Abstract

A comparison between air and helium for use as working fluids in the energy-conversion cycle of the MPBR is presented. To date, helium has been selected in the MPBR indirect-cycle working reference design. Air open- and closed-cycle variants are considered in this thesis in order to identify relative advantages in cycle efficiency, component efficiency, size, and possible development work required for deployment. The results of this comparison indicate that the helium cycle results in the smallest-sized plant, uses well-established technology, has a high busbar efficiency, and thus best meets the design priorities of the MPBR. The open-cycle-air variant employs turbomachinery components with the greatest amount of industrial experience, the least amount of development work required, and a 6% advantage in busbar efficiency when compared with the helium cycle. However, it results in a plant roughly 5 times the size of the helium plant. The closed-air cycle has a 5% advantage in busbar efficiency over the helium plant, but results in a plant roughly 2.5 times the size of the helium plant and requires approximately the same amount of development work for near-term MPBR deployment.