Modular High-Temperature Gas-Cooled Reactor Short-Term Thermal Response to Flow and Reactivity Transients

Abstract

The research repomd here has been conducted at the Oak Ridge National Laboratory for the Nuclear Regulatory Commission's Division of Regulatory Applications of the Office of Nuclear Regulatory Research. Tbe short-tenn thermal response of the Modular High-Temperature Gas-Cooled Reactor (MHTGR) is analyzed for a range of flow and reactivity transients. These transients include loss of forced circulation without scram, spurious withdrawal of a control rod group, moisture ingress, control rod and control rod group ejections, and a rapid core cooling event. For each event analyzed, an event description, a discussion of the analysis approach and assumptions, and results are presented. When possible, results of these analyses are compared with those presented by the designers in the MHTGR Preliinary Safety Information Document and in the MHTGR Probabilistic Risk Assessment. The importance of inherent safety features is illusrrated, and conclusions are presented regarding the safety performance of the MHTGR. Recommendations are made for a more indeptb examination of MHTGR response for some of the analyzed transients. The coupled heat transfer-neutron kinetics model is described in detail in Appendix A.

Physics of nuclear reactor safety

“…The fuel elements contain multiply coated fuel particles with a diameter of about 1 mm embedded in a graphite matrix. With regard to the fuel elements there are two variants: (i) in the German HTGZ design (Kugeler and Schulten 1989) fuel spheres are used with a diameter of 60 mm, forming a 'pebble bed' in the reactor core with the possibility of continuous loading and unloading of file1 spheres during full reactor operation, and compacted in a graphite matrix into fuel rods and loaded in hexagonal graphite blocks (Cleveland 1990).…”

“…Even without scramming by control of safety rods, the reactor is shut down and reaches decay power levels after a few minutes. The inherent the reactor subcritical for many hours (Nabbi 1984, Cleveland 1990). An example of the short time response to a loss of coolant flow without scram is shown in figure 14 with the separate contributions of the relevant physical phenomena leading to shutdown.…”

“…An example of the short time response to a loss of coolant flow without scram is shown in figure 14 with the separate contributions of the relevant physical phenomena leading to shutdown. Only the reactivity due to moderator heat-up is slightly positive because of the slightly positive moderator temperature coefficient at operating temperatures, but this e&ct isamply compensated by the Doppler effect i s the fuel and the xenon build-up (Cleveland 1990). The figure shows that loss of cooling leads to a very limited increase of core temperature.…”

“…Conceivable ingress rates lead to fairly slow transients and not to large power excursions. Cleveland (1990) showed that the reactor power increase for z moisture ingress event accompanied by a failure-to-scram can be terminated simply by stopping the helium circulators, making use of the inherent shutdown capability of the HTGR. Kroger and Schulten (1990) argued that water ingress is not so relevant from the reactivity point of view or the point of view of corrosion of fuel elements, but need to be considered carefully with regard to pressure increase and production of explosive gas mixtures (water gas).…”