A Flownex uncertainty analysis of a depressurised loss of forced cooling event for the PBMR

Abstract

The world is currently experiencing an energy crisis. To cope with the rising demand in South Africa, nuclear power was identified as a clean, safe and reliable source of electricity. The Pebble Bed Modular Reactor (PBMR) is an inherently safe, next-generation nuclear power plant that uses pebble fuel. In the event of a depressurized loss of coolant (DLOFC) accident, the reactor will passively cool itself, and remain within safe limits. The main purpose of this dissertation was to perform an uncertainty study on the PBMR reactor during a DLOFC accident to demonstrate this safety feature. An extensive literature survey was carried out to research the concept of uncertainty, methods for addressing uncertainty and to gather the required input data to set up a model of the PBMR reactor. The model requirements were established by use of a systematic PIRT process. A detailed model of the reactor was set up in Flownex after making the necessary assumptions and simplifications. A sensitivity and Monte Carlo sampling platform was set up in conjunction with Flownex in order to perform the uncertainty study. During the DLOFC transient, the best-estimate maximum fuel, core-barrel and RPV temperatures reached 1529, 621 and 490’C respectively. Sensitivity studies showed that the parameters that most strongly influence the results are the power profile, decay heat, pebble bed effective conductivity and the properties of the graphite reflector. Variations in fluid properties had a negligible influence on the DLOFC results. Statistical processing of the Monte Carlo simulation results provided uncertainty bands for each output. The conclusion was that with 95% confidence, there is a 5% probability of exceeding maximum fuel, core-barrel and RPV temperatures of 1582, 638 and 503 CC respectively. All three of these temperatures are below the maximum allowable temperature for each respective component. Thus all three components will stay within their code cases during the unlikely event of a DLOFC. The final effort in this study went to verification and validation (V&V) of the results. This process included V&V of the input data, software, the calculation and the model development. These processes included: a detailed internal review; comparison with analytical solutions; comparison with alternative independent calculations; and comparison with experiment. The effective pebble bed thermal conductivity is currently being validated via construction of the Heat Transfer Test Facility (HTTF). The large extent of V&V activities that have been carried out provides a high level of confidence that the results produced in this dissertation are satisfactory, if not slightly conservative.