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 HTTP has achieved the first criticality on 11/10/98. 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 to be a core type of future HTGR. Fuel loading schedule was planned based on preliminary calculations by Monte Carlo method. These calculations predicted the first criticality at 16 ± 1 columns. However, the reactor achieved the first criticality at 19 columns. The first criticality was re-predicted by comparing measured and calculated 1/M curves. The calculated 1/M curves were obtained for different critical mass adjusting some parameters such as the amount of impurities, etc. The method is called "1/M sandwich method". This method well predicted the number of fuel columns at the first criticality. The combination of this method with Monte Carlo calculation was a rational method for predicting the first criticality. It was confirmed that Monte Carlo calculation could be used for the evaluation of HTTR with < 1% of error for fully loaded core.