APPLICATION OF MCNP FOR PREDICTING POWER EXCURSION DURING LOCA IN ATUCHA-2 PHWR

This thesis consists in the developing and application of a methodology based on MCNP code to predict power and reactivity excursions depending on different representation of physical variables. The reference application of the methodology is the DEGB-LBLOCA scenario in ATUCHA-2 PHWR in the framework of the safety analysis of ATUCHA-2 nuclear power plant (NPP) (Chapter 15 FSAR). Loss of coolant accidents (LOCA) mean those postulated accidents that result from the loss of reactor coolant at a rate in excess of the capability of the reactor coolant makeup system from breaks in the reactor coolant pressure boundary, up to and including a break equivalent in size to the double-ended rupture of the largest pipe of the reactor coolant system (DEGB LB-LOCA). In the ATUCHA-2 PHWR the insertion of positive reactivity caused by the void production in coolant channels is compensated mainly by the negative reactivity inserted by shutdown system based on the fast injection of boric acid solution into the moderator tank (JDJ). Analysis of JDJ by a computational fluid dynamics code showed a complex spatial Boron distribution inside the moderator tank. The so called “Boron self-shielding effect” is indicating the over-estimation of the inserted negative reactivity due by the dilution of the highly concentrated Boron solution inserted by the JDJ when modeling its spatial distribution using thermal-hydraulics nodes of large dimensions (order of liters for the ATUCHA-2 case). The availability of calculation methods with the capability of representing the system with a level of detail beyond the actual generation of system thermal hydraulics code such the Monte Carlo based MCNP code is required in this specific event scenario. The developed methodology is focused on the set up of tools for advanced three-dimensional coupling between MCNP and a computational fluid dynamics code. A key feature of the methodology is the capability to generate MCNP based representations of complex and heterogeneous spatial distribution of physical variables. The use of this feature combined with the methodology capability to use different level of representation detail permits the investigation of power and reactivity excursions at core level in the analyzed scenario and also the prediction of JDJ inserted reactivity at the beginning of the transient. The development of MCNP5 models for simulation of ATUCHA-2 in realistic operational core conditions is a part of the methodology. Hence, research activity was focused also in the combined use of neutronic codes such as NJOY and MONTEBURNS to implement burnup effects and the thermal-hydraulic boundary conditions provided by 3D neutron kinetics coupled thermal-hydraulics (3D NK-TH) RELAP5-3D© system codes.