Analysis on temperature rise of primary upper shielding in HTTR

In a performance test of the High Temperature Engineering Test Reactor (HTTR), measured temperatures of the primary upper shielding became higher than predicted temperatures. The various causes have been examined by literature investigation, numerical computation (heat balance calculation, one-dimensional flow-network calculation, two/three-dimensional heat conduction/thermohydraulic calculations), simulation tests. Input values and boundary conditions necessary for two-dimensional thermohydraulic calculation were determined so that the calculated temperatures agreed well with temperatures measured at several points of the primary upper shielding and the standpipes. It is considered from this calculation that the temperature rise of the primary upper shielding results mainly from reduction of a purge-gas flow rate in an annulus conduit inside a standpipe. In addition, a temperature rise of the purge gas flow at the inlet, a temperature rise of a water coolant, existence of thermal gap conductance between the primary upper shielding and a cooling panel and other factors are possible to cause the temperature rise of the primary upper shielding in HTTR. Any obvious reasons why the purge-gas flow rate reduces in the annulus conduit comparing with a calculated one were not found based on phenomena and events reasonable in thermohydraulics and realistic in manufacture. Thus, the temperatures of the primary upper shielding under a normal operation condition of the HTTR have been predicted based on several hypothetical reasons of the reduction of the purge-gas flow rate in the annulus conduit. The purge-gas flow rate in the annulus conduit ranged from 0.899 g/s to 2.69 g/s, and corresponding with these purge-gas flow rates, the concrete temperatures in the primary upper shielding changed from 40degC or less to around 86degC under the normal operation condition of the HTTR. The concrete temperature is found to depend on the reason of the reduction of the purge-gas flow rate. (author)