Advanced SMRs: No Fuss, (Almost) No Muss
December 27, 2024 § Leave a comment
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
AI Will Delay the Greening of Industry
October 31, 2024 § Leave a comment
Artificial Intelligence (AI), and its most recent avatar, generative AI, holds promise for industrial efficiency. Few will doubt that premise. How much, how soon, may well be debated. But not whether. In the midst of the euphoria, especially the exploding market cap of Nvidia, the computational lynchpin, lurks an uncomfortable truth. Well, maybe not truth, but certainly a firmly supportable view: this development will delay the decarbonization of industries, especially clean energy alternatives such as hydrogen, and the so-called hard to abate commodities, steel and cement.
The basic argument is simple. Generative AI (Gen AI) is a power hog. The same query made on a conventional search uses nearly 10 times the energy as when it is using Gen AI. This presumption is premised upon an estimate made on an early ChatGPT model, wherein the energy used was 2.9 watt hours for the same query which used 0.3 watt hours on a Google search. The usage gets worse when images and video are involved. However, these numbers will improve, for both categories. Evidence for that is that just over a decade ago, data centers were the concern in energy usage. Dire predictions were made regarding swamping of the electricity grid. The power consumption in data centers was 194 terrawatt hours (tWh) in 2010. In 2018 it was 205 tWh, a mere 6% increase, despite the compute instances increasing by 550% (Massanet et al, 20201). The improvement was both in computing efficiency and power management.
More of that will certainly occur. Nvidia, the foremost chipmaker for these applications claims dramatic reductions in energy use in forthcoming products. The US Department of Energy is encouraging the use of low-grade heat recovered from cooling the data centers. A point of clarification on terminology: the cloud has similar functionality as a data center. The difference is that data centers are often physically linked to an enterprise, whereas the cloud is in a remote location serving all comers. We use the terms interchangeably here. Low-grade heat is loosely defined as heat at temperatures too low for conventional power generation. However they may be suitable for a process such as desalination with membrane distillation, and the regeneration of solvents used in direct air capture of carbon dioxide.
Impact on De-carbonizing Industry
The obvious positive impact will be on balancing the grid. The principal carbon-free sources of electricity are solar and wind. Each of these is highly variable in output, with capacity factors (the time spent by the capital in generating revenue) less than 30% and 40%, respectively. The gaps need filling, each gap filler with its vagaries. AI will undoubtedly be highly influential in optimizing the usage from all sources.
The obverse side of that coin is the increasing demand for electricity by the data centers supporting AI of all flavors. The Virginia based consulting company ICF predicts usage increasing by 9% annually from the present to 2028. Many data center owners have announced plans for all energy used being carbon-free by 2030. Carbon-free electricity capacity additions are primarily in solar and wind, and each of these requires temporal gap filling. Longer duration gaps (over 10 hours) are dominantly filled by natural gas generators. A major effort is needed in enabling the scaling of carbon-free gap fillers, the most viable of which are innovative storage systems (including hydrogen), advanced geothermal systems and small modular reactors (SMRs).
The big players in cloud computing have recognized this. Google is enabling scaling by purchasing power from the leading geothermal player Fervo Energy and is also doing the same with Hermes SMRs made by Kairos Power. An interesting twist on the latter is that the Hermes SMR is an advanced reactor of the class known as pebble bed reactors, using molten salt cooling (as opposed to water in conventional commercial reactors). It uses a unique fuel that is contained in spheres known as pebbles. The reaction products are retained in the pebble through a hard coating. This is not the place to discuss the pros and cons of the TRISO fuel used, except to note that it utilizes highly enriched uranium, much of which is currently imported from Russia. Google explicitly underlines part of the motivation being to encourage scaling SMRs. This is exactly what is needed2, especially for SMRs, whose promise of lower cost electricity is largely premised upon economies of mass production replacing economies of scale of the plant.
Microsoft has taken the unusual (in my view) step of contracting to take the full production from a planned recommissioning of the only functional reactor at the Three Mile Island (conventional) nuclear reactor facility. In my view, conventional reactors are passe and the future is in SMRs. The most recent new conventional ones commissioned are two in Georgia. The original budget more than doubled, and the plants were delayed by 7 years. Par for the course for nuclear plants. Microsoft is certainly aware of the importance of SMRs, in part because founder Bill Gates is backing TerraPower, using an advanced design breeder reactor with liquid sodium as the coolant, and molten salt for storage. The “breeder” feature3 involves creation of fissile Pu239 by neutrons from an initial core of enriched U colliding with U238 in the surrounding depleted U containment. The reactions are self-sustaining, requiring no additional enriched U. The operation can be designed to operate over 50 years without intervention. Accordingly, it could be underground. The design has not yet been permitted by the NRC but holds exceptional promise because the fuel is essentially depleted uranium (ore from which fissile U has been extracted) and the issue of disposal of spent fuel is avoided.
Impact on Green Hydrogen, Steel and Cement
Hydrogen as a storage medium, alternative fuel, and feedstock for green ammonia, has a lot of traction. One of the principal sources of green hydrogen is electrolysis of water. But it is green only if the electricity used is carbon-free.
Similarly, steel and cement are seeking to go green because collectively they represent about 18% of CO2 emissions. Cement is produced by calcining limestone, and each tonne of cement produced causes about a tonne of CO2 emissions. Nearly half of that is from the fuel used. Electric heated kilns are proposed, using carbon-free electricity. Similarly, each tonne of steel produced causes emission of nearly 2 tonne CO2. A leading means for reduction of these emissions is the use of hydrogen to reduce the iron oxide to iron, instead of coke. Again, the hydrogen would need to be green and only high-grade iron ore is suitable, and it is in short supply worldwide. A recent innovation drawing considerable investor interest is electrolytic iron production. This can use low-grade ore. But for the steel to be carbon-free, the electricity used must be as well.
The world is increasingly electrifying. It runs the gamut from electrification of transportation to crypto currency to decarbonization of industrial processes. All these either require, or aspire to have, carbon-free sources of electricity. Now, AI and its Gen AI variant is adding a heavy and increasing demand. Many of these share a common trait: the need for electricity 24/7/365. In recognition of the temporal variability in solar and wind sources, the big players are opting for firm carbon-free sources such as geothermal and SMRs. That is the good news, because they will enable scaling of a nascent industry. The not so good news for all the rest is that these folks have deep pockets and are tying up supply with contracts. Remains to be seen how a startup in green ammonia or steel will compete for carbon-free electricity.
AI could well push innovation in industrial de-carbonization to non-electrolytic processes.
Vikram Rao
October 31, 2024
1 Massanet et al. 2020 Recalibrating global data center energy-use estimates. Science, 367(6481), 984–986.
2,3 pages 53 and 12 https://www.rti.org/rti-press-publication/carbon-free-power
Research Triangle Energy Consortium 