Performance analyses of VHTR plants with direct and indirect closed Brayton cycles and different working fluids Mohamed S. El-Genk * , Jean-Michel Tournier Institute for Space and Nuclear Power Studies, Chemical and Nuclear Engineering Department, The University of New Mexico, Albuquerque, NM 87131, USA Keywords: VHTR Closed Brayton cycle Helium and binary gas mixtures Axial-flow turbo-machine Polytropic efficiency Thermal efficiency abstract This paper investigated the performance of Very High Temperature Reactor (VHTR) power plants with helium working fluid and direct and indirect Closed Brayton Cycles (CBCs), and with binary mixture working fluids of He–Xe and He–N 2 (molecular weight of 15 g/mole) and indirect CBCs. Also investigated are the effects of using low- and high-pressure compressors with intercooling, versus a single compressor, using bleed cooling the reactor pressure vessel in direct CBC helium plants, and varying the reactor exit temperature from 700  C to 950  C on the plant thermal efficiency, cycle pressure ratio and the size of and number of stages in the turbine and compressor. Analyses are performed for a shaft rotation speed of 3000 rpm, reactor thermal power of 600 MW and a temperature pinch of 50  C in the intermediate heat exchanger (IHX) for the indirect CBCs. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction A number of Very High Temperature Reactor (VHTR) design concepts are being considered for the generation of electricity at a high plant thermal efficiency (>48%) and the supply of process heat for the co-generation of hydrogen using thermo-chemical processes and for a multitude of industrial applications (INEEL, 2007; Oh et al., 2006; Johnson, 2008; No et al., 2007). Helium- cooled VHTR plants with direct and indirect Closed Brayton Cycles (CBCs) for energy conversion are being investigated in the USA, Europe, Russia, Japan and South Africa. In these plants, the reactor exit temperature could be as high as 950  C, posing many chal- lenges to the materials selection for the Reactor Pressure Vessel (RPV), heat exchangers, and the fuel performance, to mention a few. In order to use a RPV made of carbon steel alloys, with well known properties and operation experience in radiation environment, bleed cooling of the vessel would be necessary (Vasyaev et al., 2005; Yan et al., 2003). This RPV cooling uses helium bled off at the exit of the compressor, keeping the vessel wall below 371  C (644 K) (Yan et al., 2003), while operating near the plant peak thermal efficiency and at low cycle compression ratio. This cooling option of the RPV, only possible in VHTR plants with direct He- CBCs, makes it possible to use vessels made of SA 533 or SA 508 steel, which are the same materials currently been used for Light Water Reactors vessels. Another option being considered is using the returning gas from the cold leg of the recuperator (REC) in the CBC to cool the RPV wall before entering the reactor core. In this case, the temperature of the coolant entering the reactor vessel is kept at or below 490  C (763 K). This relatively high temperature requires using Mo–Cr alloys for the RPV. Such alloys need further development and testing and hence, a long lead time for deployment and eventual use. The VHTR plants being considered use both direct and indirect CBCs for energy conversion (Fig. 1a and b). With a direct CBC, the reactor coolant is also the CBC working fluid, while with an indirect CBC, the working fluid for the CBC could be the same or different from the reactor coolant. Recent studies have shown that while helium has the best transport and thermal properties of all noble gases, its low molecular weight (4 g/mole) significantly increases the size and number of stages in the CBC compressor(s) and turbine. When mixing helium with other heavier gases, such as xenon, argon or nitrogen, the resulting binary mixtures, in a certain range of molecular weights, have higher forced convection heat transfer coefficients than helium. In addition, their higher molecular weight reduces the aerodynamic loading of the blades in the CBC compressor and turbine, resulting in fewer stages in and smaller size of the CBC turbo-machinery (El-Genk and Tournier, 2008). With a molecular weight of 15 g/mole, the forced-convection heat transfer coefficients of He–Xe and He–N 2 at 1200 K are 7% and 4.6% higher than helium. The heat transfer coefficient of He–Xe (15 g/ mole) normalized to that of He is almost independent of temper- ature (El-Genk and Tournier, 2008), but that of He–N 2 increases with temperature and is 3% and 4.6% higher than for He at 800 K and 1200 K (Tournier and El-Genk, 2008b). * Corresponding author. Tel.: þ1 505 277 5442; fax: þ1 505 277 2814. E-mail address: mgenk@unm.edu (M.S. El-Genk). Contents lists available at ScienceDirect Progress in Nuclear Energy journal homepage: www.elsevier.com/locate/pnucene 0149-1970/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.pnucene.2008.11.004 Progress in Nuclear Energy 51 (2009) 556–572