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3-38
An additional factor relative to the deterrent effect is the time required to carry
out the necessary operations. This is illustrated by Table 3.3-11, which gives the dose
rates (in rem/hr) required to acquire each of three total doses within various times,
varying from a totally incapacitating 20,000 rem to a prudent individual's dose of 100
rem. Thus, to divert a small amount of fissile material to a portable, shielded container
might take less than 10 seconds, in which case a dose rate of lo7 rem/hr would be required
to prevent completion of the transfer. Only 200 rem/hr would be required, on the other
hand, to deliver a lethal dose to someone who spends five hours close to unshielded 233U
while performing the complex operations required to fabricate components for an explosive
device. The maximum anticipated concentration of 232U as projected for denatured fuel
does not provide sufficient intensity to reach totally disabling levels. Fast-reactor
bred material (depending on time after separation and quantity as well as 232U concentration)
can come within the lOO-rem/hr range.
Table 3.3-11. Gama-Ray Dose Rates for Three Levels of Total Dose vs. Exposure Timea
Time of Exposure
10 sec
1 min
5 min
30 min
1 hr
5 hr
12 hr
aFrom Ref. 18.
Dose Rate (rem/hr) Required to Deliver Total Dose of
100 rem 1000 rem 20,000 rem
36,000 360,000 7,400,000
6,000 60 , 000 1,200,000
1,200 12,000 240,000
200 2,000 40,000
100 1,000 20,000
20 200 4,000
8.3 83 1,660
The fact that the level of radiation of 232U-contaminated 233U increases with time
is a major disadvantage for a 233U-based nuclear explosive device. There is a window of
10 to 20 days immediately following chemical separation when the material is comparatively
inactive due to the removal of 228Th and its daughters. Having to deliver a device less
than ten days after fabricating it would be undesirable. While the tamper would provide
some shielding, this short time schedule would complicate the situation considerably.
For a national program it is likely that the military would want a clean 233U
weapon. This could be accomplished to a large degree by separating the 232U from the
233U using gas centrifugation. However, because the masses are only 1 amu apart this
requires several thousand centrifuges to make 100 kg of clean material per year (see Section
3.4.4). A nation possessing this isotopic separation capability would therefore probably
choose to enrich natural uranium rather than to utilize denatured fuel, thus eliminating
the 232U-induced complications.
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