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| Fig. 1: ORNL schematic of the MSBR off-gas system. [7] |
With self-regulating reaction rates and lower operating pressures than light water reactors, molten salt reactors (MSRs) are a prime candidate for generation IV reactors. MSRs use molten salt as reactor coolant, which yield a 25% greater heat capacity than water. Furthermore, MSRs are inherently practically immune to nuclear explosion - when the reactor overheats, the salt expands out of the reactor core, slowing the reaction. [1] The possibility of a steam explosion is also avoided since molten salts can have vapor pressures below 1 mm Hg at 900°C, significantly limiting the pressure within the reactor. [2] Nevertheless, MSRs have their fair share of design challenges, including the corrosion of reactor components, the production of hazardous tritium, and the difficulties of removing volatile fission products. [2] There exist two typical classifications for MSRs - solid-fuel reactors (which rely on molten salts exclusively as coolant) and liquid-fuel reactors (where fuel is dissolved directly in the salt). [1] I shall use the term "MSR" to refer exclusively to the latter.
In MSRs, after the fuel is irradiated in the core, it is possible to implement online systems to continuously remove fission products. This can not only increase the lifetime of the salt in the fuel loop but also keep the fuel composition and reactivity consistent. Ultimately, this allows the reaction to remain at criticality for an indefinite period. [3] As such, the design of these fuel reprocessing systems is an integral consideration of MSR design.
The removal of fission products from fuel salt is important for several reasons. Firstly, it improves neutron economy and therefore increases breeding ratios. Secondly, it increases the lifetime of the fuel salt and the reactor by minimizing corrosion. [3] The fission products in question depend significantly on salt chemistry, but they are generally distinguishable into groups of similar chemical characteristics. Such groupings may include gases, noble metals, nonmetals, soluble fission products, etc. [4] With the notable exception of gases (such as 135Xe), publicly available research indicates that the selective removal and recovery of fission products remains an uncertain process. [5]
In fact, the behavior of insoluble fission products is not well documented at all. There exists no published research explaining whether these products will remain suspended, agglomerate on reactor materials, or enter off-gas systems as aerosols. [5] Given this uncertainty, there is no indication that a reasonably effective method for the removal of insoluble fission products has been developed. [5]
The removal of gaseous fission products, however, has been investigated to a significant extent. Typically, a cover gas is circulated through the reactor headspace to both remove fission products and maintain an inert atmosphere. Fission products tend to transfer to the gaseous phase due to low solubility within the salt. [6] In the Oak Ridge National Laboratory (ORNL)'s Molten Salt Breeder Reactor (MSBR) experiment, after removal of the gas products, the salt is returned to the primary fuel loop through a pipe with solid particle traps. [3]
Fission products such as 135Xe are referred to as "nuclear poisons" due to their high neutron capture cross-sections. Removing noble gases from the fuel salt leads to a significantly reduced neutron poison load in the core. One problem arises from the trapping of 135Xe within the graphite moderator. To ensure that most of the products diffuse to the cover gas, the graphite can be treated with a pyrolytic coating. [6] More research is needed concerning the parameters of the cover gas insertion (including the gas bubble size) before an off-gas system can be designed.
A simple implementation of an off-gas system can be seen in the Molten Salt Reactor Experiment (MSRE) at ORNL. In this subsystem, helium gas is inserted into the pump to create a dense, inert atmosphere. When the fuel salt is sprayed through this atmosphere, helium bubbles embed themselves in the salt, subsequently trapping the insoluble gaseous fission products. [3] The MSBR features a significantly more complex off-gas system (as illustrated in Fig. 1). Upon separation of the gas and salt phases via buoyant forces, the gas is retained for 47 hours to allow 97% of 135Xe to decay. The gas is then passed through particle traps to remove solid products, at which point it consists mainly of krypton, xenon, and tritium. Half of the gas is then returned to the primary system, while the other half is sent to the gas cleanup system. This system is designed with a high degree of redundancy given its critical effects on the reactivity and lifetime of the fuel salt, and consequently the criticality of the nuclear reaction. [3]
The implementation of MSRs will require significant revisions to all facets of nuclear energy production infrastructure. In particular, fuel recycling will need to be completely redesigned since present spent fuel recycling is not capable of processing molten salt fuel streams. [2] The effects of radiolysis in MSR fuel are also not very well known. While the effects of radiolysis in chloride salts are understood to some degree, there is not a sufficient amount of research concerning the stability of molten salts under the extreme conditions of nuclear fuel irradiation. [2]
Furthermore, design of MSRs must be coordinated with the International Monitoring System (IMS), which measures radionuclide traces worldwide to detect nuclear explosions. Gaseous isotopes such as 135Xe are of particular interest to the IMS, and given that such isotopes must be continuously extracted from MSR fuel via off-gas subsystems, there may exist a conflict between the implementation of these two processes. [4] Indeed, research from the Pacific Northwest National Laboratory has shown that xenon and iodine signatures from continuously reprocessed fuel salt is indistinguishable from a nuclear explosion. The study showed that, in the absence of xenon abatement, MSRs could release 133Xe radiation on the order of 1016 Bq/d - many orders of magnitude greater than the release threshold proposed by the IMS of 5×109 Bq/d. Reducing this number requires a gas hold-up time of ~100 days, depending on the reactor design. [4]
Perhaps most importantly, the removal of soluble fission products remains largely undiscussed in publicly available literature. [2,3,5] While there exists some vague discussion regarding electrochemical removal processes, there is very little readily available information discussing a concrete, feasible method. [5] This poses a significant hurdle since the majority of fission products are, in fact, salt soluble. [2]
Thus, despite the clear benefits offered by MSRs as Generation IV nuclear reactors, the reprocessing of fuel salt remains, among other things, in need of much more research before a feasible fuel cycle can be implemented.
© Isaac Goodman. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] M. Cooper, "Molten Salt Reactors," Physics 241, Stanford University, Winter 2020.
[2] R. Roper et al., "Molten Salt For Advanced Energy Applications: A Review," Ann. Nucl. Energy 169, 108924 (2022).
[3] P. Jr. Vicente Valdez et al., "Modeling Molten Salt Reactor Fission Product Removal with SCALE," Oak Ridge National Laboratory, ORNL/TM-2019/1418, February 2020.
[4] C. Johnson et al., "Modeling of Fission and Activation Products in Molten Salt Reactors and Their Potential Impact on the Radionuclide Monitoring Stations of the International Monitoring System," J. Environ. Radioact. 234, 106625 (2021).
[5] M. A. Rose and D. Ezell, "Molten Salt Reactor Fuel Cycle Chemistry Workshop," Argonne National Laboratory, ANL/CFCT-23/50, September 2023.
[6] H. B. Andrews et al., "Review of Molten Salt Reactor Off-Gas Management Considerations," Nucl. Eng. Des. 385, 111529 (2021).
[7] R. C. Robertson, "Conceptual Design Study of A Single-Fluid Molten-Salt Breeder Reactor," Oak Ridge National Laboratory, ORNL-4541, June 1971.