Behavior of a model VVER fuel assembly under the conditions of an unanticipated accident with cooldown by pouring water from above

Abstract

Investigations of the behavior of model VVER-1000 fuel assemblies have been performed on the PARAMETR bench under conditions of an unanticipated accident with reserve water poured from above. The results of materials engineering investigations of a model fuel assembly, using optical and electron microscopy, as well as x-ray microspectral and x-ray structural analysis are presented. The results of these investigations are necessary for developing and verifying methods of computing improved evaluations for the purpose of modeling accidents with substantial core damage.Ensuring safety is one of the main problems in designing, building, operating, and decommissioning nuclear power plants. Substantiating safety includes analysis of anticipated and unanticipated accidents, including accidents with serious damage to and melting of the core.An unanticipated accident is an accident caused by initial events which are not taken into account for anticipated accidents or an accident accompanied by additional, as compared with anticipated accidents, failures of safety systems above a single failure and implementation of incorrect decisions by personnel. According to the normative documents [1], the consequences of an unanticipated accident must be decreased by taking actions that are directed toward preventing the development of an anticipated accident into an unanticipated accident and toward decreasing the consequences of an unanticipated accident. According to existing ideas about limiting the development of an unanticipated accident, the core is cooled by pouring reserve water from above and below. Investigations of the behavior of the structural components of a reactor core under conditions of an unanticipated accident with reserve water poured from below have been performed for a long time (CORA and QUENCH [2] experiments), but the effect of pouring water from above on the behavior of overheated fuel has still not been studied experimentally. In this connection, the objective of the present work is to investigate the effect of cooling by pouring water from above on melting and fracture of a model VVER fuel assembly heated above 2000°C.The tests of a 19 fuel element VVER-1000 model fuel assembly with a 1275 mm long heated part and total length 3300 mm were performed on the PARAMETR bench [3]. The model assembly was made of standard structural materials. Tantalum heaters were placed inside fuel elements and isolated by UO 2 pellets from cladding made of E-110 alloy.The experiment consists of four main stages:• preparatory stage -stabilization of a prescribed argon flow rate of 2 g/sec and a steam flow rate of 3 g/sec with fuel assembly temperature 500°C and heating the fuel assembly up to 1200°C in the hottest zone; • pre-oxidation -holding a fuel assembly at 1200°C in the hottest zone for 3450 sec; • fast heating -the temperature in the hottest section of the fuel assembly is increased up to 1800-2000°C and higher; • water poured from above -water with flow rate to 40 g/sec is poured for 500 sec on the fuel assembly from ...