A few reactor physics experiments related to the HTGR safety at VHTRC

Reactor physics experiments have been widely conducted at an HTGR-type critical assembly (Very High Temperature Reactor Critical Assembly, VHTRC) for the purpose of verifying the neutronic design of the High Temperature Engineering Test Reactor(HTTR) and also for improving the accuracy of safety-related reactor physics calculations of HTGRs. This paper introduces recent experimental works at the VHTRC. These works are of significant importance on the design and operation of practical HTGRs. The first subject describes an experiment simulating water ingress into an HTGR. Foamed polyethylene pellets were inserted in void regions in the core and reflector. Reactivity effects of the pellets were measured by a criticality method. The experimental results were compared with calculation results obtained by using SRAC code with the ENDF/B-IV based nuclear data library. The second subject describes an experiment on the VHTRC core partially inserted with an HTTR-mockup control rod. Thermal neutron flux distributions were measured by the foil activation method. The distribution were distorted strongly by the strong neutron absorption of the rod. Besides, Critical mass of 235U for the core was also compared between experiment and calculation. The results are summarized in the following: 1. The effect of water ingress into core region was well-estimated by the calculation. The effect of water ingress into the reflector region was very small. The calculation gave a comparatively poor guess to this effect. 2. Thermal neutron distribution was distorted very strongly by the partially-inserted control rod. The calculation gave a satisfactory guess. 3. Reactivity effect of partially-inserted control rod was predicted well by the calculation in spite of the uneven distribution of strong neutron absorption. (author)