#xiv Beyond Citations | Molten salt reactor: Made in China, conceived in the US
Insights from a 1970 research paper documenting how the US conceived molten salt nuclear reactor. Nearly five decades after its abandonment by the US, China has resurrected it in the Gobi desert.
For six decades India has sold the ultimate nuclear dream of harnessing potentially limitless energy for thorium. But the three-stage programme conceived to utilise thorium still remains a dream. As I describe in my latest opinion piece for Moneycontrol:
In the last six decades, India has perfected the first stage by establishing and running about 20 PHWRs. As far as the second stage is concerned, India is building a 500 MW prototype fast breeder reactor (PFBR) in Kalpakkam. It is set to become operational only in September 2026. The third stage remains in the R&D phase.
The dragon in India's neighbourhood on the other hand is taking significant steps to exploit thorium. On 17 April, the South China Morning Post (SCMP) reported that China had successfully demonstrated running a 2 MW (thermal) molten salt reactor running on thorium.
The experimental reactor, located in the Gobi Desert in China’s west, uses molten salt as the fuel carrier and coolant, and thorium – a radioactive element abundant in the Earth’s crust – as the fuel source. The reactor is reportedly designed to sustainably generate 2 megawatts of thermal power.
Zichen Wan — a China expert with about a decade of experience working with the state-run Xinhua news agency — has argued that recent science and tech reporting by the SCMP has been problematic. As such, this latest report by SCMP should be approached with some caution. However, China’s strides in thorium exploitation via the molten salt route is not a one-off — it has been covered not just by SCMP but by other publications over the years as well.
The latest SCMP report also showcases how Chinese researchers benefited from research into molten salt reactors by the US in 1960s:
American scientists pioneered molten salt reactor technology – including building a small test reactor in the 1960s – but the project was shelved in favour of uranium-based systems.
“The US left its research publicly available, waiting for the right successor,” Xu was quoted as saying. “We were that successor.”
Why was the US researching and developing molten salt reactor in Oak Ridge? Let us examine a piece of literature stemming from the Oak Ridge National Laboratory that the Chinese may have also possibly looked at:
M. W. Rosenthal, P. R. Kasten & R. B. Briggs (1970) Molten-Salt Reactors—History, Status, and Potential, Nuclear Applications and Technology, 8:2, 107-117, DOI: http://dx.doi.org/10.13182/NT70-A28619
Rosenthal et al, associated with the Oak Ridge National Laboratory in Tennessee at the time of publication, discuss the evolution of molten salt reactor technology from the late 1940s to the late 1960s. The origin of the molten salt reactor did not lay in solving the energy problem for masses. Instead, in the late 1940s the US was looking to develop nuclear powered airplanes. Molten fluoride salt was considered suitable for running a reactor powering an airplane.
The fluorides appeared particularly appropriate because they have high solubility for uranium, are among the most stable of chemical compounds, have very low vapor pressure even at red heat, have reasonably good heat transfer properties, are not damaged by radiation, do not react violently with air or water, and are inert to some common structural metals.
In the early 1950s, Aircraft Reactor Experiment (ARE) reactor was built in Oak Ridge with beryllium oxide as moderator and a fuel salt mixture consisting of sodium fluoride, zirconium tetrafluoride and uranium tetrafluoride. In 1954, the reactor hit a milestone when it operated successfully for nine days producing power up to 2.5 MW (thermal). In the same decade, the US scientists began thinking about harnessing the molten salt reactors for commercial energy as well. But unlike India, the US wasn’t necessarily worried about thorium utilisation. Thorium fuel cycle was chosen because it ‘was found to give better performance in a molten-salt thermal reactor than a uranium fuel cycle …’
However, the Molten Salt Reactor Experiment (MSRE) that started at Oak Ridge in the early 1960s did not initially bred thorium in the reactor or even use U-233. The initial fuel mixture consisted of uranium, lithium-7, beryllium, and zirconium fluorides. The reactor hit criticality in 1965. In a big milestone, the fuel was changed to U-233 (that was obtained by transmuting fertile thorium-232) in 1968. While the ultimate goal was to showcase the commercial exploitation of thorium was feasible, the MSRE was shut down in 1969, ostensibly around or after the time the authors had submitted their revised manuscript to the journal.
The authors largely hail the success of the MSRE. This is how they end their paper:
Much remains to be done before economic molten-salt breeder reactors can be achieved, but the basic virtues of the molten-salt system and the success of the MSRE offer promise that the breeder development program will meet with early success.
What happened in 1969 when the MSRE was shut down? What happened in the early 1970s that the US abandoned research into molten salts reactors altogether? Well this is a topic for a future edition of Beyond Citations.