Large-Break LOCA Assessment for the Highly Advanced Core Design

Abstract

Over the course of the years, a conceptual highly advanced core (HAC) reactor has been designed for Japan's Electric Power Development Company Limited (EPDC). The HAC reactor, which is capable of generating 1326 MW of electrical power, consists of 640 CANDU-type fuel channels with each fuel channel containing twelve 61-element fuel bundles. As part of the conceptual design study, the performance of the HAC reactor during a large loss-of-coolant accident (LOCA) was assessed with the use of several computer codes. The SOPHT, CATHENA, ELOCA and ELESTRES computer codes were used to predict the thermalhydraulic behaviour of the circuit, thermalhydraulic behaviour of single high-power channel, thermal-mechanical behaviour of the outer fuel elements contained in the high-powered channel, and the steady-state fuel-element conditions respectively. The LOCAs that were analyzed include I 00% reactor outlet header (ROH) break, and a survey of reactor inlet header (RIH) breaks ranging from 5% to 25%. The conceptual feasibility of the HAC design was evaluated against two criteria; namely, maximum sheath temperature less than 1200 degrees C and AECL's 5% sheath straining criterion to assess failure by excessive straining. For the cases analyzed, the analysis predicted a maximum sheath temperature of 820 degrees C and a maximum sheath strain of 1.5% (the maximum pressure-tube temperature was 515 degrees C). Although the maximum element-burnup of the HAC design is extended beyond the CANDU 6 burnup, the maximum linear power of HAC (40 kW/m) is significantly lower than the maximum linear power of a CANDU 6 reactor (60 kW/m). The reduced element-power level in conjunction with internal design modification for the HAC design has resulted in significantly lower internal gas pressures under steady-state conditions, as compared with the CANDU 6 design. During a LOCA, the low linear powers and zero-void reactivity associated with the HAC design has increased the safety margin. In addition, the cases analyzed indicate that the reactor conditions (reactor power transient and flow conditions) during the transient do not lead to any significant increase in fuel volumetric-average temperature relative to the steady-state.