Safety characteristics of mid-sized MOX fueled liquid metal reactor core of high converter type in the initiating phase of unprotected loss of flow accident. Effect of low specific fuel power density on ULOF behavior brought by employment of large diameter fuel pins

Safety characteristics in core disruptive accidents (CDAs) of mid-sized MOX fueled liquid metal reactor core of high converter type have been examined by using the CDA initiating phase analysis code SAS4A. The design concept of high converter type reactor core has been studied as one of options in the category of sodium-cooled reactor in Phase II of Feasibility Study on Commercialized Fast Reactor Cycle System. An unprotected loss-of-flow accident (ULOF) has been selected as a representative CDA initiator for this study. A core concept of high converter type, which employed a large diameter fuel pin of 11.1 mm with 1.2 m core height to get a large fuel volume fraction in the core to achieve high internal conversion ratio was proposed in JFY2001. Each fuel subassembly of the core (abbreviated here as UPL120)was provided with an upper sodium plenum directly above the core to reduce the sodium void reactivity worth. Because of the large fuel pin diameter, average specific fuel power density (31 kW/kg-MOX) of UPL120 is about one half of those of conventional large MOX cores. The reactivity worth of sodium voiding is 6$ in the whole core, and -1$ in the all upper plenums. Initiating phase of ULOF accident in UPL120 under the conditions of nominal design and best estimate analysis resulted in a slightly super-prompt critical power burst. The causes of the super-prompt criticality have been identified twofold: (a) the low specific fuel power density of core reduced the effectiveness of prompt negative reactivity feedback of Doppler and axial fuel expansion effects upon increase in reactor power, and (b) the longer core height compared with conventional 1m cores brought, together with the lower specific power density, a remarkable delay in insertion of negative fuel dispersion reactivity after the onset of fuel disruption in sodium voided subassembly due to the lower linear heat rating in the top portion of the core. During the delay, burst-type fuel failures in sodium un-voided ro partially voided fuel subassemblies caused a high ramp rate insertion of sodium void reactivity due to fuel-coolant interactions, resulting in super-prompt critical power burst. Taking into account the ULOF analysis result of the UPL120 core, two alternative concepts of mid-sized core of high converter type were proposed in JFY2002, increasing the average specific fuel power density to 43 kW/kg and reducing the core height to 1 m and 0.8 m. These two cores, abbreviated here as Fsm 100 and Fsm 80, have core void reactivity worth of 5.7$ and 4.9$, respectively. The ULOF behaviors in initiating phase of both cores resulted in a mild power burst with net reactivity less than 1 $ under nominal design and best estimate analysis conditions, and even under conditions with a conservative 2σ uncertainty on Doppler constant, or on sodium void reactivity worth, the cores showed a mild power burst without exceeding prompt-criticality, though the conservative conditions resulted in a severer power burst. Hence the initiating phase of these alternative cores develops into a transition phase of ULOF scenario. Another parametric study showed that if the specific power density of these alternative cores was to be reduced by 20% from its 43 kW/kg to 34 kW/kg, the ULOF power burst became super-prompt critical. This study has shown that in the core design of high converter type with a large diameter (>10 mm) fuel pins, the specific power density of fuel is an important design parameter which affects the safety characteristics of the core in ULOF events, and the above mentioned ULOF results should be taken into account in the continuing core design study. (author)