Neutron Flux in ITER: Benefits & Containment Challenges

[gildomar]

I was reading that the ITER will take advantage of the energetic neutron flux by using them and lithium 6 to breed tritium for more fuel, and to use multiple heat exchangers to grab their energy. And just to make sure that I'm reading it right, the main problem with the neutrons will be their effect on the containment walls that are between the fusion reaction and the lithium, with them knocking atoms out of place (thus lessening the integrity of the walls)?

 

[the_wolfman]

A D-T fusion power plant will have to breed tritium using the neutron lithium reaction. The reaction works with both Li-6 and Li-7. And in theory we can adjust the relative concentration of the two lithium isotope to fine tune the breeding ratio.

ITER however is not a power plant. It is a physics experiment. ITER will have some test blanket modules where scientist will perform tritium breeding experiments. Also while it has cooling lines that remove the heat from the reactor. They are there to prevent things from melting. ITER will not use this heat to produce electricity.

The neutron interaction with the walls and other structures will cause material damage which has a number of negative consequences. For example in addition to decreasing the strength of the materials, neutron damage can reduce the thermal conductivity of a material. In ITER this will make it harder to cool the first wall.

 

[DEvens]

Additional problems with neutrons hitting the walls include such things as material of the wall getting knocked off by the neutrons. That means you get atoms of the wall floating into the plasma, which strongly tends to slow down the reaction.

Then there are a variety of radiation protection issues. The neutrons must be shielded, which requires some meters of whatever you are going to use. And that shielding has to be an efficient neutron absorber that won't produce a lot of secondary radiation. Or that will absorb the secondary radiation itself. It probably should not be lead, for example, because lead has the annoying habit of producing (n,2n) reactions. That is, you start with one neutron and end up with two lower energy neutrons.

And anything that gets exposed to neutrons tends to become radioactive itself. In addition to affecting the structural characteristics of the material, you wind up with radioactive material that must be itself shielded. And possibly disposed of. And that frequently means you have hard choices as to material you might use in constructing components of the reactor. Just as one example, you probably have to keep cobalt out of the mix, since activated cobalt is particularly nasty. So many grades of steel are probably off your list because they can contain cobalt. But there are many other potential problem materials in a neutron flux.