The Severe Reactor Accidents of Three Mile Island, Chernobyl, and Fukushima

Abstract

Three major severe accidents with core meltdown/core disruption occurred at Three Mile Island (USA) in 1979, Chernobyl (Ukraine) in 1986 and Fukushima (Japan) in 2011.

The LWR of Three Mile Island was a two loop PWR with 880 MW(e) output. The accident started with technical problem in the feedwater loop for the steam generators. As the steam generators were not able to remove the heat, the pressure in the primary system increased and the safety valve of the pressurizer opened thereby releasing steam. The reactor was shut down because of too high pressure. When the pressure in the primary system dropped the safety valve did not shut again and remained open. The operators were given the opposite information by the instruments in the control room. The high pressure emergency core cooling systems started to feed water in the reactor pressure vessel. But the water in the pressure vessel rose too high and the operators throttled the emergency cooling systems. As the primary pumps started to vibrate the operators also shut down both primary pumps. As a consequence the cooling water in the pressure vessel started to boil. The zirconium claddings started to chemically react with water: hydrogen was formed. The reactor core began to melt down. The silver-indium-cadmium control rods did melt. Part of the molten core collected at the bottom of the pressure vessel. A hydrogen explosion occurred in the reactor building. Only the radioactive noble gases and a small part of the fission products iodine and cesium were able to penetrate the filters of the reactor building. The radioactive exposure of the population was therefore very small. Cost for decontamination of the plant and disposal of the destroyed core were very high. The Three Mile Island accident was classified a level 5 accident on the International Nuclear Event Scale (INES).

The Chernobyl accident occurred in one of four RBMK1000 reactors at the Chernobyl site 100 miles north of Kiev. The operators were preparing an experiment in which the energy of rotation of the turbine during shut down should produce emergency electrical power for the support of the diesel generators. Unexpectedly the experiment had to be interrupted for some time to comply with electricity supply which led to the buildup of the fission product Xe-135 (neutron poison). When the experiment could be continued the power level dropped to about 30 MW(th) because of operator error. This led to additional buildup of Xe-135 (neutron poison). As a consequence the operators had to withdraw the control rods manually to their upper limits after they had shut off the automatic control system. The RBMK1000 was known to have a positive coolant temperature coefficient. This gave rise to instabilities in power production, coolant flow and temperatures in the low power range.

Then the experiment began at the power level of 200 MW(th). Steam to the turbine was shut off. The diesel generators started and picked up loads. The primary coolant pumps also run down. However this led to increased steam formation as the coolant temperature was close to its boiling temperature. With its positive coolant temperature coefficient the RBMK1000 reactor now was on its way to power runaway. When the SCRAM button was pushed the control elements started to run down into the reactor core. However, due to a wrong design of the lower part of the control elements (graphite sections) the displacement of the water by graphite led to an increase of criticality. A steep power increase occurred, the core overheated causing the fuel rods to burst, leading to a large scale steam explosion and hydrogen formation. The reactor core was destroyed and the top shield cover and the fuel refueling machine were lifted up. Fuel elements and graphite blocks were dispersed outside the reactor core. The reactor core was now open to the atmosphere. Fission products and fuel aerosols were distributed over the Ukraine, Belarus, Russia and Europe. Very high radiation doses were received by fire fighters, operators, helicopter pilots and members of the emergency team. Approximately 800,000 military people were involved in rescue teams receiving various levels of high radiation doses. About 135,000 people were evacuated rather late. In total about 3,000 km² of land were contaminated with more than 1,500 Bq/m², roughly 7,200 km² with 600–1,500 Bq/m² and about 103,000 km² with 40–200 Bq/m² of Cs-137. The Chernobyl accident was classified a level 7 accident on the International Nuclear Event Scale (INES).

The severe reactor accidents at Fukushima occurred in 2011 after a severe earthquake with intensity 9 (Richter scale) close to the northeastern coast of Japan. The earthquake was followed by a tsunami wave which hit the six BWRs of the Fukushima-Daiichi plant with a water level up to 14 m. Unfortunately the Fukushima-Daiichi plant was only protected up to a tsunami wave level of 5.7 m. Only three BWRs of the six BWRs of the Fukushima-Daiichi plant were in operation when the earthquake and the tsunami wave hit the reactor site. All BWRs were duly shut down by the seismic instrumentation and changed into the residual heat removal mode. However, the tsunami wave flooded the two diesel generators of each of the three reactor units 1–3, located in the lowest part of the turbine building. The diesel generators and the battery systems failed. The external grid power and heat exchangers transferring afterheat to the ocean water had already been destroyed by the earthquake. In unit 1 due to the lack of electrical power the high pressure coolant injection system did not work. The steam driven isolation condenser system worked only partly in time and failed. The primary coolant system could not be depressurized due to lack of electrical power and pressurized nitrogen. Low pressure emergency pumps, therefore, could not feed water in the primary coolant system. The primary coolant system heated up and exceeded its design pressure. The core became uncovered, the zirconium claddings of the coolant system chemically reacted with water and formed hydrogen. The core melted down. The pressure in the pressure vessel was relieved into the primary containment because core melt penetrated the lower bottom wall. The pressure in the primary containment led to release of hydrogen and fission product gases into the upper reactor building, where a hydrogen explosion occurred destroying the upper building structures.

In units 2 and 3 the accident developed in a similar pattern, though with a larger shift in time. However a hydrogen explosion only occurred in unit 3 (BWR) not in unit 2 (BWR). However, a hydrogen explosion also occurred in unit 4 (BWR) due to a backflow through the common gas treating system. The hydrogen explosion destroyed the upper structures of the reactor building. The spent fuel pools of unit 1, 3 and 4 had to be cooled part time by concrete pumping trucks, water cannons or helicopters dropping water, but no damage occurred to the fuel in the spent fuel pools. After detailed measurements of the radioactivity released into the environment the Japanese government evacuated about 200,000 people. Four persons of the operating crew were killed by the earthquake and the tsunami wave. Some 20 staff members were injured by the hydrogen explosions. Out of the about 23,000 emergency workers 12 received effective radiation doses up to 700 mSv and 75 workers received <200 mSv. The radiation dose of all others was <10 mSv. The contamination of land was measured. About 2,200 persons would not be allowed to return to a no-entry zone because of too high radiation exposure. The Fukushima severe reactor accident was classified level 7 on the International Nuclear Event Scale (INES).