Feasibility of innovative design concepts of Burnable poison pins for 24-month cycle PWR

Abstract

In this study, two innovative burnable poison (BP) design concepts were introduced for an extended cycle of PWR. They are Double Tube Burnable Poison (DTBP) and Burnable poison Attached to Guide tube (BAG). DTBP is specifically designed for the 17 × 17 Westinghouse (WH) fuel assembly, and BAG is for the 16 × 16 Combustion Engineering (CE) fuel assembly. The DTBP design has two tubes inserted into each other, made of different absorber materials, and contained as a single tube by covering them with the cladding material. The inner tube is made of Al2O3-B4C with natural boron, and the outer tube is made of a low concentration of Gd2O3 homogeneously dispersed in ZIRLO. On the other hand, the BAG design contains four thin wires made of Al2O3-B4C with natural boron. These wires are attached to the outer wall of guide tubes. High-Assay Low Enriched Uranium (HALEU) fuel with enrichment of 6.96 w/o was used instead of the standard fuel to simulate a 24-month cycle length of PWR core. DeCART2D code was used to perform assembly calculations for both designs. Only DTBP design was further investigated by DeCART2D-MASTER code to perform core calculation and MCNP analysis to check the distribution of thermal neutrons flux and the depletion of poison materials at different layers of this design. After comparing these options with conventional BPs, the results showed that the DTBP and BAG + IFBA cases reduced the initial excess reactivity by about 44% and 43%, respectively. Also, they maintained the k-infinite letdown curve as the flattest line among all cases and obtained the lowest residual reactivity penalty with the values of 2806 pcm and 1898 pcm, respectively. Although the DTBP design has a slow depletion rate, the depletion of all the poison isotopes was complete before the end of the cycle, as proved by MCNP analysis. The DTBP + Erbia core achieved the 24-month cycle length without violating the design constraints of power peaking factor and moderator temperature coefficient (MTC).