ThorCon nuclear reactor

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See also: Nuclear_power_reconsidered
© Image: ThorCon USA Inc
Major components of the ThorCon reactor "can"[1]
(CC) Image: ThorCon USA Inc
ThorCon power train. An extra heat exchange loop ensures no leakage of radiation. Overall efficiency is still above 46%.[2]
(CC) Image: ThorCon USA Inc
Raw material flows and waste at a ThorCon fuel processing plant. Enriched uranium powers the cycle, but most of the energy comes from the more abundant thorium.[3]

Thorcon nuclear reactors are molten salt reactors with a graphite moderator. These reactors (and the entire power plant) are to be manufactured on an assembly line in a shipyard, and delivered via barge to any ocean or major waterway shoreline. The reactors will be delivered as a sealed unit and never opened on site. All reactor maintenance and fuel processing will be done at a secure location.

This article provides brief answers to the questions raised in Nuclear power reconsidered. For more details see the ThorCon documents[4] and ThorCon's Status Report to the IAEA.[5]

Safety

Accidental overheating. There are salt plugs at the bottom of the reactor vessel that will melt if the reactor gets too hot, allowing the fuel to flow out of the reactor into drain tanks, where the fission reaction stops, and the decay heat is absorbed by a "cold wall".[6] Safe shutdown does not depend on any mechanical, electrical, or computer systems.[7] No operator action is required, and there is nothing an operator can do to stop a safe shutdown. Reactors that meet these requirements are called "walk-away safe".

Leakage of Radioactivity The molten salt circulates at low pressure, and any leakage from the reactor will quickly solidify. The most troublesome fission products, including iodine-131, strontium-90 and cesium-137, are chemically bound to the salt. There is no pressurized water near the reactor vessel. Leakage to the environment is blocked by three gas-tight barriers - the Can, the Silo, and the Hull.[8] Tritium is captured by getters in inert gas in the power module hall and in the secondary heat exchanger cell. A third salt loop allows any tritium penetrating both heat exchangers to be captured in the third loop.[7] Xenon and krypton bubble out in the header tank, are held in storage tanks until they have decayed to harmless levels, and are then cooled, compressed and stored.[9]

Sabotage The hull is a 10ft thick wall of sand with an inch of steel on each side, capable of blocking a jumbo jet with eight-ton engines.[7] Reactivity can be increased only by adding fuel slowly through an orifice inside the silo, out of reach of any rogue operator. The maximum rate of increase in reactivity is enough for load following, but never enough that the reactor can go prompt critical.[7] Should a group of terrorists seize control of the plant and attempt to remove fuelsalt, they would require use of the 500-ton deck crane, which could be easily disabled with small artillery.[10] There is no vulnerability to cyber attack for any safety-critical system.

Waste Management

The "waste" in the ThorCon fuel cycle is actually valuable fuel for future reactors. Like other moderated reactors, a once-through fuel cycle uses less than one percent of the available energy in the mined uranium. Adding thorium and reprocessing the spent fuel can double this efficiency, but realizing the full potential of fission energy will require fast neutron reactors capable of burning thorium and depleted uranium. Spent fuel salt is an easily stored solid.

Average per year for a 500 MW plant:[3]
High Level Waste: 13,400 kg to dry-cask storage [11]
Medium and Low Level Waste: 343 tonnes of irradiated steel (one of the 4 "cans") shipped out for refurbishment.
Recycled Fuel: 650 kg of 19.7% U-235 (33% of total U consumption)
Other: (Medical isotopes, etc.)

Weapons Proliferation

The reactors are delivered as sealed cans and never opened on site. All reactor maintenance and fuel processing is done at a central secure location.

There is no online chemical processing to remove fission products or anything else, and no highly enriched material anywhere — none above 20% U-235.

The sealed cans are inside a high-radiation silo under a heavy concrete lid. Any attempt to get inside the silo can be detected by sensors and security cameras and stopped by local police or military.

Uranium is always low-enriched. Plutonium is always diluted with thorium, in fuel salt with hazardous fission products.[7] Making bombs from this material would be far more difficult than starting from uranium ore.

Cost

(CC) Image: ThorCon
Two 500 MW ThorCons and a can ship.

ThorCon claims the expected cost of a complete power plant will be less than a coal plant of equal power.[12] Everything except the structure itself is replaceable. After four years of operation and four years of cooling, the sealed reactor can with the entire primary loop is returned to a centralized recycling facility, decontaminated, disassembled, inspected, and refurbished. Incipient problems are caught before they can turn into casualties. Thorcon plants with replaceable sealed reactors are expected to operate for 80 years; but if a ThorCon is decommissioned, the process is little more than pulling out but not replacing all the replaceable parts.[13]

Specs for a 500 MW plant:[7]
Plant cost per kW: $1200
Operating cost per kWh: $0.03 (including $0.006 for fuel)
Fuel consumption per day: 5.3 kg of 19.7% enriched uranium plus 9.0 kg of thorium.[3]
-145 tonnes of natural uranium per GW-year compared to about 250 tonnes for a standard light water reactor
- future re-enrichment of spent fuel will cut this uranium requirement by a third
Initial fuel load (2 cans): 78,000 kg NaF-BeF2-ThF4-UF4 (76-12-10.2-1.8 mol %)

The main challenge in designing a low-cost molten salt reactor is the lifetime of components exposed to high temperature and high levels of neutron irradiation. Expensive steels were used in early reactor experiments [14] to resist corrosion by the flowing molten salt. ThorCon uses standard steels.[15] Components with thin steel, like the heat exchangers, can be replaced. The reactor vessel and anything with thicker steel can be reused. At end-of-life, the slightly radioactive steel can be melted down and recycled for new reactors.

Lifetime of the reactor is actually limited more by the graphite moderator than by degradation of the steel. ThorCon’s graphite needs are similar to those of an HTGR (High Temperature Gas-cooled Reactor). Life expectancy is 4 years at 680C and total irradiation 3e22 n/cm2 by high-energy neutrons.[16] The steel and graphite for the reactors add a little more than 50 million dollars to the cost of a 500 MW plant.[15]

Design Notes

  • For a complete presentation of this design, see the ThorCon documents.[4]

New reactor designs have been evolving for decades, and may continue for many more. ThorCon chose a design using existing well-tested technology that could be rapidly deployed while other companies perfected their more sophisticated designs. They chose a simple upscale of the MSRE [17], a Molten Salt Reactor that had been tested at Oak Ridge National Laboratory in the 1960's. In choosing this simpler design, ThorCon avoided many problems of more advanced designs, including the need for expensive materials to ensure long lifetime, worries about diversion of fissile material, and unexpected costs that might come up with less proven newer technologies.

ThorCons are designed to solve the immediate CO2 emissions crisis, but there are better solutions for centuries down the road. Fuel efficiency is better than existing PWRs (Pressurized Water Reactors), but still far short of what can be achieved in an FNR (Fast Neutron Reactor). ThorCons are not breeder reactors, making more fissile material than they consume. They require an ongoing supply of enriched uranium. Heavy-water reactors can run on natural uranium.[18] FNRs can burn anything - spent fuel from PWRs, old bomb cores, thorium, even depleted uranium.[19] ThorCon's temperature is much higher than existing PWRs and high enough for some process heat applications, but if we want to stop burning fossil fuels entirely, even higher temperatures [20] could provide "green hydrogen"[21] for a truly carbon-free economy.

Notes and References

  1. Fig.10 from Section 1.2 in ThorConIsle
  2. ThorCon Power Conversion.
  3. 3.0 3.1 3.2 ThorCon Fuel Cycle
  4. 4.0 4.1 ThorCon Isle
  5. IAEA Advanced Reactors Information System (ARIS) ThorCon_2020.pdf 2020/06/22.
  6. The wall is kept "cold" by water that is replenished from a storage tank above the reactor. Circulation is maintained without pumps, because the hot water (or steam) rises in the space around the cold wall.
  7. 7.0 7.1 7.2 7.3 7.4 7.5 ThorCon SpecSheet7
  8. See section Release resistance in ThorCon Safety
  9. ThorCon Drain tank
  10. Section 6.2 in ARIS Status Report
  11. ThorCon power plants can store up to 80 years of used fuel onboard, using passive air cooling. ThorCon Fuel
  12. See ThorCon Economics for a detailed analysis of cost.
  13. "ThorCon is Fixable" p.1 in ThorConIsle
  14. The Molten Salt Reactor Experiment used an expensive alloy Hastelloy-N.
  15. 15.0 15.1 ThorConIsle 2019-03-03 p.26
  16. ThorCon graphite specification 2019-04-19
  17. See ThorConPower.com/history for a brief history of what many believe would have been a safer and more economical choice for commercial nuclear power.
  18. https://en.wikipedia.org/wiki/Pressurized_heavy-water_reactor
  19. FC-MSR Nuclear Reactor
  20. High Temperature Gas-cooled Reactors like China's HTGR may provide the kind of heat needed for production of zero-carbon hydrogen from water.
  21. The Sulfur-Iodine Process S-I Diagram requires temperatures near 1000C to run efficiently.