Transient events modeling for the 400 MW pebble bed modular reactor

Full text of publication follows: The Pebble Bed Modular Reactor (PBMR) is a high temperature, helium cooled, graphite moderated pebble bed reactor, using a multi-pass fueling scheme. An important indicator of the design safety margin is the maximum fuel temperatures reached during a variety of transient conditions. Possible severe transient events include Pressurized and De-pressurized Loss of Forced Cooling (PLOFC and DLOFC), as well as fast reactivity insertions such as Total Control Rod Withdrawal (TCRW) scenarios. Analysis of operational modes such as a 100%-40%-100% load follow pattern can also be used to assess the stability of the design in regard to potential Xenon oscillations. The aim of this paper is to present the PBMR transient results for these four main events, and to quantify the effects of changes in the values of various thermal hydraulic and material parameters on the transient results. The overall temperature changes in graphite moderated HTGR's are normally very slow during transients, due to the large heat capacity of graphite. However, due to the large negative fuel temperature coefficient any changes to the UO2 kernel temperature will result in an immediate feedback to the global nuclear behavior of the reactor. The coupled neutronic and thermal-hydraulic code TINTE was specifically developed to assess the complex transient behavior of pebble bed high temperature reactor designs. TINTE calculates time-dependent neutron fluxes, heat source distributions and heat transfer rates between solids and gases in a 2-D r-z geometry to obtain the global transient core temperature behavior. The code was developed by KFA (Kernforschungsanlage), today Forschungszentrum Juelich, over many years and obtained by PBMR (Pty) Ltd under a license agreement, and incorporates numerous material property correlations for graphite and other core structures, including the temperature and fast-fluence dependence of the pebble bed effective thermal conductivity. A number of DLOFC calculations were also performed to assess the effects of changes in the values of various thermal hydraulic and material parameters on the transient evolutions, including the maximum and average fuel temperatures during a DLOFC. Parameters investigated include the change in thermal conductivity caused by a variation in the fast-fluence exposure of the graphite reflectors, and the effect of variations in the thermal conductivity, emissivity and specific heat capacity of the helium coolant, stainless steel and graphite structures. Another important contributor influencing the transient behavior is the pebble bed effective thermal conductivity, and in this study the Zehner-Schluender, Robold and Breitbach-Barthels correlations are used to quantify the importance of this parameter for a DLOFC transient. The results indicate that the DLOFC maximum fuel temperatures remain below acceptable levels for all the transient cases, and that the axial Xenon oscillations are strongly damped. (authors)