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crossover period

Page history last edited by Andrew Alder 9 months ago

Definition

The crossover period is the length of time it takes the nuclear waste produced by a nuclear reactor to decay to the point that it is less radioactive than the material used to produce and fuel the reactor would have been had the reactor not been built or operated.

Why have such a term?
1. A revealing anecdote
2. Is there a better term?
Nonsense
The nature of nuclear waste
Recycling
Thorium fuel
Nuclear fusion
See also
More to follow

Why have such a term?

Good question! In fact many are surprised to find that such a concept even exists. But for all fission reactors yet built or proposed, it's well-defined and finite and rather useful politically.

And I mean "politically" in the best sense, of helping people to understand the issues. In the nuclear debate there hasn't been a lot of this, frankly. On both sides, there has been a lot of really bad politics.

I'm not going to even speculate on why the antis have indulged in such bad politics. But I have seen the bad politics of the pro-nuclear side from the inside. Often without meaning to, often with the best of naive intentions, they have held the public in contempt. They have just said "trust us", believing that the public should and would. Bad, bad mistake.

See the pathetic PR record of nuclear power.

Although it's obvious I think from that, I should explicitly disclose that I am a pro-nuclear greenie, and was raised to be one by my late father, who was also a pro-nuclear greenie and one of Australia's more prominent nuclear engineers. And that for eight years I worked for the Australian Atomic Energy Commission among many of like mind. We certainly weren't doing it for the money; A year after I left there to work for a bank instead (I was a computer person, not a nuclear expert) my take-home pay had doubled, almost to the cent.

At the age of ten I helped Dad build our own solar hot water service. When compiling his eulogy, I discovered that at a similar age, he had built his first wind turbine. He was dedicated to providing safe, clean energy; That, sailing and Mum were his lifetime passions. With some minor distractions such as his sons, Tchaikovsky and single malt whiskies.

But like all nuclear experts of his vintage, he made some colossal mistakes in the political arena. Sorry Dad.

A revealing anecdote

While working at AAEC I attended a lunchtime staff seminar given by Bill Gemmell, a nuclear engineer whose particular interest and expertise was the fast reactor.

I was working an afternoon shift operating the computer. (I was supervising the shift, actually. When I started as a trainee I was the very lowest paid of the technical and scientific staff, and Dad was the highest, but I had moved up a little by that time. I was still earning a lot less than the other operators I was supervising. Government work can be like that.) I came in a few hours early to attend. At the end questions from the floor were invited.

I asked:

"Bill, you've said that the fast reactor can get the period of time before the waste is less radioactive than what we dug up down to under three hundred years. But as I understand it, that's operating the reactor, reprocessing etc to get the maximum amount of energy from each pound of uranium. What if we instead optimise for minimum waste? How much can we further reduce this period of time then?"

He answered "It would be to a little under seventy years."

But he then added "But I don't think anyone is interested in doing that."

And at the time, he was right on both counts.

Is there a better term?

I would like to think there was one. It's such an obvious and important thing so far as helping the public to understand the pros and cons of nuclear power.

But I have not found it. You'll notice that in the question I asked Bill above, I had to spell out at length what I meant. He had mentioned the concept too, but hadn't given it a name.

Everyone knew it was an important concept, and key reason that nuclear power was feasible. But nobody seemed interested in explaining it to the public in terms they would understand.

So I propose "crossover period". The period of time after which what you have to dispose of is less radioactive than what you dug up to create it was.

Nonsense

Whenever I introduce the term, the reaction is often "Nonsense!" And it's not hard to see why. If, in the long term, the nuclear industry makes the world less radioactive rather than more radioactive, that puts a significantly different light on waste disposal. It actually sounds as if it might not be as much of a problem as some have claimed. And most important, it gives a realistic figure of how much storage time is required. And this figure is likely to be far less scary than the meaningless and ill-informed name-dropping of selected half-lives.

Another anecdote. Years ago while attending Macquarie University I attended a front lawn meeting where students discussed topical issues. This one was on nuclear power, or more precisely, on why it was so very, very bad. And again, after the speakers had done their thing (the antis very effectively, the pros pathetically) questions were invited.

I made my way to the queue, against the advice of my fellow Mathematics and Physics students who feared for my safety. I asked one of the anti-nuclear speakers:

"From what you've said, Plutonium has a much longer half-life than Uranium, that's what makes it so dangerous, is that correct?"

With only a little hesitation (perhaps while he reflected that he didn't really know, but more likely while he worked out what the consequences were to his arguments if it wasn't true) he answered "Yes, that's correct." He then collected his political wits and yelled "Of course it's correct! What are you trying to pull?" and was going to go on but I said "Thank you" and left... as did all the science students. We were outvoted and knew it.

Just for the record, the naturally occurring (and longest-lived) isotopes of uranium are U-235 (half-life 700 million years) and U-238 (half-life 4.5 billion years) while the longest-lived isotope of plutonium is Pu-244 (half-life 80 million years), but Pu-244 is very rare and the longest lived significant isotope is Pu-239 (half-life 24,100 years).

So to say or even assume that Plute has a longer half-life than Uranium is not a simple mistake but profound and disturbing ignorance.

Plute is dangerous stuff. But not for the reasons that the antis give, and probably not nearly as dangerous as they claim and sincerely believe. And more to the point, if its dangers are understood, then and only then can reasonable decisions be made.

The nature of nuclear waste

Nearly all the nuclear waste produced by a uranium (and/or plutonium) fueled nuclear power station is one of three things:

Fission products from splitting the fuel atoms.
Transuranic elements formed when what was mined as a uranium atom absorbs a neutron and is transmuted.
Left over Uranium that hasn't fissioned or been transmuted.

All this material started out being mined as uranium.

There is also radioactive waste produced by activation of the coolant, the reactor structure, anything that the neutrons from the core can touch. But it is insignificant compared to the fission products and transuranics.

And so, the amount of radioactive material we have after operating a nuclear reactor is very nearly the same in terms of weight as the amount we had beforehand. It's maybe a very little less because of e=mc², or a little more because of neutron damage to the reactor itself. But not much, either way, because nearly all of the waste is the same stuff we dug up as mildly poisonous, slightly radioactive uranium. It's just been transformed into other elements, with different half-lives and chemical properties.

Now the stuff we mined was dangerous. A State Cabinet minister was touring Rum Jungle and had been told not to pick up anything. He took a piece of interesting looking rock back to his office anyway, and he was a VIP so nobody stopped him. It stayed in his office and was proudly showed to visitors for some years, it was relevant to his portfolio and nobody else had anything like it. Then somebody finally said "I hope you're washing your hands after handling that and making sure the office is well ventilated, that is pitchblend." So they got a geiger counter, and then evacuated the office until someone competent came and took it to the AAEC, and last I knew it was behind four inches of lead.

That was natural uranium. So why is the waste even more dangerous, if its half-life is very much less?

Let me say this carefully: The waste is more dangerous because its half-life is shorter. That is what the front lawn speaker did not know.

And this shorter half-life means that it is decaying more quickly. Which has two inevitable consequences:

More radiation in the short term.
Less radiation in the long term.

And that is why there is a crossover period. And after this period, the earth is permanently less radioactive as a result of the nuclear industry.

The crossover period is a long time, but not nearly as long as some assume. Current generation of PWRs, 2000-3000 years. IFR, less than 70 years. Some nukes are better than others. (And the tragedy of the nuclear debate is that it's been so dominated by polarised will we/won't we discussion that this has been overlooked, which led to the lack of criticism of the irresponsibly dangerous Chernobyl plant, and will lead to other accidents as the world inevitably goes nuclear.)

Recycling

By far the best way to deal with long-lived nuclear waste is to recycle it. The more radioactive component of fission waste is the fission products. They are predominantly neutron-rich and don't readily absorb neutrons (although they could be destroyed by a proton accelerator and this has been proposed) but the transuranics (the longer-lived component of the waste) are good fission fuel, and can and should be recycled. That's how the FBR reduces the crossover period!

Thorium fuel

I haven't mentioned Thorium fuel above just to keep it simple. Thorium is the other fission fuel, and there's three times as much Thorium in the world as Uranium, so it is the future and the Thorium fuel cycle is being actively developed by India and China, but currently all power reactors use Uranium and/or Plutonium that has been produced from Uranium.

Exactly the same logic applies. The crossover periods will depend on what the technology looks like. We have no commercial Thorium-fueled reactors yet, while we have hundreds of Uranium fueled reactors including a few FBRs to provide data. (Most of the FBRs have been decomissioned, but there's one still running in France and both Russia and China are building new ones.)

Nuclear fusion

Ah, I hear someone say, but what about nuclear fusion? Its advocates say it won't create large amounts of waste. Shouldn't we wait for that? ITER is being built in France at this very moment, and the plan is that it will be followed (somewhere) by DEMO, a still larger plant to actually generate a little bit of electricity, and PROTO, a prototype fusion power plant.

A fusion plant such as ITER has no crossover period. Or at least, not a finite one. The earth is permanently more radioactive as a result of nuclear fusion. The fuel (lithium, from which tritium is produced, and deuterium) was not radioactive in the first place, but the waste is. This applies to all proposed fusion technologies, not just to tokamaks such as JET (the Joint European Torus in the UK), ITER, DEMO and PROTO.

Some hope that the waste will be less in the short term than that produced by fission reactors. But nobody yet knows how to achieve even this. One of the key problems with the Fast Breeder Reactor has been developing materials to withstand its neutron flux, which is ten times that of the current generation of PWRs but still a tenth of ITER. So similarly, one of the key problems still to be solved for ITER is how to make a lining material that will withstand this sort of physical damage. That's any sort of lining material, radiologically nasty or not. And while FBR fuel is surrounded by coolant, providing some shielding, there's nothing at all between the lining material and the fuel of ITER other than a magnetic field, which contains the plasma fuel but is completely and utterly transparent to neutrons.

Some authorities (including the website of ITER itself) avoid the issue by talking of "long-lived" rather than "high-level" waste. It's true that the waste from fusion so far has been short-lived, like fission products, rather than long-lived, like transuranics. It still needs some form of disposal, and the Tritium to be used in ITER all came from fission reactors which did create long-lived waste. So however you look at it there's a problem still to be solved.

JET was built without remote handling equipment, on the assumption that it wouldn't become highly radioactive. But they decided after building it to use deuterium-tritium fuel rather than just pure deuterium as originally planned, and it did become radioactive as a result, and the retrofitting of remote handling then cost an enormous sum. ITER will use the deuterium-tritium reaction (after some initial tests with D+D just to help tune the apparatus before making it radioactive), and will need and have remote handling facilities from day one, just like a fission reactor.

So others, and I am one I'm afraid, think that the radiological properties of the lining material will not in practice be a major factor in choosing it, because just getting any sort of lining material to work is hard enough. And that as soon as it becomes obvious that fusion plants are creating far more radioactive waste than fission for the same amount of electricity produced (which will happen at about the time of the first lining replacement of PROTO I guess) the idea of building or operating fusion power stations on earth will be abandoned, indefinitely (and perhaps unfairly).

But ITER is a good thing. Its research will help us to build all sorts of better things, just as the Apollo moon program did. Better fission reactors may even be one of them. ITER will be a much better source of very fast neutrons than we have ever had, unless the International_Fusion_Materials_Irradiation_Facility has an unexpected breakthrough. The work on its superconducting coils is well funded and world class.

And if anyone eventually restarts the deuterium-deuterium research program, maybe fusion will come of age. There are no plans to do so. It proved far too hard. Not only is it far harder to get the D+D reaction started in the first place, but once you do start it there's a side reaction, a minority reaction, which produces... Tritium! Which then almost immediately fuses with the hot Deuterium all around it, producing of course the same high-energy neutrons that are the problem with D+T. That's another reason JET went to D+T... there was nothing left to lose! So "aneutronic" fusion is an equation, but not yet a technology and may never be one. But even so, a D+D reactor may be less neutronic.

Some nukes are better than others.