Radioactivity of fusion reactor walls vs. fission

[Jeremy Thomson]

I'm wondering if a fusion reactor would make its walls more radioactive thru neutron activation than a fission reactor, for a given amount of energy. It seems to me that (hydrogen) fusion produces most of its energy as neutrons that are unlikely to absorbed by the sparse near vacuum plasma. Fission produces most (90%?) of its energy to the fission products and neutrons are absorbed by the fuel to further fission and transmute uranium into plutonium. So I figure a lot more neutrons are hitting the wall in a fusion reactor and at higher energies than the moderated neutrons of a fission reactor. I guess a LOT depends on what the walls are made of. Fission plants are are of steel and I believe iron doesn't readily absorb neutrons or produce long enough lived radioisotopes to be a problem. Fusion reactor walls need to absorb the neutrons and turn them into heat. I believe lithium is proposed because it would allow breeding of Tritium, creating more fuel.

 

[Astronuc]

The first wall of a fusion reactor is expected to experience are relatively high fluence, particularly with respect to fast neutrons > 1 MeV, and consequently a higher damage (dpa = displacements per atom) than a fission reactor pressure vessel. I'd have to find the numbers for an ITER type system.

In fission reactors, utilities have adopted so-called low leakage or low-low leakage patterns/cores, in order to minimize neutron fluence of the reactor baffles, surrounding structure and pressure vessel. Low-leakage patterns place depleted high burnup fuel, which operates at about 0.3-0.4 of core average power, with the outer faces lower. Some LWR utilities may employ natural U fueled rods, stainless steel or Hf (power suppressor) rods to reduce fluences near certain locations of the pressure vessel.

 

[mfb]

Fission plants are are of steel and I believe iron doesn't readily absorb neutrons or produce long enough lived radioisotopes to be a problem.

2.7 years for Fe-54 that becomes Fe-55, 44 days for Fe-58 (rare) that becomes Fe-59, if it captures another neutron before it decays it becomes Fe-60 with a half-life of 2.6 million years. Pure iron is a bad material, however, and the usual additives can be problematic in terms of activation.

 

[the_wolfman]

I'm wondering if a fusion reactor would make its walls more radioactive thru neutron activation than a fission reactor, for a given amount of energy. It seems to me that (hydrogen) fusion produces most of its energy as neutrons that are unlikely to absorbed by the sparse near vacuum plasma. Fission produces most (90%?) of its energy to the fission products and neutrons are absorbed by the fuel to further fission and transmute uranium into plutonium. So I figure a lot more neutrons are hitting the wall in a fusion reactor and at higher energies than the moderated neutrons of a fission reactor. I guess a LOT depends on what the walls are made of. Fission plants are are of steel and I believe iron doesn't readily absorb neutrons or produce long enough lived radioisotopes to be a problem. Fusion reactor walls need to absorb the neutrons and turn them into heat. I believe lithium is proposed because it would allow breeding of Tritium, creating more fuel.

D-T fusion produces around 3-5 times more neutrons per MW than fission. This is complicated by the fact that the D-T fusion neutrons are more energetic than fission neutrons. As Astronuc stated, neutron damage to the first wall is a major concern. Activation of the first wall is also an issue. This first wall will be the primary source of radioactive waste in a fusion reactor.

However, I'm not sure I understand the point of the comparison. In fission the primary source of radioactive waste is the spent fuel. The spent fuel is going to be more radioactive than the fusion first wall, and it's going to be much much longer lived.

Also you don't need to absorb neutrons to capture their heat. Neutrons will scatter off of atoms. In these billiard ball like collisions the neutrons will transfer some of their energy to the atoms they scatter off of.

A D-T fusion reactor will have a lithium blanket to breed tritium. But the lithium blanket is not the first wall. Instead the lithium will be in a solution (like FLiBe) that we will pump it though channels in the first wall.

 

[Jeremy Thomson]

...
However, I'm not sure I understand the point of the comparison...

You often read of how fusion is clean and has no radioactive waste. How much of a simplification is that? I believe spent fuel from fission is about half transuranics and half fission products. Am I right in that fusion 'spent fuel' will be isotopes of helium? Which if radioactive have very short half lives. So the 'products' of fusion aren't a problem but the activated reactor walls will be just as much a problem as a fission reactor when it comes to decommissioning. But will it be worse, the same or better. I suspect worse because with fusion you actually want the neutrons to hit the walls and make heat.

 

[the_wolfman]

You often read of how fusion is clean and has no radioactive waste. How much of a simplification is that? I believe spent fuel from fission is about half transuranics and half fission products. Am I right in that fusion 'spent fuel' will be isotopes of helium? Which if radioactive have very short half lives. So the 'products' of fusion aren't a problem but the activated reactor walls will be just as much a problem as a fission reactor when it comes to decommissioning. But will it be worse, the same or better. I suspect worse because with fusion you actually want the neutrons to hit the walls and make heat.

I agree that the statement that fusion produces no radioactive waste is wrong. Perhaps a better statement is that fusion produces no "long-lived" radioactive waste. Here "long-lived" means a half life greater than 1000 years. However, most people miss this subtle distinction, so when comparing fusion to fission it's probably best to talk about the differences in the amount and life-time of the radioactive waste produced.

One by-product of most fusion reactors is He-4. It's not radioactive period. Often a neutron is the other by-product. And as I stated before this neutron has the potential to activate the material surrounding the plasma. We also use these neutrons to bread tritium. These are the primary sources of radioactive waste is a fusion reactor.

If you want to do a fair comparison between to fusion and fission you should consider all sources of radioactive waste. Such a detailed accounting can be difficult, so as a 0-th order approximation you could compare the primary source of waste for fusion with the primary source of waste for fission. The primary source of waste for fission is the spent fuel. Whereas the primary source of waste in fusion comes from breeding and activation of the reactor wall. If you want to do a fair comparison then you should compare the the amount and life time of the spent-fuel from a fission reactor to the amount of waste from a fusion reactor. Comparing the activation of the fission reactor components to the activation of the fusion reactor components ignores the elephant in the room. It's not a fair comparison.

 

[Astronuc]

I believe spent fuel from fission is about half transuranics and half fission products. Am I right in that fusion 'spent fuel' will be isotopes of helium? Which if radioactive have very short half lives. So the 'products' of fusion aren't a problem but the activated reactor walls will be just as much a problem as a fission reactor when it comes to decommissioning. But will it be worse, the same or better. I suspect worse because with fusion you actually want the neutrons to hit the walls and make heat.

Spent LWR/Candu/AGR fuel consists of unused UO₂ (or MOX), which is the majority and the fission products (FP) and transuranics (TU). The amount of FP and TU depends on burnup (energy/mass) of the fuel and time. It now common to have fuel with burnups of 45 to 60 GWd/tU, which is approximately 4.5 to 6% of the initial fuel consumed (4.5 to 6% fima), and I believe a lesser fraction of TU. I'll have to check on the TU fraction as a function of burnup.

The structure will become activated, so that would become radwaste, when the structure is removed. It has to be stored somewhere such that the radioactive material does not enter the environment/biosphere.

Adding to what the _wolfman has mentioned, He-4 and He-3 are not radioactive, so they are not a problem. Any T bred has to be collected and its release mitigated.

One could do useful comparison of a fission and fusion system on the basis of 1 GWe. However, that requires some details of each system, e.g., fuel cycle and operating cycle, as well as detailed designs.

For fusion systems, one goal is to identify low-activation, radiation-resistant structural materials.

This reference provides some information on fission products, transuranics and activation products in LWRs - Radiochemistry in Nuclear Power Reactors
http://www.nap.edu/download.php?record_id=9263

One example for UO₂ shows (Figure 2-4) that at a burnup of 35 GWd/tU, or ~3.5% fima, the Pu content is about 1% of the fuel metal with about half being Pu-239, one-third Pu-240, and 10% Pu-241. Less than 0.1% consists of Am-241, Cm-242 and Cm-244.

Figure 2-5 shows that for 2.5% at 40 GWd/tU, U-235 account for less than 20% of fissions, while Pu-239 represents slightly more than 50% and Pu-241 about 20% of fissions. U-238 undergoes fast fission, and it starts at about 7% of fissions and increases to about 8-9% of fissions. For higher enrichments, the U-235 fraction of fissions would not decrease as quickly, and Pu-239, Pu-241 would show a slower increase, which is due to self-shielding.

 

[Astronuc]

Since the early 1980s, there have been programs to identify 'low activation' materials for nuclear applications. Work to develop 'low activation' materials began following investigations in the UK and US into the decay characteristics of the radionuclides that would be produced in a fusion reactor environment in various element constituents of numerous structural alloys.

A good reference on the subject is ASTM STP 1047.
http://www.astm.org/DIGITAL_LIBRARY/STP/SOURCE_PAGES/STP1047.htm

The overview begins - "Some of the most serious safety and environmental concerns for future fusion reactors involve induced radioactivity in the first wall and blanket structures. One problem caused by the induced radioactivity in a reactor constructed from the conventional austenitic and ferritic steels presently considered as structural materials would be the disposal of the highly radioactive structures after their service lifetimes. To simply the waste-disposal process, "low-activation" or "reduced-activation" alloys are being developed. The objective for such materials is that they qualify for shallow land burial, as opposed to the much more expensive deep geologic disposal, or alternatively, they can be recycled after a suitable period of storage."

The book covers the fourth day of the 14th International Symposium on the Effects of Radiation on Materials held in Andover, Ma, June 27-30, 1988. It is a bargain for US$60. The first three days are covered by a companion volume, ASTM STP 1046.