Assessment of nominal and off-nominal parameters of a soluble boron free small modular reactor design

Boric acid is traditionally used for reactivity control in pressurised water reactors. However, it increases the rate at which reactor components corrode. This leads to increased operational radioactive waste produced through neutron-activation of corrosion fragments. During reactor operation, radioactive corrosion product deposits, or so-called crud, accumulate on fuel cladding in regions of high burnup. Boron compounds have been detected in such deposits, and they cause axial-offset anomaly, i.e. deviation in axial power prediction. Accumulation of boron-laden deposits can reduce plant efficiency to about 70 % over time. This work presents the design process and analysis of nominal and off-nominal parameters of a soluble boron-free small modular reactor core. The design, and analyses were carried out using CASMO4e, CMSLINK, and SIMULATE3 codes. In addition, a borated variant of the target boron-free core was designed and used as reference. Equilibrium cycles were achieved for both the reference and the boron-free core. The limiting FΔH during the transition to equilibrium was found to be 1.754 and 1.651 for the reference and boron-free cores. The equilibrium FΔH were found to be 1.624 and 1.57 compared to the design limit of 1.7. The hot zero power MTC for the boron-free core at beginning of cycle was found to be –22.67 pcm/K compared to −3.64 pcm/K for the reference. The removal of boric acid made the MTC coefficients more negative and the FΔH lower compared to the reference. The boron-free core had enough rod-worth for power and isothermal defect counteraction and reactor shut down.