On 20 May Tepco published its latest report on the 2011 Fukushima nuclear disaster (Report on the Investigation and Study of Unconfirmed/Unclear Matters in the Fukushima Nuclear Accident, Progress Report No.3) which has shed more light on events subsequent to the earthquake, revealing a failure in one of the reactors' pressure relief systems.
The new report looks into data collected by a robot probe sent into the No 2 reactor building in October. It measured radiation levels and studied the conditions of leakage control systems. The main focus of the investigation is the reactor's containment system, which is equipped with a pressure release system designed as a last resort to vent radiation into the atmosphere. During March 2011 earthquake, pressure was rising at the damaged reactor, and staff opened the pressure release valves to send excess contaminated steam to an exhaust tower.
However, it appears that the system failed, owing to the failure to operate of a rupture disk between the valves and the exhaust outlet. The robot's readings showed no increase in radiation levels near the disk, which means it did not burst. The Tepso report suggests that gases from reactor 1 flowed back into the reactor 2' system, pressuring the disk from the opposite side. Both reactors use the same exhaust conduit in their venting systems. The report also suggests that only about a quarter of the release valves were found open. Commenting on the findings, Tepco stated that they are "not definitive" and more inquiries will be needed to find out exactly what happened.
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