On March 12, 2011, a nuclear leak occurred at the Fukushima Daiichi and Dai-ichi nuclear power plants as a result of a massive earthquake measuring 9.0 on the Richter scale. As a result, the Japanese government made an emergency decision to evacuate residents in a 10-kilometer radius, and as of March 13, the Japanese government has expanded the evacuation area to 20 kilometers.
The Atomic Energy Safety and Security Agency (AESA) said in a statement that radiation levels in the central control room of Unit 1 at Tokyo Electric Power Co.'s Fukushima Daiichi nuclear power plant, which was automatically shut down by the March 11 earthquake, had reached 1,000 times the normal value. The latest bulletin said radiation levels near the plant's main gate continued to rise, reaching more than 70 times the normal level at 9:10 a.m. on March 12, the company said.
This is the first time Japanese authorities have confirmed that radioactive material from a nuclear power plant has leaked to the outside. On the morning of March 12, all residents of Futaba, Okuma and Tomioka towns in the vicinity of the first and second nuclear power plants belonging to Tokyo Electric Power Co. in Fukushima Prefecture, Japan, began evacuating outside the designated danger zones, totaling about 20,000 people.
The Atomic Energy Safety and Security Agency (AESA) has ordered Tokyo Electric Power Co. to release steam from the reactor vessels at Units 1 and 2 of the Fukushima Daiichi plant to the outside in order to prevent a rise in air pressure inside the vessels housing the nuclear reactors, which would make them unable to withstand the pressure and break.
On the morning of March 15, an explosion occurred at Unit 2 of Japan's Fukushima Daiichi nuclear power plant, damaging the pressure control pool. The winds are now facing north for the day, with winds blowing inland from the Pacific Ocean, and are estimated to have a more severe impact on Japan, according to Japanese television station NHK.
Tokyo Electric Power Co. said on the morning of the 16th, 16 5:45 (4:45 GMT) Fukushima Daiichi nuclear power plant suffered another fire that morning. The company also confirmed that two nuclear power plant staff members are unaccounted for and have yet to be found.
According to the latest news from the **** same news agency on the 16th, Tokyo Electric Power Co. revealed that it is presumed that as of 3:30 p.m. (local time) on the 15th, 70% of the fuel in unit 1 of the Fukushima Daiichi nuclear power plant was damaged, and 33% of unit 2 was damaged.
The U.S. media reportedly pointed to the Japanese government's confusing and inconsistent distribution of information in the early stages of the nuclear crisis, as well as its lack of coordination. Some in the nuclear energy industry believe that the authorities in preventing nuclear fuel meltdown "has nearly lost control". The Tokyo Electric Power Company (TEPCO) is preparing to release steam from Unit 1, the most serious of the three reactors at the Fukushima Daiichi plant, first. TEPCO will do the same for units 2 and 3 if the cooling reactors are unable to resume functioning as soon as possible.
TEPCO noted that steam from the reactor vessel at Unit 1 of the Fukushima Daiichi plant will be passed through a huge pool of water before being released through the vent. When passing through the water, the radioactivity will be reduced to a certain extent, while the staff will always observe the amount of radioactive material at the exit of the exhaust pipe.
In addition, the Fukushima Daiichi plant has lost its cooling function, and Tokyo Electric Power Co. has begun releasing steam from the reactor vessels at Fukushima Daiichi Units 1 and 2 to reduce vessel pressure and prevent a larger breakout. The company is also preparing to release steam from two other reactors at the plant to the outside.
This is the first time Japan has taken the emergency hedge measure of opening valves at a nuclear power plant to release steam to the outside. Although the move could also lead to leakage of radioactive material into the outside environment, it would prevent the plant from losing its containment function due to vessel breakage. Japan's Minister of Economy, Trade and Industry, Mari Kaieda, said that according to a prior assessment, even if radioactive material is released, it will be in trace amounts. The security agency noted that it would be able to ensure the safety of residents as the government has decided to expand the evacuation area and the wind is blowing towards the sea. Preliminary analysis of the accident at Japan's Fukushima nuclear power plant
0 Background of the accident
On the afternoon of March 11, 2011, a massive earthquake measuring 9.0 on the Richter scale struck off the eastern coast of Japan and triggered a tsunami. The Fukushima Daiichi nuclear power plant, located on the eastern coast of Japan's Honshu Island, was shut down and several units suffered loss of cooling, with Unit 1 exploding on the afternoon of March 12, and Unit 3 exploding twice on March 14th. Japan's Ministry of Economy, Trade and Industry (METI) Atomic Safety and Security Agency (ASSA) acknowledged that radioactive material had leaked into the atmosphere, and residents within a few kilometers were evacuated (the evacuation area has been expanding).
1 Overview of the Fukushima Nuclear Power Plant in Japan
The Fukushima Daiichi Nuclear Power Plant (Fukushima Daiichi Atomic Power Plant) in Japan is located off the coast of Okuma-cho, Futaba-gun, Fukushima Prefecture. Fukushima Daiichi has six units, Unit 1, 439 MW, is a BWR-3 type unit, which was connected to the grid in the second half of 1970 and put into commercial operation in 1971; Units 2 to 5 are BWR-4 type, 784 MW, which were put into operation from 1974 to 1978; and Unit 6 is BWR-5 type, 1067 MW, which was put into operation in 1979. The six units are at the same site, all boiling water reactors, and all are owned by Tokyo Electric Power Company.
(The above account may seem like a list of data, but it sets the stage for the accident in the first place: Unit 1 had been in operation for 40 years, and it was time to retire.)
The map shows units 1 through 4 from right to left, with units 5 and 6 a little farther to the north.
Another Fukushima second nuclear power plant, the two days of the explosion is the Fukushima first nuclear power plant, not related to the second nuclear power plant, not table.
2 Boiling Water Reactor Preparatory Knowledge
Considering that there are only Pressurized Water Reactors (PWRs) and Heavy Water Reactors (CANDUs) on the Chinese mainland (note that it is the Chinese mainland, and Taiwan's is a Boiling Water Reactor (BWR), and Taiwan's under-construction Lungmen plant is a little bit more advanced ABWR), a brief introduction to Boiling Water Reactors (BWRs) is in order here.
Boiling water reactors and pressurized water reactors are both light water reactors, and both rely on H2O as a moderator and coolant. Both use low-enriched uranium as fuel. Of the more than 400 nuclear power units in the world today, more than two hundred are pressurized water reactors and nearly one hundred are boiling water reactors.
The following diagram shows the schematic of Unit 1 of the Fukushima I nuclear power plant: the basic operating process of a boiling water reactor:
Feedwater from the turbine system (dark blue tube) enters the reactor pressure vessel, descends along the annular space between the core envelope and the vessel wall, enters the lower chamber of the pile under the action of the jet pumps (the start of the white arrows), and folds upward to flow through the core, where it is heated and partially Vaporization. After the steam-water mixture is separated by a steam-water separator (the process of steam-water separation is similar to that of a pressurized water reactor steam generator), the steam (light blue piping) goes to a turbine generator (several yellow blocks are the high-pressure cylinders, three low-pressure cylinders, and the generator, the same as in the AP1000), which does work to generate electricity. The steam pressure is about 7MPa, and the dryness is not less than 99.75%. Turbine spent steam is condensed, purified, heated and then sent to the reactor pressure vessel by the feed pump, forming a closed loop. The recirculation pumps (two pumps on both sides of the core) serve to form a forced circulation in the reactor, with the feed water taken from the bottom of the annular space, pressurized and then sent into the reactor vessel as the driving stream for the jet pump. The ABWR (Advanced BWR Advanced Boiling Water Reactor), currently developed by Hitachi and GE, replaces the recirculation pump and the ejector pump with an in-reactor circulation pump.
Similar to pressurized water reactors, boiling water reactors have several safety barriers: one, the fuel cladding, which is different from the zirconium-niobium alloy of the AP1000, who uses zirconium-2. two, the pressure vessel. This is the same as the pressurized water reactor. Three, the dry well, also called the first layer of containment. That is the black pear-shaped shell in the picture above.
There is also the square cement shell outside as the fourth boundary, in fact, the cement shell is only wind and rain, can play a small role, but not very big.
Compared with pressurized water reactors, boiling water reactors have the following characteristics:
1. The control rods are inserted from below the core
Because there is a vapor separator above the core, and the upper portion is steam-dominated, there is insufficient slowing down of neutrons. However, the problem is that the rods cannot be dropped by gravity after the loss of power as in the case of pressurized water reactors, and the expected transient accident probability of failing to shut down the reactor is increased, which requires a higher level of reliability for the control rod drive mechanism.
The control rods are electrically or mechanically driven in normal operation, and in the event of a power loss the control rods are jacked up by backup hydraulics. Each set of control rods, or two sets of control rods, has a separate hydraulic drive.
This is not the biggest feature of boiling water reactors, but it's necessary to list it first here. Because some analyses on the internet mention the inability to drop rods, etc., and there is no such thing as that. According to the news on the IAEA official web site, the reactor was automatically shut down at that time (All four units automatically shut down on March 11), there is no mention of control rod failure. And if the control rods did fail, there's no reason why the operators couldn't have injected boron water into it.
2. Boiling-water reactivity is not chemically compensated by boron
Pressurized-water reactors have a boric acid solution in the first circuit, but boiling-water reactors have fresh water flowing through the core.
Because it's normally fresh water, once boron water is injected, it can have a big impact on the future operation of the reactor (assuming, of course, that if the reactor survives this time unscathed.) , to put it more seriously, injecting boron water would basically render the reactor unusable as well. However, the advantage of injecting boron water is to ensure a high margin of shutdown while cooling. For example, AP1000, CMT (core makeup tank) boron concentration of 3400ppm, ACC (an injection box) 2600ppm, IRWST (built-in recharge water tank) 2600ppm, anyway, for pressurized water reactors, as long as needed after the accident, the first time to the core of the injection of concentrated boron water.
Actually, boiling water reactor nuclear power plants, in general, are stocked with boron water. When the accident occurred, the operator has two choices: one is to inject water, in case of lucky escape later can be used again, this is more conservative. The second is to inject boric acid, the reactor may not be able to be used again, but it can be cooled down better than water, and it can also ensure the margin of shutdown.
This feature set the stage for a second deterioration of the accident later.
3. Boiling water reactors normally operate in a boiling state
This statement is basically the equivalent of nonsense, of course boiling water reactors are in a boiling state.
But this also dictates that the accident conditions of a boiling water reactor are similar to normal conditions, whereas a pressurized water reactor normally operates in a subcooled state, with boiling occurring in the event of a water loss accident, which is much different from normal conditions.
This feature will make the operator take greater chances.
4. Unloading method is different from pressurized water reactor
Pressurized water reactor also has the problem of core overpressure. But for the second generation of pressurized water reactor, a circuit overpressure, can be introduced through the regulator top of the pilot-operated safety valve pressure relief box. The pressure relief tank is not very large and does not contain much water, but it is still inside the containment vessel. For AP1000, the overpressure of the first circuit is led to the atmosphere in the containment through the spring-loaded safety valve and bursting membrane on the top of the regulator, and the bursting valve of the fourth ADS stage is also led to the atmosphere in the containment. And if the first three levels of ADS action, it is to the built-in material exchange tank. In short, regardless of the second generation or AP1000, after depressurization, the radioactivity is still contained within the containment.
The boiling water reactors are different. Note the torus under the pear shape in the picture above, which is a tank with a volume of about 4000m³, equivalent to two large AP1000 built-in recharge tanks. But this chiropractic tank is not in the pressure boundary, when the pressure is removed, the steam passes directly through the two barriers of the pressure vessel and the dry well. For pollutants with long half-lives, this is almost equivalent to direct emission into the atmosphere. This feature set up a third deterioration of the accident that followed.
5. Boiling water reactors are highly economical
Boiling water reactors save investment by eliminating the need for pressure regulators and steam generators. At the same time, because the steam pressure can be higher than the pressurized water reactor, so the thermal efficiency is also higher. But this feature has nothing to do with accident analysis, purely as background knowledge. No table.
6. steam plant radiation
And not to mention the fission products, the light activation product N16 is enough to suffer. So pressurized water reactor operation into the containment = he killed, boiling water reactor operation into the turbine plant = suicide. Irrelevant to the accident, not tabulated.
Other preparatory knowledge:
1. About nuclear power plant diesel engines
The second generation of nuclear power plants, whether boiling water reactors or pressurized water reactors, have a problem. In the event of a severe accident accompanied by a plant-wide loss of power, emergency diesel engines are needed to start up quickly within 20 seconds to provide power to safety-related systems. The main one is the an injection system, which injects water into the reactor to ensure that the core is cooled and not exposed.
The reliance on the diesel engine laid the fourth seed for the accident.
2. On the sources of hydrogen in nuclear power plants
Generally speaking, there are the following sources of hydrogen in a nuclear power plant: ① Generator stator cores and rotor windings require hydrogen for cooling, but in the turbine plant. ② Hydrogen is added for one circuit to suppress the oxygen content. However, people with common sense know to put the hydrogen away from the pressure vessel. The hydrogen addition to the AP1000 chemical capacity system is placed in the auxiliary plant. ③ Hydrogen is produced when charging batteries, but the amount is relatively small. ④After the accident, the zirconium-water reaction between the exposed fuel cladding zirconium-2 and the steam will generate a relatively large amount of hydrogen.
This zirconium-water reaction laid the fifth ambush for the post-accident explosion. It can even be said to be the culprit.
3 How the Accident Happened and Worsened
1. On the afternoon of March 11, 2011, an earthquake struck, the control rods were inserted, and the reactor was safely shut down. The thermal power of the core dropped from its normal 1400 megawatts in a matter of minutes to only about 4 percent of the remaining heat, which was still falling but at a slower rate.
2. After stopping the reactor should ensure that the plant power source is not lost, by the an injection system to the core to replenish water to ensure that the core cooling to prevent overpressurization, but the earthquake destroyed the power grid, the power supply outside the plant is not available; emergency diesel engine is very competitive up to the core of the injection of fresh water. Note that it was fresh water, not boron water; in other words, the operators took a more conservative approach.
3. It didn't last long, the tsunami came, the diesel engine room was flooded, and the emergency diesel engine was unavailable. Fortunately, there were still batteries, albeit of smaller capacity, which did something to cool the pressure vessel for eight hours after the accident.
4. With the batteries looking like they were going to run out, there was good news and bad news: the good news was that trucks were arriving with mobile diesel engines, the bad news was that the diesel generators' interfaces were incompatible with those of a nuclear power plant! Core cooling is temporarily halted.
5. And in order to save the pressure vessel, it had to be depressurized to prevent it from overpressurizing and exploding. And the operators did. Thus, on March 12, the Japanese government admitted that radioactive iodine and cesium had been measured. On the one hand, it shows that the operator had started to unpressurize long ago, and on the other hand, it shows that there was already damage to the fuel cladding.
6. Tragically, early in the morning of the 12th, Naoto Kan was coming to inspect ......
Based on the preparatory knowledge just mentioned, the radioactivity in the environment would have risen if the pressure was unloaded, and even though Kan was inspecting the plant from the air, it still wasn't a good thing for the Japanese Prime Minister, who didn't have a protective suit on, so according to Japanese certain forums (not officially confirmed), the depressurization was temporarily suspended due to this inspection. But the residual heat waits for no one, and the temperature and pressure inside the containment are still rising.
7. After Kan left, the operator began to continue releasing the pressure inside the pressure vessel. At this point, the temperature inside the pressure vessel was about 550 degrees Celsius, and the core was already exposed and generating large amounts of hydrogen. Therefore, the steam containing hydrogen was discharged into the plant atmosphere by simple cooling and filtration of the pressure relief tank.
8. At about 3 p.m., with a loud bang, the roof of the reactor plant was completely destroyed by the explosion, leaving only the steel structure.
Before and after the explosion
The photo above shows a schematic of the reactor plant, with the reactor pressure vessel, still intact, in the tan center.
Slightly outside the circle of pressure type for the dry well, also called primary containment, after the explosion is also still intact, after all, is 15 centimeters thick stainless steel plus a meter thick cement. That means the third barrier is still intact.
Hydrogen gas exploded in the upper part of the plant, causing the concrete in the upper part of the plant, which is not very strong, to blow away completely, leaving only the steel structure.
9. And at this point, the reactor's cooling problems had still not been solved. Exactly what difficulties were encountered is not yet clear why.
After the explosion, fire pumps were used to inject seawater (with boron) directly into Unit 1, where the fuel meltdown occurred, for cooling. It's not clear exactly where the seawater was injected, but it's safe to say that Unit 1 can be stabilized as long as there are no new disasters. Although depressurization work may still have to be done, meaning that steam containing iodine 131 and cesium 137 will still have to be discharged to the outside world.
The accident at Unit 1 is over for now, but the crisis at Units 2 and 3 is still not over. Currently, there is also an explosion in Unit 3, with consequences similar to those of Unit 1. At 8 p.m. on the night of the 14th, the core of Unit 2 had been fully exposed to the water and was in a dry burn state.
4 Lessons from the Accident
1. Questions about what measures to take
Throughout the process, operators had been taking a more conservative approach to cooling. Although they had the opportunity, they did not inject boron water into the core until the explosion. On the one hand, they didn't want the reactor to be scrapped, and on the other hand, they were taking a chance on the reactor's ability to withstand the blast. Objectively, the operators were maximizing the protection of the reactor, but not maximizing the protection of the public.
Some say the accident was a man-made disaster in which TEPCO lost sight of its own interests, and in that sense it is not unreasonable.
2. On the issue of decommissioning years
By March 26 this year, Fukushima Daiichi nuclear power plant unit one will soon celebrate his 40th anniversary of commercial operation. Forty years would have meant the end of the plant's life, but TEPCO decided to extend Unit 1's life by 20 years in view of its economic interests. And ironically, the life extension approval was just received in February this year.
While the accident occurred within the 40-year lifespan and had nothing to do with the life extension, the accident is a wake-up call for nuclear power plants that are in the process of extending their lives or are about to do so. Because after all, Unit 1 has had a steady stream of accidents in recent years due to aging equipment issues.
3. On the issue of improving the cooling method of in-service nuclear power plants
Currently in service second-generation nuclear power plants, including the construction of the third-generation EPR and has been put into operation third-generation ABWR, after the accident, without exception, need emergency diesel engines to do security. The active nuclear power plants, including China's second generation plus, diesel engines are low arrangement, even put the fuel tank is still underground, mostly unable to withstand tsunami attacks. Not to mention the insulation of the cable after the sea water recedes, alone a diesel engine into the water is enough to headache.
And the unavailability of a diesel engine often means that the core is not far from overheating and overpressurizing. While it's completely technically impossible to convert all active plants to non-energized, it's possible to consider adding other cooling measures, or adding backup power.
4. Questions about radiation monitoring
I don't know if Vladivostok, which is separated from China by a mountain, has any radiation-monitoring stations, but it seems that Yanbian in Jilin, which is the closest in a straight line to China, and Mudanjiang in Heilongjiang do not. Changchun and Shenyang have them, but it seems a bit late if the big cities are monitored. With North Korea's nuclear power plant coming on line doesn't seem too far away, more radiation monitoring points at certain borders are still necessary.
5. On the issue of external rescue
Japan's nuclear power plant accident, although most of Japan's local nuclear power plants to take care of themselves, but the U.S. nuclear aircraft carriers played a relatively large role. At present, although China has many nuclear power plants, but there are many types of piles, and there is little communication between the companies that belong to them. If an accident occurs at a nuclear power plant, whether it is possible to organize an orderly and effective rescue of other nuclear power plants is still a relatively serious problem.
5 Follow-up Impact:
1. First of all, the impact of this accident on the world's nuclear energy industry will be quite far-reaching. The following is just a brief analysis at a lower level.
2. Anti-nuclear demonstrations increase in countries around the world. Resistance to the nuclear power development process (although it may not affect the pace of development in some countries).
3. As Lu Qizhou, a member of the National Committee of the Chinese People's Political Consultative Conference (CPPCC) and general manager of China Power Investment Corporation (CPIC), gave a graphic analogy to the AP1000 in front of the national media: "'Non-energy-driven' systems are like flush toilets, topped by a large tank, and do not rely on energy power. " It is to be expected that the AP1000 will be slightly more recognized by everyone.
4. The people's radiation protection ability is further strengthened. Iodine tablets and other anti-radiation medicines have become the regular medicines for some nuclear energy workers and their families.
5. The world's nuclear safety history has been rewritten. The Fukushima nuclear power plant will be printed in the new edition of nuclear power textbooks, along with Three Mile Island and Chernobyl.
6. The world's nuclear safety regulatory system is further strengthened, and the protection level of new nuclear power plants is further enhanced.