Duty cycle reactivity control and xenon spatial oscillation stability analysis of a boron-free small modular reactor core

One of the challenges associated with operating a boron-free Small Modular Reactor (BFSMR) is increased use of rod cluster control assemblies (RCCAs) for duty cycle operations and power manoeuver, and burnable poisons for excess reactivity control. Frequent RCCA movement may result in increased xenon spatial oscillations. In this work four LPs with similar thermal output of 1429.51 MW were assessed to determine a candidate LP for the BFSMR core. CASMO4, ROSA and SIMULATE3 were used for fuel assembly modelling, core loading and 3D core simulation and assessment. The LP with 32 feed fuel assemblies and a maximum of 56 ${{Gd}_{2}O}_3$ doped fuel pins in a fuel assembly was adopted as the best LP. A RCCA insertion pattern for duty cycle operation of the candidate LP and the power-dependent rod insertion limit (PDIL) for power manoeuver have been established. In addition, free-xenon stability of core and RCCA movement-initiated xenon transient were assessed. The peak linear heat rate ($PLHR$), enthalpy rise hot channel factor ($F_{\Delta H})\text{,}$ and the heat flux hot channel factor ($F_q)\text{,}$ were used as a criterion for acceptance of RCCA insertion pattern at each burnup step and each power level associated with power manoeuver. The target $PLHR\text{,}$ $F_{\Delta H}\text{,}$ and $F_q$ were set at 42.12 kW/m, 1.65, and 2.6, respectively. The maximum values for $PLHR\text{,}$ $F_{\Delta H}\text{,}$ and $F_q$ associated with the RCCA insertions insertion pattern were found to be 38.2 kW/m, 1.608, and 2.38. The maximum $PLHR\text{,}$ $F_{\Delta H}\text{,}$ and $F_q$ values associated with power manoeuver for the power range 80%–100% were found to be 33.01 kW/m, 1.62, and 2.26 respectively. These values were associated with the 8.5 MWd/kg burnup step – a burnup step at which ${Gd}_2{O}_3$ depleted and the core had the maximum $F_{\Delta H}$ value. The core was found to be stable with respect to free-xenon oscillations and xenon transients, as respective stability indices were found to be negative.