Coupled Neutronic And Thermal Hydraulic Modelization Applied To The Fast Injection Of A Slug Of De-borated Water Into The Core

During the last years, two major sets of studies concerning: - heterogeneous dilution (coupled neutronic and thermal hydraulic modelization applied to the fast injection of a slug of de-borated water into the core), - TMI 1 Rod ejection benchmark, in collaboration with the US NRC and the Kurchatov Institute, have been performed by CEA on the request of IPSN. The aim is to provide informations to support IPSN safety assessment of the new methodology proposed by the French Utility. The present paper will focus on the heterogeneous dilution studies. After having described the main sequences which could lead, with an important calculated probability, to a rapid sweeping of the core by a slug of pure water, the main results and conclusion obtained from TRIO-VF calculation will be presented. These studies have also pointed out the importance of the three-dimensional aspects of the transfer of a de-borated water slug into the core. In order to evaluate the consequences of this transient on the core, it is necessary to use three-dimensional neutronic computer codes (CRONOS 2). According to the results obtained, which pointed out the risk of reaching prompt criticality, IPSN reiterated the importance to be attributed to prevention. In particular, it considered that, regarding the large uncertainties associated to the mixing studies, the subcriticality of the core should be increased from 1000 pcm to 2000 pcm in the shutdown state in order to avoid any return to criticality during an heterogeneous dilution transient (considering a de-borated water slug up to a 3 m3 in the cold leg). Electricite de France has also been encouraged to continue the improvement on the mixing calculations tools including international comparison. (authors)