12 October 2016
Nuclear fuel mixed with debris left inside three crippled reactors at Tokyo Electric Power Co.’s Fukushima Daiichi plant after their meltdowns in the 2011 accident is estimated at 880 tons, including concrete and other structural materials. The fuel debris is believed to be about three times as heavy as fuel proper. The findings were based on data on conditions inside the reactors immediately after the accident, the proportion of fuel to total debris and other figures obtained by a computer analysis system of the International Research Institute for Nuclear Decommissioning (IRID), a government-business think tank tasked with studying ways of decommissioning the plant. The debris weight is expected to be some key reference data in the lead-up to its future removal, regarded as the most difficult phase of decommissioning work.
The weight of nuclear fuel left inside the Nos. 1-3 reactors at the time of the accident was estimated at 69 tons, 94 tons and 94 tons, respectively, adding up to 257 tons. The mixture of fuel and debris was estimated to weigh 279 tons, 237 tons and 364 tons, respectively, for a total of 880 tons, or 2.5 times to four times the weight of fuel. The fuel is believed to have mixed with stainless steel pressure vessel components, zirconium fuel rods and the concrete bottoms of containment vessels when it melted down.
Of the fuel debris in the Nos. 1 and 3 reactors, nuclear fuel and stainless steel -- the main component of the debris -- accounted for about 30% each, and concrete materials around 40%, according to the IRID analysis. In the No. 2 reactor, fuel and stainless steel constituted a total of some 70% and concrete elements the rest.
The IRID analyzed data on conditions inside each reactor right after the accident, including pressure, temperature and the amount of water injected, as well as the results of muon tomography scans inside the No. 1 reactor that used muon beams stemming from cosmic rays. Multiple computer analysis programs were used for the study to evaluate fuel debris movements in a comprehensive manner, and the figures thus obtained are as close to reality to the maximum possible extent currently, according to IRID. In the case of the No. 2 reactor, however, there is a possibility of discrepancies being caused in weight data between the estimated and actual figures because the amount of water injection is not known, making it difficult to presume fuel debris movements.
(Translated by Kyodo News)