- High-temperature operation naturally presents design challenges.
- LFTR technology base has largely stagnated for 40 years.
- LFTR technology is very different from the water-cooled, solid-fueled reactors that are the basis for current nuclear power generation, and is not yet fully understood by regulatory agencies and officials.
“Only Single Fluid graphite designs do not require new materials to be verified in a strong neutron fluance.” LeBlanc TEAC3
“The design weakness of the two-fluid design [at Oak Ridge National Labs in the 1960s] was its complex plumbing. The design used brittle graphite pipes to hold the fuel salt. The pipes separated the fuel salt and breeding salt, so they were essential. The problem is that graphite expands under intense neutron bombardment. So, graphite pipes would change length, crack and become very leaky… In modern research, copper-reinforced graphite fiber cloth seems theoretically suitable, but no physical tests have been done.” Wikipedia
We have computer modeling, design, and testing methods not available in the 1960s; we’ll likely find a better material and/or better core designs.
[So at worst, we go with graphite and the reactor would be periodically shut down, like current reactors needing to be refueled? “Typical graphite lifetimes of 4 years” — while we’d love to have a reactor that runs non-stop for 50 years, would 4 years be okay? These pipes would only be in the reactor core. So drain the fuel, swap in new pipes (are designs so would be easy), and start it up again. These graphite pipes cracking wouldn’t be catastrophic, they are only separating liquid salt containing uranium (and reaction byproducts) from liquid salt containing thorium, primarily for ease in separating out reaction byproducts.]
Solving the “two fluid plumbing problem” that ORNL had in the 1960’s: Make the reactor core a small (less than 1 meter diameter) elongated cylinder. A single barrier separates core and blanket regions (not intermixing regions in the core with complex pipes). The entire volume of the core is power-producing salt; the carrier salt itself is a fairly effective moderator, so no graphite moderator is needed. Increase power generated without intermixing by extending the length of the core.
This reactor design has a “potential lower limit of start up fissile inventory of a mere 150 kg/GW(e) with 400 kg/GW(e) being a more conservative goal. For comparison ORNL Two Fluid work was about 700 kg/GW(e), ORNL Single Fluid 1500 kg/GW(e), an LWR is 3-5 tonnes/GW(e) and liquid metal cooled fast breeders 10-20 tonnes/GW(e).” A reactor with a 70 cm wide cylindrical core 6.6m long with a steam cycle generator, gives 224MW electricity. “Including a meter thick blanket and outer vessel wall still results in a simple to manufacture design that can fit within a tractor trailer for transport”. D. LeBlanc / Nuclear Engineering and Design 240 (2010) p. 1644-1656
Need more research on materials for in the core, or improvements on the Hastelloy-N alloy for use in the reactor core: “It is likely though that Hastelloy N has a limited lifetime if used within the full neutron flux of the core. Use in the outer vessel walls and heat exchangers should pose little problem but substantial work will be required in order to qualify any new alloys for ASME Section III use.” D. LeBlanc / Nuclear Engineering and Design 240 (2010)
“Potentially a much superior metal barrier is a high molybdenum alloy which is known to have a much greater tolerance to neutron damage (Zinkle and Ghoniem, 2000).” D. LeBlanc / Nuclear Engineering and Design 240 (2010)
There is a long Core-Blanket barrier materials discussion at D. LeBlanc TEAC3
“…the Oak Ridge National Laboratory prototype LFTR showed some signs of corrosion after four years’ operation. Hence this would be a technical challenge that needs to be addressed if LFTRs are to be constructed and have an expected 50-year operational lifetime… The corrosion problems are potentially soluble simply by employing sufficiently thick pipe and chamber walls fabricated from Hastelloy-N, or alternatively developing further improved corrosion-resistant metal alloys, says Dr. Norris.” [Dr. Timothy Norris, European Patent Attorney at Norway’s ACAPO] — Nuclear Energy Insider
What is Needed Short Term: Fuel Salt chemistry and corrosion studies of various carrier salts and materials for heat exchangers or potential 2 Fluid barriers. Non-nuclear component testing of pumps, valves, heat exchangers etc. LeBlanc TEAC3
… we believe a small prototype plant should be built to provide experience in all aspects of a commercial plant. The liquid nature of the molten salt reactor permits an unusually small plant that could serve the role just so that the temperatures, power densities, and flow speeds are similar to that in larger plants. A test reactor, e.g., 10 MWelectric or maybe even as small as 1 MWelectric would suffice and still have full commercial plant power density and therefore the same graphite damage or corrosion limited lifetime. Supporting research and development would be needed on corrosion of materials, process development, and waste forms, all of which, however, are not needed for the first prototype. Thorium-Fueled Underground Power Plant, Moir and Teller, 2005
We need to show adequate long corrosion lifetime for nickel alloy resistant to the tellurium cracking observed after the past reactor ran for only 4 yr. If carbon composites are successful, corrosion will likely become less important. We want to prove feasible extraction of valence two and three fluorides, especially rare earth elements, which will then allow the fuel to burn far longer than 30 yr (200 yr). We need to study and demonstrate an interim waste form suggested to be solid and liquid fluorides and substitute fluorapatite for the permanent waste form of fission products with minimal carryover of actinides during the separation process. This solution holds the promise to diminish the need for repository space by up to two orders of magnitude based on waste heat generation rate. We need a study to show the feasibility of passive heat removal from the reactor after-heat and stored fission products to the atmosphere without material leakage and at reasonable cost. Another study needs to show that all aspects of the molten salt reactor can be done competitively with fossil fuel. Thorium-Fueled Underground Power Plant, Moir and Teller, 2005