Nuclear power systems capable of outputting low powers (< 100 MWth) are increasingly receiving interest internationally for deployment not only as electricity production systems, capable of operating off-grid, but also as systems able to provide industrial process heat. These ‘micro-reactor’ concepts must demonstrate economic competitiveness with other potential solutions capable of providing similar power outputs. With this in mind, reactor technologies that offer inherent advantages associated with improved power density and simplified operation, both of which are important attributes that determine economic competitiveness, are reviewed in the context of the fundamental safety functions provided by the IAEA. The reactor technology chosen based on the results of the review were: low vapour pressure coolants like molten salt or liquid metal; solid moderator material; and conventional solid UO2 fuel. Initial infinite lattice neutronic studies indicated a series of positive reactivity coefficients. A finite system was also modelled using a molten salt as the coolant. When modelling the finite system the coolant temperature reactivity coefficient became negative, the void coefficient strongly negative and moderator temperature coefficient negative to weakly positive. Given that a number of reactivity coefficients were negative to strongly negative in the finite system, the weakly positive moderator temperature coefficient is not thought to be prohibitive. Thus the design should exhibit acceptable safety performance. Whilst the importance of leakage in fast reactor cores is well known, a key outcome from this study is the strong influence of leakage on all safety related parameters for the thermal reactor designs considered here with solid moderator material. Thus it seems that safety studies for such small cores should be based on full core calculations instead of the traditional infinite lattice studies for fuel assemblies.

Introduction

There is an increasing interest, globally, in small reactors (< 300 MWe) that are designed to be assembled, as far as is practical, in a factory setting. These so-called Small Modular Reactors (SMRs) have been discussed extensively elsewhere (OECD-NEA, 2016; Vujićet al, 2012; IAEA, 2016). Briefly, apart from being ideally suited to customers with smaller power requirements, the benefits SMRs may offer are: increased flexibility with respect to siting; improved safety performance; reduced construction times; and reduced upfront investment requirements. The challenges facing SMRs relate to development costs; uncertainty surrounding licensing (especially for innovative technologies that regulators are less familiar with); and uncertainties surrounding economic competitiveness, in terms of cost per kWe. Many of the SMRs have power outputs ∼۱۰۰ MWe, with the intention of placing multiple units together to allow for electricity production around 500 MWe and sharing of facilities (such as turbo-generator units) to reduce costs. Therefore, the primary purpose of these systems is to provide electricity to the grid. It should be noted that as power plants are grouped together, this limits siting flexibility. However, to operate systems with a small combined power output (< 200 MWe), bespoke turbo-generators would be required. There has also been recent interest in mobile floating nuclear power plants. Two recent noteworthy examples are: the ACPR50S, which is a 60 MWe reactor, being developed for the supply of electricity, heat and desalination; and the Russian Akademik Lomonosov plant which uses two 35 MWe reactors (WNN, 1301). Besides the Akademik Lomonosov plant, several new designs are investigated for autonomous power supply in Russia (Goltsov et al, 2016). Some SMRs are focused on producing even lower power outputs and are targeted at industrial power facilities or remote locations where there is no grid available. Furthermore, these low powered systems intend to take the safety performance benefits of many SMRs further by achieving indefinite decay heat removal. These smaller variants of SMRs are sometimes termed micro-reactors (NNL, 2014).