Fukushima Daiichi Nuclear Plant Weekly Review

This week, Tokyo Electric Power Co. released an analysis of several unanswered questions raised by the cascade of events in March of 2011 that destroyed four reactors at the Fukushima Daiichi nuclear plant in Japan. Among its conclusions:

Fire trucks inject water into Fukushima reactors March 16, 2011. Source: TEPCOCore Damage More Extensive in Unit 3

Where previous estimates placed the amount of damaged fuel in unit 3 at about 60 percent, further study suggests that nearly all of the fuel melted, and most of it likely settled in the bottom of the reactor's containment outside of the reactor pressure vessel. TEPCO said it changed its assumptions because the unit's high pressure coolant injection system is now believed to have stopped working several hours earlier than first assumed. The report estimates is stopped injecting water at about 8 p.m., March 12, when RPV pressure was no longer high enough to sustain its operation. TEPCO plans further study of how far into the concrete the fuel penetrated, but it is not believed to have breached the containment structure.

Water Injection Accelerated Fuel Melt in Unit 2

TEPCO also researched unexplained fluctuations in RPV pressure at unit 2. The report suggests that after the pressure dropped steeply on March 14, water levels in the reactor fell and exposed the core. Pumping from fire trucks then refilled the RPV with water that reacted with superheated zirconium fuel cladding, creating hydrogen and large amounts of additional heat that contributed to fuel melting and a subsequent increase in pressure. When pumping was suspended, the pressure dropped.

Unit 3 RCIC Tripped

Researchers also studied why reactor core isolation cooling systems operated for different lengths of time in different damaged units. While unit 2's RCIC operated for several days, the RCIC at unit 3 failed after 20 hours with charge remaining in backup batteries. The investigation suggested a damage prevention device shut it off to protect the system from high turbine exhaust pressure. The report noted that the finding will likely encourage other plants to create a way to override the shutoff function in an extreme emergency.

The entire report is available here.

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