Flownex analysis of high temperature test reactor thermo-hydraulic benchmarks

Abstract

The High Temperature engineering Test Reactor (HTTR) is an experimental High Temperature Gas-cooled Reactor (HTGR) built by the Japanese Atomic Energy Research Institute (JAERI) to facilitate tests of HTGR technology. One of these test activities involves the validation and verification of thermo-hydraulic codes used in the design of similar HTGR plants. This report details the benchmarking of the Flownex simulation package as used by PBMR (Fly.) Ltd., a South African company developing another type of HTGR known as the Pebble Bed Modular Reactor. The benchmark is of a loss-of-off-site-power event that was tested at the HTTR facility. The event involves a cut of the electric power supply to the circulators, a reactor SCRAM and the activation of the Auxiliary Cooling system to remove decay heat. The need for verification of thermodynamic software is very important in modem nuclear power plant designs, as so much depends on the results produced. Any errors in these results can have serious economic and safety consequences. This report firstly discusses the background of the study, elaborating on the need for the work and the benefit that can be derived from it. Thereafter the process of software verification and validation (V&V) is discussed so that the need for V&V may be clearly understood. Various modelling and simulation methods are then compared, to provide an idea of the work already done in this field. Following this more detail is given on the HTTR test plant and how it is modelled in Flownex. This model is then used for both steady-state and transient simulations, the results of which are then compared with test data. With some exceptions, the study shows that the simulation results are very close to the measured data. Differences are of such a magnitude that they may be attributed to instrumentation inaccuracies. The study contributes to the field in that the methodology of analysing thermo-hydraulic systems is further broadened. The conclusions drawn from this study are aimed at the simulation design engineer, to help him or her understand similar problems and to find solutions faster.