Investigation on Thermal-hydraulics and Core Degradation Issues of Fukushima Accident by relap5-scdap code

摘要

This paper presents the analysis of the accident occurred at Fukushima Unit-3 Nuclear Power Plant, taking into account the latest and more detailed description of the events available. To this aim, the accidental sequence has been simulated by RELAP5/SCDAP code. The analysis is focused on the phenomenology that had a predominant role into the accident evolution at Unit 3. The RELAP5/SCDAP 3.5da version is used on the basis of the Laguna Verde BWR plant nodalization made available by Innovative Systems Software (ISS) Group, chaired by Prof. Allison. The nodalization has been modified with respect to the main systems in service during the sequence. The modifications to the nodalization have been performed also for reducing the error mass in the calculation of the very long thermal-hydraulic transients, like that occurred at Fukushima Unit 3. The good results obtained also for this phase of the accident, which lasted about 2 days, will be presented. However, the most interesting part of the analysis is obviously that of core damage. The coupling of RELAP5 with SCDAP models allows simulating the core degradation progression and the hydrogen production, starting from the condition of "core isolation phase", until resumption of the coolant level through Fire Extinguishing Line was actuated. The calculation results are discussed on the basis of the available plant data, even though incomplete. In fact, the real state of core degradation remains unknown and requires direct investigations in situ. Only once a reactor examination program will be harried out, an extensive and comprehensive data base will be available on the phenomena occurred during the core melt scenario progression, including aspects related to material relocation processes inside the vessel and consequent radioactivity releases. However, it is already possible to evaluate the influence of some uncertain core degradation parameters on core melt progression, like fuel rod slumping criteria with associated heat transfer phenomena. Some considerations on the lessons learned from the Fukushima accident conclude the paper.