Nuclear Accident at Fukushima #1Nuclear Power Station

Transcription

1 Nuclear Accident at Fukushima #1Nuclear Power Station By Prof. Dr. Yoichi Fuji-ie, Representative Director of Nuclear Salon Fuji-ie 1. Brief Observation of Fukushima#1 Nuclear Accident On March , an unprecedented earthquake with the magnitude 9 occurred in the Tohoku and Kanto districts. It is reported on March 24 that about 10,000 people are confirmed to be dead and more than 17,000 people are missing. Details are still unclear now. The massive Tsunami associated with the earthquake expanded the disaster and made it difficult to grasp the damage. In Japan, all the nuclear power plants are locating on the coast to adopt a seawater cooling system. In Fukushima area, Tokyo Electric Power Company (TEPCO) has two nuclear power stations called Fukushima #1 with six Boiling Water Reactors (BWRs) as shown in Photo1, and Fukushima #2 with four BWRs which are about 250 km away from Tokyo. Following the earthquake of magnitude 9.0, a huge Tsunami with an estimated height of 14 meters or above rushed to the power station. Both were beyond the range expected and responded by the safety design of the nuclear power plants, whose design bases were magnitude 8.0 and wave height of 6m, respectively. As a result, with regard to the basic functions to ensure the reactor safety, i.e., "reactor shutdown, reactor cooling(decay heat removal) and containment of radioactive materials, the emergency reactor shutdown was successfully achieved and avoided any nuclear reactivity insertion. The emergency power supply systems were also activated, since another external power supply system failed because of the earthquake, and then the decay heat removal functions were well launched as well. However, 50minutes later, a huge Tsunami arrived at the Fukushima area and it led to the loss of decay heat removal functions due to failure of emergency power supply systems. Consequently, zirconium in the 1

2 cladding was supposed to be overheated and caused the reaction with water, which produced hydrogen gas to result in hydrogen explosion. This rapid hydrogen burning or explosion resulted in the destruction of the reactor buildings. In this stage, the most important thing is to ensure the decay heat removal, which is the most important to hold the radioactive materials in the fuel anyway. To maintain the safety of light water reactors, "decay heat removal" should be reminded as the first priority. Since the emergency power supply was unusable, the pool water temperature increased gradually. The pool sloshing due to the earthquake, in particular, contributed to a decrease in the water amount of the pool, and this could enhance the temperature increase afterwards. Certain amounts of fuels failed due to the loss of cooling and resulted in the hydrogen generation in the spent fuel(sf) pool, and the situation was similar in the reactor core. This accident also caused the release of radiation and radioactive materials. Therefore, "Evacuation Area" was laid out within 20 km from Fukushima #1 NPS and "Indoor Save Area" between 20 and 30 km from the NPS. Because of the fear of radioactivity and also the lack of articles necessary for daily life, many people and even a town itself in these area are moving the outside of the prefecture. Many foreign embassies call for return to home country or call for move to Kansai area and they are even preparing charter flights. However, such kind of behavior should not be blamed, although the spatial dose rate in Tokyo is even far smaller than that people can be exposed to during the flight. The Japanese government(embassy) would also take such actions if it might happen outside the country. On March 19, some kinds of vegetables and milk as well as tap water near the Fukushima(Tohoku) and Kanto districts are found to contain radioactive materials such as Iodine-131 more than the reference value. Although the use of these products will not affect the human body, people are asked to dispose of them. On March 22, it was found that 210Bq Iodine-131/kg water was 2

3 contained in tap water in Tokyo. According to Japanese national standards under the Food Sanitation Law, infants are allowed to drink tap water under 100Bq/kg, and adults, 300Bq/kg. Therefore the Tokyo municipal government recommended the infants not to drink tap water. However, the content of radioactive material, especially Iodine-131, decreased rapidly from 210 (March22) to 190(March 23) and 76Bq(March 24), and the recommendation was canceled afterwards. Following the accident chronology, it is better to follow the accident from the change of radiation level including some measurement in vegetables and milk. Concerning the degree of radiation-induced problem to the public health, several decisions were made. 2. Accident Chronology (Shut-down, Cool-down and Containment) 2.1 Plant Specification and its Layout There are six nuclear power plants of a BWR type at Fukushima#1 Nuclear Power Station(NPS). Units from No. 1 to No. 3 were operated at that time, and the rest, i.e., units from No. 4 to No. 6, were under planed shut down conditions due to regular inspection and fuel exchange. Figure. 1 shows the bird-eye view of units No.1 to No.4 with fuel assemblys and decay heats in SF pools. The reactor power of unit No. 1 is 460MWe and the reactor has been operated since March 1971, and the outputs of units No. 2 and No. 3 are 784MWe and those reactors have been operated since July 1974 and March 1976, respectively. As for unit No. 4, the reactor power is 784MWe and it has been operated since October 1978, and the regular inspection was in progress and thus all the reactor fuels were removed into the SF pool and also the new fuels for exchange were also stored. The temperature increase rate in the SF pool was higher and the damage of fuels became worse than the one in other SF pools. 2.2 Safety Design of Japanese BWR All the plants were designed based on the defense-in-depth principle as similar to other Japanese LWRs. Conservative safety design is taken against Design 3

4 Basis Accidents. The Emergency Core Cooling System (ECCS) is designed to be redundant with sufficient diversity in order to ensure the reactor core cooling and the containment integrity with high reliability. A severe accident management(am) program was investigated and planned by TEPCO after the licensing procedure. This AM program was suggested or recommended to develop for all existing nuclear reactor plants. This procedure is not addressed as a regulatory requirement. However the effectiveness of the AM procedures was reviewed by the regulatory members and experts and then have been improved and updated so far. In addition, the seismic design was strictly assumed, referring to the historical data. The assumed maximum acceleration at the ground level is about 600 Gal based on magnitude 8.0 as the level of the earthquake origin. Reported acceleration at the plant site was about 450 Gal, which is lower than the assumed acceleration. On the other hand, Tsunami was assumed to be about 6m in height and the diesel generator(dg) system was installed at the height of about 10m. Reported Tsunami at the plant site was about 14m in height, which is higher than the assumed height of Tsunami. 2.3 Event Progression (see Table 1) At 14:46 of March 11, the Tohoku-Kanto Earthquake occurred and all the NPS was safely shut down. All of the external power was lost due to the earthquake. Then the emergency diesel generators were successfully launched and the decay heat removal was assured in units No1, No.2 and No.3. However, 50minutes later, an extremely huge Tsunami had attacked the DGs at Fukushima#1 NPS and thus all the DG systems lost functions. Fuel tanks for emergency DG systems had been drift away and were failed due to the submerged conditions. An alternative external power supply line was also lost and it was not possible to ensure the electric power supply. As a result the Emergency Core Cooling System( ECCS) lost its functions. 4

5 The situation continued without cooling capability and it is reported hydrogen detonation and burning occured as shown in Table 1. Concerning the accident sequences, we will make analysis afterward and send you next messages when the detailed information will be clarifed. The sincere efforts made by related persons joining to Fukushima nuclear power station made the cooling capacity improve gradually and it is reported that the water temperature is maintained at less than 100 degrees C except for 128 degrees C at unit No 3. As a result of the efforts to secure the external power supply, something good is in prospects 10 days after the earthquake. The situation is considered to be going in the right direction. The external power supplies were secured for all power plants on March 24. After the recovery of the external power supply together with the cooling functions, the exchange of sea-water to normal-water will be performed in order to ensure the stable cooling for the fuels both in the reactors and the SF pools. 3. A Message from Nuclear Salon Fujiie 3.1 Accident Control to Safe Situation by Cooling We have no evidence now whether the core-melt has occurred or not in each reactor. However, the dose rates measured by the monitors of various places have basically decreased to the rate of I-131 half-life, i.e. eight days, since after the radioactive substance discharged along with the hydrogen detonation observed at March 15 as shown in Fig.3. The neutron beam has not been detected. The temperature and pressure were lowered by injecting seawater in the reactor containment vessel and the reactor pressure vessel at units No. 1, No.2 and No.3. Now 300 hours have passed from the reactor shut down. The heat from the reactor core also decreased to about 3MW for unit No.1 and 5MW for units No.2 and No.3, and the measured temperature of the pressure vessel and infrared rays thermometer from helicopter shows less than 100 degree C. Continuous efforts are made to secure "Cooling down" by pouring the water. If it would be possible to cool enough, the accident consequences would be minimized. 5

6 It is being confirmed that the water temperature of the SF pools of units No.3 and No.4 have fallen by pouring seawater by the fire engine etc., and if it is possible to cool them enough, the minimization of the accident can be achieved. 3.2 Radiation Effect to Public The success in the reactor shutdown had contributed to preventing the massive melting of the reactor fuels, and thus the massive release of radioactive materials never occurred. It is speculated that major radioactive nuclides in the Kanto-district would be Iodine and Cesium, which are volatile fission gases and exists in the fuel and cladding gap. These nuclides could be released by fuel failures and pressure release from the reactor vessel to the containment and the resultant containment venting. The hydrogen explosion could contribute to disperse these nuclides. Further efforts will be devoted to cool down the degraded core to prevent additional release of fission products from melted fuel. It is also observed that the decay of the spatial dose rate is corresponding to the decay curve of Iodine-131. Cesium could increase the dose rate in the basement, since the halving time is 30yrs. Even though increases in the spatial dose rates were observed in the Kanto-district, but these values are less than the spatial dose rate of that people are exposed to during the flight abroad. Thus it is not reasonable to evacuate from the Kanto-district. The nuclear safety shall achieve the zero fatalities of the public by direct radiation exposure. This goal has been achieved and it would be continued. Then to minimize the effect of radiation on the public health, evacuation and indoor save area were taken to the residents in the neighborhood of Fukushima #1 NPS. Finally radiation protection has been extended to control irradiated foods such as milk, vegetables and water, as mentioned at the end of section References: [1] Report from Government Nuclear Emergency Response Headquarter [2] NISA Press Release [3] TEPCO Press Release 6

7 5. Members who joined for the preparation of this report are: Dr.Tamotsu Ishii, Mr.Masato Idesawa, Mr. Shoji Kotake, Ms. Michiko Yugami, Ms. Tomoko Kinoshita and Prof. Yoichi Fujiie 7

8 Table 1 Event chronology and dose rates of the Fukushima#1 NPP (Unit No. 1, No. 2, No. 3, and No. 4) Date and Time Event [1] TEPCO Press Release Unit 1 (460MWe) Unit 2 (784MWe) Unit 3 (784MWe) Unit 4 (784MWe) [2] Government Nuclear Emergency Response Headquarters :46 Automatic shut down Automatic shut down Automatic shut down Outage [3] NISA Press Release :36 All ECCSs were unavailable.[2] All ECCSs were unavailable.[2] :30 The containment pressure increased and then the containment venting started.[2] :36 Hydrogen explosion occurred and white-colored smoke (steam) was observed.[2] :20 Injection of seawater with boric acid was started.[2] Events in Reactor :10 All ECCSs were unavailable.[2] The water injection function for the reactor was lost. [3] :41 The containment venting started. [2] Events in SF pool The pressure inside the containment vessel was :00 decreased due to the manual opening of the safety release valve, and water with boracic acid was injected.[1] :12 Seawater injection to the reactor was started. [2] :10 Seawater injection to the reactor was stopped. [2] Seawater injection to the reactor was stopped. [2] :01 The panel of the reactor building was opened to prevent hydrogen explosion. [2] :34 Seawater injection to the reactor was started. [2] :17 The water level came to the top of the fuel rod.[2] :06 The safety relief valve was opened. The water level in the reactor recovered to -1100mm from -1500mm. Sea-water injection was stopped due to the high reactor pressure (5.4MPa). [2] Hydrogen explosion occurred.[3] : :30 The reactor water level : -3700mm. The fuel rods became completely exposed. [2] As the water level was decreased and the pressure was increased in the reactor, a measure was taken to decrease the pressure and inject seawater. Then, the water level and the pressure recovered.[1] :00 The containment venting was started. [2] : : :10 The drywell pressure was beyond the design pressure. A measure was taken to decrease the pressure and inject water.[2] Abnormal sound was heard at the suppression pool and the pressure decreased in this pool.[2] :14 [Pool]White-colored smoke (steam) was observed.[2] :20 A large sound was heard, and damage of the reactor building was observed.[2] A large sound was heard, and damage of the reactor building was observed. [2] The wall of the operation floor was partly damaged, and the pressure in the suppression pool decreased.[3] :38 Fire occurred.[3] :00 [Pool]Dose around the building:400msv [1] Dose around the building:100msv [1] :00 Natural fire extinction was confirmed. [3] :25 Fire extinction was confirmed. [2] :45 Fire was observed. [1] :15 Fire was not observed. [1] :34 [Pool]White-colored smoke (steam) was observed. [2] :48 [Pool]Water was dropped by helicopter. (~10:00) [2] :05 [Pool]Water was poured. ( ~19:15) [2] :35 [Pool]Water was poured. ( ~20:09) [2] :00 [Pool]Water was poured. ( ~14:38) [2] :42 [Pool]Water was poured. ( ~14:45) [2] :30 [Pool]Water was poured. ( ~0:50) [2] :05 [Pool]Water was poured. ( ~3.20 3:40) [2] :20 Water was poured. (81ton, ~9:29) [2] :08 [Pool] Water was poured. (40 ton, ~17:20) [2] :46 Electricity was accessed. [1] :22 Water was poured. (81 ton, ~19:43) [2] :30 Water was poured. (1,137 ton, ~3.21 3:58) [2] :37 Water was poured. (91 ton, ~8:41) [2] :55 Gray-colored smoke was observed.[1] :20 White-colored smoke (steam) was observed.[1] :00 Reactor Conditions Electricity was restored after checking some equipments. [2] Water Level: (A)&(B) -1750mm; RPV-Pressure:(A) 0.3MPaG,(B) 0.27MPaG; CV-Pressure:0.26MPaabs Electricity was restored after checking some equipment. [2] Water Level: (A) -1300mm; RPV-Pressure:(A)&(B) MPaG; CV-Pressure: 0.11MPaabs Water Level: (A)-1900mm, (B)-2300mm; RPV- Pressure: (C)-0.10MPaG, (A) 0.036MPaG; CV- Pressure: 0.10MPaabs Pool Condition No Available Information 51 deg.c No Available Information No Available Information

9 Photo 1 Layout of Fukushima#1 Nuclear Power Station

10 BWR Crane Spent Fuel Pool Spent fuel Steam to Turbine Fuel rods Containment vessel Reactor building Reactor pressure vessel Control rods Water Suppression pool References: 1.Pictures related to earthquake disaster in the eastern Japan The Mainichi Daily News. Online 25 March etsu/21.html 2.Fukushima I nuclear accidents Wikipedia,the free encyclopedia. Online 25 March ear_accidents Fig. 1 Bird Eye View of Fukushima#1, units No.1 to No.4

11 Sv/h :00 6:20 1:20 23:05 9:30 4:00 4:10 5:10 6:10 6:50 7:50 8:50 9:50 3/12 3/14 3/15 3/17 Time 3/21 3/23 3/25 3/12-3/25 Sv/h :00 22:20 23:20 0:10 1:00 2:40 6:00 11:40 13:20 3/14 3/15 3/16 15:00 16:40 Date, Time 3/14 21:00-3/16 15:50 Fig. 2 Spatial Dose Rate at the front gate of Fukushima#1 NPS 20:30 23:30 1:00 2:40 6:40 8:20 10:00 11:30 13:10 14:50