Large eddy simulation based fire modeling applications for Indian nuclear power plant

Full text of publication follows: The Nuclear Power Plants (NPPs) are always designed for the highest level of safety against postulated accidents which may be initiated due to internal or external causes. One of the external/internal causes, which may lead to accident in the reactor and its associated systems, is fire in certain vital areas of the plant. Conventionally, the fire containment approach and/or the fire confinement approach is used in designing the fire protection systems of NPPs. Indian NPPs (PHWRs) follow the combined approach to ensure plant safety and all newly designed plants are required to comply with the provisions of Atomic Energy Regulatory Board (AERB) fire safety Guide. In respect of older plants, the reassessment of adequacy of fire safety provisions in the light of current advances has becomes essential so as to decide upon the steps for retrofitting. Keeping this in mind the deterministic fire hazard analysis was carried out for the Madras Atomic Power Station (MAPS). As a part of this exercise, detailed fire consequences analysis was required to be carried out for various critical areas. The choice of CFD based code was considered appropriate for these studies. A dedicated fire hazard analysis code Fire Dynamics Simulator (FDS) from NIST was used to perform these case studies. The code has option to use advanced fire models based on Large Eddy Simulation (LES) technique/ Direct Numerical Simulation (DNS) to model the fire-generated conditions. The LES option has been extensively used in the present studies which were primarily aimed at estimating the damage time for important safety related cable. Present paper describes the salient features of the methodology and important results for one of the most critical areas i.e. cable bridge area of MAPS. The typical dimensions of the cable bridge area are (length x breadth x height) of 12 m x 6 m x 2.5 m with an opening on one side of the cable bridge area. With almost equal gap, six numbers of cable trays running longitudinally and arranged equidistant along the vertical direction occupy one side of the room. The other side contains three more cable trays running along the length of the room. All the cable trays contains only the safety related control cables. There is no power cable in the room. Hence pilot fire was assumed to be a trash fire in the centre of the room. The important parameters like temperature, oxygen concentration and velocity transients were monitored at various planes. The temperatures were also monitored on surfaces of all the safety related cables at various locations. The geometry under consideration was discretized into a mesh of 16 X 48 X 12. The analysis raveled that assumed trash fire could damage the nearest cable in about 50 seconds and may generate the secondary cable fire if the damage temperature of 200 deg C is assumed for the PVC cables. However the secondary cable fire has not been modeled, only the minimum damage time for any safety related cable was required to be assesses for subsequent fire PSA studies. The paper briefly describes the field model based fire modeling used in the code FDS along with its advanced features. The cable bridge area of MAPS is subsequently depicted in terms of methodology of analysis and the results obtained. The studies carried out and reported in this paper were subsequently used for the detailed fire PSA of MAPS. (authors)