Expert Report

Lessons Learned from the Fukushima Accident for Improving Safety and Security of U.S. Nuclear Plants: Phase 2 (2016)

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The 2011 Fukushima Daiichi nuclear accident should serve as a wake-up call to nuclear plant operators and regulators on the critical importance of measuring, maintaining, and restoring cooling in spent fuel pools during severe accidents and terrorist attacks. This final phase of a 2-part study on lessons learned from the Fukushima Daiichi nuclear accident provides findings and recommendations for improving U.S. nuclear plant security and spent fuel storage.

The report finds that spent fuel storage facilities at the Fukushima Daiichi nuclear plant -- both spent fuel pools used to store fuel under water and casks used to dry-store fuel -- maintained their containment functions during and after the March 11, 2011, Great East Japan Earthquake and tsunami.  However, one of the two gates separating the Unit 3 spent fuel pool from the adjacent reactor well was damaged during the accident.  Also, water appeared to have leaked around the gate seals in the Unit 4 spent fuel pool, allowing water to flow into the pool from the reactor well.

This water leak was accidental but also fortuitous, because it replenished water lost from the Unit 4 pool by evaporation, likely preventing water levels from dropping to the tops of the racks where the spent fuel was being stored.  Keeping the fuel covered with water is essential for cooling and radiation shielding.  Uncovery of the fuel would have substantially increased radiation levels above and around the pool, limiting personnel access to the pool and nearby areas, and could have resulted in severe damage to the fuel, increasing the potential for large radioactive material releases into the environment. 

The report's authoring committee recommends that the U.S. nuclear industry and the U.S. Nuclear Regulatory Commission (USNRC) improve the ability of plant operators to measure real-time conditions in spent fuel pools and maintain adequate cooling of stored spent fuel during severe accidents and terrorist attacks. These improvements should go beyond the current post-Fukushima response to include hardened and redundant physical surveillance systems such as cameras, radiation monitors, pool temperature monitors, pool water-level monitors, and means to deliver pool makeup water or sprays, even when physical access to the pools is limited by facility damage or high radiation levels.

The committee also found that extreme external events and severe accidents can cause widespread and long-lasting disruptions to security infrastructure, systems, and staffing at nuclear plants. Such disruptions can create opportunities for malevolent acts and increase the susceptibility of critical plant systems to such acts. The committee recommended that nuclear plant operators and their regulators upgrade and/or protect nuclear plant security infrastructure and systems and train security personnel to cope with extreme external events and severe accidents.

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