CANDU 3 Reactivity Mechanism Deck Shielding Analysis

Abstract

The reactivity mechanism deck of the CANDU 3 reactor is a part of the top primary shield system. It complements the shielding provided by the water in the shield tank extension, a vertical cylindrical tank above the reactor calandria vessel, which contains reactivity mechanisms. The dose rates above the reactivity mechanism deck are mainly set by the neutron and gamma radiation streaming through the reactivity control unit (RCU) penetrations, although radiation transport through the bulk shield makes a small contribution. The reactivity mechanism deck is designed for dose rates less than 250 uSv/h to allow operator access for inspection of reactivity mechanism drives during reactor operation. This paper presents the methodology used to calculate these dose rates. The shielding cod ANISN, DOT 4.2, ORIGEN, QAD-CG, and the standard calculational techniques have been used far this purpose. The calculation indicated that the reactivity mechanism deck, with the RCU penetrations and their shielding arrangements, provides a satisfactory top shielding configuration and reduces dose rates to acceptable levels on the reactivity mechanism deck during reactor operation.