"Post-test analysis of International Standard Problem 25 by Relap5/mod3 code" (in Italian)

The document deals with the description of results obtained by the Relap5 code in the post-test open (numerical) simulation of the reflood phase of a LOCA experiment performed in the Pressurized Water Reactor (PWR) experimental simulator Achilles installed at the Winfrith Research Center in United Kingdom. The Relap5 is the well-known computer code developed at Idaho National Laboratory in US: the code is in use at UNIPI since more than a decade. The Achilles loop is a Separate Effect Test Facility (SETF) simulating with full height (full pressure), full linear power a US type PWR. The concerned test was selected as International Standard Problem 25 (ISP 25) by OECD/NEA/CSNI (Organization for Economic Cooperation and Development / Nuclear Enrgy Agency / Committee on the Safety of Nucear Installations). Following the results of the pre-test analysis, the present document describes the results of the post-test calculation submitted (by UNIPI) to UKAEA (UK Atomic Energy Agency) after the execution of the test. This is called open post-test. It allows an evaluation of the capabilities of the computer code and of the code user team in predicting the scenario of an accident. This is relevant for demonstrating the capabilities in evaluating safety margins of existing NPP, with main reference to PWR reflood (in this case).