Talk:ThorCon nuclear reactor

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 Definition A molten salt reactor (MSR) using a mix of uranium and thorium in a fluoride salt with a graphite moderator. [d] [e]
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 Workgroup categories Physics and Engineering [Categories OK]
 Subgroup category:  Nuclear Engineering
 Talk Archive none  English language variant American English

Nuclear power is a controversial topic, and some of the controversies remain unsettled, even after the facts in the article are agreed on. This Discussion page will provide a concise summary from each side of these unsettled issues. Much of this discussion is collected from internet forums, and we welcome updates to improve these summaries.


Radioactive gases

Critiques of MSR safety on p.91 of Lyman 2021:
"MSR developers often say that cesium-137 and certain other troublesome fission products do not present a problem because they remain chemically bound in the liquid fuel and are not released. However, this is misleading because it only applies to those isotopes that are generated directly in the fuel from fission. It does not apply to isotopes that are produced indirectly by the decay of noble gas fission products only after the gases are released from the fuel.
For instance, in addition to being produced directly by fission, cesium-137 also results from the decay of short-lived xenon-137, which has a 3.82-minute half-life. Indeed, nearly all of the cesium-137 generated in a nuclear reactor is produced through xenon-137 decay rather than directly by fission."
"MSR developers say little about this issue and how they expect reactor operators to manage and dispose of large quantities of high-level cesium-137 waste."
"Another troublesome radionuclide, tritium, with a halflife of 12.5 years, is highly mobile and cannot be effectively captured. Even with a costly off-gas control system, MSRs would almost inevitably discharge far more tritium and other radioisotopes into the environment during normal operation than solid-fueled reactors."
Replies from ThorCon's 2020 IAEA report:
p.11 "The off-gas processing system uses helium sweep gas to entrain Xe and Kr gasses, passing them slowly through the off-gas cooling tanks within the Can where most of the Xe-135 decays to Cs-135 that is trapped there. The He, Xe, and Kr gas mixture then flows from the Can through two hold-up tanks and a charcoal delay line in the secondary heat exchanger cell. The gas flow continues to a cryogenic gas processing system to separate the gasses, storing stable Xe and radioactive Kr-85 in gas bottles and returning He for reuse as a sweep gas."
p.13 "Inert gas in the annulus between the cold-wall and Can captures tritium that may permeate the Can or Fuelsalt Drain Tank. A getter system removes the tritium from the inert gas."
p.13 "The solar salt loop captures any tritium that has made it to the secondary loop, and ensures that a rupture in the steam generator does not release harmful chemicals."
p.34 "The hold-up tanks contain no radioactive materials because once the off-gas leaves the Can, the daughter products of the gases are not radioactive. So the only radioactive materials are tritium and Kr-85. The Kr and Xe gas bottles will be removed and sold."
Reply to a question in Renewables vs Nuclear Debate How does Cesium get “released from the core” in your design?:
"In general it doesn't but any losses would likely be as CsI into the offgas system, where a chiller condenses/captures it."
Editor's Note: ThorCon's 2020 filing does not directly address the concern about Cs-137 in Lyman's 2021 report. The comment about CsI being captured in the offgas system is for a different reactor.
Comments from a discussion in
"As for the gas issue, this stinks as a red herring, as it insinuates that off-gassing is something that hasn’t been considered to date, which would only be the case if the engineers attached to these projects were incompetent fools. This would have to be extended to the personnel of the several national agencies in various countries that have given initial approval to some of these SMRs."
"Tritium is produced by CANDU reactors in volumes enough that it has to be dealt with and it is to the point where this has become the top global source for this isotope and a bit of a side hustle for CANDU operators."

Waste Management

For more discussion see the Discussion page of Nuclear waste management

Increased waste from Small Modular Reactors

National Academy of Sciences report:
Nuclear waste from small modular reactors, L.M.Krall,, PNAS, 31 May 2022,
From the abstract:
"the intrinsically higher neutron leakage associated with SMRs suggests that most designs are inferior to LWRs with respect to the generation, management, and final disposal of key radionuclides in nuclear waste."
"water-, molten salt-, and sodium-cooled SMR designs will increase the volume of nuclear waste in need of management and disposal by factors of 2 to 30."

Reply from Jack Devanney, Principal Engineer, ThorCon USA Inc:
"All fission reactors produce essentially the same amount of fission products per thermal energy produced. This is immutable physics. Designs with higher thermal efficiency produce less fission products per kWh electricity produced, but this difference is less than a factor of two from the clunkiest LWR to the fanciest paper HTGR. Either way the amount of spent nuclear fuel represents a beautifully small problem."

For a critique of the NAS report see:

What about non-fuel waste

Question in the FaceBook group Renewable vs Nuclear:
"all the stuff on the primary loop. Pipes, pumps, wiring, blah blah. Obviously there is going to be mechanical and chemical wear on parts and they will need to be replaced like any power plant relying on heat -> steam -> turbine -> generator process. Parts will be be additionally exposed to neutrons and presumably undergo nuclear reactions and I assume some of those reactions will compromise their mechanical properties... eventually. How big of a deal is this? Is it slow, fast? What is the balance of fuel waste to non-fuel waste? Where do these parts go?"

Answer from World Nuclear Association Recycling and reuse of materials from decommissioning:
Recycling materials from decommissioned nuclear facilities is constrained by the level of radioactivity in them. This is also true for materials from elsewhere, such as gas plants, but the levels specified can be very different. For example, scrap steel from gas plants may be recycled if it has less than 500,000 Bq/kg radioactivity. This level however is one thousand times higher than the clearance level for recycled material from the nuclear industry, where generally anything above 500 Bq/kg may not be cleared from regulatory control for recycling.

Answer from Captain Roger Blomquist, United States Navy (retired) 8 Feb 2022:
"There are small concentrations of activated structural elements like cobalt. These typically have half-lives of years, not multiple decades. If they are recycled, then the workers doing the recycling will need to take (sometimes expensive) precautions to minimize their radiation exposure. I doubt that any such exposures would be harmful, although some might be. The precautions are quite likely far in excess of what is needed to prevent actual health effects."

Question on
How big a problem is irradiated steel and other non-fuel waste from a nuclear power plant?

Add image caption here.

Answer from a World Nuclear Association discussion of recycling and reuse of materials from decommissioning:
Decommissioned steam generators from Bruce Power in Canada
"These steam generators were each 12m long and 2.5m diameter, with mass 100 tonnes, and contained some 4g of radionuclides with about 340 GBq of activity. Exposure was 0.08 mSv/hr at one metre." This compares to a chest x-ray (0.020 mSv) or the minimum exposure to show a measurable increase in cancer risk (100 mSv) XKCD Radiation Chart

Answer from Lyle McElhaney 30 March 2022:
Iron is an element that is difficult to make radioactive.

This table shows that iron-56, which is almost 92% of all naturally occurring iron, requires three neutron absorptions before it becomes radioactive, and two absorptions for another 2% of the material. A single absorption is a low probability event for any given iron nucleus; absorbing three is a low probability to the third power. If it does happen, it results in iron-59 which beta decays to cobalt-59 (stable) with a half-life of 45 days.

Of course, it also shows that about 6% of iron will become unstable with the absorption of a single neutron. The resulting isotope decays by electron capture, which does not emit a particle other than a neutrino, and results in manganese-55, which is stable. So, no harm done by that other than possibly some gamma-rays.

The cobalt-59 resulting from a triple neutron capture could catch another neutron, becoming the dreaded cobalt-60, nemesis of the cobalt bomb. Cobalt-60 has a powerful gamma-ray emission as it beta decays with a half-life of around 5 years. This requires, as noted, 4 successive neutron captures with an intervening beta decay after the third. It is a very small probability event in concept; I don’t know what it is in practice.

Other materials - some do become radioactive when drenched with neutrons for an extended time. One would need to know what materials to analyze what happens.

Risk of proliferation

Union of Concerned Scientists report: "Advanced" Isn't Always Better, Edwin Lyman (2021).
“We studied the most prominent 'advanced' nuclear reactor designs. Unfortunately, few are safer or more secure than current generation reactors.”
From the Executive Summary:
“All MSRs chemically treat the fuel to varying extents while the reactor operates to remove radioactive isotopes that affect reactor performance. Therefore, unlike other reactors, MSRs generally require on-site chemical plants to process their fuel. MSRs also need elaborate systems to capture and treat large volumes of highly radioactive gaseous byproducts.”

Reply from Jack Devanney, Principal Engineer, ThorCon USA Inc:
“For the record, ThorCon does no chemical processing online to remove fission products or anything else. Xenon and krypton bubble out in the header tank, are held in storage tanks until they have decayed to harmless levels, and then cooled, compressed and stored. There's nothing elaborate or complex about the process.”


Limited material resources

Comment on corrosion in a discussion of Molten Salt Reactors at
A basic problem with MSRs is that the materials used to manufacture the various reactor components will be exposed to hot salts that are chemically corrosive, while being bombarded by radioactive particles. So far, there is no material that can perform satisfactorily in such an environment."
Reply from Jack Devanney, Principal Engineer, ThorCon USA Inc:
The salt is not corrosive provided proper control of impurities and proper redox control. For SUS316 [stainless steel], this is backed up by over one hundred thousand hours of testing.
Editor's Note:
For more on this question, see Ch.12 - Corrosion in Molten Salts, in Molten Salts Chemistry, Lantelme & Groult, Elsevier 2013.

Question: in the FaceBook group Americans for Nuclear Energy
I've heard that the steel gets brittle. Is that just the surface? Can it be annealed with a torch and returned to service, or do we have to discard 343 tons of radioactive steel every 8 years?
Answer: from Ed Pheil, Chief Technology Officer, Elysium Industries
There are multiple types of steel damage. The type limiting Thorcon to 8 years is fluoride salt corrosion. Less limiting are neutron displacement damage. This is bulk steel damage, not surface. It can be partially annealed by heating the stainless steel to 650C for hours. Another damage is an (n, alpha) reaction in the nickel creating bulk nickel. Annealing makes the helium migrate to grain boundaries, making it more brittle. Also not limiting.
Further there is creep damage, due to high temperature, not neutrons, also not limiting.
The question is the rate of accumulation of each type of damage. Fluoride salt is more corrosive in a radiation field due to radiation creating free Fluorine ions (F-). They chose to use type 316 stainless steel because it is qualified & cheap, unlike Hastalloy-N. Enough cheaper to warrant 8 yr replacement, but also they would have a qualified material to be able to build "today".

Comment in a discussion of the ThorCon design at
"ThorCon has 12 mol% Beryllium in its salt mix. There is only one large Beryllium mine in the entire world. From ThorCons' numbers I calculate that a single 1,000 MW plant would use approximately 2.5 tons of Beryllium to start up. Since total world production of beryllium is about 260 tons/year and ThorCons have to be replaced every 4 years, 400 1,000 MW ThorCons (approximately current world nuclear reactors) would use up the entire world supply."
Reply from Lars Jorgensen, Engineer, ThorCon USA Inc:
Beryllium is a common mineral. Yes the number of mines in the world is limited, but easily expanded if there is a market.
Also the beryllium is recyclable in the future should the need arise.
Only a very small fraction of the beryllium is consumed in the fission process (through (n,2n) reactions).
ORNL's conclusion was that there wasn't a resource problem.
Beryllium is the 47th most abundant element found in Earth’s crust (Emsley, 2001), and it is widely distributed in many rock types (Hörmann, 1978).

Editorial discussion

see the ../editorial page for more discussion on editorial issues.