Development of accident tolerant control rod for light water reactors

For an accident tolerant control rod (ATCR) designed to improve LWR (light water cooled reactor)core safety, the following inherent characteristics are required: sufficient neutron absorption, high-temperature stability and high miscibility with fuel materials to avoid uncontrollable recriticality. In the present work an ATCR is being developed to improve the safety of light water reactors under all conditions including severe accidents. Promising neutron-absorbing materials for the ATCR include rare-earth sesquioxides (RE₂O₃), hafnium dioxide (HfO₂) and mixtures of these materials. A reactivity worth analysis of these materials in representative light water reactors revealed that a conventional control rod can be replaced by an ATCR without significant impact on the reactor operation conditions. A high-temperature compatibility of the candidate ATCR materials and iron, which is a main component of control rod cladding, was examined experimentally, and no significant reaction occurred upon heating to 1600 Celsius degrees for 1 hour. That is, the ATCR is expected to have no significant damage prior to extensive fuel pin failure in severe accidents, thus avoiding uncontrollable recriticality after reactor shutdown even in case of injection of non-borated or insufficiently borated water for emergency cooling of the reactor