Air ingress during the primary pipe rupture accident of an HTGR

The present experiments have been carried out to investigate the air ingress process during the primary pipe rupture accident of a HTGR with a primary focus on the 1st stage of the accident. The experiments were performed with a test facility simulating the HTTR. The following conclusions are derived from the present experimental study. (1) There are two stages in the primary pipe rupture accident. (2) In the 1st stage of accident, air enters the reactor by (a) molecular diffusion, (b) the very slow natural circulation and (c) ordinary natural convection in local space. Of these three processes, (a) and (b) are the rate control process to determine the air ingress rate, which results in a small amount of air ingress rate in the 1st stage. In the 2nd stage of accident, air flows into the reactor by ordinary natural circulation throughout the reactor, which results in a large amount of air ingress rate in the 2nd stage. (3) The duration time of the 1st stage (or the onset time of the second) was 100-300 hours for the present experimental facility. It decreases with an increase of the temperature of the reactor core simulator. (4) In the 1st stage of accident, air does not react with graphite when the temperature of graphite pipes is bellow about 400degC. Only CO2 is generated by the O2-graphite reaction in a 600-800degC temperature range of the graphite pipes. CO as well as CO2 is generated in a 850-1000degC temperature range. The generation ratio of CO to CO2 increases with an increase in the graphite temperature. CO is only generated in a temperature range above 1050degC. (5) In order to predict the concentration of gas species in the reactor, it is necessary to take into account the CO-O2 reaction as well as O2-graphite reaction. (author)