Potential application of Rankine and He-Brayton cycles to sodium fast reactors

Abstract

Traditionally all the demos and/or prototypes of the sodium fast reactor (SFR) technology with power output, have used a steam sub-critical Rankine cycle. Sustainability requirement of Gen. IV reactors recommends exploring alternate power cycle configurations capable of reaching high thermal efficiency.

By adopting the anticipated working parameters of next SFRs, this paper investigates the potential of some Rankine and He-Brayton layouts to reach thermal efficiencies as high as feasible, so that they could become alternates for SFR reactor balance of plant. The assessment has encompassed from sub-critical to super-critical Rankine cycles and combined cycles based on He-Brayton gas cycles of different complexity coupled to Organic Rankine Cycles. The sub-critical Rankine configuration reached at thermal efficiency higher than 43%, which has been shown to be a superior performance than any of the He-Brayton configurations analyzed. By adopting a super-critical Rankine arrangement, thermal efficiency would increase less than 1.5%. In short, according to the present study a sub-critical layout seems to be the most promising configuration for all those upcoming prototypes to be operated in the short term (10–15 years). The potential of super-critical CO₂-Brayton cycles should be explored for future SFRs to be deployed in a longer run.

Introduction

Fast reactors are one of the main lines of research and development under the frame of the so called Generation IV International Forum (Bouchard, 2008). Their breeding potential and capability to implement a closed fuel cycle strategy by burning minor actinides can be further supported by reaching an outstanding safety level and high thermal efficiencies. Nonetheless, in order to become a technological reality, these systems have to demonstrate their feasibility and economic competitiveness with respect to other energy generation technologies.

Sensitive to this situation, the European Commission has launched a number of projects within the 7th Framework Program of EURATOM to address potential fast reactor systems. In particular, the sodium fast reactors are being investigated within the European Sodium Fast Reactor project (ESFR), comprehensively described elsewhere by Fiorini (2009). The many decades of research behind this concept have provided a sound technological basis. However, the need to comply with all those demanding requirements inherent to sustainability, highlighted a number of issues worth to be explored, from the reactor configuration (i.e., pool vs. loop reactor) to the cost, going through the fuel cycle, safety, balance-of-plant (BOP), etc.

A number of sodium fast reactor prototypes and demonstration plants (IAEA, 1999) have been built and operated (i.e., Phenix, SuperPhenix, BN-600, etc.). In those cases in which some power output was pursued, all of them used a sub-critical Rankine cycle. Since the start of operation of all these reactors, a significant experience has been built in operating super-critical power cycles in the fossil power industry (Susta, 2004). This fact and the progress achieved in areas like materials and components reliability during the 90s, have eventually resulted in a significant improvement in their thermal performance (Leyzerovich, 2009), so that when conceiving a new generation of SFRs the potential of this type of power cycles should be explored. In addition, given the advances accomplished in the field of gas turbo-machinery in the last two decades, the suitable thermal and chemical properties of some gases, like helium, and the advantage of preventing Na–H₂O reaction, Brayton cycles have been paid attention as alternate cycles for Gen IV reactors (Chang and Richard, 2005, Herranz et al., 2007a, Herranz et al., 2007b, Herranz et al., 2007c, Herranz et al., 2008) and, particularly, for SFRs (Peterson, 2003, Zhao and Peterson, 2008, Saez et al., 2008). However, given the relatively low temperatures of the coolant anticipated at the reactor exit (around 545 °C), which prevent attaining high thermal efficiencies, specific studies have been devoted to assess the potential benefits of complex configurations (Zhao et al., 2009) or alternate gases or gas mixtures (El-Genk and Tournier, 2007).

Two sodium reactor architectures have been traditional in SFR technology: pool and loop reactor configurations. The former has been the configuration adopted by countries like France, Russia, India and China and it has been tested for years in demonstrators and/or prototypes (i.e., Phenix, Super-phenix and BN-600). The latter has been developed by Germany and Japan, the Joyo and the Monju reactors being examples. Both design configurations are being considered within the ESFR project with an additional intermediate loop (consisting of 6 loops working in parallel) between the reactor coolant system and the power cycle. It is this secondary loop the one which will behave as the heat source for the power cycle through an intermediate heat exchanger (IHX). This intermediate loop avoids any potential chemical interaction between the reactor cooling Na and the power cycle fluid, which in the case of water/steam would result in a hazardous violent explosion. No matter which reactor configuration is eventually developed within the ESFR project, a set of common characteristics have been set and are taken as a reference in this study. Table 1 summarizes the most significant SFR parameters for thermal efficiency assessment (Saez et al., 2008).

This paper explores the thermal performance of Rankine and Brayton configurations based on the postulated SFR reactor parameters in Table 1 (Saez et al., 2008). As for the Rankine layouts, given the existing experience with sub-critical power cycles in preceding and current SFRs, a sub-critical configuration has been adopted as a reference for comparison purposes; additionally, based on the current trends in the power industry, two super-critical cycles will be proposed and their advantages with respect to the reference layout discussed. Several Brayton cycles have been studied, starting from the most basic CBTX (Hawthorne and Davis, 1956) and extending the study to assess the effect of inter-cooling, re-heating and coupling to an Organic Rankine Cycle (ORC); although the base fluid considered in the Brayton analyses has been helium (He), an additional study has been conducted to assess the effect of using three gas mixtures of helium with nitrogen (N₂), argon (Ar) and xenon (Xe). Despite the potential of super-critical CO₂ Brayton cycles for SFR BOPs, they have been kept out of the scope of this study to be a specific matter of an additional study. Therefore, this study should be seen as a search for power cycle configurations that could become alternates to those based on super-critical CO₂ Brayton layouts.