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Conference Article: Core physics tests of THTR pebble bed core at zero power

Abstract

In September 1983 first criticality of the THTR 300 was reached with 198,190 spherical elements. The core was fully loaded with 674,200 spherical elements in October 83. During this period core physics tests verified the core design and the shut down margins of the reactor at ambient temperature and in 1 bar air. The following tests were performed: Core loading until first criticality. Height-dependent reactivity. Reactivity worth of reflector rods in these conditions. Core fully loaded and critical rod configuration obtained. Reactivity worth of reflector rods with core fully loaded. Incore neutron detectors were used together with an incore neutron source to monitor the criticality conditions. The measurements were performed with the method of subcritical multiplication and the inverse kinetic method. Special problems had to be taken into consideration, for instance a higher density of the pebble bed in the lower part of the core. The initial criticality could be predicted better than 0,004Δk. The core physics tests at zero power were finished in January 1985. During this period the following tests were performed: Temperature coefficient until 240 deg. C. Excess reactivity of the core at ambient temperature. Reactivity worths of different rod configurations. Reactivity worth of helium. The methods of measurement were the same as in the previous period. Additionally nitrogen was used as an absorber gas. By determining the nitrogen reactivity worth coefficient it was possible to measure the excess reactivity of the core at ambient temperature and different rod configurations by adding nitrogen to the core up to 16 bar

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Reference:

Specialists' meeting on safety and accident analysis for gas-cooled reactors. Oak Ridge, TN (USA). 13-15 May 1985

International Atomic Energy Agency, Vienna (Austria)

IAEA-TECDOC--358, pp:285-298