Calculation of the moderator temperature coefficient of reactivity for water moderated reactors

An American nuclear standard has been recently issued, providing guidance and qualitative recommendations on how the moderator temperature coefficient (MTC) of reactivity should be determined through reactor calculations in water moderated (and cooled) reactor cores (ANS, 1997. Calculation and measurement of the moderator temperature coefficient of reactivity for water moderated power reactors. American Nuclear Society, American National Standard ANSI/ANS-19.11-1997). The present work provides quantitative information on areas of concern and effects addressed in this standard, namely, the effect of the core reflector region and the effect of the magnitude of the moderator temperature change. The calculations consist of a sequence of static criticality analyses, and are performed with a purposely developed two-dimensional reactor model based on two neutron energy groups. The numerical results indicate that the thickness of the reflector has a measurable effect on the accuracy of the MTC value only for a small core, introducing uncertainties of the order of 10%. Instead, the effect of temperature change is found to be negligible within the recommended range of 3 to 5°C.