Conceptual analyses of equilibrium conditions to determine a long-term fuel management strategy for research reactors Afshin Hedayat Reactor Research and Development School, Nuclear Science and Technology Research Institute (NSTRI), End of North Karegar Street, P.O. Box 14395-836, Tehran, Iran article info Article history: Received 30 June 2013 Received in revised form 6 October 2013 Accepted 30 October 2013 Keywords: Research reactor Fuel Equilibrium Core Safety Tehran research reactor abstract We present conceptual analyses of equilibrium conditions for research reactors to determine periodic and long-term requirements for a practical fuel management strategy (FMS). The MTR-PC package developed for research reactors is used for FMS simulation. A baseline is calculated and is in satisfactory agreement with previous calculations and the operating state of the Tehran research reactor. Funda- mental concepts, design criteria, and safety issues for research reactors are discussed for development of a practical FMS. Fast calculations and simplified models of an operational FMS are considered before final planning and simulation. This includes pre-estimation of the fuel assemblies required, fast FMS simu- lation assuming ideal equilibrium conditions, and verification of operating safety limits and conditions. Semi-equilibrium behavior due to a partial refueling task and equilibrium reflexes for the core param- eters are also studied. The results show that the simplified models and methods are in accordance with the operating states and conditions. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Research reactors are necessary tools for nuclear energy devel- opment and evolution. A significant number of operating research reactors have become multi-purpose facilities that produce radio- isotopes and perform neutron radiography, semiconductor doping, and neutron activation analysis for a wide range of users while continuing their traditional role in education and training (IAEA, 1999, 2007). Fuel management is defined as the placement, movement, and discharge of fuel assemblies. Each time a reactor becomes subcritical, a decision is required regarding which fuel elements (FEs) should be discharged, reshuffled, and replaced by fresh FEs (IAEA, 2008b). Each refueling task affects the transformation of states (burn up) both immediately and in all successive stages, since some fuel remains in the core from stage to stage (subcycles). The set of all decisions (a complete set of refueling tasks) is called a fuel management strategy (FMS) or operating policy (Mahlers, 2003; Wall and Fenech 1965). Utilization of a multi-purpose research reactor via a FMS re- quires specific neutronic conditions within the reactor operating limits and conditions (OLCs). There are two general approaches for core management for a research reactor:  Optimization of each core configuration independently to pro- vide different desired conditions during reactor operation (Hedayat et al., 2009); and  Introduction of an equilibrium core or periodic refueling chains chosen for a long operating time (Lerner et al., 2004; Villarino and Padilla, 2011). The first method can be useful when each operating core is chosen and arranged independently; it can be optimized to improve cycle length, neutron fluxes at irradiating locations, irra- diating volume, and safety margins for the installation of experi- mental devices. Conversely, a periodic FMS can be scheduled for particular irradiating tasks according to the general characteristics of each reactor type. Since FEs remain in the core until they are extracted, an overall decision can be made sequentially for refuel- ing processes and final discharge. This should provide operating requirements within the reactor OLCs. This approach introduces an equilibrium core via a periodic refueling pattern as a long-term FMS for research reactors. The reactor power, fuel type, safety features, and moderator and reflector types are the most important pa- rameters for general decision-making criteria. Here we investigated the appropriate number of FEs and the periodic cycle length (T c ) for both desired equilibrium discharge burn-up and end-of-cycle (EOC) reactivity. Research reactors typically use from one to three fresh FEs to feed an outein (OI) refueling pattern (IAEA, 2005a; Lerner et al., E-mail addresses: ahedayat@aeoi.org.ir, af.hedayat@yahoo.com. Contents lists available at ScienceDirect Progress in Nuclear Energy journal homepage: www.elsevier.com/locate/pnucene 0149-1970/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pnucene.2013.10.020 Progress in Nuclear Energy 71 (2014) 61e72