HTGR Knowledge Base

Conference Article: Physical status of the 10 MW test module.

Abstract

Like most graphite moderated HTR systems, the 10 MW Test-Module reactor is undermoderated. This means if the water ingress is into the pebble bed that this system will gain reactivity as moderator is added to the core. The reactivity increase caused by water ingress strongly depends on the geometry of the core, the temperature, the burnup status and especially the moderator to fuel ratio (the metal content per fuel element). For the equilibrium core 5 gram of heavy metal per fuel element is chosen in order to limit the effect of water ingress. Another important effect of water ingress is to reduce the worth of control rods which are usually located in the reflector. The distance between core and control rods in the reflector are carefully calculated. In the 10 MW Test-Module reactor the core shutdown capability of 12 control rods are selected such that any accident reactivity can be counterbalanced and only half of the rods have to be available to render the reactor down from normal operation to the cold, permanent subcritical condition.

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Reference:

Uncertainties in physics calculations for gas cooled reactor cores. Proceedings of a specialist's meeting held in Villigen, Switzerland, 9-11 May 1990

International Atomic Energy Agency, Vienna (Austria). International Working Group on Gas-Cooled Reactors

IWGGCR--24, pp:66-72