When A Nuclear Reactor Breaks Down

By Justin H. Joe, Ph.D.
Mount Kisco, NY

In 1986, in a city named Chernobyl in Northern Ukraine, a poorly designed test of a nuclear reactor cooling system caused a huge power surge that initiated a massive explosion in the reactor [1, 2]. There was no containment vessel, and radioactive gases and materials were released into the atmosphere.

While human error was chiefly to blame for the Chernobyl disaster, it was the hand of nature that started the latest nuclear crisis. On March 11, 2011, all six units of the reactors at the Fukushima Daiichi power station automatically shut down during a 9.0-magnitude earthquake, the strongest-ever recorded in the country’s history. The enormous tsunami caused by the earthquake quickly knocked out the plant’s emergency cooling pumps. As workers continue to struggle to bring the reactors under control from hydrogen explosions and partial meltdown of fuel core, an understanding of what occurred inside the station may help in drawing a better picture of the events now taking place in Japan.

Inside a nuclear reactor are fuel rods consisting of outer tubes made of Zircaloy, or zirconium alloy, in which are housed uranium fuel pellets. In a shutdown, control rods can be placed between the fuel rods to stop the nuclear chain reaction by absorbing thermal neutrons [3]. While this stops the reaction, fuel rods continue to give off enormous amounts of heat at around 6% of full operational power. The rods must be kept submerged in water in order to keep them from overheating. An electric pump pulls heated water from the reactor vessel to a heat exchanger, and cool water from a river or an ocean is brought in to draw off the heat [2]. In Japan, there was insufficient electricity to run the cooling system following the earthquake. Emergency diesel generators failed quickly as the tsunami washed over them, and backup batteries were depleted shortly afterwards. Operators added water in an attempt to vent the steam and replace the water as it evaporated. However, the water began boiling away faster than they could replace it after losing electricity. Fuel rods were exposed to air, heating up quickly. As temperatures spiked, the cladding would oxidize, and the fuel would begin to melt.

Hydrogen can be formed from the steam and the Zircaloy cladding interaction [4]:
H2O +Zr –> ZrO +H2

Accumulating hydrogen without venting or a controlled burn, a pressure pulse can be produced from the hydrogen interaction with the oxygen in the containment atmosphere:
2H2 + O2 –> 2H2O

In the process, water abruptly evaporates into steam and expands into a large volume. This can be analogous to the sudden flashing into steam when the lid of an automobile radiator is unintentionally opened under pressure. When the pressurized water senses the lower atmospheric pressure than what is normally present in the pressurized radiator, it flashes into steam, and the coolant in the radiator is lost.

As the result of the Fukushima event, radioactivity levels at monitoring posts in the vicinity of the plant were found to be elevated. Consequently, evacuation of local residents within a 20-kilometer radius around the plant was implemented. Units 1 to 4 were involved in the accident accompanying hydrogen explosions. The sound of an explosion was heard at Unit 1 along with a plume of white smoke, which is thought to have resulted from a hydrogen explosion that occurred on March 12, 2011. The fission products Cs136 (T1/2=13.1 d) and Y91(T1/2=58.6 d) were found in the water at the turbine hall. Lights were turned on in the control room on March 25, 2011. Subsequently, coolant pumps came back online. Water samples from Unit 2 revealed the existence of Co60(T1/2=5.27 y) and Mo99(T1/2=66.03 h) which are radioactive nuclides that could have leaked from a condensate polisher in the basement of the turbine building or piping.

As the crisis continues, careful accident analysis and closer environmental monitoring should be carried out in order to minimize the public health hazard, accompanied by a long-term plan designed in cooperation with international nuclear experts [5].

Fukushima is not another Chernobyl. For the Japanese, earthquakes are a part of life. Tremors occur in Japan every five minutes on the average, swaying buildings that are designed to withstand the strongest of Earth movements. This record-setting earthquake is a test of the endurance and persistence of the 127 million people of Japan, who will undoubtedly recover, restore, and continue to move forward.

Reference

1. Eerkens, J.W., Safety Considerations in Nuclear Operations, in The Nuclear Imperative2010, Springer Netherlands. p. 135-156.
2. Murray, R.L., Reactor Safety and Security, in Nuclear Energy (Sixth Edition)2009, Butterworth-Heinemann: Boston. p. 289-321.
3. Reyes, J.N. and J.B. King, Nuclear Engineering, in Encyclopedia of Energy, J.C. Cutler, Editor 2004, Elsevier: New York. p. 315-331.
4. Ragheb, M., FUKUSHIMA EARTHQUAKE AND TSUNAMI STATION BLACKOUT ACCIDENT. Click here for the pdf , 2011.
5. Saji, G., A new approach to reactor safety goals in the framework of INES. Reliability Engineering & System Safety, 2003. 80(2): p. 143-161.