System and safety studies of accelerator driven transmutation ... - SKB

Table 6 shows how the differences in ψ* observed
in helium- and LBE-cooled cores disappear in a
homogenous core model.
He- LBE- He- LBE-
MOX MOX AcN AcN
Heterogeneous 18.5 16.8 16.8 15.2
Homogeneous 16.8 16.9 15.2 15.3
-15% 1% -10% 1%
Table 6: ψ* for the four cases in heterogeneous and homogenous
core models. The homogenous model is too simplified to model
helium-cooled cores accurately, while giving accurate results for
LBE-cooled cores. The last row indicates the deviation of the
results from using a homogenous core model to using a
heterogeneous model.
5.6. Effects of changing the coolant fraction
The geometrical distribution of source neutrons in
the fuel is dependent on the coolant fraction,
especially for gas-cooled cores. To examine whether
this has an impact on ψ*, cores with different pinpitch
over diameter (P/D) were modeled for the He-
MOX case. The level of subcriticality was kept
constant by altering the radius. The results are
displayed in Table 7.
P/D 1.291 1.4 1.5 1.7 1.9
ψ* 16.8 18.6 18.5 20.1 20.1
- 10% 10% 19% 19%
Table 7: ψ* for the He-MOX case, when P/D and consequently the
coolant fraction is varied. A larger coolant fraction leads to higher
proton source efficiency.
When P/D is increased, the helium-filled paths to
the outer parts of the core are widened, increasing the
probability of source neutrons reaching further away
from the spallation target leading to higher source
efficiency. These observations confirm the results
found when comparing the heterogeneous and
homogenous core models.
Elsevier Science 7
6. Conclusions
Americium introduction in the fuel of a subcritical
core decreases ψ*. This is why ψ* is around 10%
lower in a core loaded with americium based fuels
than in a core fuelled by MOX-fuel. ψ* is reduced by
another 10% when helium is replaced by LBE.
Both of these effects are due to the strong
dependence of fission and capture cross-sections on
incident neutron energy in the 1 MeV region in even-
N nuclides and to their appearance in slightly shifted
energy bands.
Acknowledgments
This work was financially supported by the
Swedish Centre for Nuclear Technology and by SKB
AB (Swedish Nuclear Fuel and Waste Management
Co).
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