Tools for supercritical carbon dioxide cycle analysis and the cycle's applicability to sodium fast reactors

Abstract

The Sodium-Cooled Fast Reactor (SFR) and the Supercritical Carbon Dioxide (S-C0₂) Recompression cycle are two technologies that have the potential to impact the power generation landscape of the future. In order for their implementation to be successful, they must compete economically with existing light water reactors and the conventional Rankine cycle. Improvements in efficiency, while maintaining safety and proliferation goals, will allow the SFR to better compete in the electricity generation market. These improvements will depend on core design as well as the balance of plant, including the choice of steam or C0₂ as the working fluid. This work has developed some of the tools necessary for evaluating different design core and balance of plant options. Much of it has concentrated on the S-C0₂ Recompression cycle. S-C0₂ promises to be useful as a working fluid in high-efficiency power conversion systems for SFRs because it achieves higher efficiencies at the high temperatures associated with SFRs. The recompression cycle is capable of operating with very high efficiencies due to the low compressor work needed when C0₂ approaches its critical point at the compressor inlet. The potential of this cycle to meet the needs of next-generation plants must be investigated across the entire range of operations and within each component of the system. A steady-state code for analysis of the recompression cycle was previously developed at MIT in the form of CYCLES II, but the present work has made significant improvements to this code that make the new version, CYCLES III, more versatile.

(cont.) This code can help to size components of the system and predict the costs and performance of the system at steady -state. Coupling of the primary and secondary loops is a major concern, the construction of the intermediate loop and associated heat exchangers (IHX) being critical to cost, efficiency, and safety. Furthermore, there is little experience in industry with large-scale compressors for S-C0₂. The experience that has been gained is typically proprietary. Most existing C0₂ compressors do not operate near the critical point and therefore, perform much like any other semi-ideal gas compressor. Accordingly, consistent, usable models of non-ideal gas compressors have been developed in the present work to produce preliminary designs and performance maps for the compressors in S-C0₂ recompression cycles. Compressor designs were developed for a 500 MWth S-C0₂ recompression cycle. The main compressor achieves an operating point total-to-static efficiency of 90.4 % and the recompressing compressor achieves 91.4 %. Further work can continue once these areas have been developed, including transient analysis, the effects of impurities on the system, and investigation of cycles which operate on other working fluids. Additionally, changes in the intermediate loop, the arrangement of the reactor vessel, and in-core changes will affect the efficiency of the SFR. These include the option of diluent grading in the fuel, flattening of the core outlet temperature profile, choosing Rankine or S-C0₂ for the balance of plant, and heat exchanger design. All these have been evaluated for their impact on plant efficiency.

(cont.) It has been determined that the S-C0₂ recompression cycle can provide efficiency benefits over conventional Rankine cycles for SFRs with core outlet temperatures at or above 510 oC. With the S-C0₂ cycle, SFRs can achieve thermal efficiencies of ~42 %.