Research Triangle Energy Consortium

The potentially catastrophic condition that a nuclear reactor can encounter is overheating leading to melt down of the core. Conventional reactors need active human or automatic control intervention. These can go wrong, as they did in the 3 Mile Island accident. Small modular (nuclear) reactors (SMRs) are designed to share the trait of passive cooling down (automatic, without intervention) in the event of an upset condition. SMR designs to achieve this control differ, but all fall in the class of intrinsically safe, to use terminology from another discipline. This is the no fuss part.

The muss, which is harder to deal with, entails the acquisition and use of fissile nuclei (nuclei which can sustain a fission reaction), and then the disposition of the spent fuel. Civilian reactors use natural uranium enriched in fissile U-235 to up to 20%. At concentrations greater than that, theoretically a bomb could be constructed. The most common variant, the pressurized water reactor (PWR), uses 3 to 6% enrichment. Sourcing enriched uranium is another issue. Currently, Russia supplies over 35% of this commodity to the world. The US invented the technology but imports most of its requirement.

In all PWRs and most other reactors, nearly 90% of the energy is still left unused in the spent fuel (fuel in which the active element is reduced to impractical concentrations) in the form of radioactive reaction products. Recycling could recover the values, but France is the only country doing that. The US prohibited that until a few decades ago, for fear that the plutonium produced could fall into the wrong hands. Geological storage is considered the preferred method but runs into local opposition at the proposed sites, although an underground site in Finland is ready and open for business.

One class of reactors that defers the disposal problem, potentially for decades, is the breeder reactor. The concept is to convert a stable nucleus such as natural uranium (U-238) or relatively abundant thorium (Th-232) to fissile Pu-239 or U-233, respectively. The principal allure, beyond the low frequency of disposal, is that essentially all the mineral is utilized without expensive enrichment. In both cases, the fuel being transported is more benign, in not being fissile. One variant uses spent fuel as the raw material for fission. The reactor is the recycling means.

At a recent CERA Week event, Bill Gates drew attention to TerraPower, an SMR company that he founded. For the Natrium (Latin for sodium) offering, which combines the original TerraPower Traveling Wave Technology (TWR) with that of GE Hitachi, the coolant is liquid sodium (they are working on another concept which will not be discussed here). Using molten metal as a coolant may appear strange, but the technical advantage is the high heat capacity. The efficacy of this means was proven as long ago as 1984, when in the sodium-cooled Experimental Breeder Reactor-II at Idaho National Laboratory, all pumps were shut down, as was the power. Convection in the molten metal shut down the reactor in minutes. That reactor operated for 30 years. So, that aspect of the technology is well proven. TerraPower’s 345 MWe Natrium reactor, which broke ground in Wyoming earlier in 2024, is not technically a breeder reactor, although it utilizes fast neutrons, which is helped by the coolant being Na, which slows neutrons down less than does water (the coolant in PWRs). Natrium uses uranium enriched to up to 19% as fuel.

Natrium has two additional distinguishing features. The thermal storage medium is a nitrate molten salt, another proven technology in applications such as solar thermal power, where it is an important attribute to provide power when the sun is not shining.  For an SMR, the utility would be in pairing with intermittent renewables to fill the gaps. Their business model appears to be to deliver firm power with a rated capacity of 345 MWe and use the storage feature to deliver as much as 500 MWe for over 5 hours. In general, the unit could be load following, meaning that it delivers in sync with the demand at any given time.

The most distinctive feature of the Natrium design is that the nuclear portion and all else, including power generation, are physically separated on different “islands”. This is feasible in part because the design has the heat from the molten sodium transferred by non-contact means to the molten salt, which is then radiation free when pumped to the power generation island. The separation of nuclear and non-nuclear construction ought to result in reduced erection (and demobilization) time and cost. Of course, sodium-cooled reactors are inherently less costly because they operate at ambient pressures, and the reactor walls can be thinner than they would be for an equivalent PWR.

The separation of the power production from the reactor ought also to lend itself to the reactor being placed underground and less susceptible to mischief. This is especially feasible because fuel replacement ought not be required for decades. This last is the (almost) no muss feature. Disclaimer: to my knowledge, TerraPower has not indicated they will use the underground installation feature.

The “almost” qualifier in the “no muss” is in part because, while the fuel is benign for transport, the neutrons for reacting the U-238 are most easily created using some U-235. Think pilot light for a burner. Natrium uses uranium enriched to 16-19% U-235. However, as expected for a fast reactor, more of the charge is burnt. Natrium reportedly produces 72% less waste. These details support the fact that, their other attributes notwithstanding, SMRs do produce spent fuel for disposal although with less frequency in some concepts, especially breeders, and this is the other reason for the “almost” qualifier.

As in all breeders, no matter what the starting fuel is, additional fuel could in principle be depleted uranium. This is the uranium left over after removal of the U-235, and it is very weakly radioactive. Nearly a ton of it was used in each of the old Boeing 747s for counterweights in the back-up stabilization systems. It was also used (probably still is) in anti-tank missiles because the pyrophoricity of U caused a friction induced fire inside the tank cabin after penetration. Apologies for the ghastly imagery, but war is hell.

Advanced SMRs could play an important role in decarbonization of the grid.  My personal favorites are those that use thorium as fuel, such as the ThorCon variant which they are launching in Indonesia. Thorium is safe to transport, relatively abundant in countries such as India, and the fission products do not contain plutonium, thus avoiding the risk of nuclear weapon proliferation.

As in most targets of value, we must follow the principle of “all of the above”*.

Vikram Rao, December 26, 2024

*All together now, from All Together Now, by The Beatles (1969), written by Lennon-McCartney

Tagged: energy, news, nuclear, nuclear energy, nuclear-power