A Neutronics Exploration of LWR Fuel Assembly Using FCM Fuels for SMR Application

The objective of this work is to explore the new LWR fuel assemblies that are feasible in terms of neutronic parameters for SMR (Small Modular Reactor) application with FCM (Fully Ceramic Micro-encapsulated) fuels. The main design goals are given as follows : 1) long cycle length (Fuel residence time with two-batch refuelling more than 1460 EFPDs (Effective Full Power Day) with a large reactivity margin of 4.76 percent D rho at the discharge) in fuel assembly level depletion calculation, 2) Negative MTC (Moderator Temperature Coefficient) over the fuel residence time, and satisfaction of some thermal-hydraulics constraints. The FCM fuel where TRISO fuel particles are distributed in SiC matrix of high thermal conductivity has been considered as one of ATF (Accident Tolerance Fuel) concepts. However, the low heavy metal density nature of the FCM fuel requires fat pellet design and high density kernel. Thus, particular focus was put on large diameter FCM fuel pins in configurations of the 13x13 and 11x11 rather than conventional 17x17. Also, we considered several different claddings that are being considered as the advanced claddings for ATF fuels. These candidate claddings considered in this work includes FeCrAl, SiC, and SiC with Cr coating. The candidate fuel assembly (FA) designs were selected and screened using results of the preliminary neutronics and TH scoping calculations on the 17x17, 13x13 and 11x11 FAs. Fuel design parameters considered include the fuel rod diameter, clad material (FeCrAl and SiC) and thickness, gap thickness, TRISO packing fraction, and kernel diameter and enrichment, etc. It was found that the 13x13 FA design provides wider feasible range in view of neutronics and TH performance than 11x11 and 17x17 FA designs. The candidate FCM FA designs were further optimized through detailed neutronics exploration at FA level with various BP (Burnable Poison) materials. Sensitivity study was carried out on the impact on the burnable poison with different BP materials (Gd, Er, etc.), BP types (BISO or admixed) and the BP locations in a FA. The results show that FCM FA designs with scattered BPs (Gd2O3, B4C, Er2O3 BISO particles) around water holes can also provide acceptable fuel residence times with acceptable level of maximum pin power peaking factor and negative MTCs.(author).