Development of Advanced Cross-Section Model including Control Rod Depletion for Soluble Boron-Free SMR Operation

Abstract

An advanced cross-section feedback method and control rod depletion model are developed for soluble boron-free operation of SMR based on the nodal diffusion code RAST-K. Recently, the demand for small module nuclear reactors (SMRs) has increased, and soluble boron-free operation is widely considered. Since boron cannot be used to control excess reactivity for soluble boron-free operation, the control rod must be inserted into the core at all times. In this case, neutron spectrum is hardened due to control rod insertion, thereby changing nuclear fuel depletion. Additionally, the depletion of the control rod material decreases its value, so it is necessary to consider the effects of fuel depletion with the insertion of the control rod and the depletion of the control rod material. In reactor physics, two-step code systems have been widely used for nuclear reactor core design and analysis with reliable accuracy of solution even with small computational cost. The nodal diffusion code RAST-K has been developed for pressurized water reactor using the group constants generated by neutron transport code STREAM. The latest methodology and technique are adopted for supporting various demands of analyzing the newly reported phenomenon like axial offset anomaly. In addition, the multi-physics coupling is performed based on RAST-K. The verification and validation of RAST- K is also performed for various PWR benchmarks and especially for the cycle depletion of commercial PWR in South Korea. Because traditional cross-section feedback method cannot cover the effect of the rodded fuel depletion, the new cross-section feedback method and control rod depletion model is developed in this research. The microscopic cross-section and number density is extracted from residual macroscopic cross-section to track the effective control rod isotope for independent control rod node with fuel node. In order to consider the effect of control rod material depletion, the branches in terms of control rod depletion quantity are added. Additionally, the effect of spectrum hardening due to control rod insertion can be considered by adding the cross-section set from the rodded fuel depletion. The two sets of cross- sections from the unrodded and rodded depletion are combined using the history index variable. The newly developed cross-section feedback method and control rod depletion model is verified for the multi-cycle depletion calculation of SMR core with respect to direct whole core transport code STREAM3D. The spectrum hardening due to control rod insertion gives negative effect on reactivity, and the depletion of control rod material gives positive effect on reactivity. Therefore, the newly developed method produces similar accuracy on reactivity with traditional method due to error cancellation. However, the accuracy of other variables such as control rod worth and power distribution is significantly improved. The daily load-follow and reactivity initiated transient simulations are also performed using newly developed method.