Proposal of safety demonstration test plan of HTTR by cold test of loss of forced cooling with vessel cooling system inactive

The safety demonstration tests have been carried out using HTTR to demonstrate inherent safety of HTGRs. The loss of forced cooling (LOFC) test with vessel cooling system (VCS) inactive was planned at the initial reactor power of 30% (9 MW), in which all of 3 gas circulators are tripped to stop the forced cooling and make the VCS inactive. In this condition the decay heat can not be removed from the reactor core through the reactor pressure vessel (RPV) by any active and passive equipment. In this test, the temperatures of water cooling tubes of VCS are expected to increase. The safe and promising test procedure to carry out the test was proposed by investigating the method to make the temperature of water tube not exceed the operational limit, and technical items were extracted. The cold test simulating the LOFC test with VCS inactive was carried out to investigate the following three items. (1) To acquire test data such as temperatures of water cooling tubes of VCS to improve prediction by numerical analysis to reflect to improve test plan. (2) To confirm the safe and promising test procedure which was proposed to carry out the test. (3) To investigate natural convection of cooling water in VCS to lower temperatures of water cooling tubes such as header and biological shielding. The cold tests were carried out without nuclear heat in core to simulate the LOFC with VCS inactive, in which the two water cooling trains were stopped, to acquire the temperature data of such components as water cooling tubes as header and biological shielding, under the conditions of 120 and 150°C at the reactor inlet temperature measuring position. The safe and promising test procedure to carry out the LOFC test with VCS inactive, which was the technical item to propose the safety demonstration test plan, was verified through the cold test. (author)