Integral simulation of the first 3 weeks of severe accident at Fukushima Daiichi Unit 1 with SOCRAT code. Part I – Qualification of model and simulation of the initial phase

Abstract

This paper is the first part of the summary overview which presents the results of IBRAE's work over the last 11 years to study the severe accident at Unit 1 of Fukushima Daiichi Nuclear Power Station. The paper is dedicated to the description of the integral model of the Unit 1 with assumptions made, and to its qualification with the available data. The qualification includes the comparison of simulation results with measurements for the initial phase of the accident which starts with the initiating event (earthquake) and ends with tsunami arrival. The important issues discussed are the direct modeling of isolation condenser performance and verification of the possibility to use an integral approach to estimate the nuclide inventory in the core. These questions are of methodological importance for modeling the main phase of the accident.Direct modeling of isolation condenser as part of integral calculation allows verifying the hypothesis about the isolating valves left open inside containment after tsunami arrival and station blackout. The isolation condenser system could influence the development of severe accident by limiting the pressure peak in the reactor at the time of melt relocation into the lower plenum. Thus, verification of the isolation condenser model at the initial stage of the accident justifies the correctness of results obtained for the core degradation phase of the accident.The integral approach in nuclide inventory estimate presumes the need to mix the fuel with different burn-ups in the effective groups of fuel assemblies, which may bring an uncertainty in the nuclide inventory prediction. Quantification of this uncertainty is important for assessment of prediction accuracy for the source term.