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IRSN, Institut de radioprotection et de sûreté nucléaire

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Enhancing Nuclear Safety


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HEMERA: a 3D coupled core-plant system for accidental reactor transient simulation


Congress title :ICAPP seventh International Conference on Advances in Nuclear Power Plants
Congress location:Nice
Congress date :13/05/2007

Summary

A Nuclear Power Supply System (NPSS) is driven and regulated by several diversified and complex phenomena which are distributed in space and coupled in time in different and somewhat varying ways. In a first approximation, they depend on the reactor type, the core design and lay-out, the fuel features, the coolant, the loading strategy and cycle, the operating mode and, more generally, on the whole status of the system. The main coupling agent in a reactor system is the temperature field inside the fuel, the core and its immediate surroundings. The temperature affects the neutron behaviour, both in normal operation and during the transients, through the cross-sections, which account for the probability of neutrons to interact with matter in every zone and at any time. Thus, temperature is always relevant to normal reactor operation and control, but it may become extremely important and sometime decisive in the transients, mainly the reactivity driven ones, which are characterized by very short response-time and severe power variations. In the framework of their joint venture to develop a system to study reactor transients in “safetyrepresentative conditions”, IRSN and CEA have launched the development of a fully coupled 3D computational chain, called HEMERA (Highly Evolutionary Methods for Extensive Reactor Analyses), based on the French SAPHYR code system, constituted by APOLLO2, CRONOS2 and FLICA4 codes, and the system code CATHARE. It includes cross sections generation, steady-state, depletion and transient computation capabilities in a consistent approach. Multi-level and multi-dimensional models are developed to account for neutronics, core thermal-hydraulics, fuel thermal analysis and system thermal-hydraulics. Currently rod ejection and main steam line break accidents are investigated. The HEMERA system is applied to French PWR. A selection of results for main steam line break conditions are presented along with a comparison of the available levels of simulation, ranging from 0D to 3D and from assembly-wise to pin-wise in he core. The present paper outlines the main physical phenomena to be accounted for in such a coupled computational chain with significant time and space effects and outlines the main assumptions and options adopted in its programming.

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