I’ve often found myself trying to find a good source of microscopic cross-sections for nuclear isotopes, and today I found a nice source:
On here, I found the thermal cross-sections I’ve been looking for that would allow me to compare the behavior of protactinium and uranium in the blanket salts to thorium. Here are the thermal (2200 m/s) cross-sections for several blanket isotopes, with units in barns:
TOTAL ELASTIC FISSION CAPTURE half-life
Li-7 1.015 0.97 - 0.045 stable
Be-9 6.1586 6.1510 - 0.0076 stable
F-19 3.643 3.652 - 0.0096 stable
Th-230 32.32 9.774 0.0 22.55 75,400 yr
Th-232 21.11 13.70 0.0 7.40 14e9 yr
Th-233 1478. 13.0 15.0 1450.0 22.3 min
Pa-231 210.69 9.954 0.02 200.72 32,700 yr
Pa-232 1176.2 12.23 700.0 464.0 1.31 day
Pa-233 53.051 13.021 0.0 40.031 27.1 day
U-232 162.3 10.79 76.66 74.88 69.8 yr
U-233 588.38 11.97 531.16 45.25 159,000 yr
U-234 119.2 19.41 0.0062 99.75 245,000 yr
U-235 698.2 15.03 584.4 98.81 704e6 yr
The nominal blanket composition would be as follows:
From these cross-sections, you can see that thorium-232 has a moderate cross-section for absorption, but there’s so much of it in the blanket that it does almost all the neutron-absorbing (as we would want).
After absorbing a neutron, the Th-232 becomes Th-233, which has a monster absorption cross-section (almost 200x that of Th-232) but its half-life is so short (22 min) that it isn’t around very long to absorb a neutron.
Once it turns into Pa-233, the absorption cross-section is still over 5 times greater than the Th-232. That is one of the basic reasons why it’s so important to isolate the Pa-233 from the blanket–in order to prevent another neutron absorption. This is a key step that you just can’t do in a solid-core reactor that’s trying to “burn” thorium (and achieve a conversion ratio of > 1.0).
Finally, the Pa-233 decays to U-233 in 27 days. The U-233 has a huge cross-section, mostly for fission (531 barns) but with a lot of absorption (45 barns). Thus, uranium-233 left in the blanket will really want to gobble up blanket neutrons and cause fission. That leads to even more trouble, because that will deposit fission products in the blanket, complicating reprocessing and making the blanket “hot” with radiation from fission products.
All of these factors argue for getting protactinium of out the blanket and letting it decay to U-233 outside of the neutron flux. The U-233 can then be removed by fluorination to UF6 and adding it back to the core salt by reduction to UF4. Continuous refueling of the core means that excess reactivity in the core can be held to almost nothing, an extremely important consideration for safe operation that is very difficult to achieve in a solid-core reactor.