Search - OECD Nuclear Energy Agency

This problem can be solved by using a combined approach to protect MOX-fuel, namely:
combination of 232 U admixture to fuel with short-term irradiation of manufactured FAs in ADF. Timedependent
recession of RED from the FA irradiated for 90 days in ADF blanket [2] is shown in
Figure 2. It can be seen that during the first two years the RED of fission products accumulated in
irradiated FA is substantially larger than 100 rem/h while during the next two years the RED remains
at only 50% of this protection level. But, in combination with the RED from 232 U decay products, total
RED becomes sufficient to ensure radiation protection of MOX-fuel at a level no less than 100 rem/h
for a 25-year period.
Figure 2. RED at 1 m distance from irradiated FA with MOX-fuel containing 0.01% 232 U
100000
10000
Rate of exposure dose, rem/h
1000
100
10
Total
RED of fission products
1
0 10 30 50
Time, yr
As mentioned above, the level of radiation protection barrier (100 rem/h) is defined by permissible
232
U concentration in fabricated fuel and, finally, by capabilities of the technology used for MOX-fuel
fabrication. For example, at MOX-fuel fabrication plant in Sellafield (United Kingdom) a semi-distant
technology is applied. Such a technology allows to fabricate MOX-fuel and to manufacture FAs with
RED at 1-m distance from ready-made FA about 0.6 rem/h.
Of course, if it is necessary, this level of protection barrier may be heightened up to the required
value. However, it leads to proportional upgrade in radiation level at stage of MOX-fuel fabrication
and requires proper application of sophisticated technology. For instance, if the value of 1 000 rem/h
for RED at 1-m distance from ready-made FA was chosen as criterion for reliable radiation
protection, then the 232 U content in fuel may be increased up to 0.1%, and RED at the end of FA
manufacturing will be about 6 rem/h. So, the value of RED adopted as protection criterion and RED
from ready-made FA differ approximately by a factor of 170.
Here, it is appropriate to underline once more an advantage of 232 U as compared with γ-active
radionuclides ( 137 Cs, for instance). This advantage is related with the delayed nature of γ-radiation
emitted by 232 U. Intensity of this radiation, negligible at initial stage, increases along with gradual
accumulation of γ-active isotopes in the decay chain of 232 U, attains a peak value and only afterwards
942