Safety Analysis for Hot Gas Duct Vessel in HTR-PM

Abstract

The hot gas duct vessel (HGDV) is an important part of the high-temperature reactor–pebble-bed module (HTR-PM) primary loop pressure boundary system. It connects the reactor pressure vessel (RPV) and steam generator pressure vessel. Because the dimensions of the HGDV are smaller than those of the other two vessels, it is often considered the weakest of the three vessels. Therefore, the safety of the HGDV has become one of the most important issues in the design of the HTR-PM. In the present paper, a comprehensive safety analysis of the HGDV in the HTR-PM was performed with an emphasis on the structural features. The designs of the HGDV and the support system of the primary loop pressure boundary are first described. A preliminary safety analysis of the HGDV, including the stress calculations and leak-before-break (LBB) analysis, is then presented. The results show that the stress levels of the HGDV under various accidents have a safety margin of at least 55.3% compared with the stress limits specified in American Society of Mechanical Engineers code, and the stress intensity factor at the postulated flaw is less than the critical stress intensity factor. The LBB analysis indicated that the leak detection system is capable of detecting leaks caused by a postulated through-thickness crack in the HGDV before it reaches the critical size. Although the preliminary analysis has proved the safety of the HGDV, the consequences of a hypothetical HGDV double-ended break accident were also studied to further investigate the safety features of the HTR-PM. Several mitigation measures were employed based on the original design. The structural integrity of the support system, the reactor internals, and the containment under double-ended break accident were evaluated. The results show that these main structures could maintain integrity under the HGDV double-ended break accident.