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On March 11, 2011 the Fukushima Daichi and Daini nuclear plants were severely impacted by the Great East Japan earthquake. Seismic damage to the transmission system led to the loss of nearly all offsite power. Internal flooding damage from the concomitant tsunami resulted in loss of all medium voltage emergency AC power buses at the four (4) original units at the Daichi site. These units were then subjected to Station Blackout (SBO) conditions for an extended period. Several units at the Fukushima Daini site were similarly subjected to SBO conditions. Following the events described above, the nuclear industry and regulators have issued many analyses and reports which focus on Fukushima specific design margins, safety features, and operator actions. However, little attention is paid to the near complete loss of instrumentation and control power which made operator monitoring and response to the accident significantly more difficult. Despite heroic efforts to restore power to the DC instrument buses (e.g., using car batteries connected in series) operators were ‘left in the dark’. This paper examines proposed designs for direct onsite charging of DC instrumentation bus batteries (independent of the AC medium voltage buses) which utilize decay heat as a source of energy.

Developing Installed DC Power Backup System for the APR1400 for Station Black-Out (SBO) Coping.