DARPA wants to bypass the thermal middleman in nuclear power systems

Nuclear batteries with beta emitters driving some kind of semiconductor have been around for a while, but they're very low power.[1]

"... Betavolt's team of scientists developed a unique single-crystal diamond semiconductor that is only 10 microns thick, placing a 2-micron-thick nickel-63 sheet between two diamond semiconductor converters to convert the decay energy of the radioactive source into electric current to form an independent unit."

"... 100 microwatts, a voltage of 3V, and a volume of 15 X 15 X 5 cubic millimeters ..."

3-4 orders below the power requirements for a phone. An AirTag-type intermittent device, though...

So, can those be scaled up? Are all those little beta-emitters in coin cell form factor going to be a problem? Nickel-63 has a half life of 100 years, so they'll be active for a while. Not dangerous unless broken up and ingested, but need to be kept out of the food chain.

[1] https://www-betavolt-tech.translate.goog/359485-359485_64506...

In fission, it seems that most of the energy release is in the form of the kinetic energy of the daughter nuclei rather than gamma radiation or the kinetic energy of neutrons (from Wikipedia: For uranium-235 (total mean fission energy 202.79 MeV), typically ~169 MeV appears as the kinetic energy of the daughter nuclei, which fly apart at about 3% of the speed of light, due to Coulomb repulsion. Also, an average of 2.5 neutrons are emitted, with a mean kinetic energy per neutron of ~2 MeV (total of 4.8 MeV.) The fission reaction also releases ~7 MeV in prompt gamma ray photons.)[1]

Given this, I'm guessing that, for direct conversion to be at all efficient here, a significant fraction of this energy would have to be converted into electrical potential energy rather than be dissipated as heat in collisions between these nuclei and any part of the apparatus. Are there any nascent technologies of this sort?

[1] https://en.wikipedia.org/wiki/Nuclear_fission

This is one of the reasons why, as an undergrad, I switched from nuclear engineering to physics: at the end of the day, we're still using heat and turbines, just with extra, more dangerous steps; although the materials engineering aspect is recognizably challenging, I found it not particularly thrilling.

At other, far end of the scale, if Hawking radiation does exist, black holes could be considered converters of mass to energy, skipping all of the conservation of baryon and lepton numbers ... although at very large timescales until you have a fizzy, spicy nano black hole on hand.

Controlled capture of the various types of radiation (sometimes I find that word to be sloppy) to extract the kinetic energies does not seem to be physically impossible, but I have oft wondered how as I think about various nuclear batteries which have existed. Indeed, the article doesn't even break it down enough: beta ought to be split into beta-plus (positrons) and beta-minus (electrons), and they skipped some 2p emissions. My guess is that not only will each need its own approach, but that each of those would be subdivided into different energy bands, not unlike having different compounds for chlorophyll-A and chlorophyll-B, only for, say, fast neutrons versus thermal neutrons.

And I think that's gonna be materials engineering again. Whoops!

Helion https://www.helionenergy.com/technology/ is a commercial fusion company working on a design that theoretically would use direct energy capture from the magnetic fields generated during the fusion event. They made some headlines last year. Not clear if their approach will be successful but it certainly is an interesting approach.

I always thought super conducting ccds with plasma scintillating cell intermediaries were the way to go.

Capture the alpha and beta radiation with the plasma scintillators. Plasma being ideal because it wont degrade with bombardment.

Capture the em radiation with ccds.

We normally think of ccds as low power capture devices for cameras. Theres no reason they couldn't be scaled up to handle the power requirements. Perfect use case for super conductors.

This of course for moderate to large scale fusion reactors where cost is a negligible object.

Of course the dream is solid state Hau arrays. Which Dr Lene Hau postulated 15 years ago… but thats a whole other story.

If photovoltaic cells can create power from radiation in the visible light range, I suppose there might be radiovoltaics that can do something similar. But I wonder if they can capture the high power fluxes from a modern core.

Nuclear generation didn't always require a steam turbine. Radioisotope thermoelectric generator is an old tech that doesn't need any kind of turbines or liquids to function. It's still being used on the two Voyager spacecrafts. Of course it doesn't meet DARPA's requirement of bypassing the thermal middleman but it can be scaled to be reasonably small and can generate a few hundred watts.

What's the typical efficiency of a nuclear power plant? i.e. what fraction of the energy from the fission reaction gets converted into electricity?

One of the previous attempts at that, with a "power density of a gasoline tank", according to the authors: https://en.wikipedia.org/wiki/Optoelectric_nuclear_battery

This is just a speculation. What if we were to use the heat from fission to generate hydrogen instead of electricity? Would that help at all?

For instance a high enough temperature will cause water to dissociate.

Aug 30 seems like a ridiculously short deadline for proposals considering that this is as cutting edge as it gets.

This is pretty much the only hope for nuclear power in the future. Current reactors are way too expensive, and they do not get cheaper the more we build of them.

Miles upon miles of pipes with high-performance welds meant to last decades is no way to build a cheap and cost-effective electrical generation system. We need something better.

Also, getting off a thermodynamic heat engine means the chance for far greater efficiency. Going through a heat cycle is hugely inefficient.

For example, just extending the lifetime of the Diablo Canyon reactor pair in California, for five years extra life from 2025 to 2030, is expected to cost a minimum of $8.3B. That's the utility's claim before the work has been done, and life all nuclear/construction projects, it will almost certainly balloon midway.

TL;DR nuclear needs a tech breakthrough like direct conversion.

Makes the use of depleted uranium look benign

Imagine thousands of soldiers with battery powered tools that must not(!!!) be dismantled

So long as it happens "over there" I guess