Evaluation on seismic integrity of HTTR core components

Japan Atomic Energy Agency (JAEA) has carried out research and development to establish the technical basis of High Temperature Gas-cooled Reactor (HTGR) by using High Temperature engineering Test Reactor (HTTR). The HTTR is graphite-moderated and helium gas-cooled reactor with prismatic fuel elements and hexagonal blocks. Here, the graphite block is brittle materials and might be damaged by collision of neighboring blocks by the large earthquake. A seismic observation system is installed in the HTTR site to confirm a behavior of a seismic event. Seismometers are installed in the surrounding foundation at the reactor building. On March 11th, 2011, off the Pacific coast of Tohoku Earthquake (2011 Tohoku Earthquake) of magnitude 9.0 occurred. After the accident at the TEPCO's Fukushima Daiichi Nuclear Power Station, the safety of nuclear reactors is the highest importance. To confirm the seismic integrity of HTTR core components, the seismic analysis was carried out using the evaluation waves based on the relationship between the observed earthquake motion at HTTR site and frequency transfer function. In parallel, confirmation tests of primary cooling system on cold state and integrity confirmation of reactor buildings and component support structures were also carried out. In the evaluation a simple evaluation method was applied by using the response magnification factor analysis based on the calculated stress at the design stage. Because graphite is a brittle material, the stress is assumed in the linear range. To calculate the stress, the calculated stress at the design stage was multiplied to the ratio of the analytical results of seismic force to those at the design stage. The seismic acceleration and the impact force between the graphite blocks are calculated by the seismic analysis of the graphite structure. The calculated stress by the response magnification factor analysis was compared with the criteria. As a result, it was found that the stress values of graphite blocks satisfied an allowable value, and the integrity of the HTTR core components was ensured. The integrity of HTTR core components was also supported by the operation without reactor power in cold conditions of HTTR. The obtained data was compared with the normal plant data before the earthquake. As the result, the integrity of the HTTR facilities was confirmed. (author)