The principal sources of activity in irradiated and chemically purified thorium are 234 Th and its short-lived daughter 234mPa, and 228 Th and its daughters. Beta and gamma activity from these

tCorresponding to a weekly total of 0.100 rem for continued exposure.

nuclides constitute the greatest danger in external exposure; neutrons from (a, n) reactions with light contaminants are relatively unimportant in this regard. Prediction of activities due to 234Th is similar to the analyses of 237U activity in Sec. 2.2.

Nuclides in the 234 Th chain reach equilibrium concentration during irradiation exposures of a few months or greater, with the concentrations given by

Nоз^оз ~ МогОоіФ (8.28)

and ^04X04 = Маіо0зФ (8.29)

where 002 is the equivalent thermal cross section for (n, 7) reactions in 232 Th, and is greater than the true thermal value to allow for absorption of resonance neutrons. By combining Eqs. (8.28) and (8.29), the equilibrium concentration of 234Th is

■Wot _ ОсдОозФ2 ,й ,пч

JV02 Л04Л0З

If the 232Th is irradiated in a neutron flux with a negligible component above 6.37 MeV so that no 232U-228Th are formed, postirradiation cooling can reduce the beta activity to a tolerable level. Even if 228Th is present, preprocessing decay of 234Th may be useful to aid beta decontamination of the separated thorium product. From Eq. (8.32) the time required for the 234Th-234Pa beta activity to reach the beta activity of natural thorium of 4.37 X 1СГ7 Ci/g is given by

Tc = 34.8 In (6.17 X 1O2OOo20o302) days (8.33)

where аф are expressed in reciprocal seconds. ‘

For the uranium-thorium-fueled reactor of Fig. 3.33, am = 6.1 b, о0з = 520 b, and 0= 1.2 X 1014 rt I (cm2 ‘s), resulting in 234Th-234Pa beta activity at discharge of

®^ = 1.2X 10-2 Ci/g 232 Th Л’02

The time for this to decay to the equilibrium beta activity of the 232Th daughters is

Tc = 356 days

Cooling for this length of time will ensure that in chemical reprocessing thorium can undergo total beta decontamination to twice the level of natural 232 Th. The decontamination can be verified with total beta monitoring. For shorter cooling times beta discrimination techniques must be used to ensure that long-lived beta contaminants are not present in the separated thorium.

218 Th is also present in Eradiated thorium and is accompanied by beta-emitting daughters in its decay chain. These daughters are removed from thorium in chemical reprocessing, but they appear again in the separated thorium, growing with a time constant of about 4 days. Thereafter, the beta activity in the separated thorium approaches the level in secular equilibrium with 228 Th. It is therefore important that monitoring for beta decontamination of thorium separated in fuel reprocessing be carried out promptly after the separation is performed.