUniversity of Washington's colleaguesCharles M. Hornberg(Charles M. Hohenberg).But before using his instrument, we must first extract xenon from the sample.Usually, scientists only need to heat the host mineral above its melting point, and the rock will lose its crystal structure and can no longer retain the xenon stored inside.In order to obtain more information about the origin and storage process of this gas, we have adopted a more sophisticated method - laser extraction, which can specifically release xenon from individual particles of mineral samples without touching other parts around.
The only ore fragment we can use is only 1mm thick and 4mm wide. We apply this technology to many tiny spots on the fragments.Of course, we first need to decide where to focus the laser beam.In this regard, Hornberg and I received great help from our colleague Olga Pravdivtseva, who took a detailed X-ray photo of our sample and identified the candidate minerals.After each extraction, we will purify the gas obtained, and then put xenon into Hornberg's mass spectrometer, which will display the atomic number of each isotope.
The location of xenon surprised us. It is not as we imagined that it is widely distributed in uranium rich mineral particles. The largest amount of xenon is aluminum phosphate particles that do not contain uranium at all.It is obvious that all theNatural mineralThe xenon concentration in these particles is the highest.The second surprise is that the extracted gas is significantly different in isotopic composition from the gas normally produced by nuclear reaction.Nuclear fission will certainly produce xenon 136 and xenon 134, but in the Orkro ore, these two isotopes seem to be seriously missing, while the content of other lighter xenon isotopes changes little.
How did this difference in isotopic composition come about?Chemical reactions cannot provide an answer because all isotopes have identical chemical properties.Can nuclear reactions, such as neutron capture, be explained?After careful analysis, my colleagues and I also ruled out this possibility.We have also considered the physical separation process of different isotopes: heavier atoms move slower than lighter atoms, and sometimes they will separate from each other.The uranium enrichment plant uses this process to produce reactor fuel, but it requires a fairly high level of technology to build such industrial equipment.Even if nature can miraculously create a similar "device" on the micro scale, it is still unable to explain the xenon isotope proportion mixed together in the aluminum phosphate particles we studied.For example, if physical sorting did occur, considering the existing content of xenon 132, the absence of xenon 136 (4 atomic mass units heavier than xenon 132) should be twice that of xenon 134 (2 atomic mass units heavier than xenon 132).But in fact, we have not seen such a pattern.
After racking our brains, we finally figured out the reason for the abnormal xenon isotope composition.All xenon isotopes we measured are not direct products of uranium fission.On the contrary, they are the products of the decay of radioactive iodine isotopes. Iodine comes from the decay of radioactive tellurium, and tellurium comes from the decay of other elements. This is a famous nuclear reaction sequence. The final product is stable xenon.
Our breakthrough is that we realize that different xenon isotopes in the Oklo sample were produced at different times, and the timetable they follow is determined by the half-life of their parent element iodine and the next generation element tellurium.The longer certain radioactive precursors (i.e. intermediate products of a series of reaction processes) exist, the longer their xenon formation process will be delayed.For example, xenon 136 began to form only about 1 minute after the self-sustaining fission reaction of Oklo began;An hour later, the lighter stable isotope xenon 134 appeared;Then, a few days after the beginning of fission, xenon 132 and xenon 131 appeared on the scene;Finally, xenon 129 was formed millions of years later - by this time, the nuclear chain reaction had stopped for a long time.
If the Orkro ore vein has been in a closed state, the xenon gas accumulated during the operation of its natural reactor will maintain the normal isotope ratio produced by nuclear fission and has been preserved to this day.However, scientists have no reason to believe that this system will be closed.In fact, there are sufficient reasons to suspect that it is not closed.The simple fact that the Oklo reactor can regulate nuclear reaction by itself in some way provides indirect evidence.The most likely regulation mechanism is related to the activity of groundwater: when the temperature reaches a critical point, the water will be boiled and evaporated.Water acts as a neutron moderator in the nuclear chain reaction. If the water disappears, the nuclear chain reaction will be temporarily stopped.Only when the temperature drops and enough groundwater infiltrates again, the reaction area will continue to undergo fission.
This statement on how the Oklo reactor works highlights two main points: first, nuclear reactions are likely to occur intermittently in some way;Second, there must be a lot of water flowing through these rocks - enough to flush out some xenon precursors, such as tellurium and iodine, which are soluble in water.The presence of water helps to explain why most xenon is currently retained in aluminum phosphate particles, but not in uranium rich minerals. It is important to know that the fission reaction originally generated those radioactive precursors here.Xenon will not simply migrate from one group of minerals that already exist to another - aluminum phosphate minerals probably did not exist before the Ocro reactor began operating.In fact, those aluminum phosphate particles may be formed in situ. Once the water heated by nuclear reaction is cooled to about 300 ℃, aluminum phosphate particles will be formed.
During each active period of the operation of the ORCRO reactor and the subsequent period when the temperature is still high, a large amount of xenon (including xenon 136 and xenon 134 with relatively fast formation speed) will be driven away.When the reactor cools down, the half-life is longerXenon precursor(That is, xenon 132, xenon 131 and xenon 129 radioactive precursors with relatively rich content will be produced finally) will preferentially combine with the aluminum phosphate particles being formed.As more water returns to the reaction area, neutrons are properly moderated, and the fission reaction resumes again, making this cycle of heating and cooling repeat again and again.The result is the strange xenon isotope composition that we have observed.
What force can keep xenon in aluminum phosphate mineral for 2 billion years?Furthermore, why was the xenon produced during one reactor operation not removed during the next operation?We haven't found the exact answers to these questions.It is speculated that xenon may be trapped in the cage structure of aluminum phosphate minerals, which can accommodate xenon generated in the cage even at very high temperatures.Although the details are still unclear, regardless of the final answer, one thing is clear: the ability of aluminum phosphate to capture xenon is amazing.
Batch reactor
Ancient nuclear reactors are like today'sGeyser, has a naturally formed self-regulation mechanism.They provide scientists with new ideas in nuclear waste disposal and basic physics research.
After understanding the observed basic process of xenon isotope production in aluminum phosphate, my colleagues and I tried to establish a mathematical model for this process.This calculation reveals more information about reactor operating time, and all xenon isotopes provide roughly the same answer.The Oklo reactor we studied was "turned on" for 30 minutes each time, and then "turned off" for at least 2.5 hours.Such a pattern is like some geysers we see. First, they slowly heat up, and then in a spectacular eruption, they boil out all the groundwater stored, and then re store it to start a new cycle, which continues day after day, year after year.This similarity supports the view that the groundwater flowing through the Orkro ore vein not only acts as a neutron moderator, but also evaporates from time to time, forming a regulatory mechanism to protect these natural reactors from self destruction.In this regard, this regulatory mechanism is very effective, and there has not been a meltdown or explosion for hundreds of thousands of years.
One might imagine that engineers working in the nuclear power industry might learn one or two things in Oklo.They can really learn something, but it is not necessarily about reactor design, but perhaps more important is the way to dispose of nuclear waste.After all, Oklo has operated for such a long time like a geological repository, which is why scientists need to investigate in detail. They want to know how various fission products migrate out of these natural nuclear reactors.They also carefully examined another similar ancient nuclear fission area, which was found by exploration drilling in a place called Bangombe, about 35 kilometers away.The Bangobe reactor is particularly remarkable because its burial location is shallower than the open-pit uranium mines in Oklo and Oklo Bando regions, so there is more water flowing there.In a word, the conclusion of the investigation has doubled our confidence: a variety of dangerous nuclear waste can be successfully isolated underground.
OkloIt also demonstrated a way to store nuclear waste that was once believed to cause environmental pollution.Since the advent of nuclear power generation, a large number ofradioactivityXenon 135, Krypton 85And other inert gases are released into the atmosphere.naturalfission reactor It shows that aluminum phosphate mineral has a unique ability to capture and store these gaseous wastes for billions of years. It may be feasible to seal these waste gases in this mineral.
The Orkro reactor also revealed to scientists that they once believed thatBasic physical constantsAlpha (alpha, which controls cosmic parameters such as the speed of light) may have changed.Over the past 30 years, the Oklo phenomenon, which occurred 2 billion years ago, has been used to refute the view that α has changed.But in 2005, Los Alamos National LaboratorySteven K. Ramono(Steven K. Lamoreaux) andJustin R. Togson(Justin R. Torgerson) inferred from the Oklo phenomenon that this "constant" has indeed changed significantly (and strangely, the direction of change of the constant they obtained is opposite to that of others).For Ramono and Togson's calculations, some details of the operation process of Oklo are very important. From this perspective, the work done by my colleagues and I may help clarify this complex problem.
Are these ancient reactors in Gabon the only natural reactors ever seen on earth?Two billion years ago, the conditions required for self-sustaining fission were not very rare. One day, we may be able to find other natural reactors.I think that a bit of revealing xenon gas will greatly help this search.
principle
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Principles of Nuclear Reactor
Nuclear reactors arenuclear power plantHeart of[1]Its working principle is as follows:
atomfromNucleusIt is composed of extranuclear electrons.The nucleus is composed ofprotonAndneutronform.WhenUranium 235When an atomic nucleus is bombarded by foreign neutrons, one atomic nucleus will absorb one neutron and split into two atomic nuclei with smaller mass, and release 2-3 neutrons at the same time.The neutrons produced by this fission will bombard another uranium 235 nucleus, causing newfission。If it continues like this, it is fissionchain reaction。Chain reaction generates a lot of heat energy.The heat can be taken away by circulating water (or other substances) to prevent the reactor from burning down due to overheating.The exported heat can turn water into steam and drive the steam turbine to generate electricity.It can be seen from this that the most basic composition of nuclear reactor is fission nucleus+heat carrier.But only these two items cannot work.Because a large number of high-speed neutrons will fly away, it is necessary to slow down neutrons to increase the chance of collision with atomic nuclei;The working state of a nuclear reactor should be determined according to people's wishes, which requires control facilities;Uranium and fission products are highly radioactive and will cause harm to people, so reliable protective measures must be taken;When a nuclear reactor accident occurs, it is necessary to prevent radiation leakage under various accident conditions, so the reactor also needs various safety systems.In conclusion, the reasonable structure of nuclear reactor should be: nuclear fuel+moderator+heat carrier+control facilities+protective devices+safety facilities.
It should also be noted that,uraniumOre cannot be directly used as nuclear fuel.uranium oreIt is required to go through the procedures of selection, grinding, acid leaching, concentration, etc. to produce a certain uranium content, a certain geometric shapeuranium barOr spherical fuel can participate in reactor work.
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Classification by use
Inside nuclear reactor
According to the use, nuclear reactors can be divided into the following types[10]
① SetneutronBeams are used for experiments or nuclear reactions using neutron beams, including research reactors, material experiments, etc.
⑤ Nuclear reactions that generate heat for power generation are called power reactors.
⑥ Nuclear reactors used to propel ships, aircraft, rockets, etc. are called power reactors.
In addition, nuclear reactors are divided intonaturalUranium reactor, enriched uranium reactorthoriumHeap;According to neutron energyFast reactorandThermal reactor；according tocoolant(Heat carrier) materials are divided into water cooled reactor, gas cooled reactor, organic liquid cooled reactorliquid metalCold reactor;According to the moderator, it is divided into graphite pilewaterCold reactor, organic reactor, molten salt reactor, sodium cooled reactor;According to neutron flux, it can be divided into high flux reactor and general energy reactor;According to thermal state, it can be divided into boiling reactor, non boiling reactor and pressurized water reactor;According to the operation mode, it can be divided into pulse reactor and steady reactor, etc.Nuclear reactors can have more than 900 designs in concept, but they are very limited in reality.
Classified by historical age
Nuclear reactor perspective
The former Soviet Union built the world's first atomic power station in 1954, opening a new page in the peaceful use of atomic energy.Britain and the United States built atomic power stations in 1956 and 1959 respectively.By September 28, 2004, there were 439 power generation stations in 31 countries and regions in the worldatomic reactorIn operation, the total capacity is 364.6 million kilowatts, accounting for about 16% of the world's total power generation capacity.Among them, France has built 59 atomic energy reactors for power generation, accounting for 78% of its total power generation;Japan has built 54, and the atomic power generation accounts for 25% of its total power generation;104 buildings have been built in the United States, and the atomic power generation accounts for 20% of its total power generation;Russia has built 29, and the atomic energy power generation accounts for 15% of its total power generation.China built its first atomic power plant in 1991. Including this one, there are currently nine nuclear power reactors in operation, with a total capacity of 6.6 million kilowatts.Another two reactors are under construction in China.Our country alsoPakistanBuild an atomic power station.
The first generation (GEN-I) nuclear power plant is an early prototype nuclear power plant, that is, light water reactors (LWR) nuclear power plants developed from 1950 to early 1960, such as Shippingport pressurized water reactor (PWR)Dresden(Dresden) boiling water reactor (BWR) and Magnox in the UKGraphite gas cooled reactorEtc.
The second generation (GEN - Ⅱ) nuclear power plant is a large-scale commercial nuclear power plant developed and constructed on the basis of the first generation nuclear power plant from the late 1960 to the early 1990, such as LWR (PWR, BWR), Canada Candu reactor (CANDU), Soviet pressurized water reactor VVER/RBMK, etc.Until 1998, most nuclear power plants in the world wereSecond generation nuclear power plant。
The third generation (GEN - Ⅲ) refers to advanced nuclear power plants that meet higher safety indicators, and the safety indicators are required to meet the requirements of URD.Third generation nuclear power plantAdopt standardized, optimized design and saferPassive safety system, such as advanced boiling water reactors (ABWR), system 80+, AP600, European pressurized reactor (EPR), etc.
The fourth generation (GEN - Ⅳ) is a nuclear power plant with higher safety to be developed. Its goal is to reach the practical level by 2030. Its main characteristics are high economy (comparable to natural gas thermal power plants), good safety, small waste generation, and the ability to prevent nuclear proliferation.
GIF held in Tokyo from September 19 to 20, 2002（International Forum on the Fourth Generation Nuclear Energy SystemAt the Generation IV International Forum (GIF) meeting, the 10 participating countries agreed to develop the following six fourth generation nuclear power plant concept reactor systems on the basis of 94 concept reactors.
Classification according to cooling mode
Gas cooled fast reactor
The gas cooled fast reactor (GFR) system is a fast neutron spectrum helium cooled reactor, which uses a closed fuel cycle. Composite ceramic fuel can be selected as the fuel.It uses direct cycle helium gas turbine for power generation or its process heat for thermochemical hydrogen production.Through comprehensive utilization of fast neutron spectrum and complete recycling of actinides, GFR can minimize the generation of long-lived radioactive waste.In addition, its fast neutron spectrum can also make use of existing fissile materials andConvertible materials(including depleted uranium).The reference reactor is a 288 MW helium cooling system with an outlet temperature of 850 ℃.
Liquid metal cooled fast reactor
The lead alloy liquid metal cooled fast reactor (LFR) system is a fast neutron spectrum lead (lead/bismuth eutectic) liquid metal cooling reactor, which uses a closed fuel cycle to achieve effective conversion of convertible uranium and control actinides.The fuel contains convertible uranium andTransuranic elementMetal or nitride.
The LFR system is characterized byPower plantFor example, LFR system can be a large overall power plant of 1200 MW, or a combination of module system with rated power of 300~400 MW and a 50~100 MW with a long refueling interval (15~20 years).LFR is a turnkey power plant manufactured by a small factory, which can meet the demand for small grid power generation in the market.
The liquid cooled fast reactor (SFR) system is a fast neutron spectrum sodium cooled reactor. It uses a closed fuel cycle that can effectively control the conversion of actinides and convertible uranium.SFR system is mainly used to manage highRadioactive wasteEspecially in the management of plutonium and other actinides.The system has two main schemes: medium scale nuclear power plant, namely, 150~500 MW power, and uranium plutonium fuel-minor actinides -Zirconium alloy;Medium to large-scale nuclear power plants, that is, 500~1500 MW, use uranium plutonium oxide fuel.
The system has good safety performance due to its long thermal response time, large margin for coolant boiling, primary system operating at near atmospheric pressure, and intermediate sodium system between the radioactive sodium in the circuit and the water and steam in the power plant.
Molten salt stack system
Molten salt reactor (MSR) system is a super thermal neutron spectrum reactor, and the fuel is a circulating liquid mixture of sodium, zirconium and uranium fluoride.Molten salt fuel flows through the graphite channel in the core and generates an epithermal neutron spectrum.The liquid fuel of MSR system does not need to manufacture fuel elements, and it is allowed to add actinides such as plutonium.Actinides and most fission products form fluoride in liquid coolant.Molten fluoride has good heat transfer characteristics and can reduce the pressure on pressure vessels and pipes.The power level of the reference power station is 1000 MW, the coolant outlet temperature is 700~800 ℃, and the thermal efficiency is high.
Cold reactor system
The very high temperature reactor (VHTR) system is a graphite moderated helium cooled reactor with a once through uranium fuel cycle.Thisreactor coreIt can be prismatic block core (such as Japan's high-temperature engineering test reactor HTTR) or spherical bed core (such as China's high-temperature gas cooled test reactor HTR-10).
The VHTR (Ultra High Temperature Gas Cooled Reactor) system provides heat. The core outlet temperature is 1000 ℃, which can produce hydrogen or process heat for petrochemical or other industries.Power generation equipment can also be added to the system to meet the needs of cogeneration.In addition, the system has flexibility in using the uranium/plutonium fuel cycle to minimize waste.The reference reactor uses a 600 MW core.
Supercritical water cooled reactor
Supercritical water cooled reactor (SCWR) system is a high-temperature and high-pressure water cooled reactor, which operates above the thermodynamic critical point of water (374 ℃, 22.1 MPa).Supercritical water coolant can increase the thermal efficiency to about 1.3 times that of light water reactor.The characteristic of this system is that the coolant does not change its state in the reactor and is directly connected to the energy conversion equipment, so the auxiliary equipment of the power plant can be greatly simplified.The fuel is uranium oxide.There are two schemes for core design, namely thermal neutron spectrum and fast neutron spectrum.Reference systemThe power is 1700 MW, the operating pressure is 25 MPa, and the reactor outlet temperature is 510~550 ℃.
Composition
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overview
There are many types of reactors, but they are mainly composed of active zone, reflector, external pressure shell and shielding layer.The active region consists of nuclear fuel, moderator,coolantandcontrol rodEtc.In reactors currently used in atomic power stations,PWRIs the most competitive reactor type (about 61%),Boiling water reactorAccounting for a certain proportion (about 24%),Heavy water reactorLess (about 5%).
PWR
The main features of PWR are:
1) Use cheap and widely available ordinary water as moderator and coolant,
2) In order to keep the cooling water with high temperature in the reactor liquid, the reactor operates under high pressure (water pressure is about 15.5MPa), so it is called pressurized water reactor;
3) Since the water in the reactor is in liquid state, the turbine is drivenGenerator SetThe steam of must be generated outside the reactor;This is achieved bySteam generatorIt is realized that the cooling water from the reactor, that is, the primary circuit water flows into one side of the heat transfer tube of the steam generator to transfer heat to the secondary circuit water on the other side of the heat transfer tube, so that the latter can be converted into steam (the secondary circuit steam pressure is 6-7MPa, and the average steam temperature is 310 ℃, taking Daya Bay Nuclear Power Plant as an example);
4) Since ordinary water is used as moderator and coolantNeutron absorptionBecause of its large cross section, it is impossible to use natural uranium as nuclear fuel. Enriched uranium (the content of uranium-235 is 2-4%) must be used as nuclear fuel.BWR and PWR belong to the same categoryLight water reactorLike the PWR, it also uses ordinary water as the moderator and coolant. The difference is that steam is generated in the BWR (the pressure is about 7MPa) and directly enters the gas turbine for power generation. No steam generator is required, and there is no distinction between the primary circuit and the secondary circuit. The system is particularly simple, and the working pressure is lower than that of the PWR.However, the steam of boiling water reactor is radioactive, so shielding measures should be taken to prevent radioactive leakage.The heavy water reactor uses heavy water as moderator and coolant. Because its thermal neutron absorption cross section is much smaller than that of ordinary water, natural uranium can be used as the nuclear fuel of the heavy water reactor.The so-called thermal neutrons refer to the fast neutrons emitted during the fission of uranium-235 atomic nucleus, whose velocity is slowed down to 2200m/s and energy is about 1/40eV.The possibility of the fission of uranium-235 caused by thermal neutrons is 190 times greater than the possibility of being captured by uranium-238 nuclei.In this way, the nuclear fission chain reaction can continue in the heavy water reactor with natural uranium as fuel.Since the moderated neutron of heavy water is not as effective as that of ordinary water, the core of heavy water reactor is much larger than that of light water reactor, which makes it difficult to manufacture pressure vessels.The heavy water reactor still needs to be equipped with a steam generator. The heavy water in the primary circuit brings heat to the steam generator and transfers it to the ordinary water in the secondary circuit to generate steam.The biggest advantage of the heavy water reactor is that it uses natural uranium as nuclear fuel instead of enriched uranium, but one of the important reasons hindering its development is that it is difficult to obtain heavy water, because heavy water in natural water only accounts for 1/6500.
Moderator
Moderator
Neutrons released by fission reaction of nuclear fuel areFast neutron, while in thermal neutron orIntermediate energy neutronSlow down neutrons should be used in the reactor to maintain the chain reaction,ModeratorIt is used to reduce the energy of fast neutrons and slow them down to neutrons or intermediate energy neutrons[11]。There are many different requirements to consider when selecting moderators.The first is the core characteristics: good slowing performance and as low as possibleNeutron absorptionsection;The second is pricemechanical propertiesandIrradiation sensitivity。Sometimes moderators are used as coolants. If not, they are closely related in design.The most widely used solid moderator is graphite, which has the advantages of good moderating performance and machining performance, small neutron capture cross section and low price.Graphite is one of the two moderators that can use natural uranium as fuel;The other is heavy water.Other moderators must use enriched nuclear fuel.From the perspective of nuclear characteristics, heavy water is a better moderator, and because it is liquid, it can also be used as coolant. The main disadvantage is that the price is expensive, and the system design needs strict sealing requirements.Light water is the most widely used moderator. Although its moderating performance is not as good as that of heavy water, it is cheap.heavy waterIt has a common disadvantage with light water, that is, radiation decomposition, hydrogen and oxygen accumulation and recombination.
control rod
control rod
It plays the role of compensating and regulating neutron reactivity and scram in the reactor[12]。makecontrol rodThe thermal neutron absorption cross section is large, but the scattering cross section is small.Good control rod materials (e.gcadmium, silver, indium, etc.) still have large thermal neutron absorption cross section after neutron absorption, so the service life is very long.The control rod materials commonly used in nuclear power plants are boron steel, silver indium cadmium alloy, etc.Among them, boron containing materials are widely used due to their rich resources and low price, but they are prone to radiation embrittlement and size change (swelling).Silver indium cadmium alloy is the main control material of light water reactor because of its large thermal neutron absorption cross section.The PWR adopts rod bundle control, and the control material is made into rod shape. Each rod bundle consists of 24 control rods, which are evenly distributed among 17 × 17 fuel assemblies.The nuclear power plant adjusts the depth of the control rod inserted into the fuel assembly through a special drive mechanism to control the reactivity of the reactor. In an emergency, the control rod is used to shut down the reactor (at this time, the control rod material absorbs a large amount of thermal neutrons, making the self-sustaining chain reaction unable to maintain and stopping).
coolant
coolant
Driven by the main circulating pump, circulating in the primary circuit[13]The heat is taken away from the core and transferred to the working medium in the secondary circuit, so that the steam generator can generate high-temperature and high-pressure steam to driveturbogeneratorelectricity generation.coolantIt is the only reactor component that works both in the core and outside the reactor, which requires that the coolant must be stable in high temperature and high neutron flux fields.In addition, most suitable fluids and the impurities they contain will be radioactive under neutron irradiation, so the coolant should be contained in radiation resistant materials and shielded with materials with good ray blocking ability.The ideal coolant should have excellent moderator nuclear characteristics, large heat transfer coefficient and heat capacity, oxidation resistance and no high radioactivity.Liquid sodium (mainly used in fast reactor) andSodium potassium alloy(Mainly used for space power reactor) has large heat capacity and good heat transfer performance.Light water has advantages in price, treatment, oxidation resistance and activation, but its thermal characteristics are not good.Heavy water is a good coolant and moderator, but it is expensive.Gas coolant (such as carbon dioxide and helium) has many advantages, but it is required thatLiquid coolantHigher circulating pump power and higher system sealing requirements.Organic coolantThe outstanding advantage is that the activation activity in the reactor is low, because the neutron capture cross section of all organic coolants is low, and the main disadvantage is that the radiation decomposition rate is high.Most widely usedPWR nuclear power plantUse light water as coolant and moderator.
Shielding layer
For protectionneutron、Gamma rayandthermal radiationShielding must be provided around the reactor and most auxiliary equipment.Its design should strive for low cost and space saving.For gamma ray shielding, steel, lead and ordinaryconcreteandHeavy concrete。The strength of steel is the best, but the price is higher;The advantage of lead is high density, so the thickness of lead shield is small;Concrete is cheaper than metal, but its density is smaller, so the thickness of shielding layer is larger than other materials[9]。
The intensity of gamma rays from the reactor is very high, which will generate heat after being absorbed by the shield, so the gamma ray shielding layer close to the reactor is often equipped withCooling water pipe。There is a thermal shield between the core and the pressure shell of some reactors to reduce the radiation damage of the pressure shell caused by neutrons and the heating of the pressure shell caused by radiation.
Neutron shielding requires materials with large neutron capture cross-section elements, usually containing boron, and sometimes concentratedBoron-10。Some shielding materials emit gamma rays after capturing neutrons, so there should be a layer of gamma ray shielding outside the neutron shielding.Generally, when designing the outermost shielding, the radiation should be reduced below the allowable dose level for humans, which is often calledBiological shielding。The outermost shielding of nuclear power plant reactor is generally selectedordinary concrete Or heavy concrete.
Traveling wave reactor
nuclear powerTraveling wave reactorThe name ofTraveling wave tubeBut the nature of physics is quite different.The traveling wave tube uses the electron beam emitted by the electron gun to transmit energy to the microwave transmitted in the same direction in the focusing system, so as to amplify the microwave signal.The nuclear traveling wave reactor is a fast neutron bombardment produced by the fission of a small amount of high concentration uranium 235 at the starting endDepleted uranium(almost exclusively uranium 238) to produce plutonium 239.Plutonium 239, after capturing neutrons, fissioned to produce as many as 300 kinds of intermediate mass atoms, and produced an average of 2.5 neutrons and 200 million electron volts of energy.fissionIt can be absorbed by liquid metal sodium or other heat carrying media to generate electricity, and the newly generated neutrons will keep the nuclear reaction in the core moving forward until the whole core is "burned".So the traveling wave reactor is named[10]。
main features
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Nuclear power has an important advantage - very clean.Compared with thermal power plants, nuclear power plants are simply the ultimate in terms of environmental protection.Thermal power plants release more radioactive substances into the atmosphere than nuclear power plants, and they also release a large amount of carbon, sulfur and other elements into the atmosphere.
Unfortunately, there are also some serious problems in the operation of nuclear power plants:
Uranium mining and purification is not a very clean process.
Abnormal nuclear power plants can cause major problems.ChernobylDisaster is a recent example;On March 12, 2011, the earthquake caused JapanFukushima PrefectureNuclear leakage occurred at the first and second nuclear power plants.
Spent fuel of nuclear power plant[14]They have been toxic for hundreds of years, and there are no facilities in the world that can safely and permanently store them.
Transportation of nuclear fuel to and from nuclear power plants poses some risks, but so far, no such accident has occurred in the United States.
To a large extent, these problems have caused the attempt to build new nuclear power plants in the United States to deviate from the normal track.Because the society seems to generally believe that the risk of building nuclear power plants exceeds the return.
advantage
1) Energy is highly concentrated, fuel cost is low, and comprehensive economic benefits are good.One kilogram of uranium-235 or plutonium-239 provides energy equivalent to 2300 tons of anthracite in theory.In the current practical application, 1 kg of natural uranium can replace 20-30 tons of coal.Although the one-time infrastructure investment of atomic power generation is large, the cost of nuclear fuel is much cheaper than that of coal and oil.Therefore, the total cost of atomic power generation is lower than that of conventional power generation.
2) Due to the small amount of fuel required, it is not limited by transportation and storage.For example, a conventional power plant of 1 million kilowatts needs to burn 3 million tons of coal a year, and on average, a 10000 ton ship is needed every day to transport coal.However, using atomic energy to generate electricity only requires 30 tons of nuclear fuel a year.
3) The pollution environment is relatively light.Atomic power generation does not emit CO, SO2, NOX and other harmful gases and solid particles, nor does it emit greenhouse gasescarbon dioxide。The daily emission of radioactive waste gas and liquid waste from the atomic power station is very small, and the radiation dose to the surrounding residents is less than 1% from the natural background.The probability of serious accidents that release a large amount of radioactive materials is extremely low. In the 10000 reactor year operation history of the world, only one accident that affects the outside of the plant has occurredchernobyl accident It is caused by operators' illegal operation and reactor design defects (lack of necessary safety barriers).You may have heard of the accident at the Three Mile Island Nuclear Power Station in the United States. This accident was caused by human negligence and equipment failure.Because the reactor has several safety barriers, no one died in the accident, and the average radiation dose received by 2 million people within 80 km is less than that received by wearing a luminous meter for a year.
Security
nuclear reactor
The reactor will not explode for at least three reasons:
1) More than 90% of the nuclear fuel used in atomic bombs is fissile uranium-235, while only 2-4% of the nuclear fuel used in power reactors is fissile uranium-235;
2) The reactor is equipped with control rods made of materials easy to absorb neutrons, and the speed of nuclear fission reaction is controlled by adjusting the position of control rods;
3) The coolant constantly brings out the huge heat generated by the fission reaction of the reactor core, so that the temperature in the reactor is controlled within the required range.
Some people may also want to ask why some countries do not easily transfer atomic power generation technology?This is because while the reactor is used for power generation, a certain amount of plutonium-239 is also produced in the reactor (except that most neutrons bombard the uranium-235 nucleus to make it fissionable, there is still a part of neutrons captured by the uranium-238 nucleus to make the latter become plutonium-239. About 50% of the plutonium-239 generated in the reactor is then bombarded by neutrons to make it fissionable, releasing energy and making nuclear fuel proliferate;Less than 50% of the remaining plutonium-239 remains in the reactor.),After reprocessing, plutonium-239 can be extracted and used to make atomic bombs.The plutonium-239 produced by the heavy water reactor is about twice that of the pressurized water reactor.
application area
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Nuclear fission releases a lot of energy and neutrons.Nuclear reactors have many uses, but to sum up, one is to use fission nuclear energy, the other is to useFission neutron。Nuclear energy is mainly used for power generation, but it is also widely used in other fields.for exampleNuclear heating, nuclear power, etc.
Nuclear heating is a new technology developed in the 1980s. It is an economic, safe and clean heat source, and has been widely valued in the world.In terms of energy structure, the heat source used for low temperature (such as heating) accounts for about half of the total heat consumption. Most of this heat is obtained directly from coal combustion, which causes serious environmental pollution.In China's energy structure, nearly 70% of the energy is consumed in the form of thermal energy, and about 60% of it is low-temperature thermal energy below 120 ℃. Therefore, the development of nuclear reactor low-temperature heating is of great significance in easing the supply and transportation tension, purifying the environment, reducing pollution and other aspects.Nuclear heating is a promising way of nuclear energy utilization.Nuclear heating can be used not only for residential heating in winter, but also for industrial heating.especiallyHigh temperature gas cooled reactorIt can provide high temperature heat source and can be used in coal gasification, ironmaking and other heat consuming industries.Since nuclear energy can be used for heating, it must also be used for cooling.Tsinghua UniversitySuccessful tests have been carried out on a 5MW low-temperature heating reactor.Another potential big use of nuclear heating is seawater desalination.Among various desalination schemes, nuclear heating is the most economical one.In the Middle East and North Africa, due to the lack of fresh water, seawater desalination is in great demand.
Nuclear energy is also a power with unique advantages.Because it does not need air to support combustion, it can be used as a special power for underground, underwater and space without air environment;And because it consumes less material and has high energy, it is a special power that can supply energy for a long time after one loading.For example, it can be used as a special power for rockets, spaceships, artificial satellites, submarines, aircraft carriers, etc.Nuclear power may be used for interstellar navigation in the future.At present, human space exploration is still limited to the solar system, so the aircraft needs little energy, and solar cells can be used.If you want to explore other galaxies outside the solar system, nuclear power is probably the only option.United States, Russia and other countries - always engaged inNuclear powered satelliteThe purpose of the research and development is to install power generation equipment with power generation capacity of hundreds of kilowatts on satellites.With high-power power supply, the power of satellites in communication, military and other aspects will be greatly enhanced.The Cassini nuclear powered space exploration spacecraft launched by NASA on October 15, 1997, will fly to Saturn for seven years, covering a long journey of 3.5 billion kilometers.
nuclear reactor
Nuclear propulsion[15], mainly fornuclear-powered submarine, nuclear aircraft carriers and nuclear icebreakers.Due to the high energy density of nuclear energy, it only needs a small amount of nuclear fuel to run for a long time, which has great military advantages.in especialNuclear fission energyNo oxygen is needed for the production of the, so the nuclear submarine can sail underwater for a long time.Because nuclear power propulsion has such great advantages, the number of nuclear reactors that have been manufactured for ship propulsion in the world over the past decades has reached hundreds, exceeding the number of reactors in nuclear power plants (of course, its power is far less than that of nuclear power plant reactors).At present, nuclear aircraft carriers, nuclear destroyers, nuclear cruisers and nuclear submarines have formed a powerful maritime nuclear force.
nuclear reactor
The second major use of nuclear reactors is to use a large number of neutrons released in chain fission reactions.There are many uses in this regard. We will just give a few examples here.We know that if the nucleus of many stable elements absorbs another neutron, it will become a radioisotope.Therefore, reactors can be used to mass produce various radioisotopes.The wide use of radioisotopes in industry, agriculture and medicine is almost universally known.In addition, the current industry, medicine and scientific research often need a film with extremely small holes to filter and remove extremely small impurities or bacteria in the solution.Bombard with neutrons in a reactorFilm materialExtremely small holes can be generated to meet the above technical requirements.The neutron in the reactor can also be used to produce high-quality semiconductor materials.We know that a small amount of other materials must be doped into monocrystalline silicon to become a semiconductor, such as phosphorus.Generally, the method of diffusion is used to let phosphorus vapor infiltrate through the surface of silicon wafer in the furnace.However, the effect of this is not ideal. The phosphorus concentration in silicon is uneven, and the concentration inside becomes lower when the surface concentration is higher.Neutron doping technology is currently available.Put monocrystalline silicon in the reactor to be irradiated by neutrons. After silicon captures a neutron, it will become phosphorus after decay.Since neutrons are uncharged and easily enter the interior of silicon wafer, the silicon semiconductor produced by this method has excellent properties.Cancer can be treated with neutrons produced by reactors.Because many cancer tissues have more absorption of boron, and boron has a strong ability to absorb neutrons.After boron is absorbed by cancer tissue, after neutron irradiation, boron will become lithium and emit alpha rays.Alpha rays can effectively kill cancer cells, and the therapeutic effect is much better than that of external gamma rays.Neutrons in the reactor can also be usedNeutron radiographyOr neutron imaging.Neutron is easy to be scattered by light substances, so neutron radiography is particularly effective for detecting light substances (such as explosives, drugs, etc.). If X-ray or ultrasonic imaging is used, it cannot be detected.
matters needing attention
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Critical state
When uranium-235 atoms split, two or three neutrons will be released (depending on the way of splitting).If there is no uranium-235 atom nearby, these neutrons will fly away in the form of neutron rays.If the uranium-235 atom is part of a piece of uranium - there are other uranium atoms nearby - then the following three situations will occur:
·If, on average, one free neutron hits another uranium-235 nucleus and causes it to undergo fission each time, then the mass of this uranium is considered critical.Its mass will maintain a stable temperature.The nuclear reactor must be maintained in a critical state.
·If, on average, there is less than one free neutron hitting another uranium-235 atom, then this mass is subcritical.Eventually, the induced fission of matter will end.
·If, on average, more than one free neutron hits another uranium-235 atom, then the mass of this uranium is supercritical.Uranium will heat up.
aboutNuclear bombIts designers require that the quality of uranium far exceedsSupercritical massIn this way, all uranium-235 in the fuel block can undergo fission in a very short time.In nuclear reactors, the reactor core needs to be slightly supercritical, so that staff can control the temperature of the reactor.Workers absorb free neutrons by operating control rods to maintain the reactor at a critical level.
The critical condition of uranium is determined by the content (enrichment level) of uranium-235 in the fuel and the shape of the fuel block.It is conceivable that if the fuel is thin and flaky, most free neutrons will fly out rather than hit other uranium-235 atoms.A sphere is the best shape.The amount of uranium-235 gathered together in spherical shape to realize critical reaction is about 0.9 kg.This quantity is therefore calledCritical mass。The critical mass of plutonium-239 is about 283 g.
Development prospect
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sustainable development
Through the effective use of nuclear fuel, to achieve the means of providing sustainable energy production;Minimize the amount of nuclear waste, strengthen management, reduce long-term management affairs, ensure public health and protect the environment.[16]
Improve security
Ensure higher safety and reliability;Greatly reduce the probability and degree of core damage, and have the ability to quickly restore reactor operation;Cancel the necessity of taking emergency measures outside the plant site.
Improve economy
Power generation costSuperior to other energy sources;The risk level of capital can be compared with other energy sources.
Prevention of nuclear proliferation
Using the characteristics of the reactor system itselfnuclear fuel cycle Materials that have passed the treatment in are more resistant to nuclear proliferation and are difficult to be usednuclear weaponOr is stolen;In order to evaluate the nuclear nonproliferability of nuclear energy, DOE is developing a quantitative evaluation method for preventing nuclear proliferation for the fourth generation nuclear power plants.
Huaneng Shandong Shidao Bay Nuclear Power Plant High Temperature Gas cooled Reactor Nuclear Power Plant Demonstration Project has been constructed since 2008, and the project will be put into operation in November 2013.This is the first nuclear power plant in China to use the fourth generation nuclear reactor, which uses the fourth generation high-temperature gas cooled graphitePebble bed reactor, referred to as pebble bed pile.[17]
Fourth generation reactorAmong the six configurations of, the high-temperature gas cooled reactor is a very promising scheme. The current high-temperature gas cooled reactor has two schools: graphite pebble bed and cylindrical fuel. The former is used by China andSouth AfricaThe latter are the United States, Russia and Japan.[18]
The graphite pebble bed pile is also called pebble pile. It was first built in Germany in the 1960s. Due to the limitations of technology and demand, there was no major development in 30 years. Until the 1990s, the pressure of the international energy crisis became increasingly serious. South Africa and China began to study and explore the modernization and practicality of this technology,PBMR (400MW thermal power) designed by South Africa's state-owned power design and China Institute of Atomic Energy TechnologyHTR-PM（460MW）。Both designs have been basically completed, during which China has completed the construction and operation of Tsinghua University's 10MW principle reactor (HTR-10), and HTR-10 has been connected to the grid for a long time.
On November 5, 2018, the first nuclear reactor project for scientific research in Saudi Arabia was started.[8]
France
On November 27, 2018, French President Makron said that by 2035, 14 of the 58 nuclear reactors operated in France will be closed, of which 4 to 6 will be closed by 2030.[19]
Belgium
On September 23, 2022 local time, the Belgian nuclear power operator Engie Group said that Belgium would officially close the "Doel 3" nuclear reactor later that day.Belgium now has two nuclear power plants and a total of seven nuclear reactors, which will be the first time that the country has terminated the operation of nuclear reactors.[21]
Accident record
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1979 Three Mile Island Nuclear Power Plant Accident
On March 28, 1979, a serious accident occurred at the Three Mile Island Nuclear Power Plant in the United States, where part of the reactor core melted and collapsed.However, due to the containment effect of the pressure boundary of the primary circuit and the containment, the radionuclides leaked into the surrounding environment are very small, and there is no harm to the environment and public health. Only three plant workers are exposed to radiation slightly higher than the quarterly dose management limit.Among the 2 million residents with an area of 80 kilometers, the average radiation dose received by each person is less than that received by wearing a night light meter or watching a color TV set for a year.
1986 Chernobyl nuclear power plant accident
On April 26, 1986, the Chernobyl nuclear power plant accident occurred in the former Soviet Union.This is the most serious nuclear accident in the history of peaceful use of nuclear energy.Among the nuclear power plant staff and accident rescue personnel, 28 people died due to very high radiation doses.In order to prevent the public from large radiation exposure, 116000 residents near the power station were evacuated.There are two main reasons for the accident.First, in the process of testing how long the generator rotor can continue to supply power by its own rotational inertia under power failure conditions, the operator seriously violated the operating procedures and cut off all safety control systems, resulting in the failure of the safety protection system to start.Second, there are serious defects in the safety design of the reactor (pressure tube graphite moderated boiling water reactor).[22]
Natural nuclear reactor
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The discovery of prehistoric natural nuclear reactors,
An examination of the ore extracted from a mine in Gabon found that it contained U-235, a fissile type, with a lower abundance.
Today all natural uranium contains 0.720% uranium-235.This is the case whether uranium is extracted from the crust, lunar rocks or meteorites.But the rock in Auckland is only 0.717%.
After supplementary analysis, Perrin and his colleagues confirmed that the uranium ore is completely natural.They found traces of fission products in ores.The conclusion is that uranium ore is natural and has experienced fission.There is only one possible explanation - rocks are evidence of natural fission that occurred more than 2 billion years ago.
Peter Woods, head of the IAEA's uranium production team, said: "Just like the artificial light water nuclear reactor, if nothing can slow down the speed of neutrons, the fission reaction can only be stopped"."Water acts as a moderator in the rock of Oklo, absorbing neutrons and controlling chain reaction." The specific geological background of Gabon today also provides help.The total chemical content of uranium (including U-235) is high enough, and individual deposits are thick and large enough.Ocelor has successfully survived the passage of time.Experts suspect that there may be other such natural reactors in the world, but these reactors must have been damaged, eroded or submerged by geological processes, or have not been found at all.[23]
