[jamest (OP)]

Dave writes in with, 'Is there any other way to produce electricity from nuclear fission (or fusion) apart from producing heat and using it to create steam to drive a generator?'

[paul cotter]

Fission produces energy in the form of heat. Although not very efficient a steam cycle is the only realistic way of producing electricity. Thermoelectric converters rarely reach 10% efficiencies. Magnetohydrodynamic(MHD) converters have been proposed as being more efficient but the technical problems involved have prevented their use.

[alancalverd]

At 30% efficiency, a thermoelectric generator could prove more costeffective over a lifetime than a steam turbine. However it is also worth considering the end use of the electricity: a considerable amount is used to generate low-grade heat for cooking and space heating, or for stationary motors, so a local nuke, say in a village, factory or hospital, might be used simply to provide steam at ~300 C and avoid all the losses of electricity production.

An old bee in my bonnet is neatly summarised by Wikipedia:

The current interest in nontraditional methods for the generation of hydrogen has prompted a revisit of radiolytic splitting of water, where the interaction of various types of ionizing radiation (α, β, and γ) with water produces molecular hydrogen. This reevaluation was further prompted by the current availability of large amounts of radiation sources contained in the fuel discharged from nuclear reactors.

[paul cotter]

30% with thermoelectric converters? Are you hiding some advanced tech, Alan?

[alancalverd]

There are already some promising semiconductor thermocouples with > 20% efficiency, apparently.

[paul cotter]

20% would be quite good. I'm not up to date on these developments.

[SeanB]

The one that has the longest life is to use beta emitters and a tiny capsule, isolated in a vacuum, that relies on the beta particles, which are electrons, to provide a very tiny current to power ultra low power equipment Think of things like low power clocks that run on nanowatts of power when the equipment is shut down, and also powering static memory that stores data and configuration. Expensive, only made by a few niche manufacturers, and with ITAR regulations associated with the export of them.

https://en.wikipedia.org/wiki/Betavoltaic_device

For more information.

[evan_au]

beta emitters ... to power ultra low power equipment

For maximum power output, the ideal operating voltage for a typical beta emitter is around 500,000 Volts,
- which is a bit high for low-power electronics (which typically operate at < 1V)

See a typical beta particle energy spectrum here: https://en.wikipedia.org/wiki/Beta_decay#Description

[alancalverd]

The trick with a beta energy source is charge separation. Consider a metallic beta source surrounded by a conducting sphere with an air gap. Even at 500 keV, beta radiation is mostly absorbed in a few millimeters of copper, so you now have a self-charging capacitor C. V = Q/C, so the available voltage between the source and the absorber can be anything from zero to 500 kV depending on the ratio of beta activity to current drain.

This technique has the advantage over plutonium thermoelectric generators that it doesn't need a cold sink. PuTEGS work OK in space where you can radiate heat to the 3K background, or in the human body where you have 70 - 100  kg of water at a constant  310K, but if you want the simplest possible source of, say, 100 volts, with a wide range of operating ambient you might consider a beta capacitor.

[evan_au]

Researchers investigated nuclear-powered propulsion, where:
- In the atmosphere, the nuclear reactor heats the air, which expands and propels the airplane
- In space, the nuclear reactor heats a propellant, which expands and propels the spaceship
       - Water would not make a good propellant, as it's latent heat is too high

Nuclear-powered airplanes were not put into production, as an airline crash would spread nuclear waste across a wide area.
- A nuclear-powered spacecraft would be less of a problem, provided the reactor was not fired up before reaching orbit (ie only low-radioactive materials present), and they were very confident it would never return to Earth.

See: https://en.wikipedia.org/wiki/Nuclear_thermal_rocket

[evan_au]

Many nuclear reactors use water for cooling, but other designs have been used:
- Gas-cooled reactors https://en.wikipedia.org/wiki/Gas-cooled_reactor
- Liquid-metal cooled reactors (eg using liquid sodium or liquid lead as a coolant) https://en.wikipedia.org/wiki/Liquid_metal_cooled_reactor

One proposed design for a helium gas-cooled reactor would use the Helium gas to directly turn a gas turbine, potentially achieving high efficiency. https://en.wikipedia.org/wiki/Gas_turbine_modular_helium_reactor
- But as far as I know, most reactors separate the nuclear cooling circuit from the power generation circuit with a heat exchanger; this assists maintenance of the turbines since they should not become radioactive. Normally, water/steam is the working fluid feeding the power-generating turbines.

[alancalverd]

My father worked for the UK Central Electricity Generating Board from 1945 (coal) to the late 60s when power reactors were diversifying from the original Magnox design. His comment on the entire program was "a very complicated way to boil water".

His favorite boiler actually used slurry pumped out of coal mines and coal washing facilities.

[paul cotter]

Yep, the coal burning plants don't burn coal as in an open domestic fire, they blow coal dust into the combustion chamber.

[Petrochemicals]

The Americans and Soviets had "atoms for peace" programmes. It was harnessed largley in the method of excavations. But I assume the op is trying to convert nuclear energy into electrical?

Thermoelectric converters rarely reach 10% efficiencies

I'd this the same  device that powers space probes such as Pioneer which is why it is still going strong?

[evan_au]

Pioneer (Thermoelectric)... why it is still going strong?

The Voyager 1 & 2 spacecraft had 3 Thermoelectric generators each, powered by Plutonium-238 (half-life 88 years).

Each generator:
- Produces 2400W of heat (halving every 88 years)
- Generates 157W of electricity (halving every 88 years)
- It was launched in 1977, so it hasn't reached its half-life yet
- but NASA has had to power down some instruments in order to keep others operating at full power.
https://en.wikipedia.org/wiki/MHW-RTG

The more modern device on the Mars Curiosity rover supplies both:
- electricity (to power the electronics)
- and heat (to keep the electronics warm during the cold Martian night).
https://en.wikipedia.org/wiki/Multi-mission_radioisotope_thermoelectric_generator

[evan_au]

The NS episode broadcast with this QOTW answer described an intriguing Diamond/Carbon 14 battery with no moving parts (similar to Alan's description above).

I infer from this description:
- an artificial diamond is grown (using Chemical Vapour Deposition, CVD?), containing a layer of carbon-14 mere micrometers thick
- Beta particles are emitted by the C-14, with average energy 156 keV
- These Beta particles (very high-energy electrons) tear electrons off thousands of nearby atoms
- Surrounding the C14 are doped regions forming a semiconductor diode, which corral the liberated electrons into a DC electronic circuit
- The doped regions produce a DC voltage of around 2V, at low current  (the voltage would be below 5.5V, which is native diamond's band-gap)
- 2 to 5V DC is compatible with low-power electronics
- The surrounding diamond keeps the beta particles away from nearby sensitive electronics or living cells
- It doesn't need a vacuum chamber
https://www.thenakedscientists.com/articles/interviews/radioactive-diamond-battery-will-last-thousands-years

There is an environmental angle to this:
- The UK has a lot of graphite blocks from decommissioned nuclear reactors, which they want to dispose of safely
- The neutron radiation in the reactor turns normal C12 into C13 and C14, mostly on the side nearest the nuclear fuel.
- The C14 makes the blocks radioactive, and thus harder to manage
- By shaving off the C14-contaminated parts of the blocks, the blocks are easier to handle
- This demonstration diamond battery is a possible use of the recovered C14 (after enrichment)
- An earlier prototype used radioactive Nickel-63
https://www.bristol.ac.uk/cabot/what-we-do/diamond-batteries/

[alancalverd]

Now that's clever!

An even simpler use for high-level nuclear waste would be to sterilise drinking water, or make a chlorine-free swimming pool. There's a lot of free energy in radioactive junk!