Integrally Estimated Nuclide Inventory and Decay Heat from Potential Nuclear Waste Represented by the Spent Fuel from the Fukushima Daiichi Unit 1

Abstract

Assuming an accident that may damage spent fuel assemblies at an NPP, nuclear fuel in the spent fuel pool can be considered as radioactive waste. Data on its nuclide composition and heat generation is seen important for the safety assessment of its storage and potential radiological consequences in case of an accident. The SOCRAT/V3 integral code designed to simulate severe accidents at nuclear power plants with VVER power units is able to estimate the nuclide inventory of irradiated fuel based on a one-through calculation and a built-in model. This code was used for a computational analysis of the spent fuel from the Fukushima Daiichi Unit 1, namely, evaluated was its nuclide composition and its decay heat release. The results were compared with the reference data from JAEA (Japan). The calculations demonstrate that in case of severe accident simulations, the use of an integral approach causes no large errors in the estimated nuclide inventory. For most dose-forming radionuclides in the spent fuel, relative deviation of activity from the reference data does not exceed 15%, which appears to be lower than the typical uncertainty under source term predictions. Despite certain overestimation tendency that can be noticed, the decay heat levels calculated for the spent fuel pool appear to be quite similar to those presented in the JAEA report differing by no more than 16% in the first years after the accident, and by 5% ten years after the accident.