Comparative study on nuclear characteristics of APR1400 between 100% MOX core and UO2 core

Recently, APR1400 won the European Utility Requirements (EUR) certification proving the capability of 50% Mixed Oxide (MOX) core design with 18 months cycle length. Several researches show that nuclear characteristics of 30% MOX core is similar to UO2 core. Nonetheless, neutron spectrum hardening effect in MOX core would change many nuclear design parameters related to reactivity in adverse direction as MOX core loading increases up to 100%.

This paper investigates the performance of APR1400 with 100% MOX fuel, regarding reactivity related nuclear design parameters such as Moderator Temperature Coefficient (MTC), Fuel Temperature Coefficient (FTC) and ShutDown Margin (SDM). The investigation begins with evaluating the nuclear design parameters of 16 × 16 MOX fuel assembly, with respect to Moderator to Fuel Ratio (MFR) and compares with the nuclear design parameters of UO2 fuel assembly.

APR1400 performance with 100% MOX fuel is also investigated by evaluating the nuclear design parameters of an initial cycle and an equilibrium cycle satisfying nuclear design requirements. For this purpose, loading patterns for the initial cycle and the equilibrium cycle are developed using CASMO-4 and SIMULATE-3. This research reveals that MOX core has larger optimum moderation point, more negative MTC. Furthermore, neutron spectrum hardening effect make BA and control rod worths smaller than UO2 core and thus SDM becomes the most limiting nuclear design requirements.

Finally, this research proves that 18 months cycle with 100% MOX core can be design for APR1400, without breaking all design requirements: 18 months cycle length, pin peaking factor less than 1.55, negative MTC and FTC, and SDM greater than 5500 pcm.

Abbreviations
APR1400, advance power reactor 1400; BA, burnable absorber; BOC, beginning of cycle; CBC, critical boron concentration; CEA, control element assembly; EOC, end of cycle; EUR, European utility requirement; FTC, fuel temperature coefficient; KUR, Korean utility requirement; LWR, light water reactors; MFR, moderator to fuel ratio; MOX, mixed oxide; MTC, moderator temperature coefficient; OMP, optimum moderation point; PWR, pressurized water reactor; RIA, rod insertion allowance; SDM, shutdown margin; HFP, hot full power
Keywords
Mixed oxide (MOX); APR1400; Shutdown margin