Swedish antis & hypocrisy

Date: Oct 6, 2004

Swedish antis & hypocrisy

Date: Oct 16, 2004

An update on Sweden's "nuclear phase-out" :

NUCLEONICS WEEK OCTOBER 14, 2004
Sweden's Barsebaeck shutdown decision sparks widespread protest
Protest over a forced shutdown of Barsebaeck-2 has poured in from almost
all sides since the Swedish government announced that intention last week,
with electricity-intensive industry threatening to move production
abroad. Only anti-nuclear groups appear pleased with the decision.
In addition, leaders of the Center Party, which is a participant in the government's
three-party energy agreement covering the shutdown, said earlier this
week they want to discuss alternatives with the other parties to the right of
the minority Social Democratic government.
Center Party Secretary Joeran Haegglund claimed such discussions are
not the first step to withdrawing from the agreement. "We have to fulfill the
decision to close Barsebaeck. But long term, we want to do something else,"
he said. The Center Party has a history of opposing nuclear power.
Haegglund said discussions with the conservative-bloc parties would focus
on the future of Sweden's 10 other operating reactors rather than on
Barsebaeck-2. Barsebaeck-1 was shut in 1999 by government order.
Minister for Trade, Employment & Communications Leif Pagrotsky
announced last week that the government intends to order
Barsebaeck-2 closed next year (NW, 7 Oct., 1). In addition to
the Center Party, the government's energy policy is supported
by the Left, under an agreement reached in 1997. The
government must still make the formal shutdown decision
and set a closure date.
To save money, the government had hoped to work out
an agreement with the nuclear industry covering all nuclear
power units, with Barsebaeck-2 being shut early and the others
basically allowed to run to the ends of their lifetimes. But
the government's negotiator said the two sides were too far
apart to continue talking, although nuclear industry representatives
claimed the two sides were coming closer.
Neither side will discuss specifics. But sources suggested
to Nucleonics Week that the government's announcement
is a negotiating tactic intended to make it clear to the
nuclear industry that a better deal on shutdown will not be offered.
Swedish unions blasted the government's decision. "It is
unacceptable that a Social Democratic government has an
energy policy that threatens employment and jeopardizes
Swedish electricity supply," Janne Ruden, chairman of the
Union of Service & Communication Employees, and Sune
Ekbaage, chairman of the Swedish Paper Workers Union,
said in a joint statement. They called for the government to
keep Barsebaeck-2 running.
Jan Johansson, chief executive of mining company
Boliden, said that a Barsebaeck-2 shutdown "can make it
problematic to justify future investments in Sweden."
Johansson is a former Vattenfall vice president. He noted
that the energy situation in both neighboring Norway and
Finland is more stable and that Boliden is more likely to
invest in those countries given the uncertainty in Sweden.
Magnus Hall, chief executive of the Holmen forest products
company, said, "The most serious thing with the
Barsebaeck decision is the uncertainty it creates. We would
like to invest in Sweden, but the uncertainty makes it difficult."
Jan Magnusson, director of Swedish grid company
Svenska Kraftnaet (SVK), warned that a Barsebaeck shut-down
will force SVK to negotiate more agreements with
electricity-intensive industrial companies on voluntarily
stopping production when it is extremely cold.
SVK has agreements with three companies, including
Holmen, which give it the right to cut electricity supply to
their factories if there is a risk of electricity shortage for
heating households and providing power to public buildings
such as hospitals. Sweden is heavily dependent on electric
heat. Such agreements are expensive for SVK because it
has to compensate the companies for their lost production time.
"If it's a really cold winter, we just manage as it is today,"
Magnusson said. "It will be worse without Barsebaeck."
Prime Minister Goeran Persson said that electricity-intensive
industry needs to "ensure that they have the most energy-
conserving processes and modern technology." Persson
said, however, that he wouldn't predict when nuclear power
in Sweden would be completely shut down. "It will take
time because we need to find good alternatives, both from
an environmental viewpoint and economically."
But not all Social Democrats favor a shutdown. Benita
Vikstroem, a Social Democratic municipal politician who
leads a coalition of 65 communities with energy-intensive
industry, called the Barsebaeck decision "deeply worrying,"
and said she supports the Liberal Party's energy policy,
which includes building new reactors.
On Oct. 12, the Liberals arranged an energy seminar in
Malmoe, close to Barsebaeck. Following that, Liberal Party
Vice Chairman Jan Bjoerklund toured the plant. He is the
latest in a series of pronuclear politicians to visit Sweden's
nuclear plants in the past few months. In some cases, politicians
have spent several days observing operations and have
even sat in on control room shifts.-Ariane Sains, Stockholm

Date: Oct 16, 2004

I am not sure what the Swedes will do if the rabid environmentalists force them to stop burning oil and natural gas--I guess they will just have to freeze in the dark in winter (I bet it gets pretty cold over there!)

Sweden will have to burn more oil and gas to make up for the power shortfall from the closure of the nuke plant--and this will cause more emissions which the environmentalists oppose...

It is this precise hypocrasy that I find most dangerous with the rabid enviromental groups.

Literally, to do as the REL (rabid environmentalist lobby) proposes we would have to stop driving, stop eating, stop burning petroleum--and shut down all nuclear plants. Sit quitely in the dark and starve to death.

Well, atleast as far as my family is concerned--this is just NOT an option. And anyone proposing legislation otherwise will have a fight on their hands...

Date: Oct 19, 2004

In order to cover the energy needs, Sweden will be forced to buy electricity from Finland that has built a new (modern) nuclear power plant. This is the hypocrisy: swedish people will use nuclear energy but let other countries take care of the waste.

Indeed, nuclear waste is a real ****. Nobody wants it and nobody knows what to do with it. Since oil is running out and coal is an even grater ****, which other alternative do you propose else than starting to reduce the consumption, i.e., spare energy?

Date: Oct 19, 2004

Since oil is running out and coal is an even grater ****, which other alternative do you propose else than starting to reduce the consumption, i.e., spare energy?

Zero point energy extraction.

Date: Oct 20, 2004

"nuclear waste is a real ****. Nobody wants it and nobody knows what to do with it."

Planet Earth is made of nuclear waste.... we rae made of nuclear waste.... all that is, except the unburned hydrogen : stellar nuclear furnaces created all the heavier elements & isotopes. Some of them are still radioactive -- including the Potassium-40 in our blood, and the uranium and thorium & decay daughters, which still heat the ground we stand on, replenishing the life-giving atmosphere through volcanism, which is constantly being lost to space....

The uranium and thorium waste has billion-year half-lives. Burning them in fission reactors converts them to fission products with half-lives several orders of magnitude shorter. Having thus reduced the long-term radioactivity of the surface of the earth, "nobody knows what to do with it."

Brilliant, huh ?

Date: Oct 20, 2004

It takes about 100000 years for the spent fuel to come down to natural uranium activity. The reason is that in a fission reactor, fission is not the only process that occurs: neutron capture is very common, and leads to the creation of heavier transuranics elements like plutonium, neptunium, americium, curium, that have very long half-lives and can cause serious damages if released in the environment before the 100000 years are completed.

Date: Oct 20, 2004

Look, those heavier transuranics elements like plutonium, neptunium, americium and curium are fuel for reactors, just like uranium -- except it doesn't have to be mined from the ground.

The only difference is that one fuel - uranium - stays around for billions of years, while the other stays around for only hundreds of thousands of years.

What's your point ?

Date: Oct 21, 2004

There are major problems!

1) The heavier transuranics do not have the nice features uranium have for intrinsic safety of a water-moderated reactor. I am talking about the fraction of delayed neutrons and the strong absorption resonances of 238U, the first making possible to run a reactor critically without the chain reaction to get out of control, and the second being responsible for the "negative temperature coefficient", that is, if the temperature of the core would increase, more neutrons would be absorbed in the resonances, resulting in an automatic power regulating. Plutonium has less delayed neutrons than uranium, and americiem and curium have almost no delayed neutrons at all, making it impossible to burn them in an efficient way in standard reactors. Burning plutonium in standard reactors (MOX fuels) results in even more americium production. Plutonium is therefore preferably burned in fast reactors, but they are more costly. Highly specific reactors, that must run undercritically, would be necessary for getting rid of the americium. The best concept we know is accelerator-driven systems (ADS) for which design studies are being made and that are expected to be very costly. We are far from commercialisation of ADS.

2) Chemical separation of all these elements from the spent fuel is VERY tricky. It involves many steps, and many precautions because the whole stuff is highly radioactive. It results a high cost.

That is essentially why I am so sceptical toward nuclear energy for other applications than space.

Date: Oct 21, 2004

By the way, I prefer low activity during billons of years (like uranium) than high activity during hundreds of thousands of years (like americium)

.

Date: Oct 21, 2004

Hmmm -- the Americium in the smoke detectors in my home will only be around for hundreds of years - not hundreds of thousands of years. I know of many homes where people have died because they didn't have Americium (smoke detectors).

As you say, "Chemical separation of all these elements from the spent fuel is VERY tricky," which is why other, less costly, dry processes are being developed. But even the chemical separation method has been used successfully in a number of countries around the world. While recycling spent nuclear fuel is more expensive at this time than using fresh fuel, the incremental cost to the overall cost of nuclear power plant operation is quite small. There are other instances where society has decided to pay the extra cost of recycling, rather than generating mountains of garbage, despite the fact that manufacturing new products is often cheeper. Its about time we got over the antinuke hangup.

Your point about delayed neutrons is correct, but very much over-stated : the higher transuranics are only present in trace amounts, and plutonium use in reactors is old hat: our CANDU reactors use fuel bundles that produce some 60% of their power from in-situ produced Pu near the end of their in-core life without even using MOX fuel, while several other countries use MOX in their LWRs on a routine basis. Of course the suggestion that no technological advancement will occurr in the coming thousands of years is ridiculous. Just a few hundred years ago, people using harse-drawn buggies would have found little use for gasoline or electricity......

Date: Oct 21, 2004

I checked my data. I were overestimating the half-life of americium: it may be around one thousand years and not one hundred thousands. This is even worse: a shorter half-life means a higher activity, and one thousand years is still beyond what we can foresee (about the half-life of a human civilisation). After plutonimum, americium is the next real problem: even if it is produced in small amounts, after one thousand years, 241americium alone is still responsible for an activity by more than one order of magnitude higher than the original uranium activity of the fresh fuel.

I think separation processes are successful at low cost for plutonium extraction but not for americium. Like you, I believe this will become better in the future, but research costs money. CANDU is a heavy water moderated reactor, I think it produces relatively large amounts of americium.

I insist: nobody knows what to do with the spent nuclear fuel. At the moment, it accumulates in temporary disposals, waiting for solutions about the final disposal. Yucca mountain (60000 tons) is already full. Nobody wants to live close to the disposal and this leads to political problems, not to talk about the plutonium still present and potential bomb material. It is simply accumulating too fast for us to handle. Even with significant thechnological advances, I doubt our descendants will be willing (and able) to dig in our disposals and transmute such huge amouts.

Date: Oct 21, 2004

I checked my data. I were overestimating the half-life of americium: it may be around one thousand years and not one hundred thousands. This is even worse: a shorter half-life means a higher activity, and one thousand years is still beyond what we can foresee (about the half-life of a human civilisation). After plutonimum, americium is the next real problem: even if it is produced in small amounts, after one thousand years, 241americium alone is still responsible for an activity by more than one order of magnitude higher than the original uranium activity of the fresh fuel.

No offsense, but radiation is a fact of life. You, me, the planet, the Sun, we're all radioactive. Americanium is NOT a problem, because it is useful in industry and not particularly dangerous. (Mostly Alpha, some Gamma decay IIRC.) Why proclaim that we need to bury it just because it's radioactive? It's not hurting anyone by being put to good use. Heck, it's even saving lives. Not to mention that there are some studies that show some forms of background radiation can actually improve human health!

This whole idea of radioactivity == evil has got to go. It's a cancer on our society and has only served to cause a decline in the quality of life.

Date: Oct 22, 2004

Good post AKAImBatman ! ....thanks !

I would only add that I hope we can get as much Americium as possible -- with further irradiation in a fast neutron reactor, we can turn a good part of it into Americium 242m, the "holy grail" of nuke fuels for space rockets & power reactors alike. At ~7000 barns of thermal neutron fission x-section (and a several hundred barns at epithermal energies -- corresponding to temperatures of ~50,000 K, with correspondingly high Isp), there is no other fission reactor fuel that even comes close.

Date: Oct 22, 2004

By the way.... regarding AKAImBatman's remark that "radiation is a fact of life," I should add that that the "10kBq" in my subscriber name refers to the ~10,000 Becquerels of radioactivity in my (and everyone else's) body. That's 10,000 nuclear disintegrations per second, every minute, every day, and every year of our lives, for as long as we live. About two-thirds of that is due to radioactive Potassium 40 (K-40), one of the "primordial" nuclear waste products of stars, with a half-life of nearly two billion years. The rest is largely due to the radioisotopes from the uranium decay chain (including Polonium, Radium, etc.), which we get in our food (picked up by plants from the ground), and also due to a few "cosmogenic" radioisotopes like Carbon 14.

Date: Oct 22, 2004

Thanks, but I already knew all that about natural radioactivity.

The question is about the LEVEL of radioactivity, especially released in the environment. Swedish people have a good experience of that because radioactive clouds fall down to north Sweden after the Chernobyl accident, and still today it is not clear whereas it is safe to fish or pick mushrooms around some lakes there.

I know americium has many uses (I even use it at my work), always in sub-kilogram quantities (it is hot and must be well confined). What I am worried about is the accumulation of radioactive stuff. We have about 500 nuclear power plants in the world today, each of them producing about 26 tons of radioactive waste per year, consisting usually of 96% uranium, 1% plutonium, 0.1% other transuranics and 3% fission products. Suppose you remove and recycle successfully the uranium and the plutonium (which far from all actually do), there is left 26 kg of other transuranics and 780 kg of fission products. Multiply this by 500 power plants: 13 tons other transuranics and 390 tons fission products per year. I expect more nuclear power plants to be built in the near future and the nuclear age to last for about one more century. At the end, we are left with thousands of tons of other transuranics that must be either safely burried for the next hundred thousand years or fissionned in an ADS. But the real perspective is much worse than that: in reality, fuel reprocessing plants are few and many countries find more advantageous not to touch the spent fuel at all (like it has been in USA for decennies). I would expect tens of thousands of tons of plutonium to be accumulated here and there in the world, mixed with thousands of thousands of tons of uranium and hot fission products. Do you think we will keep control of all that? Do you think it will be cheap for the next generations to burry it in final disposals or to reprocess it?

Date: Oct 22, 2004

Working on the assumption that each nuclear reactor churns out 26 tonnes each year (a gross simplification as the size varies and newer reactors are more fuel efficient). That means that with nearly 500 reactors around the world, the annual worldwide output of HLW is 13,000 tonnes. Is that a lot for a year's worldwide output? Heck no! A few coal power plants together put out that much highly toxic waste per day.

That's the good thing about nuclear waste. It may be rather unnice, as are many materials used in other industries, but they are produced in such a small quantity that they are highly manageable. With all this controversy over Yucca Mountain, has anyone wondered what has been done in the fifty years prior? The answer is that waste has been stored on site at the power stations that generated it. There's that little of it that there has been no need for a waste dump until now. That's the one thing that never gets mentioned. In Britain waste is still being stored in Sellafield alone and most of the waste there is from the only Magnox weapons program.

In any event, new reactors will be much better. I believe higher levels of enrichment are all the rage as are higher burnups so there is less of the long lived transuranics in the spent fuel.

10kBq Jaro, is that figure right? That's an awfully low activity for an entire body.

Date: Oct 22, 2004

You have to multiply the quantity of toxic material with the time it will be around -> 100000 years for transuranic elements. This is a very new problem we have to face with long-lived radioactive waste, the time scales that go beyond our experience as a human culture

. Please not compare what is not comparable: radiotoxicity is very different from chemical toxicity. But I agree, we have to stop burning coal.

Date: Oct 22, 2004

If we talking about toxicity-years, then we have to multiply the millions of tonnes of waste produced from various other industries all the time by infinity because chemical waste is toxic permanently. The beauty of radioactive waste is that is does eventually decay and no longer poses a problem.

But the problem with worrying about what's happening in ten thousand years time is that we assume nothing will be found to deal with the waste. Already accelerator driven systems are under development (especially in India where they want to take advantage of their vast thorium reserves), which will help deal with it. Because nuclear waste is so small in quantity, we have breathing room to come up with solutions.

Chemical and radiotoxicity may be different in nature but you're making an appeal to the Radiation Boogey Man. You'll die from a lethal dose of mercury just as easily as you'll die from a lethal dose of radiation and there are a thousand and one chemicals that are just as carcinogenic, more so in some cases like asbestos, than radiation. Would you rather ingest uranium dust or cyanide? If you're smart, you'll choose uranium dust, although frankly I'd rather neither. We have to think proportionately about things. Highly toxic chemicals are used in vast quantities, producing vast amounts of waste in all other industries. Just because the nature of their toxicity is different from nuclear waste, which is primarily a threat through radiotoxicity, it does not mean that it is any less important to contain and handle that waste properly. The difference is that nuclear waste is handled with great regulation while waste from other industries is not nearly as well supervised.

I'm not trying to say nuclear waste is not hazardous. I'm saying that nuclear waste must be treated in perspective. It is small and containable, which gives it the edge over many things. It is also potentially useable. Nuclear decay is a potent source of energy after all. Geologically repositories are no worse than the huge landfills and waste sites for all manner of chemical nasties except that they needn't be as large. Besides, transuranics are reactor useable and sooner or later, humanity will use them. They're the problem. If we want to thing long term about this, like what will happen in 10,000 years, we have to remember that we can progress a lot as well in handling and making use of what we now call waste.

Date: Oct 22, 2004

Chemical waste is toxic permanently

I would need explanations. I thought that toxic chemicals were at least partially neutralized after some relatively short time.

Of course all sorts of waste are harmful. I use to say that everything has a price, even cheap things, because beyond the low price there are very often environment-harmful methods used. A simple example is the use of oil in a car. It is cheap and convenient, so everybody does it, but when millions of cars are used dayly in a big city, not only becomes it noisy and difficult to breathe, but it contributes to global effects for which we have no idea the price will be. I therefore encourage poeple to take the bike for the few kilometers trip to work. The same concerning the excessive use of electricity.

Date: Oct 22, 2004

RE: 10KBqJaro

Date: Oct 22, 2004

RE: Swedish antis & hypocrisy

Date: Oct 22, 2004

quote:
Originally posted by: Glom
The only reason nuclear waste gets a focus is because its form is unique to one minority industry so it is more easily made a target than chemical waste which is produced all over the place."

It is also vulnerable to a common human perception error that was commented on by Stalin.

"One death is a tradgidy, a million is merly a statistic"

"Nobody" died at TMI, yet it was called a disaster and is credited with virtually killing the industry. *Thousands* of peopl have been killed by Hydro plant failures yet nobody has ever sugested that hydro electric generation should be abandonned.

Dusty

Date: Oct 22, 2004

I think there's an even better quote for Nuclear fear than Stalin's:

"We only fear that which we do not understand." -Unknown

Violence, Death, Combat, these are all things man has known for thousands of years. But radiological effects are scary, because there's no one to look in the face and defeat. It kills silently, without any warning to those it takes.

Of course, once you understand it, radiation is not all that scary. You simply realize that it is very difficult to receive a lethal dose in most cases, and get on with your life. Unfortunately, the public does not yet understand this.

Date: Oct 22, 2004

That quote may be a paraphrase of one of Marie Curie's.

Getting back to the issue of recycling spent nuclear fuel, here's a little bulletin from Japan :

Japan govt panel recommends reprocessing spent nuclear fuel, not burying it
AFX Asia, 22 October 2004

TOKYO (XFN-ASIA) - The Atomic Energy Commission today issued a draft revised long-term national nuclear policy plan, which recommends reprocessing spent nuclear fuel rather than burying it. The draft says Japan might not be able to maintain nuclear power as a key power source if the government abandons its current plans for reprocessing nuclear fuel. It also says the advantage of reprocessing spent nuclear fuel would be limited if a fast-breeder reactor that uses recycled plutonium were not built as planned. Fast-breeder reactors can burn plutonium to produce electricity while breeding more plutonium. In its report, the Atomic Energy Commission estimates that reprocessing all the spent nuclear fuel over a 60-year period to 2059 would cost 42.9 trln yen, while burying it would cost 30.0-38.6 trln yen. This was the first time the panel ever issued an estimate of the total cost.

<SNIP>

Date: Oct 23, 2004

In its report, the Atomic Energy Commission estimates that reprocessing all the spent nuclear fuel over a 60-year period to 2059 would cost 42.9 trln yen, while burying it would cost 30.0-38.6 trln yen. This was the first time the panel ever issued an estimate of the total cost.

There is one suspicious thing about those cost estimates. Is the 42.9 trin (trillion?) yen gross or net cost? Because most countries sell their power. So while it may not be 100% cost effective, it may be cheaper than just burying it.

Date: Nov 3, 2004

Plutonium has less delayed neutrons than uranium, and americiem and curium have almost no delayed neutrons at all, making it impossible to burn them in an efficient way in standard reactors. Burning plutonium in standard reactors (MOX fuels) results in even more americium production.

This statement, by the way, is wrong.

Its been bugging me for a while, but until today, I haven't really had time to give it much thought.

We (Canadian Nuclear Society) got a question from a college student earlier today about Americium, so I had to go dig into my files a bit.

Delayed neutrons are called that because instead of coming out at the moment of fission (the so-called "prompt neutrons," which comprise nearly 100% of all neutrons produced as a result of fission), they are produced by certain specific fission product nuclei, long after fission has taken place (mostly from seconds to minutes later).

There are litterally hundreds of different fission product nuclei (or isotopes), but there is a characteristic double-hump distribution, with the two mass peaks centered roughly around strontium-90 (the low-mass hump) and cesium-137 (the high-mass hump). The delayed neutron precursors are a tiny, but very important part of the fission product distribution, because they make nuclear reactors controllable : if criticality is dependent on both prompt and delayed neutrons, then in effect the tiny minority of delayed neutrons has "veto power" (to use political terms).

Anyway, the point is that while there are slight differences in the shape of the double-hump fission product distribution between nuclear fuels like uranium, plutonium, americium, etc., the differences in delayed neutron production are relatively minor : saying that "americiem and curium have almost no delayed neutrons at all" is just plain wrong -- in fact they all have roughly the same amount of delayed neutrons.

The big difference arises from the question of what happens next -- what the delayed neutrons do in the reactor.

In a uranium-fueled reactor they participate in propagating the fission chain reaction -- even though their energy at birth is very different (much lower) than that of the "fast" prompt neutrons. That's because uranium-235 (and plutonium-239) has a good probability of fission when hit by a low-energy neutron (i.e. they are said to have a high thermal or epithermal neutron fission cross-section). By contrast, Americium-241 has a neutron fission cross-section similar to those of U-235 & Pu-239 at high energy (the prompt neutrons), but virtually zero for low energy neutrons -- either delayed neutrons or moderated prompt neutrons. So Am-241 acts as though the delayed neutrons didn't exist at all. So it would certainly be correct to say that a reactor fueled with pure Am-241 would only be possible in the fast (unmoderated) version, and that it would be uncontrollable, because of the ineffectiveness of delayed neutrons in maintaining "veto power."

But -- and this is a big but -- americium will never be more than a small percentage of the fuel in any reactor, so the delayed neutrons from its fission products will propagate the fission chain reaction in the other components of the fuel (U-235 or Pu-239, etc.), thus maintaining the "veto power" necessary for effective control using safe, slow-acting, man-operated mechanisms, with power ramp rates on the order of a few percent of max. power per hour.

As for producing more Am-241 in MOX-burning reactors -- yes, but only up to a point : the build-up curve approaches an equilibrium value that is still very far from anything like pure-Am-241 fuel. But its certainly true that if one wants to burn the heavy trans-uranics, rather than produce them, then fast-neutron reactors are the way to go. I think a typical scenario is to have a bunch of MOX-burning LWRs feeding heavy trans-uranics to one or two large fast reactors. In the longer-term, these fast reactors will include breeding blankets, surrounding the core, for producing additional plutonium or U-233 (from natural thorium) as feedstock for LWR MOX.

In conclusion, the problem is not nearly as intractable as Phil makes it out to be. In fact, it does not even require accelerator-driven transmutation.

Date: Nov 3, 2004

Yes, you are right! Thank you for your lighting. The amount of dalayed neutrons is about the same, but in the case of Americium which has a high-threshold fission cross section, they are uneffective. Wonderful that I can understand that at last.

Now I have another point that is still unclear. I think that Plutonium has a lower fission cross section than 235U in the energy region of the delayed neutrons. How do you make a fast reactor fueled with large amounts of Pu and event certain amounts of Am safe?

Date: Nov 4, 2004

I think that Plutonium has a lower fission cross section than 235U in the energy region of the delayed neutrons.

This is too vague -- like the prompt neutrons, delayed neutrons scatter down (or up) to the energy of their environment. This means that for moderated reactors like LWRs, all the neutrons that avoid getting absorbed (or leak out of the reactor) in the process, end up as thermal neutrons. Plutonium-239 has a slightly higher fission x-section, compared to U-235, for BOTH fast AND thermal neutrons, so the differences are fairly subtle. For commercial plants, the most important difference is the fuel enrichment level, which for uranium is typically about 3 - 4 %. So to produce MOX fuel with comparable reactivity -- which is what you need if you want to avoid making extensive control system modifications -- the U-235 content must be reduced by roughly the increase in plutonium content. The replacement ratio is not exactly one-to-one -- particularly if the plutonium is a mix of various isotopes plus americium -- but you get the picture....

For fast neutron reactors the picture is similar, but the differences between fuels are somewhat greater, because thermal neutron x-sections don't count in this case, and because Pu-239 actually has a slighly lower fission x-section in the intermediate energy range (including the lowest energy reached by both prompt and delayed neutrons) ......plus some other subtle differences which must be taken into account when designing fast reactors. This is not to say that "making a fast reactor fueled with large amounts of Pu and even certain amounts of Am safe" is particularly daunting compared to ones fueled with enriched uranium -- as long as the equivalent enrichment level with plutonium in the fuel is about the same, and as long as all the other, more subtle effects, are taken into account in the reactor physics calculations.

Compared to LWRs, the minimum enrichment level of fast reactors is much higher - usually in excess of 12 - 15%, but often closer to 20%. That's because the fast neutron fission x-section of ALL fissile fuels is much lower here, than in the low energy (moderated) region -- on the order of ~300 times lower in fact (i.e. ~2 barns versus ~600 barns).

On the other hand, the parasitic neutron absorbtion x-section of fission product "poisons" is also very much lower in fast reactors vs. LWRs -- in some cases dramatically so, by factors of many thousands (particularly in the case of xenon). This is a very important difference, because it means that fast neutron reactors don't suffer from post-shutdown "poison-out" conditions, which prevent LWR restart for nearly a day (time for the xenon to decay away) unless a large amount of excess reactivity is used to override it. Also, the long-term gradual buildup of all the rest of the fission products has a much greater negative reactivity effect in LWRs than in fast reactors. This is compounded by fissile fuel burn-up, which in fast reactors is compensated much better by breeding fissile fuel from U-238 or Th-232 (to the point of producing a surplus, if so desired....).

The net result is that fast reactors are always designed with much less initial excess reactivity than LWRs -- providing an equally long (or better) run time between refueling outages, as well as easily handling shutdowns & restarts. This is particularly important in reactors for ships and for space power applications, where load-following operation is often required, making fast reactors an especially appealing choice..... Lower initial core load excess reactivity is of course traditionally associated with increased safety, for obvious reasons....

Hope this answers your question.

Date: Nov 4, 2004

If I understand nuclear technology--it should be possible to engineer an overall fuel cycle in which the spent fuel is reprocessed to remove fission products with relatively short have lives, and then send back the transuranics to a plant for further 'burning.' In this way plutonium, americium and other elements are continually recycled and consumed, while the shorter lived fission products can be stored long enough to decay down to reasonable levels (and not withing hundreds of thousands of years.)

Constant reprocessing also extends the useful energy production of uranium stocks we do have, and provides a useful way of disposing surplus weapons grade isotopes (by introduction and dilution in the fuel stream servicing utility reactors.) The only issues I see with reprocessing is security of weapons grade materials coming into the plant and safety of incoming spent fuel and outgoing fuel. This shouldn't be too difficult to achieve at a high security installation.

Now if I understand Pebble Bed Reactors--they can achieve the high-burnups necessary to consume most of the insitu-bred plutonium as well as the high thermodynamic efficiency necessary to run an economic power generating plant. How about reprocessing the spent fuel from a high-temperature gas-cooled pebble bed reactor? Will the high-burnup levels mean the spent fuel will be 'unusually' hot (relative to spent fuel from a typical light-water reactor?) I would imagine that the mechanical stability of the carbon-carbon spherules in a Pebble Bed Reactor would mean that it should be pretty safe to transport them in casks. The casks would probably have to have an active cooling system--such as helium gas blown through the cask and then cooled with water run through a convection cooler (like an automotive radiator.) Although a cask cooled by a heat-pipe using water might also be practical.

The point is that once the weapons-grade materials were consumed and processed into useable fuels, they're gone. They become a part of the fuel stream and at that point it should be pretty difficult to reprocess them back to their former weapons use. I'm not sure how much of a threat such a cycle would represent from a proliferation standpoint--but in all things there is the risk that someone will abuse it. Anything to do with nuclear technology could be adapted to weapons purposes given only the desire and motivation to do so. The basic physics behind nuclear power and nuclear bombs is the same.