Re-criticality potential at Fukushima Daiichi Unit 4 following the March 2011 earthquake and tsunami

The damage to Fukushima Daiichi Unit 4 reactor buildings discovered on March 15, 2011, led to concerns that the Unit 4 spent fuel pool had leaked and might have gone dry. The possibility of a re-criticality event occurring following reflooding of a fully drained Unit 4 spent fuel pool was a concern. Uncertainties regarding the status of the fuel and the pool made it difficult to apply conventional criticality safety methods. However, EPRI was able to apply an alternative criticality safety methodology, developed for the analysis of spent fuel criticality during transportation accidents, to show that the most likely configuration of damaged fuel and pool would be a decrease in reactivity. Thus it was safe to reflood the pool. (author)