OECD CSNI ISP 18: pre-test analysis of a small break LOCA in LOBI-MOD2 facility performed at University of Pisa by RELAP4/MOD6 code

The document deals with the pre-test analysis of an experiment performed in LOBI in the configuration Mode2. LOBI was a Pressurized Water Reactor (PWR) simulator installed in the EC center of Ispra Italy financially supported by EURATOM. The configuration Mod2 of LOBI was designed as a consequence of the Three Mile accident in US and is the follow-up of the configuration Mod1. Mod2 was designed to simulate stratification phenomena and natural circulation which are important during Small Break Loss of Coolant Accidents (SBLOCA); as a difference Mod1 was designed to simulate phenomena expected during Large Break Loss of Coolant (LBLOCA) scenarios. Namely downcomer size in Mod2 was much smaller than in the case of Mod1 and a large number of Emergency Core Cooling Systems (ECCS) were installed in Mod2. The concerned experiment was selected by OECD/NEA/CSNI (Organization for Economic Cooperation and Development / Nuclear Energy Agency / Committee on the Safety of Nuclear Installations) as International Standard Problem (ISP) 18. ISP 18 was a Small Break Loss of Coolant Accident (SBLOCA) scenario without the actuation of the High Pressure Injection System (HPIS). The results of the blind calculation performed by Relap4 without the knowledge of the experimental and submitted to the ISP host organization (JRC-Ispra) are documented in this report (at the end it revealed so close to experimental data that questions were put in relation to possible knowledge by the authors of the experiment before the execution of the calculation). The result of the work were submitted to EURATOM/OECD for issuing the comparison report of ISP 18.