Friday, September 11, 2015

Transatomic’s Nuclear Molten Saltiness

transatomic-logo-2x2We’ve written a bit about NuScale over the last week, primarily to spotlight its small reactor expo. The NuScale design includes a boiling water reactor, a technology that is, in its essentials, well-understood and time tested. It’s not the only such technology, but it is used in about a third of American reactors (pressurized water reactors are the others).

Plenty of other technologies have been developed over the life of domestic nuclear energy. Canada primarily uses home-grown pressurized heavy water reactors; it has exported its CANDU technology to other countries, notably India. Russia has developed light water graphite-moderated reactors, which are similar to boiling water reactors. (World Nuclear Association has a roundup of reactor types here.)

Another design that came to life in the 1960s is the molten salt reactor. WNA describes it thusly:

In the normal or basic MSR concept, the fuel is a molten mixture of lithium and beryllium fluoride (FLiBe) salts with dissolved low-enriched uranium (U-235 or U-233) fluorides (UF4). The core consists of unclad graphite moderator arranged to allow the flow of salt at about 700°C and at low pressure. Much higher temperatures are possible but not yet tested. Heat is transferred to a secondary salt circuit and thence to steam or process heat. The basic design is not a fast neutron reactor, but with some moderation by the graphite is epithermal (intermediate neutron speed) and breeding ratio is less than 1.

The design was created by Oak Ridge National Labs in the early-mid 60s and basically proved out, though never scaled up to commercial application. WNA does not speculate why this might be, but work on it petered out around 1976. It could be that the industry had standardized around different designs by then or the government killed financing and orphaned the technology.

So it has remained – or had remained:

There is now renewed interest in the MSR concept in Japan, Russia, China, France and the USA, and one of the six Generation IV designs selected for further development is the MSR in two distinct variants, the molten salt fast reactor (MSFR) and the Advanced High Temperature Reactor (AHTR) – also known as the fluoride salt-cooled high-temperature reactor (FHR) with solid fuel, or PB-FHR specifically with pebble fuel.

And that brings us back around to small reactors, because one of those companies that has renewed interest in molten salt technology is startup Transatomic. It want to tout its technology, of course, but this comparison with light water reactors is informative:

Molten salt reactors like Transatomic Power’s are fueled by uranium dissolved in a liquid salt. The fuel is not surrounded by cladding, making it possible to continuously remove the fission products that would otherwise stop the nuclear reaction. The liquid fuel is also much more resistant to structural damage from radiation than solid materials – simply, liquids have very little structure to be damaged. With proper filtration, liquid fuel can remain in a molten salt reactor for decades, allowing us to extract much more of its energy.

All designs have their unique qualities – a molten salt reactor produces less used fuel because more of it is used. In some designs, if something goes wrong, the liquid core (the molten salt containing the fuel) can be drained as though down a drain into a shielded container. Additionally, the fuel can stay in the reactor for decades.

Still, anything with the word salt in it also must mitigate potential corrosion, especially a big pot of salty broth such as Transtomic proposes, but that issue appears not to be one – and hasn’t been since the original design from the 60s. See here for an interesting discussion.

So we wish Transatomic luck with its molten salt small reactor. This is a technology whose time has been waiting to come for 50 years and the current interest in it is striking and (we may hope) significant.


This story about European interest in molten salt reactors is interesting and worth a look. This is the bit that stuck out:

For years nuclear scientists have talked about a revival of molten salt reactors, which are powered by a liquid fuel rather than solid fuel rods, that will help spark the long-awaited “nuclear renaissance.”

The “nuclear renaissance” and what will spark it. It reminds me of magazines that interview a hot young actor and tout him as “the new Brando.” It’s a meaningless phrase to justify writing about a topic.

As far as we can tell, nuclear is doing quite well in its, shall we say, Regency period. There are challenges, as there always have been – and there are opportunities, as there always have been. Molten salt reactors have challenges and present opportunities. That’s enough to make a story.


Charles Barton said...

You have addressed a number of the issues, that I have attempted to address in my blog Nuclear Green. I must say, that I find much in your account of the MSR history to be less than satisfactory. For example, both of ORNL's MSRs were graphite moderated, and operated as thermal neutron reactors. The Transatomic power reactor might be describedas bimodal, It is critical in a moderated thermal neutron area of the core, but fast neutrons are allowed to escape the core, and breed U-238 atoms. both Pu-239 and U-235 are used as nuclear fuels.

The primary AEC motive for shutting down MSR research, was to devote the money involved to the unsucessful Clinch River Reactor. TMilton Shw justified shutting down MSR research, by arguing that it should not be developed because it required development.

I have written two posts, the first concerns criticism of Transatomics plan to protect its Zarconium Hydride moderator with a material that would be capable of resisting the combined effects of fluoride salts, heat and radiation. It is questionable if ant such material is avaliable, the oprion would be to follow the ORNL path to using any moderator, or to chuck moderation, as the French have advocated, and to run the MSR as a fast reactor.. This was discussed on Energy from Thorium,though the Transatomic Principals failed to participate. ,I consider their failure to defend their concept. This is most unfortunate. Despite the questionable future of their moderator concep, the TAP Reactor has very interesting features, and I wish them good luck in resolving any and all problems.

Anonymous said...

NuScale is a PWR with integrated steam generators, not a BWR.