ANS: Japanese Nuclear Power Plant Update

Source: American Nuclear Society

American Nuclear Society Backgrounder: Japanese Earthquake/Tsunami; Problems with Nuclear Reactors
3/12/2011 5:22 PM EST

To begin, a sense of perspective is needed… right now, the Japanese earthquake/tsunami is clearly a catastrophe; the situation at impacted nuclear reactors is, in the words of IAEA, an “Accident with Local Consequences.”

The Japanese earthquake and tsunami are natural catastrophes of historic proportions. The death toll is likely to be in the thousands. While the information is still not complete at this time, the tragic loss of life and destruction caused by the earthquake and tsunami will likely dwarf the damage caused by the problems associated with the impacted Japanese nuclear plants.

What happened?

Recognizing that information is still not complete due to the destruction of the communication infrastructure, producing reports that are conflicting, here is our best understanding of the sequence of events at the Fukushima I?1 power station.

– The plant was immediately shut down (scrammed) when the earthquake first hit. The automatic power system worked.

– All external power to the station was lost when the sea water swept away the power lines.

– Diesel generators started to provide backup electrical power to the plant’s backup cooling
system. The backup worked.

– The diesel generators ceased functioning after approximately one hour due to tsunami induced damage, reportedly to their fuel supply.

– An Isolation condenser was used to remove the decay heat from the shutdown reactor.

– Apparently the plant then experienced a small loss of coolant from the reactor.

– Reactor Core Isolation Cooling (RCIC) pumps, which operate on steam from the reactor, were used to replace reactor core water inventory, however, the battery?supplied control valves lost DC power after the prolonged use.

– DC power from batteries was consumed after approximately 8 hours.

– At that point, the plant experienced a complete blackout (no electric power at all).

– Hours passed as primary water inventory was lost and core degradation occurred (through some combination of zirconium oxidation and clad failure).

– Portable diesel generators were delivered to the plant site.

– AC power was restored allowing for a different backup pumping system to replace inventory in reactor pressure vessel (RPV).

– Pressure in the containment drywell rose as wetwell became hotter.

– The Drywell containment was vented to outside reactor building which surrounds the
containment.

– Hydrogen produced from zirconium oxidation was vented from the containment into the reactor building.

– Hydrogen in reactor building exploded causing it to collapse around the containment.

– The containment around the reactor and RPV were reported to be intact.

– The decision was made to inject seawater into the RPV to continue to the cooling process,
another backup system that was designed into the plant from inception.

– Radioactivity releases from operator initiated venting appear to be decreasing.

Can it happen here in the US?

– While there are risks associated with operating nuclear plants and other industrial facilities, the chances of an adverse event similar to what happened in Japan occurring in the US is small.

– Since September 11, 2001, additional safeguards and training have been put in place at US nuclear reactors which allow plant operators to cool the reactor core during an extended power outage and/or failure of backup generators – “blackout conditions.”

Is a nuclear reactor “meltdown” a catastrophic event?

– Not necessarily. Nuclear reactors are built with redundant safety systems. Even if the fuel in the reactor melts, the reactor’s containment systems are designed to prevent the spread of radioactivity into the environment. Should an event like this occur, containing the radioactive materials could actually be considered a “success” given the scale of this natural disaster that had not been considered in the original design. The nuclear power industry will learn from this event, and redesign our facilities as needed to make them safer in the future.