energies
energies
Article
On the Employment of a Chloride or Fluoride Salt Fuel System in Advanced Molten Salt Reactors, Part One: Thermophysical Properties and Core Criticality
Abstract: Molten salt reactors, as one of the six main technologies of Generation Four, 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 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 molten salt fast reactor designs. Two distinguished projects, EVOL based on fluoride salt, and iMAGINE based on chloride salts, were considered in order to achieve this goal as case studies. The first part of this series 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 molten salt fast reactor core has been considered for a neutronic simulation by using MCNPX version two point seven 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, vapor 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 molten salt fast reactors will be postponed for more investigation in the final part of this article series, combining the data with different potential user profiles.
One. Introduction
One. Introduction
Generation Four of the nuclear energy systems will provide sustainable energy generation that meets clean air objectives and provides long-term availability of systems and effective fuel utilisation for worldwide energy production. All Generation Four systems aim at performance improvement, new applications of nuclear energy, and/or more sustainable approaches to the management of nuclear materials; however, with differing priorities.
Molten salt fast reactors as one of the candidate systems, in addition to satisfying the Generation Four objectives, derive the benefit of operating in a "Closed Fuel Cycle"
mode without the extensive cost and challenges of solid fuelled systems. This feature can solve the main Achilles' heel of nuclear power plants-waste management and its related environmental issues-with the ability to use light water reactors' spent fuel to fabricate the molten salt fuel for use in the reactor core. Furthermore, a high breeding ratio of molten salt fast reactors makes this reactor type one of the best selections for long-term energy extraction, even without the need for refuelling.
One of the main and most fundamental challenges in the design of molten salt fast reactors is to select the appropriate molten salt fuel, as it strongly influences the neutronic, thermal-hydraulic, life cycle, shielding, breeding ratio, structural damage parameters and structural material dedication for the reactor. It can be said that it is the most fundamental and effective parameter in the design of this type of reactor programme.
A salt fuel/coolant comprises different individual salt components mixed into a multi-component system, e.g., as binary or ternary mixtures, typically an alkali metal salt and a heavy metal fuel salt. The melting points of each salt component are generally too high for coolant applications, and mixing several components into the binary or ternary systems reduces the melting point of the resulting salt system to more practical levels.
Various salt fuel/coolant systems were proposed, along with different research and projects interested during the development periods of molten salt reactors and pyro-reprocessing. One of the first successful projects was conducted by the Oak Ridge National Laboratory between nineteen sixty-five and nineteen sixty-nine, and different salt fuel systems were examined through the Molten Salt Reactor Experiments and the integrated results were delivered in the report. A quaternary fluoride-based salt fuel system "LiF-BeF-two-ZrF-four-UF-four (sixty-five point zero - twenty-nine point one - five point zero - zero point nine mol percent)" with thirty-three percent of two three five U was considered in the Oak Ridge National Laboratory studies for the first criticality, although different experiments were conducted later to recognise the best salt fuel composition. Another project was run by Oak Ridge National Laboratory in nineteen seventy-two, designing a thermal-spectrum molten salt breeder reactor by employing LiF-BeF-two-ThF-four-UF-four as the salt fuel (two three five U enrichments between thirty-three and ninety percent) to include both fissile and fertile materials in the fuel composition. Asher et al. conducted a detailed assessment of a two thousand five hundred MWe molten chloride salt fast reactor for the United Kingdom with lessons learned from the Molten Salt Reactor Experiments project from nineteen seventy-one to nineteen seventy-two. Their preliminary study had shown that a fast system using the two three three U/two three two Th cycle and fluoride salts did not indicate encouraging results, thus they considered a new ternary salt fuel, "NaCl-UC-thirteen-PuCl-three" (sixty point zero - thirty-seven point zero - three point zero mol percent), for their project.
In the last years, during two thousand ten to two thousand fourteen, the EVOL project (Evaluation and Viability of Liquid Fuel Fast Reactor System) was developed by CNRS (France), partly under a European Commission grant to innovate the molten salt fast reactor concept. As a result of this, two different quaternary fluoride-based salt fuel systems, "LiF-ThF4-UF4-PuF3 (seventy-eight point six - twelve point nine - three point five - five mol%)" and "LiF-ThF4-UF4-(TRU)F3 (seventy-seven point five - six point six - twelve point three - three point six mol%)"-with two hundred thirty-five U enrichments between five to thirty percent, depending on the core design-were proposed to be used in the MSFR (TRU: transuranium elements, which are mainly represented by PuF3). In addition, the EVOL team arranged a strong collaboration with the MARS project (minor actinides recycling in molten salts), supported by Russian agencies and ROSATOM. The European and Russian partners have conducted theoretical and experimental studies to verify the feasibility of the MOSART and MSFR systems with a different core focus. On the one hand, to reduce the long-lived waste toxicity, and on the other hand, to produce electricity simultaneously. EVOL has been followed by a later project funded by the European Commission, entitled "SAMOFAR (A Paradigm Shift in Reactor Safety with the Molten Salt Fast Reactor)", conducted during two thousand fifteen to two thousand nineteen. The main goal of the SAMOFAR project is to evaluate the safety features of MSFRs for future applications. The ternary system of LiF-ThF4-UF4 (seventy-seven point five - twenty point zero - two point five mol%) has been identified as one of the salt fuel candidates to be used in the MSFR in this follow-up project of EVOL. In addition to the past and ongoing projects in this field, some studies can be found through published literature as results of academic research. In one of the most recent studies, Faure and Kooyman have analysed the application of bromide and iodide salts as potential nuclear salt candidates in MSRs. They have concluded the analogous application of iodide salts in comparison to the fluoride and chloride ones, while bromide salt has less chance because of its high gamma emission. A review work has been published by Gakhar et al. to analyse the properties, purification and corrosion of salt fuel by focusing on the fluoride and chloride one recently.
The iMAGINE project of the University of Liverpool chased the idea of operating a reactor on spent fuel without prior reprocessing for energy production and waste management. The more detailed investigation, "Defining a Draft for a Zero Power Reactor Experiment for Molten Salt Reactors," was launched at the University of Liverpool in collaboration with other academic and company partners in June twenty twenty-one. The project focuses on developing a zero-power molten salt fast reactor to study the iMAGINE approach experimentally as the first step into a nuclear system that can employ the spent nuclear fuel extracted from light water reactors as the main fuel source and operate as a closed fuel cycle. The ternary /quaternary (adding PuCl3, latest through breeding) chloride-based salt fuel system "NaCl-UC13-UC14" with two different compositions of forty-two point five - seventeen - forty point five mol% and twenty - twenty-three point six five - fifty-six point three five mol% was considered as the starting composition (it should be noted that this composition can be changed during projects based on new findings). The project is looking to analyse/optimise the design parameters in all aspects for a zero-power molten salt fast reactor as a first step on the Gen Four MSRs ladder.
This manuscript starts the first part (part one) of a series of studies on the full-scope evaluation of employing chloride or fluoride salt fuel in MSFRs from different perspectives. In this manuscript, as the first step of the series, the pros and cons of using chloride or fluoride salt fuel in MSFR designs are investigated from a thermophysical property and core criticality point of view. In order to be as realistic as possible, a one-to-one comparative study was conducted between the EVOL-proposed fluoride-based salt fuel and the chloride-based iMAGINE systems. In addition, a pile-type cylindrical salt fuel reactor was simulated by using MCNPX V2.7, and the criticality simulation results were used to support the comparison study. Finally, a comparative table is presented that points out the results of this evaluation for a better understanding of each salt system's advantages and disadvantages.