Effectiveness of Redundant Systems Remains Uncertain at Japanese Reactor

March 14th, 2011 by CHHS RAs

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by Oleg Pelekhaty

In the wake of Japan’s largest recorded earthquake and subsequent tsunami, Japanese nuclear energy officials continue to grapple with the increasingly serious situation at the Fukushima Dai-ichi Nuclear Power Plant. The situation highlights the importance of multiple redundancies and the utility of adopting an all-hazards approach to planning and preparedness activities.

Immediately after the 9.0 earthquake, engineers performed an emergency shutdown of the reactors, known as a SCRAM. A SCRAM halts the nuclear chain reaction and attempts to absorb energy, while coolant pumps remove the heat the fuel rods continue to produce. If the heat is not removed, it will eventually melt the rods’ outer metal sheath. 

A combination of factors overwhelmed some of Fukushima’s safety systems, and highlight the need for multiple redundancies.

  • Diesel generators, intended to power the plant’s cooling systems in the event of a power outage like the one caused by the earthquake, were swamped by the ensuing tsunami.
  • An emergency cooling system powered by a bank of backup batteries took over, but the batteries were quickly drained.
  • Portable generators were brought in to power the system, but the room housing the electrical switchgear connection was flooded by the tsunami.
  • Firefighting vehicles have been working furiously to flood the reactors with seawater and boric acid to cool the rods, but water levels have dropped to dangerously low levels as only one out of five fire trucks appear to be working. Moreover, exposed uranium reacts with water, producing highly-flammable hydrogen. The build up of hydrogen caused the explosions in Fukushima’s No. 1 and No. 3 units.

Nuclear experts point out that several crucial systems have functioned properly. Though the dramatic explosions at reactors No. 1 and No. 3 blew out the buildings’ roofs and walls, injuring 11 people onsite, they occurred outside the reactors’ containment vessels and did not release significant amounts of radiation. As hydrogen continues to build up in the reactor units, however, officials will vent the steam to avoid more explosions.

Japanese and American officials have issued statements that there will not be a catastrophic release of radiation from the Fukushima reactor. Radiation levels detected by the USS Ronald Reagan were about “the same as one month’s normal exposure to natural background radiation in the environment.”

Efforts are now focusing on the emergency cooling operation at the No. 2 reactor; engineers have been struggling to keep water on the fuel rods, which at times have been fully exposed to air.

 

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