Nuclear Green interviewed Sherrell Greene of Oak Ridge National Labs about Fluoride Salt Cooled High Temperature Reactors (FHRs), Molten Salts can be used as both reactor coolants and as reactor coolants – fuel carriers and Advanced High Temperature Reactors (AHTR) are hybrid reactors which combine features of Molten Salt Reactors with features of Gas Cooled Graphite Structured or of Pebble Bed Reactors. The nuclear fuel for AHTRs is solid and embedded in graphite structures rather than a liquid salt dissolved in liquid salt coolant/carriers. SmAHTRs are Small Advanced High Temperature Reactors.
What do you view as the advantages of Molten Salt cooled Advanced High Temperature Reactors (AHTR)?
First, to avoid confusion, at ORNL we developed some terminology I would like to see adopted more widely. We liked to call liquid salt-cooled reactors, “FHRs” – Fluoride salt cooled High temperature Reactors. These are salt-cooled, but not salt fueled. Then, of course, there’s the molten salt reactors or “MSRs”, with are both cooled and fueled with fluoride (or possibly chloride) salts.
The first FHR concept ORNL developed in conjunction with SNL and others, was the Advanced High Temperature Reactor (AHTR) in the early 2000’s. The AHTR in a large GW+ class central-station electricity generator. The second concept we developed during the past year or so was the Small modular Advanced High Temperature Reactor (SmAHTR). SmAHTR is a 125 MWt ( 50+ MWe) modular FHR for both process heat and electricity production.
In my view, FHRs integrate the best attributes of liquid metal-cooled reactors (LMRs), gas-cooled reactors (GCRs), and molten salt-cooled reactors (MSRs). They are high-to- very-high temperature, low pressure systems. They employ fluoride salt coolants, TRISO particle graphite fuels, and Brayton power conversion systems. Due to their low pressure, salt coolants, and graphite fuels, they inherit the best safety attributes of LMRs, GCRs, and MSRs. They inherit the economic advantages of low pressure systems – which means thinner-walled vessels and piping.
However, FHRs have their own issues. The favored fluoride salt (FLiBe) is very expensive. The high-temperature salt-tolerant structural materials are expensive. There are tritium control issues. And since you have to make the jump from a low pressure reactor to a high-pressure power conversion system in the electricity production application, there are a number of component design and reliability challenges – particularly with regard to heat exchangers.
Having said all of that, I’m big on the promise of FHRs.
How might the development of the AHTR contribute to the development of the MSR?
In many ways. In order to successfully develop and deploy an FHR, one must solve many of the fundamental technology challenges required for MSRs: the fluoride salt supply chain, the structural materials supply chain, fluoride salt pumps and heat exchangers, tritium control, instrumentation and control technologies… just to name the more important technologies. The development of an FHR would leave the country with a robust development infrastructure for MSRs. And the successful deployment of an FHR would be a path-finder for the licensing and regulatory framework environment required for successful MSR deployment. However, the fact that MSRs have a liquid, mobile fuel will be a large, unaddressed hurdle in the regulatory arena.
What do you view as the potential uses of the AHTR?
High-to-very high temperature process heat and high-efficiency electricity production for central station and remote applications.
Does the AHTR have a potential cost advantage compared to the LWR?
Yes. Their low pressure character translates to less metal in their construction. The coolant doesn’t interact energetically with air/water. This means one is not driven to massive containments. Less concrete. SmATHR could benefit enormously from factory fabrication. However, there are off-setting issues. I’ve already mentioned that fluoride salt coolant is really expensive. The nickel alloy structural materials aren’t cheap either.
I’m actually very much on the fence with regard to LCRs. They are potentially attractive fast-spectrum systems and could offer some innovative options for burn/breed fuel cycles. But they also face much more daunting chemical compatibiliy challenges than fluoride salt-cooled concepts. I don’t consider LCRs to be in the tsame category of feasibility as liquid fluoride reactors.
Nextbigfuture contributions this week were:
Two ACP100 reactor modules have been ordered in China. These 100 to 150 MWe reactors can be built in about 30 months, which is about half the time of larger reactors in China. These first of kind units will each cost a bit less than $400 million if budgets are not overrun.
Lightbridge has expanded the nuclear fuels they have under development from thorium-uranium fuels to new fuels that will enable uprating of 17 to 30% of pressure water reactors.