The energy trilemma forms the key driver for the future of energy research. In nuclear technologies, molten salt reactors are an upcoming option which offers new approaches. However, the key would be closed fuel cycle operation which requires sufficient breeding for a self-sustained long term operation ideally based on spent fuel. To achieve these attractive goals two challenges have been identified: achieving of sufficient breeding and development of a demand driven salt clean up system. The aim is to follow up on previous work to create an initial approach to achieving sufficient breeding. Firstly, identification of a salt system with a high solubility for fertile material and sufficiently low melting point. Secondly, evaluation of the sensitivity of the breeding performance on the sort of fissile material, the fissile material loading, and the core dimension all based on a realistic salt system which provides the solubility for sufficient fertile material to achieve the required breeding in a homogeneous reactor without breeding blanket. Both points are essential to create an innovative solution to harvest the fruits of a closed fuel cycle without the penalty of the prohibitively huge investments. It is demonstrated that the identified and investigated NaCl-UCl based systems are feasible to deliver the requested in-core breeding within the given solubility limits of fertile material in the salt system using either uranium as start-up fissile component or plutonium. This result is enriched by the analysis of the achievable full power days per inserted mass of plutonium. These new insights support reactor optimization and lead to a first conclusion that systems with lower power density could be very attractive in the case of low fuel cost, like it would be given when operating on spent nuclear fuel.

Molten salt reactors (MSRs), as one of the six main technologies of Gen IV, can meet the broad area of sustainability, economics, safety and reliability, proliferation resistance and physical protection goals. One of the main and first challenges in designing molten salt fast reactors (MSFRs) is the selection of an appropriate molten salt fuel system based on the envisaged applications and objectives. In this study’s series, a full-scope evaluation has been conducted about employing either chloride or fluoride salt fuels as the main competitors’ candidates for fuel salt in MSFR designs. Two distinguished projects, EVOL (CNRS, Grenoble-France), based on fluoride salt, and iMAGINE (The University of Liverpool, UK), based on chloride salts, were considered in order to achieve this goal as case studies. The first part of this series (part 1—this article) deals with the investigation of the thermophysical properties of the salt fuel system, criticality search and neutron-flux energy spectrum. An identical typical design of the MSFR core has been considered for a neutronic simulation by using MCNPX V2.7 based on the chemical composition of the fuel salt mentioned in both projects. The thermophysical evaluation has been conducted through literature research and theoretical methods based on the experimental values for the salt component properties. The results of the study are presented for thermophysical properties, including the melting point, vapour pressure/boiling point, specific heat capacity, thermal conductivity and density, in addition to neutronic simulation for the core critical dimension and neutron-flux spectrum of both the chloride- and fluoride-based salt fuel systems. In the discussion of the results, it is concluded that both the chloride and fluoride salt fuel systems have benefits that are seen on different comparative parameters. The delivered data will provide future decision makers with evidence for the salt choice for balancing their design objectives with the opportunities and expectations. Thus, a final selection of the most appropriate salt fuel system to be used in MSFRs will be postponed for more investigation in the final part of this article series, combining the data with different potential user profiles.

Nuclear fission technologies have the potential to play a significant role in the energy mix of a net-zero and sustainable society. However, to achieve the sustainability goal two significant challenges remain: efficient and sustainable fuel usage and the minimization of long-term nuclear waste. Civil nuclear molten salt systems and technologies offer the opportunity to address both, delivering future reactors at scale for efficient and effective power production and nuclear waste burnup. Potentially, both objectives could be fulfilled in one reactor system, which could significantly improve sustainability indices. The key to this innovation is demand driven development of a significantly reduced fuel cycle with enhanced proliferation resistance which offers further potential for improvement. To achieve these goals, a transformative approach for salt clean-up during molten salt reactor operation is proposed, by concentrating on the detection and removal of key neutron poisoning elements which prevent the reactor from long-term operation. To enable this highly innovative development work, a novel analysis of the evolving elementary fuel composition, their concentrations, and their criticality influence is now provided in this work. This, combined with consideration of the oxidation states of each of these elements then provides the basis for the selection of these key poisons and the development of advanced separation processes and process monitoring. This work also discusses the importance of the effective integration of physics and chemistry when systems modelling in achieving these system development goals.

Nuclear technologies have the potential to play a major role in the transition to a global net-zero society. Their primary advantage is the capability to deliver controllable 24/7 energy on demand. However, as a prerequisite for successful worldwide application, significant innovation will be required to create the nuclear systems of the 21st century, the need of the hour. The pros (low harmful emissions, high reliability, low operational expenses, and high energy density) and cons (environmental damage, fuel waste disposal concerns, limited uranium reserves, and long construction time-frame) of nuclear are discussed and analysed at different levels—the societal and public recognition and concerns (accidents, weapons, mining, and waste) as well as the scientific/engineering and economic level—to assure a demand-driven development. Based on the analysis of the different challenges, a vision for the nuclear system of the 21st century is synthesised consisting of three pillars—unlimited nuclear energy, zero waste nuclear, and accident-free nuclear. These three combined visions are then transformed into dedicated and verifiable missions that are discussed, in detail, regarding challenges and opportunities. In the following, a stepwise approach to the development of such a highly innovative nuclear system is described. Essential steps to assure active risk reduction and the delivery of quick progress are derived as answers to the critique on the currently observed extensive construction time and cost overruns on new nuclear plants. The 4-step process consisting of basic studies, experimental zero power reactor, small-scale demonstrator, and industrial demonstrator is described. The four steps, including sub-steps, deliver the pathway to a successful implementation of such a ground-breaking new nuclear system. The potential sub-steps are discussed with the view not only of the scientific development challenges but also as an approach to reduce the regulatory challenges of a novel nuclear technology.

The energy trilemma and UN SDG 7 form the key drivers for the future of all kinds of energy research. In nuclear technology, molten salt reactors are an interesting option, since they can offer a game-changing approach to deliver an attractive, highly sustainable option for a zero-carbon society by providing sufficient breeding for a self-sustained long-term operation based on spent nuclear fuel from existing reactors while being able to be controlled ideally by inherent processes. To achieve these goals, several basic challenges have already been identified and worked on; demonstration of sufficient breeding and development of a demand driven salt clean up system. This study follows up on the opportunity for reactor control based on inherent feedback mechanisms. It is demonstrated that the investigated sodium chloride-uranium chloride-based systems can feasibly deliver a thermal feedback effect, which is strong enough, to compensate for the critically change due to breeding and fission product accumulation for two different compositions. The achieved results provide a very positive outlook on the system inherent ability for reactor self-control and even for the feasibility of a load following operation based on inherent feedback effects instead of massive and costly manual/automated control system operation.

Nuclear technologies have the potential to play a unique role in delivering low carbon energy for a future net-zero society. However, for long-term success, nuclear technologies will need to deliver innovative solutions as proposed in iMAGINE. One of the key challenges for the envisaged highly integrated nuclear energy system is the need for a demand-driven salt clean-up system. The work described provides an insight into the interplay between a potential salt clean-up system and the reactor operation in a dynamic approach. The results provided will help to optimise the parameters for the salt clean-up process by delivering a dynamically calculated priority list, identifying the elements with great influence on reactor operation. The integrated model is used to investigate the ideal time for the initiation of the clean-up as well as the effect of different throughputs through the clean-up system on criticality as well as on the concentration of the elements in the reactor salt. Finally, a staggered approach is proposed with the idea to phase in the chemical clean-up processes step by step to keep the reactor critical. The results provide an essential step for the progress of iMAGINE as well as a basis for the interdisciplinary work required to bring iMAGINE into real operation.