Swedish study reports on actinide transmutation and nuclear fuel recycling

[NNadir]

Officially, as of the 1980's, Sweden is committed to phasing out nuclear power. Increasingly this historical decision is being treated as if it were nonexistent, or at least as if it were irrelevant.

Sweden proposes to become an oil free nation, something that will not happen without the use of nuclear power. Either they will fail in their objective, or they will continue to use nuclear power.

Sweden has recently produced a report on the evaluation of partitioning and tranmutation strategies to treat and recover spent nuclear fuel.

Here are some relevant points from the document:

• While P&T will not replace the need for appropriate geological disposal of high level waste, the study has confirmed that different transmutation strategies could significantly reduce, i.e. a hundred-fold, the long-term radiotoxicity of the waste and thus improve the environmental friendliness of the nuclear energy option. In that respect, P&T could contribute to a sustainable nuclear energy system.
• Very effective fuel cycle strategies, including both fast spectrum transmutation systems (FR and/or ADS) and multiple recycling with very low losses, would be required to achieve this objective.
• Multiple recycle technologies that manage Pu and MA either together or separately could achieve equivalent reduction factors in the radiotoxicity of wastes to be disposed.
• The study shows that pyrochemical reprocessing techniques are essential for those cycles employing ADS and FRs where very high MA-content fuels are used.
• In strategies where Pu and MA are managed separately, ADS can provide additional flexibility by enabling Pu-consumption in conventional reactors and minimising the fraction of dedicated fast reactors in the nuclear system.
• In strategies where Pu and MAs are managed together, the waste radiotoxicity reduction potential by use of FRs and ADS is similar and the system selection would need to be made based on economic, safety and other considerations.
• Further R&D on fuels, recycle, reactor and accelerator technologies would be needed to deploy P&T. The incorporation of transmutation systems would probably occur incrementally and differently according to national situations and policies.
• Fully closed fuel cycles may be achieved with a relatively limited increase in electricity cost of about 10 to 20%, compared with the LWR once-through fuel cycle.
• The deployment of these transmutation schemes need long lead-times for the development of the necessary technology as well as making these technologies more cost-effective. Some of the more important technical conclusions from this study are:

• To achieve the goal of reducing the amount of long-lived radionuclides in the waste by a factor of 100 or more the losses in each partitioning cycle must be less than 0.1%.
• Transmutation of plutonium alone will decrease the long-term radiotoxicity by only a factor of five compared to spent fuel.4
• Transmutation can be achieved either by fast reactors or by different combinations of light water reactors, fast reactors and accelerator-driven systems.
• Physical limitations imply that very long time is needed to pursue a “complete” transmutation. This means that the indicated objectives can only be reached if the technology is used for at least 100 years.
• The use of pyrochemical methods implies new potential chemical and radiological risks that must be mastered.
• All transmutation strategies for light water reactors give considerable amounts of depleted and irradiated uranium that have to be managed. If this uranium not is considered as a resource for future fast reactors, the long-term radiological effects when radium, radon and its daughters grow in must be accounted for.

[amandabeech]

1. Would you please provide a translation for interested laypersons?

I'm particularly curious as to what "partitioning" is. I haven't taken chemistry since high school, so what is "MA?"

Also, do you have a link to the original in translation or otherwise? There is a forum I visit occasionally where this might be of interest. I believe that one or two members are Swedish.

[NNadir]

2. I apologize Amanda for not being clearer.

I'm very comfortable with this technology and I often forget that not many other people are. I should not assume that people know to what I am referring.

Here are more details on what is being discussed in layman's terms.

Partitioning is the separation of a mixture of chemical elements into pure elements, or, sometimes, very complex mixtures having many components into portions having only a few components.

Here's a little more detail: Generally nuclear fuel when it is placed in the reactor has only a few elements in it. Typically the elements involved are mostly uranium and oxygen, as a chemical compound, uranium oxide. Sometimes small amounts of other elements are added. For instance, sometimes the elements gadolinium or boron are added as "burnable poisons," materials that have the effect of limiting how quickly the fuel can be consumed early in the fuel cycle. (Typical fuel cycles, the period during which the reactor operates without any refueling whatsoever, are about a year and half.)

When the reactor is operating, uranium atoms are being split into smaller atoms. It does not happen that each uranium atom splits into identical fragments, nor do they even split into fragments that are roughly the same size. Depending on the type of fuel, whether the primary fissioning isotope is uranium-235, or plutonium-239 (dominating most reactors today) or uranium-233 (which will probably dominate in the future if there is a future) the uranium is split into a group of smaller elements having an atomic mass of between 85-105 and a larger fragment that have a mass distribution of around 125-150. (Atomic mass units are defined as being 1/12 of the exact mass of carbon-12). What this means that the fission products, the elements into which the uranium is split, do not consist of just a few elements, but they represent many different elements which have many different chemical and nuclear properties.

Almost all of the fission products are radioactive, but many of them decay quickly into other elements that are not radioactive. For instance, the element barium, a very common fission product (and the very first one observed upon the discovery of nuclear fission by Lise Meitner and Otto Hahn) has no radioactive isotopes that have a half-life of over a few weeks. On the other hand, the element strontium, which is also a fission product and behaves chemically much like barium, has an important radioactive isotope, Sr-90, that has a half-life of over 28 years.

This means that if you remove the nuclear fuel, and by chemical means, separate the barium from the strontium from the remaining uranium, you can effectively dispose of the barium as a non-radioactive material after you have let it stand for a few months. This reduces the mass and volume of the material which must be treated as radioactive.

Because of these effects, spent nuclear fuel contains many elements. Here is the periodic table of the elements:

http://www.webelements.com/

The vast majority of spent nuclear fuel in the world today is principally uranium. In almost all cases, the mass of uranium represents 95% + of the mass of the fuel. About 1% of the fuel is typically plutonium. (I'll explain in a bit.) Between 3% and 5% are fission products, dominated by elements 36 (the gas krypton) to element 46 (the precious metal palladium) in the smaller fragments, and element 53 (iodine) to element 64 (the metal gadolinium) in the heavier fragments. Trace quantities of other elements are found. It actually happens that some of the elements formed in the fission process actually can stop the reactor from working as they accumulate. In fact the very first experimental nuclear reactors frequently did shut down because one of the isotopes formed. The gas xenon, has an isotope, 135, that is particularly capable of absorbing the extra neutrons that keep nuclear reactors running their chain reactions. All modern nuclear reactors must compensate for this effect, which is known as "xenon poisoning." (Fortunately for the future of nuclear technology, when this first occurred, Enrico Fermi, the inventor of the nuclear reactor, was on hand to figure out and explain what was going on.) Because of effects similar to xenon poisoning, all nuclear fuel stops working before the fissionable isotopes are completely consumed. If one partitions the fuel to remove the fission products, one can under certain circumstances, reuse the fuel without further enrichment, particularly if one transfers the fuel to a different type of reactor - ideally a CANDU type reactor.

When a neutron strikes a uranium atom, it doesn't always cause the atom to split. Sometimes the neutron just bounces off. Sometimes the neutron, of course, causes the atom to split. Sometimes, however, the neutron is absorbed by the atom it strikes, becoming a part of the atom, with the isotope weighing about one mass unit more than it did before. A very common absorption reaction in nuclear fuel is the case where a neutron is absorbed by the most common isotope of uranium, U-238. Briefly a new isotope of uranium, U-239, is formed. However, uranium-239 has too many neutrons and is very unstable. It quickly decay by emitting an electron from the nucleus, converting a neutron into a proton. (This process is called "beta decay.") The result is a new element, neptunium-239. Neptunium-239 is also unstable, and after a few days, most of it decays to yet another element, plutonium-239. Plutonium-239 is thus found in all nuclear reactors after they have operated for a while. It is very suitable for use as a nuclear fuel. In fact, a considerable amount of plutonium burns in the reactor as it is formed. Much of the energy obtained from nucler fuel, although generally less than half, is in fact the result of the fission of plutonium formed during operation. Commercially, plutonium-239 is removed from spent fuel and added to uranium to make a fuel known as MOX (Mixed OXide) fuel. The "mixed oxides" are the oxides of uranium and plutonium. When plutonium is made a nucleus that is typically difficult to split U-238 is made into one that is easy to split, Pu-239. This process has always been envisioned, since the invention of nuclear power reactors, as a means of obtaining the full energy potential of naturally occurring uranium.

If you refer to the periodic table again, you will see that uranium and plutonium are found in the very bottom row of the periodic table. We call the two rows at the bottom, for technical reasons, the "f-elements." The first row of f-elements are called the "lanthanides," since they are all chemically related to the element lanthanum. We call the lowest element, the "actinides," because they have certain similarities with the element actinium. (The closeness with which the actinides behave like actinium is much less than the closeness with which the lanthanides behave like lanthanum.)

When one puts plutonium back into a nuclear reactor to recover the energy of it, a similar effect takes place as originally happened with the U-238 takes place. Most of the plutonium is fissioned, but some of it absorbs neutrons and is transformed, we say transmuted (eventually) into heavier elements. Also, some of the light isotopes of uranium, particularly U-235, also absorb some neutrons and is transformed into a long lived isotope of neptunium, Np-237.

Thus in a continuous recycling program, other actinides besides plutonium and uranium will be formed. Of the elements formed, several will accumulate in significant amounts. These are neptunium, americium (which is present in almost every smoke detector on earth), and curium. It is also possible to accumulate very small amounts of berkelium and californium, but all of the other actinides have half lives that are too short to have accumulation of visible quantities. (It takes, on average, the processing of 1600 atoms of uranium to result in the formation of one atom of californium, and such accumulation typically involves decades in a nuclear reactor.) The actinides I have listed are referred to collectively as "MA" or "minor actinides." Plutonium and uranium, along with a nuclear fuel that is actually superior to either plutonium and uranium, thorium, are considered the "major actinides."

The minor actinides contain significant energy, which can in theory be recovered. However, since there are only a few tons of them available on the entire earth, and because they are dilute, typically present in amounts of much less than 1%, it is not currently considered that it is economic to do this as a source of energy. (This may change in future decades.) It may, however, prove desirable to recover this energy as a means of "waste" minimization. Primarily the reason for doing this "waste" minimization is perceptual, not practical. People tend to think that the actinides are more dangerous than they are. The actinides are toxic mostly if you eat them. However for chemical and physical reasons, the likelihood that people will actually eat them either by accident or by deliberate means is almost vanishingly small. The consideration of this matter, the possibility that someone somewhere someday might eat some actinides, is an international fetish, the fetish itself - in light of global climate change - being far more "toxic," as a practical matter, than the fuel will ever be.

I am sorry to report that I have no idea whether the material I have linked is available in Swedish. As is the case in many countries, particularly those with small populations of speakers, much scientific discourse in Sweden probably takes place in English before translation.

I hope this clarifies the issues.

[amandabeech]

3. Thank you very much, NNadir.

That was a very lucid, lay-friendly explanation. I have a much better idea of what is involved. Somehow, it has the ring of "doability" to it.

I used to practice a very specialized area of law (and may be forced to again). Sometimes I can still go off on a diatribe that would not be particularly comprehensible to laypeople. I used to have a little stock speech that I gave just to explain what I did!

[Massacure]

4. Your posts are always so thoughtful and well written.

Out of curiosity, how long did it take you to write the post I'm replying to?

[NNadir]

5. Thanks.

:blush:

Generally a post like that takes about 20 minutes, and a few minutes of editing. (I missed some editing there.)

Sometimes, if I have something I must research, and do not produce off the top of my head, I can spend an hour on a post.

This particular post I wrote while I was killing time, waiting for a reaction in the lab (which didn't require particularly close watching) to finish.