Molten salt reactors: In the 1960s the Oak Ridge National Laboratory (USA) designed and built a demonstration MSR using U-233 as the main fissile driver in its second campaign. The reactor ran over 1965-69 at powers up to 7.4 MWt. The lithium-beryllium salt worked at 600-700ºC and ambient pressure. The R&D program demonstrated the feasibility of this system and highlighted some unique corrosion and safety issues that would need to be addressed if constructing a larger pilot MSR.
There is significant renewed interest in developing thorium-fuelled MSRs. Projects are (or have recently been) underway in China, Japan, Russia, France and the USA. It is notable that the MSR is one of the six ‘Generation IV’ reactor designs selected as worthy of further development (see information page on Generation IV Nuclear Reactors).
The thorium-fuelled MSR variant is sometimes referred to as the Liquid Fluoride Thorium Reactor (LFTR), utilizing U-233 which has been bred in a liquid thorium salt blanket.g
Safety is achieved with a freeze plug which if power is cut allows the fuel to drain into subcritical geometry in a catch basin. There is also a negative temperature coefficient of reactivity due to expansion of the fuel.
The China Academy of Sciences in January 2011 launched an R&D program on LFTR, known there as the thorium-breeding molten-salt reactor (Th-MSR or TMSR), and claimed to have the world’s largest national effort on it, hoping to obtain full intellectual property rights on the technology. The TMSR Research Centre has a 5 MWe MSR prototype under construction at Shanghai Institute of Applied Physics (SINAP, under the Academy) originally with 2015 target for operation, but now expected in 2020.
SINAP has two streams of MSR development – solid fuel (TRISO in pebbles or prisms/ blocks) with once-through fuel cycle, and liquid fuel (dissolved in FLiBe coolant) with reprocessing and recycle. A third stream of fast reactors to consume actinides from LWRs is planned.
- The TMSR-SF stream has only partial utilization of thorium, relying on some breeding as with U-238, and needing fissile uranium input as well. SINAP aims at a 2 MW pilot plant initially, and a 100 MWt demonstration pebble bed plant with open fuel cycle by about 2025. TRISO particles will be with both low-enriched uranium and thorium, separately.
- The TMSR-LF stream claims full closed Th-U fuel cycle with breeding of U-233 and much better sustainability but greater technical difficulty. SINAP aims for a 10 MWt pilot plant by 2025 and a 100 MWt demonstration plant by 2035.
- A TMSFR-LF fast reactor optimized for burning minor actinides is to follow.
SINAP sees molten salt fuel being superior to the TRISO fuel in effectively unlimited burn-up, less waste, and lower fabricating cost, but achieving lower temperatures (600°C+) than the TRISO fuel reactors (1200°C+). Near-term goals include preparing nuclear-grade ThF4 and ThO2 and testing them in a MSR. The US Department of Energy (especially Oak Ridge NL) is collaborating with the Academy on the program, which had a start-up budget of $350 million.
However, the primary reason that American researchers and the China Academy of Sciences/ SINAP are working on solid fuel, salt-cooled reactor technology is that it is a realistic first step. The technical difficulty of using molten salts is significantly lower when they do not have the very high activity levels associated with them bearing the dissolved fuels and wastes. The experience gained with component design, operation, and maintenance with clean salts makes it much easier then to move on and consider the use of liquid fuels, while gaining several key advantages from the ability to operate reactors at low pressure and deliver higher temperatures
Sources World Nuclear