External coupling of modern and legacy multi-physics codes with application to PBMR
Odendaal, Johannes Abraham
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The Pebble Bed Modular Reactor (PBMR) is a power plant that utilizes a High Temperature Reactor (HTR) and a closed-cycle gas turbine power conversion system. Many man-years of compiling a safety analysis report are involved in obtaining a licence from a national nuclear regulatory authority. Multiple computer code systems must be integrated to perform the required calculations. In PBMR (Pty) Ltd, this integration is currently performed offline, without the ability to capture time-dependent circular feedback effects. The main codes used for transient calculations involving the reactor are the 2D neutronics code TINTE and the thermal-fluids systems code Flownex Nuclear. A time history trace of thermal-fluid boundary conditions is first calculated by Flownex Nuclear and then used by TINTE. The purpose of this research was to establish an integrated time-dependent design and safety analysis capability, with the ability to perform detailed simulation and control of all reactor plant modes and transitions. The Nuclear Engineering Analysis (NEA) Simulator, which is based on existing simulator architecture, was established to integrate and coordinate these types of analyses. The NEA Simulator enabled progressive development of the integrated simulation, with a fundamental understanding of external coupling of these codes. TINTE-calculated reactor outlet temperature and total power are used to influence time-dependent system flow behaviour. Variable simulation time steps are introduced to accelerate integrated simulations. Continuous local time-step management for the TINTE temperature solver is implemented in the TINTE-NEA Simulator interface to make it resilient. Explicit external coupling of two open-loop thermal-fluid models with circular dependencies produced acceptable results for fixed pressure transients, but not when pressure was solved. The parallel coupling was the most stable and versatile coupling analysed in this research; it also works for transients where pressure is solved. Important building blocks were established to simulate plant modes and transitions continuously.
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