Contrasted to benefits from reduction in “Th activity, preprocessing cooling increases the 228Th content of irradiated thorium. The amount Noe (7) of 228Th present at the end of an irradiation period TR, due to 232 U decay, is given by applying Eq. (2.106):

Nm 1 — ■Tr 1 —

N02 ^ 21 Мп(М22 —МпХМов “Ми) M2211 — M22XM08 ~M22)

1 — Tr

Мов(Мі1 ~ МовХМ22 ~ Мов)

where Мов = 0008 + Хов (8.35)

During the preprocessing cooling period, the atom ratio of 232 U to 232 Th remains essentially constant because of the long half-life of 232 U. An equation for the activity osNos(Tc) of 228Th present after a time Tc of preprocessing cooling is obtained by applying Eqs. (2.13) and (2.27):

Хов^ов(Гс) = obNq&(Tr) е-кхтс + 0 _e-wrc) (g 36)

Nm Nm Nm

where the activities XoeN0b(Tr) and 22N22(Tr) at the end of the irradiation are obtained from Eqs. (8.34) and (8.19), respectively.

The growth of 228 Th activity during irradiation at various neutron fluxes is shown in Fig.

8.14. At a given flux time of irradiation, the 228Th activity is lower at the higher flux levels. This is because the actual time since the beginning of irradiation is shorter at the higher fluxes and less of the 232 U formed has undergone radioactive decay.

Because 228 Th is usually not in secular equilibrium with 232 U, its activity continues to grow during preprocessing cooling. Although the total of the 228 Th and 234 Th activities decreases with time, the activity from 228 Th daughters is the most troublesome when chemically purified thorium is being refabricated. The highly energetic betas from both 228 Th and 234 Th chains give large skin doses on surface contact with separated thorium, but the hard (i. e., highly energetic) gammas (2.3 MeV) from the 228 Th chain can result in serious dose rates even with semiremote fabrication techniques.

When the separated thorium is eventually to be recycled and blended with low-activity uranium streams, such as makeup 235 U, the activity of 228 Th after a preprocessing cooling time Tc and a postprocessing storage time Ts is given by

(ХЛОов = [N13(r*)XM(l — e-K«Tc)+Nm(TR)Ховє’т’]е-^т* (8.37)

where N22(Tr) = quantity of 232U in discharge fuel Noe(TR) = quantity of 228Th in discharge fuel

Thorium can be recycled for fabrication with low-activity uranium if the 228 Th activity is no more than a factor ф greater than the 228 Th activity in natural thorium,

(XN) 08 — 0(XjV)o2

Arnold [Al] suggests a value of ф = 5 for thorium to avoid the requirement of semiremote fabrication. Combining Eqs. (8.37) and (8.38), we obtain

For an HTGR [HI, P3] with discharge concentrations of (XN)22/(XN)o2 = 4.04 X 103, (XN)oe/(XN)o2 = 2.54 X 103, Tc = 150 days, and ф = 5, we obtain

T, = 21.3 years

for thorium to be used when fabricating fuel with makeup 23SU. In the HTGR about two-thirds of the thorium is used to fabricate fuel containing makeup or recycled uranium containing no 332 U, so about two-thirds of the separated thorium would be subjected to the storage time estimated above.

For that portion of the separated thorium that is eventually to be recycled and blended with the recycled bred uranium, less time for thorium storage is possible. A reasonable criterion is that the thorium be stored for a sufficient period such that its 228 Th activity is equal to the activity of 228 Th in the recycled uranium at the time of fabrication. Ignoring process losses, the recycled bred uranium contains all of the 232 U that was present in the discharge thorium. If this recovered uranium has been stored for a time Tp prior to fuel fabrication, the activity of 228 Th in the uranium is

(Хов^м)и=^22Х22(1-е-х«г^) (8.40)

Applying the above criterion, we equate the 228 Th activity in the bred uranium to the activity

Figure 8.14 228 Th concentration in irradiated thorium. Basis: ОщСп, 2n) = 0.010 b.

of 228 Th in the fraction /3 of the recovered thorium that is eventually to be recycled for fabrication with the bred uranium, i. e.,

(Ьт#ш)и = №т#я)п (8.41)

where (XoejVog)Th is given by Eq. (8.34). Combining Eqs. (8.34), (8.36), (8.40), (8.41), and

(8.37) , we obtain

, 1 , Г„ 1 — (1 “

‘ = 5£1п[/——————— i-e-КЪ——————

For the HTGR, /3 = 0.36. Assuming that Tc — ISO days and 7> = 60 days, we obtain

Ts = 4.2 years

As the prefabrication time of uranium storage increases, less time is required for thorium storage. For the parameters listed above, if the recovered uranium is stored for 312 days before fabrication, the 228 Th activity in the uranium becomes equal to that in 36 percent of the separated thorium, so no thorium storage is then required to meet the 228Th criterion.