Simulations of Air and Water Ingress Transients for the Pebble Bed Modular Reactor (PBMR) by Means of the TINTE Code

Abstract

In this study several air and water ingress scenarios for the PBMR [1] were simulated by means of the dynamic reactor code TINTE (TIme-dependent Neutronics and TEmperatures) [2]. The Power Conversion Unit (PCU) and other sub-systems cannot be modelled with the TINTE code and therefore air ingress rates were obtained from Computational Fluid Dynamics (CFD) analysis performed by utilizing the FLUENT code [3]. The use of the TINTE code was previously validated with simulations of the NACOK corrosion experiments [4], [5], [6]. The validations were performed at Forschungszentrum Juelich, however the results are not yet published. The rates of chemical reactions between graphite and gases like O2, CO2, H2O and H2 are negligible below 400°C. Air and water ingress into the PBMR core at high temperatures can result in corrosion of the PBMR fuel spheres and a possible increase in the fission product release rate. The air ingress scenarios included in this study are; a break in the core outlet pipe at the turbine inlet location, which results in air ingress from the outlet plenum, and a break in a pipe that is connected to the top of the Reactor Pressure Vessel (RPV), which results in air ingress from top of the core. For both transients it is assumed that a Depressurized Loss of Forced Cooling (DLOFC) event takes place prior to the air ingress. The DLOFC leads to high fuel and reflector temperatures that allow higher oxidation rates. The results show that the oxidation of graphite structures in the core is more severe in the case of the outlet pipe break transient. A break in the Core Conditioning System (CCS) heat exchanger circuit during a maintenance mode or following a reactor trip could result in water ingress of up to 1000 kg into the core (the primary system is depressurized at this stage). During the water ingress the CCS continuously cools down the core. Due to the low water ingress rate and lower fuel temperatures, the water ingress transient is not as severe as the air ingress transients.