Overview of gas cooled reactors' applications with CATHARE

Full text of publication follows: For about four years, CEA has launched feasibility studies of future nuclear advanced systems in a consistent series of Gas Cooled Reactors (GCR) ranging from thermal reactors, as the Very High Temperature Reactor (VHTR) for the mid term, to fast reactors (GFR) for the long term. Thermal hydraulic performances are a key issue for the core design, the evaluation of the thermal stresses on the structures and the decay heat removal systems. This analysis requires a 1D code able to simulate the whole reactor, including the core, the vessel, the piping and the components (turbine, compressors, heat exchangers). CATHARE is the reference code developed and extensively validated in collaboration between CEA, EDF, IRSN and FRAMATOME-ANP for the French Pressurized Water Reactors. CATHARE has the capabilities to model a Gas Cooled Reactor using standard 0D and 1D modules with some adaptations to treat the specificities of the GCR designs. In this paper, the different adaptations are presented and discussed. The direct coupling of a Gas Cooled Reactor with a closed gas-turbine cycle leads to a specific dynamic plant behaviour and a specific turbomachinery module has been developed. The thermal reactors' core consists of hexagonal graphite blocks with an annular-fueled region surrounded by reflectors and a special attention is paid on the thermal modeling of such a core leading to a quasi-2D thermal description. First designs of the VHTR are proposed and are based on an indirect cycle concept with a primary circuit, cooled by helium, and containing the core and a circulator. The core power is transmitted to the secondary circuit via an intermediate heat exchanger (IHX). The secondary circuit contains a turbine and a compressor coupled on a single shaft. It uses a mixture of helium and nitrogen, in order to benefit from both the favourable thermal properties of helium for the heat exchanger, and from existing experience of turbomachines using nitrogen (air). In this context, specific studies are carried out on the gas mixture physical properties to be integrated in the CATHARE code. A large number of studies corresponding to incident or accident transients have already been carried out with CATHARE. Recent studies are presented in this paper: the transient behaviour is described and analysed, the results are discussed in details. They concern both concepts of reactors: the VHTR and the GFR. Based on a first design of the VHTR, a number of possible accident scenarios have been identified, such as Loss of Feed Water, Loss Of Load, Depressurization, and Blackout. The blackout accident has been selected as it is likely to cause the maximum temperature load on the intermediate heat exchanger (IHX). Indeed, the IHX appears to be one of the major technological challenges in the concept, and the study has therefore focussed on its behaviour in accident conditions. In case of GFR, the thermal hydraulic studies are focused on the decay heat removal systems, as the active and passive options must be evaluated. One key problem for the GFR is the possibility of passive decay heat removal after a depressurization of the primary circuit. Different options for the decay heat removal systems are studied including the injection of heavy gas to improve the efficiency of natural convection for passive systems. The behavior of those systems during transient or accident situations is estimated with the CATHARE code. (authors)