Reactor waste remains hot as waste so why do we not pile it in a massive dump and cirulate water through it to raise steam or at least preheat water for heat generation?
​
Thanks everyone for an informative and interesting discussion.
​
In: 337
Recycling nuclear waste for power is a thing that’s happened for a long time. It’s absolutely possible and it works well.
It’s not done in the US for the simple reason that it’s cheaper to mine more uranium than it is to reprocess the spent fuel rods.
That said, there’s no reason why if either the economics or technology behind this changes the equation, that the spent fuel rods can’t be uncasked for reprocessing.
We kind of do!
There are newer gen reactors that can use the waste from the older style ones as fuel, or they can use less refined fuel in the first place, but it does provide a second life for that “spent” fuel.
As far as pure thermal power? There’s just not that much waste fuel to use even if you wanted to mess with handling the stuff. It only generates a useful amount of energy as reaction mass.
Reusing nuclear fuel typically also creates additional materials, such as plutonium, which can be used for bombs. Concerns with someone stealing this material to make their own bombs has made reuse for all intents and purposes illegal due to the risk.
It’s not hot enough to boil water, which is the cheapest and simplest way to generate power from heat. We could try an alternative method but it’s cheaper to just dump the waste and buy more new fuel.
At a certain point, it becomes more trouble than it’s worth. Voyager 1 and 2 both have batteries powered by radioactive decay (radioisotope thermoelectric generators) because they’re too far away for solar power to be useful. This is also the radioactive battery that Matt Damon digs up to keep himself warm in his rover in “The Martian.”
Newer reactors can use some of this waste to squeeze some more fission energy out of it, but the decay energy itself just isn’t enough.
Thorium salt reactors can have their fuel reenriched over and over again, so it produces far less waster over its lifetime. Unfortunately, most reactors are uranium based.
Totally a thing, and a viable option. But , it would require reworking most of the current plants.
It is done, just not a lot cause of cost. A company called Terrapower is working on a new type of reactor called a Traveling Wave Reactor, which, in theory, would be very efficient and exclusively use spent fuel.
It is entirely theoretical and computer models, but the models show it should work. Unfortunately, US laws were preventing them from building a test reactor. They were trying to build one in China, but Trumo killed it when he took office because of his Chinese foreign policies. Not sure where they are these days though.
France does recycle nuclear fuel by extracting the more long-lived waste stuff from old fuel (especially plutonium and left-over uranium) and mixing it with depleted uranium left over from the enrichment process (Uranium fuel needs to be concentrated from a ratio of 99.2% U-238 and 0.72% U-235 to 3-5% U-235. As you can imagine this process means you have a lot of left-over U-238, ie depleted uranium).
This fuel is called MOX (Mixed Oxides) and reduces France need of new uranium by 17% (and the use of long-term storage by a similar proportion).
It’s simple and safe to keep spent fuel in a nice stable, secure cooling pond, especially whilst the short-lived, highly-energetic (and heat generating) fission products decay.
You don’t want to pile up spent fuel ‘in a massive dump’ as there is still scope to create a criticality with the remaining fissile material, causing a massive radiation release and possible explosion or meltdown.
Some radioactive isotopes have been separated from spent fuel to generate power or heat. The most common are strontium 90 used in Soviet radio thermal generators to generate electricity for remote radio beacons or lighthouses; polonium 210 which warmed the Soviet Lunokhod lunar rovers in the 1970s; and plutonium 238 used to power America’s deep space probes to the outer planets. However, in each of these cases, cost and efficiency weren’t really important.
Most of the answers here are about reprocessing fuel. To answer your actual question:
The efficiency of extracting energy from heat is a function of the difference in temperature at the inlet and outlet of your system. In almost all cases, the outlet temperature is “the air” or “the water”, depending on what your last cooling loop is using.
Nuclear waste in a pool is simply not that hot, they are typically around 50 C. So if the other end is, say, 20 C, you’re getting 30 C of working temperature. With those limits, a Carnot engine is less than 10% efficient.
At that sort of efficiency, the cost of the equipment needed to turn it into power is greater than the lifetime income from the electricity you’d generate. So you’d be running at negative dollars. Power companies do not like that.
Latest Answers