Transient-based evaluation of safety margins and operational limits for a boron-free small modular reactor core

Boron-free small modular reactor cores (SMR) experience high thermal feedback due to their high negative moderator temperature coefficients (MTC). In this work, operational limits and safety margins are proposed for a 1,429.51 MWth boron-free SMR. The core (at full power) has a span of high negative MTC values throughout the cycle with BOC, 8.5 MWd/kg, and EOC values of –32.41 pcm/K, –32.19 pcm/K, and −37.81 pcm/K respectively. A core with a high negative MTC is prone to instantaneous reactivity insertion in an event of a drop in coolant inlet temperature. In this work, a reactivity insertion accident (RIA) defined by a rupture in the main steam line has been applied as an initiating event for a drop in coolant inlet temperature. Seven cases were simulated − five cases represented a drop in coolant inlet temperature over the range of 10–50 K, and two cases represented the normal operating condition and the drop in coolant inlet temperature of 21.67 K which was the threshold for violating the heat flux hot channel factor ($F_q)$ design limit. Parameters considered were heat flux hot channel factor ($F_q$), core thermal power, enthalpy, fuel temperature, fuel centerline temperature, and minimum departure from nucleate boiling ratio (MDNBR). SIMULATE3-K, a thermal–hydraulic code, and SIMULATE3, a neutronics code, were coupled and used to simulate the transient. The transient was performed at end of cycle (EOC) hot full power (HFP) condition, as this condition provided the most limiting parameter response to the transient. The Rohsenow-Griffith-Kutateladze (RGK) correlation was employed to calculate the critical heat flux (CHF) applicable to the fuel pin heat transfer regime. A combination of best estimate and conservative estimates was applied in establishing operational limits and safety margins. Nominal values for the $F_q$, fuel temperature, fuel centerline temperature, enthalpy and DNBR were found to be consistent and within the range of conventional light water reactor operational limits and safety margins. The most limiting parameter was observed to be the $F_q$, whose design limit of 2.6 was violated with a drop in coolant inlet temperature of 21.67 K. A conservative drop in coolant inlet temperature of 10 K was proposed as an operational limit for the boron-free SMR.