Start-up core physics tests of high temperature engineering test reactor (HTTR), (II) - First criticality by an annular form fuel loading and its criticality prediction method

The High Temperature Engineering Test Reactor (HTTR) has achieved the first criticality on November 10, 1998. The fuels were loaded into the core from outer region to inner region to obtain characteristic data of annular cores. The annular core is expected as a core type of future HTGRs. Fuel loading schedule was planned based on preliminary calculations by Monte Carlo calculations. The first criticality was predicted at 16+/-1 columns by the preliminary calculations. Nevertheless, the reactor achieved the first criticality at 19 columns. The reasons were considered that the number of fuel columns at the first criticality was sensitive to small calculational error because an increase in Iz,fr was slow near the criticality, etc. The first criticality was re-predicted by comparing measured 1/M curve and calculated 1/M curves. The calculated 1/M curves were obtained for different critical mass adjusting some parameters such as an amount of impurities, etc. The method is called as "1/M sandwich method". The number of fuel columns at the first criticality was well predicted by the 1/M sandwich method. The combination of this method and the criticality prediction by Monte Carlo calculation was rational method for first criticality prediction. Moreover, the Monte Carlo calculation was revised on amount of materials, i.e., impurity of graphite, etc. It was confirmed that Monte Carlo calculation could be used for the evaluation of HTTR with less than 1 %Delta k/k of error for fully-loaded core.