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| Fig. 1: A molten-salt-cooled reactor system diagram. (Source: Wikimedia Commons) |
In order to prolong the lifetime of current reactors, a new class of systems referred to as Generation IV systems have been under development. Many light water reactors (LWRs) in the U.S. have recently been attempting a transition to prolong reactor lifetime from 40 years to 60 years, and to achieve this many are looking to Generation IV systems. Generation IV reactors generally allow for the reactor to operate at temperatures, radiation levels, and pressures that are above the current standard. This prolongs their lifetime expectancy to roughly 60 years. [1] One such system is the Molten Salt Reactor (MSR).
A molten-salt-cooled Advanced High-Temperature Reactor is a relatively new proposed reactor design. In this design, extremely high temperatures allow for the efficient thermochemical production of hydrogen gas or electricity through electrolysis. The proposed molten-salt-cooled reactor uses a different type of fuel that is s coated-particle graphite-matrix fuel similar to that used in high-temperature gas-cooled reactors. [2] The new component in these molten-salt-cooled reactors, however, is that it uses a molten-salt coolant and a pool configuration. As a class of reactors, molten-salt-cooled reactors are defined by two characteristics: high- temperature fuel and low-pressure liquid coolant. They are beneficial also because the neutron absorption and scattering cross sections in the salts used in the reactors are much lower than that of water. [2] Fig. 1 shows the proposed design for one of these molten-salt-cooled reactors. It can be seen that the molten salt flows from the core of the reactor to an external heat exchanger. This heat exchanger would contain the required system to achieve electricity or hydrogen production. It then dumps the heat load of the molten salt and the molten salt goes back to the reactor core.
There are many challenges in ensuring the materials used in these molten-salt-cooled reactors can withstand the harsh environments. One such challenge is the ability to conduct enough research in order to understand how the materials will behave. One essential requirement is that core materials for the molten salt reactors need to be compatible with the extremely harsh environment combining neutrons, molten salts, and high temperatures in the reactor. [3] Additionally, challenges for nuclear materials research are also due to the limited availability, extreme cost, and radiation-related difficulties associated with neutron irradiation and characterization of the irradiated materials.
There are also more challenges simply related to the unknown result of extending reactor lifetime. Generally, there is concern for the potential development of new forms of degradation in the reactors which wasnt experienced before due to their shorter lifespan. Allen et al, point to the past in the realm of radiation effects, new reactor operating conditions were established at least one new radiation-induced phenomenon was found. [1] They also point to historically starting in the 1960s, irradiation-induced hardening was discovered. In the 1970s it was swelling that was a concern for fast reactors. The 1980s brought about the effect of high-temperature embrittlement due to helium as well.
In these newer Generation IV systems, it is possible that different factors which didnt necessarily cause issue previously, could now be a problem as a result of the lifetime extension of the reactor. In addition to the molten-salt-cooled reactors, there are more types of Generation IV reactors such as sodium fast reactors or gas-cooled reactors, which all face these challenges. A challenge specifically for the molten-salt- cooled reactors is that they dont form oxides with steels in the reactor core. These oxides are usually a protective sort of layer against corrosion and without this barrier, corrosion could be a major cause for concern. More generally, the harsh conditions in the molten-salt-cooled reactors could cause damage to the materials. [1] The extremely high operating temperatures of these MSRs will require improved materials. To be as efficient as possible with the reactor operation, it is in the users best interest to increase the temperature to reach the full potential of the MSR for efficient electric and thermochemical hydrogen gas production. Oxide dispersion- strengthened alloys (such as MA-754 or MA-956) and carbon-carbon composites are among the candidates for operating temperatures at these higher ranges. [2]
Molten-salt-cooled reactors have great applications for the future. The main applications is in thermochemical production of hydrogen gas which involves a series of chemical reactions with the net result of producing hydrogen and oxygen gas from heat and water. [2] Currently the world uses 50 million tons of hydrogen gas per year, primarily for fertilizer production. The demand is growing rapidly. Large-scale research and development efforts are working to develop hydrogen gas-fueled vehicles. The energy required to produce the hydrogen gas for transportation would be approximately equivalent to that used to produce electricity. By the time the molten-salt-cooled reactor could be operational, its proposed size [2000+MW(t)] will be roughly equivalent to the production capacity requirements of a conventional hydrogen gas plant. [2]
The process of attempting to prolong the operation of these older reactors requires a deep understand of the degradation of materials in the intricate functioning of the reactor materials, in-vessel structures, concrete, and primary circuit over many decades of power production, to ensure that the reactor can continue to operate safely. [3] While there is considerable experience with this molten-salt coolant in fast reactor applications in the U.S. and internationally, there is little recent experience in the actual sodium compatibility and only scarce data on new alloys currently being developed. [1] In order for these molten-salt-cooled reactors to become fully operational and safe, a new material needs to be created to withstand the harsh reactor environments while simultaneously prolonging reactor lifespan. [4]
© Olivia Sheppard. 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] T. Allen et al., "Materials Challenges for Nuclear Systems," Materials Today, 13, No. 12, 14 (2010).
[2] C. W. Forsberg, P. F. Peterson and P. S. Pickard, "Molten-Salt-Cooled Advanced High-Temperature Reactor for Production of Hydrogen and Electricity ," Nucl. Technol. 14, 289 (2003).
[3] Y. Katoh et al., "Viewpoint Set on Nuclear Materials Science" Scripta Mater. 143, 126 (2017).
[4] J. Sunde, "Material Corrosion in Molten Salt Reactors," Physics 241, Stanford University, Winter 2018.
