Thanks for the elaboration. I did fail to allow for fusion reactors.
Yet.
AM radio was the most efficient way or transmitting sound for some 60 years until FM was developed and implemented where it held sway for about the same period. Now digital is taking over. Whether some garage physicist has a Eureka moment or it’s ground out in a high dollar lab, I don’t doubt that there will eventually be a better mousetrap for generating electricity from one of the transuranics.
Which means that many people have worked over many years to get the steam-driven turbine generators very efficient. And many are still working on improving that efficiency.
So it’s smart design to take advantage of an existing, working, fairly efficient technology for part of the job. Substitute nuclear energy for the burning coal or gas heat-producing part, but keep the rest.
While this is very true, of more importance is safety, reliability and life of a nuclear power plant.
For example, a little bit of oxygen (air) in the water will wreak havoc by corroding equipment (turbines, piping, boilers) working at high temperature/pressure on steam. Therefore there are systems like De-aerators and oxygen scavengers to remove oxygen from the steam system.
Water chemistry is in itself is a big research area and improvements over the years have made boilers and the steam system safer.
One of the earlier design (maybe it was French) had the nuclear reactor transfer the energy to molten sodium which was in turn used to heat water. It was not very reliable.
France used a sodium-cooled design in their Phénix and Superphénix fast breeder reactors and while there were numerous problems the latter operated as a commercial reactor for nearly ten years. It was shut down because of proliferation and safety concerns and because the operating cost was too great. Reliability was an issue, but the bigger problem were legal and political challenges that prevented constructing improved designs based upon what was learned from that experience.
The Molten Salt Reactor Experiment (MSRE) conducted in the ‘Sixties and ‘Seventies at Oak Ridge National Laboratories and similar molten fuel fluoride salt reactors show promise for reliable and failsafe nuclear power generation using flexible mixed isotope (uranium, plutonium, thorium) fuels in a molten lithium, beryllium and zirconium fluorides inner loop and a lithium-beryllium fluoride and lithium fluoride (FLiBe) outer loop. Other designs based on a simpler sodium-potassium (NaK or “nack”) coolant are being investigated by various companies and governments including India, Japan, Russia, and China. Here is an overview on molten salt reactors.
Stranger
When I was a kid I first learned that nuclear power plants produced electricity by using the nuclear energy inn the form of generated heat to heat up water for steam turbines – and I was dreadfully disappointed* I had imagined some more direct and magical process. Using nukes to heat water just seemed so — uninspired.
Of course, as a practical matter using nuclear fission (and, eventually, fusion) to generate lots of heat is actually the reasonable way to do things. The result of zillions of fission processes manifests itself as heat, not as voltage differences or showers of directed electrons or other particles. And using the heat difference in something like a turbine to extract power is thermodynsamically much more efficient than , say, trying to extract power with thermocouples.
Shy of something like the spherical beta source Chronos described – which rely upon a particular and reliable particle source at its heart, you’re not going to find anything that will somehow give you more direct output.
Most of the heat is actually the result of kinetic energy of the split nuclei – it’s already in the form of heat. A lot of the rest is in the form of gamma rays. Even if you could harness those gamma rays in some way to make it into electric power, it’d probably be lossy. The nuclear bomb-pumped gamma ray laser actually generated its gamma rays second-hand, using the nuclear explosion to turn wires into plasma and then pumping that plasma to high energy levels, where it generated directed beams via amplified spontaneous emission along the directions of the wires (no feedback mechanism, so it wasn’t even a true coherent laser). So even extracting your nuke power in the form of light energy isn’t really a possibility.
*I think it was at the Hall of Science in Queens, New York, at the site of the 1964-5 World’s Fair, but after the Fair had closed.
So basically, you’re saying a nuclear steam plant isn’t cool enough. When this “more efficient way to capture that energy” comes along,* I’m sure it will be adopted tout suite. In the meantime there’s the adage, “Nothing’s impossible to the guy who doesn’t have to do it.”
*Assuming the Big Steam industry isn’t conspiring to stifle research.
They are such a powerful industry that they wtote the book on the topic. How can anyone compete with that?
Just as a point of nomenclature, several posters have used the term “decay” as synonymous with fission. While nuclear decay is a type of fission (spontaneous fission that occurs at a stochastically determinate rate) the fission that produces energetic yield in a nuclear reactor is primarily induced fission which produces a sufficient amount of thermal (slow moving) neutrons to produce a chain reaction of fission events at a rate that is self-sustaining, called criticality. Criticality can be achieved and maintained by several mechanisms including introducing a starter material, producing neutrons artificially and injecting them into the fuel elements, and using a moderator to thermalize (slow) fast moving neutrons to increase the capture rate. This has to be done in a controlled fashion to avoid a prompt criticality where the chain reaction becomes a runaway exponential feedback loop that causes thermal overload and mass production of ionizing radiation, e.g. a core meltdown (fuel elements melting) or detonation (nuclear bomb)
With one known exception (the Oklo and Bangombé deposits in the Franceville Basin in present day Gabon), criticalities do not occur in nature, but all unstable isotopes decay, and fissionable isotopes will decay if impinged by free neutrons.
Stranger
Succinctly put. I suppose I read too much Tom Swift as a boy. 
But does anyone believe the current technology is the penultimate method for extracting power from radioactive elements, barring minor improvements in materials and technique?
The people who think it’s the penultimate method must think that some other method is ultimate. But those folks probably think that there are a number of other ways, so it’s not penultimate, either.
But to the point: If you want to convert heat to some other, more useful, form of energy, steam is a good way to do it. There might be some better way, but it’s not going to be very much better, because our steam technology is already pretty close to the maximum possible efficiency. You could increase that maximum possible efficiency by increasing the temperature of your reactor, but that’s tough, too, since we have to make it out of something, and any material will fail if you get it too hot.
Because of the inefficiencies inherent in dealing with heat, it’s better if you can avoid ever turning your energy into heat in the first place. We’re making some baby steps on that, when it comes to chemical fuels, with things like fuel cells. But nobody has any inkling about how we could release the energy from induced fission in any form other than heat. We can do it for gradual uninduced decay, using things like the capacitor I described, but that produces a lot less power than induced fission.
Unless you have the means to transfer the momentum from neutrons—which are, as the name implies, electrically neutral—directly to some piston or rotating shaft, there will remain the need to convert their momentum into useful work via some intermediate process. Liquid water and steam are convenient for that purpose because it is readily available, its thermodynamic properties are well understood and undergo phase changes in a temperature regime compatible with common structural materials, and it can me made to be pretty efficient. Higher efficiencies can be obtained with other working fluids like helium, but they are more expensive, less readily available, and harder to contain.
Again, as addressed upthread, thermal energy from the momentum of neutrons resulting in fission not a physical ‘thing’ that can be captured or transferred from one box to another but a condition of state relative to a lower temperature reservoir. And the past efficient ways of converting thermal energy into work are heat engines which use a controlled flow from high temperature to low temperature to drive a mechanism such as a piston, fan, or turbine to do useful work. That’s not a limitation of technology or materials; it is fundamental thermodynamics. While electrical energy can be generated by thermal differences via the thermoelectric effect, the effect is inherently limited to low efficiencies. Unless we can generate a product from nuclear reactions primarily in the form of charged particles the ability to transfer momentum to act “directly” on an inductor or capacitor to produce electrical current, steam engines and other working fluid thermodynamic heat engines are the most efficiency way of converting that thermal energy to useful work.
Stranger
Is that the main trade-off when comparing gases (e.g.: carbon dioxide), liquid (e.g.:water/steam) and solids (e.g.:molten salt), the less dense and more fluid it is, the more efficient it tends to be but the more difficult it is to prevent leaks and explosions?
Would a nuclear reactor continually go through a good amount of helium as it ran?
You could in principle make the working fluid closed-cycle, but it’s really tough to prevent helium from leaking.
No, there are numerous considerations such as how much moderation or absorption a working fluid/coolant provides, what its thermodynamic properties are, whether it will be made radioactive by neutron activation, chemical reactivity and tendency to cavitation in liquid,phase, et cetera, in addition to cost. Helium has nice thermodynamic properties, is chemically non-reactive, and doesn’t interfere or transform with neutron exposure to an appreciable degree, but it’s small atomic size and weight makes it difficult to prevent leaks and the cost of replacement are significant compared to a readily available substance such as water. The primary advantages of fuel-carrying molten salts are their high thermal mass and thermal conductivity, ability to operate at ambient pressures eliminating the need for high pressure piping, and ease of rapid shutdown and failsafe simple by draining the salt into a reservoir with a geometry that prevents criticality.
There are a large number of considerations and tradeoffs with different reactor technologies and detail designs between operating and design costs, safety, and flexibility, a fact that is often lost by people who make the broad claim that “nuclear (fission) power is perfectly safe and cost-effective”. Some methods and designs are clearly better on all metrics than others, and especially in comparison to light water and pressurized water reactors that currently dominate commercial nuclear power.
Stranger
When you are looking for a fluid for this application, you want a fluid that has a great heat transfer coefficient (so that you require smaller heat exchangers/boilers) and can carry a huge amount of heat per unit mass (so things get compact )
Water is a great fluid for this. The hydrogen bonding between polar water molecules, makes it great for :
Cite for above : Latent Heat & Heat Capacity | Earth 540: Essentials of Oceanography for Educators
Bullets 1, 3, 5 and 8 from above combined with the below convective heat transfer coefficients makes water the best fluid for the job :
Cite for above : Convective Heat Transfer
Incidentally, most of those properties are due to water having a very low molecular mass, as solids and liquids go. There just aren’t very many molecules, period, that are that light, and the lighter ones (aside from ammonia) are gases.
Unless I’m reading this quote wrong, the VAST majority of the energy emitted by nuclear fission is heat (94%), not radiation.
So with that in mind, we’re back to the whole heat engine thing.
Nuclear fusion, on the other hand, should have the ability to directly convert some of that energy into electricity with 40-60% efficiency.
Fully agree. That’s the reason NH3 (Ammonia) is/was a refrigerant of choice. Except Ammonia is very hazardous and has been discontinued at many places.
The other small liquid molecule is HF, it too has hydrogen bonds but it’s the stuff nightmares are made of.
In a cold climate you can transfer heat at lower than steam temperature to provide heating for an area. It frustrates me to see dangerous amounts of reactor waste stored in cooled areas right by the reactors. When it could be transported to facilities that could provide heat for large parts of cities, greenhouses, etc. The stuff is stored near cities as it is. But the energy is wasted. And it is right by the reactor. Increasing the magnitude of an accident.
There are cities that use waste heat from electricity production as heating energy. Making money and use of waste. Do it with nuke waste as well.
The system would be similar to a geothermal system. Without the need to drill down. Instead of added cost to store and cool the waste. It is a resource.
Those “dangerous amounts of reactor waste”, e.g. spent fuel elements contain highly radioactive nuclear actinides; aside from their extreme radioactivity many are also toxic and biochemically reactive, and thus would serve very well in a ‘dirty bomb’ application. The trade of using them in a way that compromises safety and security in exchange for a modest amount of thermal energy is not a good bargain, hence why they are kept in secure wet and dry cask storage.
Stranger
At reactor sites with poor to ineffective security. Often in areas with high seismic activity. Fukushima, Coast of California. The thermal site could easily have equal or better security. Would best be situated in Midwest areas with little seismic activity and plenty long cold winters. So far…
Some are already in position, with poor security at places like New York area reactors. New York city having a waste heat recovery heating system in place too.