Transatomic Power molten salt nuclear reactor design that would generate 75 times more electricity per ton of uranium

Transatomic Power (TAP) is developing an advanced molten salt reactor that generates clean, passively safe, proliferation-resistant, and low-cost nuclear power. This reactor can consume the spent nuclear fuel (SNF) generated by commercial light water reactors or use freshly mined uranium at enrichment levels as low as 1.8% U-235. It achieves actinide burnups as high as 96%, and can generate up to 75 times more electricity per ton of mined uranium than a light-water reactor.

Transatomic Power has greatly improved the molten salt concept, while retaining its significant safety benefits. The main technical change we make is to change the moderator and fuel salt used in previous molten salt reactors to a zirconium hydride moderator, with a LiF-based fuel salt. During operation the fuel in the salt is primarily uranium. Together, these components generate a neutron spectrum that allows the reactor to run using fresh uranium fuel with enrichment levels as low as 1.8% U-235, or using the entire actinide component of spent nuclear fuel (SNF). Previous molten salt reactors such as the ORNL Molten Salt Reactor Experiment (MSRE) relied on high-enriched uranium, with 33% U-235. Enrichments that high would raise proliferation concerns if used in commercial nuclear power plants.

Transatomic Power’s design also enables extremely high burnups – up to 96% – over long time periods. The reactor can therefore run for decades and slowly consume both the actinide waste in its initial fuel load and the actinides that are continuously generated from power operation. Furthermore, our neutron spectrum remains primarily in the thermal range used by existing commercial reactors. We therefore avoid the more severe radiation damage effects faced by fast reactors, as thermal neutrons do comparatively less damage to structural materials.

A key difference between Transatomic Power’s reactor and other molten salt reactors is its zirconium hydride moderator, which we use instead of a conventional graphite moderator. Zirconium is a metal with a low absorption of neutrons and high resistance to radiation damage. Hydrogen is a highly effective moderator. The reactor core contains zirconium hydride rods. These rods are surrounded by cladding to extend the life of the moderator in the corrosive molten salt.

The available experimental data suggest that the service lifetime of the moderator rods will be at least 4 years, and could potentially last the lifetime of the plant. Additional in situ testing is needed to determine the full extent of the service lifetime.

There are three factors driving this higher electricity output: lower enrichment, higher burn-up, and better conversion of heat to electricity:

Lower Enrichment: One ton of natural uranium ore yields 88 kilograms of LWR fuel enriched to 5%. However, it yields 274 kilograms if only enriched to 1.8%. This is a factor of 3.1X more starting fuel mass for the TAP reactor.

Higher Burn-up: At 5% enrichment, light water reactors have improved their burnups from 30 Gigawatt-days per metric ton of heavy metal (GWd per MTHM), and are quickly approaching burnups as high as 45 GWd per MTHM. In contrast, the TAP reactor can achieve up to 96% burnup at 1.8% enrichment —the equivalent of 870 GWd per MHTM out of a theoretical maximum of 909 GWd per MHTM. This is a factor of 19.2X more thermal energy for the TAP reactor.

Better Conversion: Light water reactors have outlet temperatures of 290°C -330°C, and typical thermal efficiencies of about 34%. TAP reactors have an outlet temperature over 650°C with a gross thermal efficiency of about 44%. This is a factor of 1.3X more for the TAP reactor.

According to sources cited by the World Nuclear Association, proven world reserves of uranium are estimated to be 5.3 million metric tons if the market price were $130 per kilogram (current prices are about $80-110 per kilogram – at a higher price more mines are viable) and 7.1 million metric tons if the price increases to $260 per kilogram. Using light-water reactors, WNA calculates these reserves are enough “for about 80 years” especially given expected increases in energy use.

This limitation is currently not a serious problem, because it is likely that reserves could be extended by a factor of 2 or more through additional exploration. However, nuclear power’s generation share is currently only 12% of global generation. If this were to increase because of rapid energy demand of if countries turn away from fossil fuels, the relatively low burnup of light-water reactors may become an issue.

By comparison, the TAP reactor can use current known uranium reserves to supply fully 100% of the world’s electricity needs for about 4,000 years.

Techniques now under research for collecting uranium from seawater are estimated to become economically viable once uranium reaches a price of about $300 per kilogram. The TAP reactor generates enough electricity per kilogram of fuel that it remains commercially viable even at this price.

Compared to a similarly-sized light-water reactor, the annual waste stream is reduced from 10 to 0.5 metric tons – which is 95% less waste. Furthermore, the vast majority of our waste stream – the lanthanides, krypton, xenon, tritiated water vapor, noble metals, and semi-noble metals – has a relatively short half-life decay, on the order of a few hundred years or less. We believe mankind can tractably store waste materials on these timescales, compared to the hundreds of thousands of years required for waste from LWRs.

Future TAP Designs

The basic TAP reactor design described in this report will benefit from future innovations in a number of different ways. Improvements to complementary technology will become commercially available over time. These technologies include high temperature ceramics such as SiC-SiC composites for heat exchangers and other reactor internals, which will allow us to increase the reactor’s operating temperature and increase thermal efficiency. The helium sparging in the primary loop off-gas system may be replaced by more advanced cryogenic removal methods. Furthermore, we will likely be able to incorporate closed loop Brayton cycles or open loop air turbine cycles in the future.

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