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FUKUSHIMA, DESCRIPTION OF THE ACCIDENT AND CONSEQUENCES TO THE ENVIRONMENT. Dragoslav Nikezic Faculty of Science, University of Kragujevac, Serbia nikezic@kg.ac.rs http://www.pmf.kg.ac.rs/radijacionafizika/. EARTHQUAKE.
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FUKUSHIMA, DESCRIPTION OF THE ACCIDENT AND CONSEQUENCES TO THE ENVIRONMENT Dragoslav Nikezic Faculty of Science, University of Kragujevac, Serbia nikezic@kg.ac.rs http://www.pmf.kg.ac.rs/radijacionafizika/
EARTHQUAKE ON 11.03.2011 large scale tsunami, followed after strong earthquake of M=9 overflow nuclear power plant Fukushima Daiichi. The earthquake occurred under the sea about 70 km eastern of Oshika peninsula at the depth of about 32 km. It was the most powerful earthquake that had ever hit Japan, and fifth the strongest in the world since official modern record began in 1900. This earthquake triggered big tsunami which wave reach up to 40 m in Iwate prefecture. The wave height was smaller on another locations, being about 8 m in Fukushima area. Japanese authorities reported 15854 death, and 3155 missing people. In addition, about 27000 was injured.
FUKUSHIMA DAIICHI POWER PLANT Fukushima Daiichi plant comprised by six separate nuclear reactors. All reactors were water boiled type, maintained by Tokyo Electric Power Company TEPCO. The plant was protected by a seawall protection designed to withstand a 5.7 m tsunami. Obviously this was not enough high to protect against tsunami on 11.03.2011.
SITUATION AT THE TIME OF THE QUAKE At the time of the quake, reactor 4 had been de-fueled while 5 and 6 were in cold shutdown for planned regular maintenance (refueling). Reactors 1,2 and 3 were operating. At the moment of the quake, reactors were shut down automatically. Emergency generators started to run to pump the water needed to cool the reactors.
TSUNAMI The plant was protected by a seawall protection designed to withstand a 5.7 m tsunami. However, the 8-14-metre tsunami wave arrived 15 minutes after the earthquake. The entire plant was flooded, including low-lying generators and electrical devices in reactor basements. Connection to the electrical grid was broken.
HEAT PRODUCTION Although the reactors were cooled down, there were a large amount of fission products in reactor core. About 6 % of total energy of fission is released through beta and gamma decay of fission products. It amounts about 12 MeV per one fission. This energy is ultimately transformed into the heat. Although the chain reaction was stopped, it is necessary to cool down reactor core, until the radioactivity of fission products decreases significantly below certain level. Similar is true for spent fuel which must be cooled down for some period of time. Radioactivity of fission product decreases with the time in complicated manner. Many different radioactive isotopes are presented in fresh spent nuclear fuel. All of them are BETA/GAMMA radioactive.
OVERHEATING OF REACTORS AND SPENT FUEL POOL There were three independent cooling systems. All three cooling systems failed; some of them because connection to the electricity was interrupted. Independent cooling based on generators was also broken, because they were flooded by tsunami. Without any cooling, reactors and spent fuel pool started to heat due to the radioactive decay of fission products. Soon after the tsunami, evidence arose of partial core meltdown in reactors 1, 2, and 3.
Intriguing question • In many document was written that hydrogen exploded. However, there was not any explanation on the following question: • From where did hydrogen come from? • This was intriguing question for me. To resolve this issue consultations with chemists were necessary. • Need to know chemical behavior of water vapor at temperatures between 200 0C and 300 0C in contact with some metals.
INTERACTION OF WATER VAPOR WITH ZIRCONIUM AND METALS Zirconium was used in construction elements. The most important reaction was between water vapor and zirconium (and other metals presented in construction elements), which occur at high temperature. As the result of this chemical reaction water molecules were disrupted and only hydrogen remained in gaseous phase, while, oxygen atoms were combined with zirconium (and other metals).
Explosions and fires Hydrogen formed in chemical reactions exploded and destroyed the upper cladding of the buildings housing of reactors 1, 3, and 4. Explosion on reactor No3 severely damaged reactor building No4. There are speculations that hydrogen leak from reactor No3 to No4 through underground tunnels. There were explosion and fire on reactor No2. Spent fuel rods stored in spent fuel pools of units 1–4 began to overheat as water levels in the pools dropped. Constant production of heat also due to gamma and beta radioactive decay of fission product.
The most complex nuclear accident in history. All units were involved in some way. Much more complex than Chernobyl accident, where only one reactor was destroyed. Each reactor has its own story in this accident. Difficult to manage.
MANAGING OF THE ACCIDENT Evacuation of population within 20 km, immediately after the accident. The main problem was to enable cooling of all damaged reactors and spent fuel pools. All work on damaged reactors was seriously hindered by high level of radiation within reactors building and around them. It has been reported that dose was up to 400 mSv/h at some position within the power plant. Let we remind that average lethal dose is between 4 and 8 Sv.
There were some information that dose was as high as 1 Sv/h. This result was taken with reserves because instruments were calibrated up to 1 Sv/h.
In order to cool down in Units 1, 2 and 3 fresh water has been continuously injected both via the feed water system lines and the fire extinguishers lines into the reactor pressure vessel; temperatures and pressures were stabilized. Sea water was used in the first phase of the accident, but it caused contamination of sea water.
TEPCO started work on 9 May of 2011 to install a supporting structure for the floor of the spent fuel pool of Unit 4. Fresh water was injected into the spent fuel pools of Units 1 - 4. One generator at unit 6 was restarted on 17 March allowing some cooling at units 5 and 6 which were least damaged. Grid power was restored to parts of the plant from 20 March, but machinery for reactors 1–4 damaged by floods, fires and explosions remained inoperable.
Overall, the situation at the Fukushima Daiichi nuclear power plant remains very serious. TEPCO planned to completely demolish four reactors in 30-40 years
CONTAMINATION Large scale releasing of radioactivity in environment. Power plant was very contaminated, but, as it can be seen in previous images, radioactivity was airborne toward the east (Pacific ocean). Many measurements of radiation level in Japan, were performed close and further from Fukushima NPP. Fission product, 131I, 137,134Cs, 131,134Xe detected soon after the accident, constituted large part of radioactivity released in environment.
Spread out of contamination Contamination of sea water spread by ocean stream. Airborne contamination by permanent atmospheric motion.
Effective dose for population around power plant was estimated up to 30 mSv. This is larger than dose limit for professional persons which amounts 20 mSv per annum, (in average for 5 years). This is much larger than dose limit for general population ( 1 mSv per annum). Evacuation that was undertaken immediately after the accident was justified.
Contamination of the rest of the world Radioactive contamination was observed in many parts of the northern hemisphere, including Europe, Central Asia, North America. There are many papers published in last several months about radioactivity levels in different media (air, rain water, soil deposition etc. ). General conclusion is that radioactivity level in Europe is rather small, and somewhere below detection limit.
Contamination of Europe Rain water was found contaminated with 131I up to 1.0 Bq/L . Also 137Cs was below 1 Bq/L. Radioactivity in air was measured by air pumping through some filters and measuring on gamma spectrometry.
Europe contamination wit 131I.Scale mBq/m3 of air. (Bosew etc)
Dose Dose estimated for Europe is order of 1 µSv from 131I. This dose is negligible and cannot contribute to any measurable effect.
Our measurements • Radioactivity of rain water well below 1 Bq/L • Radioactivity of air, below 0.1 Bq/m3 • External doses unchanged • Comparison to Chernobyl accident • Radioactivity of 131I in rain water 6900 Bq/L • Radioactivity of 137Cs in rain water 170 Bq/L • More than 20 other radionuclides. Fall out about 11 L/m2 • External doses increased 20 times more than normal.
