Impact of different moderator ratios with light and heavy water cooled reactors in equilibrium states Sidik Permana * , Naoyuki Takaki, Hiroshi Sekimoto Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology 2-12-1-N1-17, O-okayama, Meguro-ku, Tokyo 152-8550, Japan Received 15 December 2005; received in revised form 27 February 2006; accepted 27 February 2006 Available online 18 April 2006 Abstract As an issue of sustainable development in the world, energy sustainability using nuclear energy may be possible using several different ways such as increasing breeding capability of the reactors and optimizing the fuel utilization using spent fuel after reprocessing as well as exploring additional nuclear resources from sea water. In this present study the characteristics of light and heavy water cooled reactors for different moderator ratios in equilibrium states have been investigated. The moderator to fuel ratio (MFR) is varied from 0.1 to 4.0. Four fuel cycle schemes are evaluated in order to investigate the effect of heavy metal (HM) recycling. A calculation method for deter- mining the required uranium enrichment for criticality of the systems has been developed by coupling the equilibrium fuel cycle burn-up calculation and cell calculation of SRAC 2000 code using nuclear data library from the JENDL 3.2. The results show a thermal spectrum peak appears for light water coolant and no thermal peak for heavy water coolant along the MFR (0.1 6 MFR 6 4.0). The plutonium quality can be reduced effectively by increasing the MFR and number of recycled HM. Considering the effect of increasing number of recycled HM; it is also effective to reduce the uranium utilization and to increase the conversion ratio. trans-Plutonium production such as americium (Am) and curium (Cm) productions are smaller for heavy water coolant than light water coolant. The light water coolant shows the feasibility of breeding when HM is recycled with reducing the MFR. Wider feasible area of breeding has been obtained when light water coolant is replaced by heavy water coolant. Ó 2006 Elsevier Ltd. All rights reserved. 1. Introduction Energy demand should become constant in a future equilibrium society. Similar to sustainable development, the future equilibrium state needs an energy source which can maintain long-term sustainable supply without causing any environmental problems. Nuclear power can produce enough energy for long periods, even for a million years of mankind’s utilization by using FBR with the conversion ratio (CR) of about unity and uranium from seawater (Sekimoto, 1994). In the equilibrium state, the rate of energy consumption remains constant. If the earth’s energy supply is secured by nuclear power generation, each pro- duced active nuclide density in the reactor may be also con- stant. This state is called ‘‘nuclear equilibrium state’’. The society in this condition is called the nuclear equilib- rium society. In the nuclear equilibrium state, the produc- tion rate of nuclear energy is constant, and the production and disintegration rates of nuclear materials are constant as well. Therefore, the amount of each nuclide in a reactor becomes constant if the refueling operation is continuous (Sekimoto and Takagi, 1991). We have called the nuclear fuel cycle at the nuclear equilibrium state ‘‘the equilibrium fuel cycle’’ (Sekimoto and Waris, 1999). In the future nuclear equilibrium society, only natural uranium and/or thorium are employed as supplied fuel (Sekimoto and Tak- agi, 1991). This study showed that LWR could not achieve its criticality. Therefore, charged fuel should be enriched in LWR with conventional moderator to fuel ratio (MFR = 2) (Waris and Sekimoto, 2001a). In dealing with HM confining, it seems that in the case of uranium, it is not recycled; the enrichment decreases considerably with increasing number of confined nuclides in the reactor 0306-4549/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.anucene.2006.02.013 * Corresponding author. Tel./fax: +81 3 5734 2955. E-mail address: 04d51469@nr.titech.ac.jp (S. Permana). www.elsevier.com/locate/anucene Annals of Nuclear Energy 33 (2006) 561–572 annals of NUCLEAR ENERGY