Episode 4: Barnhart, Biofuels, and Baseloads

This week Zach and Kelly talk to Dr. Charlie Barnhart about energy! ¬†You can hear more from Charlie every week on the Science…sort of podcast!

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6 thoughts on “Episode 4: Barnhart, Biofuels, and Baseloads

  1. bryan elliott

    Wanted to just interject. You’d mentioned nuclear waste as a containable item, but you thought it was liquid, and your guest did not correct you.

    Spent fuel is a solid, it comes in little ceramic pellets. There is a liquid nuclear waste – fission product tailings in solution from reprocessing – but this has a far shorter backgrounding time (300 years).

  2. Bryan Elliott

    You stumbled on a few of my geeks (all nuclear). I’m a programmer by trade, but nuclear energy is a subject I’m obsessed over – ever since I broke down and did the ideal power density calculations for various substantial energy sources (nuclear, coal, gasoline, ethanol, ammonia, dimethyl ether, transesterified fatty acids (biodiesel), natural gas, etc), and found that while most of them have densities in the 6-14 Wh/g range, nuclear sat at around 24 MWh/g.

    It’s why light water reactors get away with 1% burnup. They’re still producing a retarded amount of energy per unit mass – and that’s what drives waste mass.

    Anyway, just a few corrections.

    Depleted uranium isn’t a big contributor to birth defects or cancer, but it is a biologically toxic, aerosolised (DU bullets vaporize and oxidize on impact) heavy metal – worse than lead in that respect. It’s not terribly radioactive, though. Anyway, if you listen to Skeptoid, episode #158 makes mention of this.

    The problem in a reactor for U-238 isn’t that it produces fewer neutrons, it’s that it doesn’t fission at all. Upon neutron absorbtion, U-238 transmutes to Pu-239, which robs you of a neutron (the currency of nuclear fire) and causes proliferation concerns.

    Commercial nukes run at 4-5% enrichment (which you did describe correctly). The minimum for criticality in a solid sphere of enriched uranium is about 1.5%, with moderation. This is impractically low for a power reactor given the need to transport heat away from the core to a turbine.

    By the way, 4% U-235, 1% burnup. We waste about 3/4 of the fissile material we put into a reactor. The remaining 96% is breedable, too, and the neutron budget ideally allows it. Unfortunately, we lose neutrons to the fission products that are produced, so you’ll never get 100% burnup in a solid-fueled reactor, or so the theory goes.

    You talked briefly about thorium, which is a whole can of worms; there’s a pantheon of designs, but the two that people seem to focus on (depending on their agenda) are molten salt style thorium breeders and attempts to use thorium in a light water style reactor.

    A molten salt reactor is fairly simple. You take a salt – which is fairly chemically stable, and generally you take one that’s optimized for that and for low neutron cross section (neutrons are expensive; don’t want your coolant suckin ’em up). You take this salt, and you melt it. Then, you make a similar salt out of your fuel – generally an actinide tetrafluoride – and you dissolve it in the molten salt. You then keep this stuff flowing through a heat engine and a set of graphite rods.

    Oak Ridge built a fairly low power (<=10MW thermal) MSR in 1964. It ran intermittently (for experiments and such) for about a year and a half at full power, after which the experiment's scope ended, and no significant further research was done. I haven't seen satisfactory explanations for why that was; there are a number of political theories about it – one of which I'll mention later – but they all throw up my skeptical red flags. The most likely one I saw was that (a) the alloy they used (Hastelloy-N) to build the thing corroded, and (b) the graphite moderators would change shape in an unexpected way under high-temperature neutron bombardment.

    From what I understand, the corrosion issue has been since solved by the company that produces Hastelloy-N, Haynes International. The graphite deformation, while unexpected initially, was geometrically predictable, which implied it could be pre-stressed, as it would settle into a final shape. This would either make fabrication more expensive, or force a "breaking in" of the reactor, but neither option seems insurmountable.

    The prevailing implementation of this tech among thorium fanboys (including China's Academy of Sciences and a US company called Flibe Energy, Inc.) is what's being marketed as the LFTR. This is essentially a uranium-fueled MSR with a blanket of thorium tetrafluoride-laced salt wrapped around it, to absorb escapee neutrons. Th-232, when it absorbs a neutron doesn't fission, but it does breed to the fissile U-233. This salt is continuously scrubbed via fluoridation and reduction* to transport the U-233 into the core, effectively making the reactor thorium-fueled. There's also a system to use a similar process to scrub the fission products from the core stream via fluoridation and distillation of what remains, in a more batched process.

    Detractors criticise LFTR for producing U-233, which they consider weaponable, but which has never been pursued as a weapons tech, due to the presence of U-232 (as the result of rare, but measurable (n,2n) reactions in neutron scattering events that don't result in absorbtion or fission), which decays to a few hard gamma emitters. This makes weapons production dangerous for the makers and weapons shinily visible to enemies.

    Promoters claim (though I'm unsure about how valid it is) that LFTR was abandoned because it didn't produce plutonium, which our weapons program desired at the time, and because it also didn't provide an excuse for extended development of plutonium separation tech – something that was (and is) required if we're to recycle our spent fuel in conventional reactors.

    TWR is an interesting device as well.

    I believe it was originally Edward Teller's idea, but the gist of it is that you have a canister of breedable fuel (fuel that can absorb a neutron and transmute to something fissile), and you place a critical charge of fissile material at one end, shaping the canister such that as the fissile fuel burns out, fuel "ahead" of it breeds, creating a travelling wave of power production.

    Terrapower's suggested implementation is slightly different, in that breedable material is shuffled in ahead of the fissile, essentially turning the travelling wave into a standing wave. Shuffling small, uniform pellets of nuclear fuel in and out of place helps to allow the most problematic of fission products – Xe-135 – to come out of the core quickly, where it can't absorb neutrons and drown the reactor.

    Designing the container for this is computationally hard, and has been the major stumbling block to development for years. Recently Bill Gates took an interest, ostensibly saying, "Oh, hey, you need computers? I think I did something with those a while back. Lemme see what I've got."

    One of the problems facing new nuclear technologies is the regulatory environment. I'm not talking about the rules on safety; they're a good thing, and the costs incurred by the NRC safety regs are considered by the industry to be an internalization of potential externalities – an attitude I'd love to see adopted by the various fossil fuel industries.

    I'm talking about the fees. The NRC charges a single, enormous fee at a per-power-plant level, which incentivizes building plants as large as possible. They have similar fee schedules for design safety evaluation. There's work being done to try and streamline NRC processes to accommodate the new smaller reactors, but like anything, progress is slow.


    * At the temperature ranges in question (400-900C), UF4 will fluoridate to UF6, while ThF4 will not undergo a similar reaction. UF6 is gaseous, so will bubble right out of the blanket salt. It can then be reduced with hydrogen gas, and introduced to the core. The resulting HF can then be electrolyzed and recycled.

  3. Adam

    Dinorwig is awesome, i was thinking about when i went on a primary school trip there just before he started talking it about it, its inside a freaking MOUNTAIN (also wales and england are seperate countries)

  4. David

    You were discussing the toxicity of sulfates and nitrates after being dumped into the environment with a sense of unknowing. As in, it exists naturally so where’s the line where it gets harmful?
    I do environmental soil chemistry up here in Canada and have studied the effects of the Alberta oilsands on environmental toxicity. I think the key to determining toxicity is concentration, i.e. yes, these compounds occur in nature but they do so as a balanced equilibrium with many other ions. In terms of the mentioned compounds, these are among the major ions occuring naturally in waters and soils but when present in higher concentrations than normal they can restrict the amount of bioavailable cations necessary for plant growth. Nitrates hold another special concern in that as part of the nitrogen cycle they can amplify the proliferation of certain bacteria resulting in explosions of cyanobacteria that suck up oxygen and nutrients from surface waters leaving unfavourable conditions for higher organisms and leaving deadzone like ecosystems. This is more a problem with fertilizer runoff, but the key in all of this is the concentration of the contaminant in the system.

  5. Rob Weber

    How can a segment on energy be complete without mentioning cold fusion?!?!?!

    In seriousness though, Kelly, I really love the episodes where you dissect recently published research, it makes me really excited to go read the article. Please bring back the lit talks!

  6. Tom Korber

    Disposing of nitrate solution shouldn’t be too difficult. In fact we dispose of huge amount of nitrates everyday though our sewerage.. Depending on what is in the nitrate solution besides the nitrates it can be processed biologically in a sewerage treatment plant or something very much like it. Nitrate can be reduced into nitrogen gas by anaerobic bacteria in a biological reactor without too much trouble. In fact there is research into using it to even produce power (Microbial fuel cells).


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