Burnable poison designs for a soluble-boron-free civil nuclear marine PWR core

In an effort to de-carbonise commercial freight shipping, there is growing interest in the possibility of using nuclear propulsion systems. Nuclear-powered propulsion allows ships to operate with low fuel costs, long intervals between refueling, and minimal emissions; however, currently these systems remain largely confined to military vessels. It is highly desirable that a civil marine core not use soluble boron for reactivity control, but it is then a challenge to achieve an adequate shutdown margin throughout the core life while maintaining reactivity control and acceptable power distributions in the core. We have considered several potential and novel burnable poison (BP) design strategies for reactivity control in this study: (Case 1) a composite BP: gadolinia (Gd₂O₃) or erbia (Er₂O₃) with 150 μm thickness ZrB₂ integral fuel burnable absorber (IFBA); (Case 2) Pu-240 or Am-241 mixed homogeneously with the fuel; and (Case 3) another composite BP: Pu-240 or Am-241 with 150 μm thickness ZrB₂ IFBA. The results are compared against those for a high-thickness 150 μm 25 IFBA pins¹ design from a previous study. We arrive at a design using 15% U-235 fuel loaded into 13 × 13 assemblies with Case 3 BPs for reactivity control. Taking advantage of self-shielding effects, this design maintains low and stable assembly reactivity with minimal burnup penalty. Case 3 provides ∼10% more initial reactivity suppression with ∼70% less reactivity swing compared to the IFBA-only design for UO₂ fuel while achieving almost the same cycle length. Finally, optimized Case 3 assemblies were loaded into a 3D reactor model in PANTHER. The PANTHER results show that the designed core can achieve the target lifetime of 15 years while minimizing the reactivity swing to a greater extent and providing a ∼30% lower radial form factor and ∼28% lower total peaking factor compared to the IFBA-only core.