Review of Molten Salt Reactor Physics Calculations [Disc 2]

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6. TWO-DmIONAL CAICULDIONS
The reactor was described in R-2 geometry as shown in Fig. 6.1.
(All dimensions on this figure are in feet.) Forty-eight mesh lines
were used in the radial direction and 67 axially. me region compositions
are listed in Table 6.1 and thelmain core densities are listed in Table
6.2. Region U, the center control rod, was treated as a typical fuel
cell (Figure 5.1) with the fuel stream replaced by graphite. In the
lower axial blanket (region U), each fuel tube was considered to be
loo$ INOR-8 between the core and the lower plenum. Region 18 is a
vacuum region which is used to impose a black" boundary condition on
the adjacent fuel and INOR boundaries.
Region 19 is an annulus of fuel
representing the four s*ets of entrance and exit fuel pipas at the bottom
of the reector. Where possible, reactor dimensions and volume fractions
were made the same as in the previous calculations (case 555).
The nine-group neutzon diffusion equations were solved with the
ASSAUIIT code.' A value of = Oig5 was obtained. The decrease in
\
bff of 0.05 compared to the previous calculations is entirely attributable
to a 215 higher thorium resonance integral used in our calculations.
A f'uel search calculation was then made on the 23% concentration
in the fuel stream until the reactor was just critical. A 23% concentration
increase of 13.s was required to raise %ff to unity. A segionby-region
neutron balance forthe critical configuration is given in
Table 6.3. Only about 0.006 of the absorptions per fissile absorption
occur outside the sllrroundhg blankets, indicating optically thick blankets.
In fact, the axial leakage is so small that the axial blankets could be
made samewhat thinner without affecting the neutron economy. However,
about 0.005 neutrons absorbed in the lower axial bLanket are captured
by INOR. This loss in breeding ratio might be avoided if it is possible
to extend the graphite f'uel tubes below the core about 6-12 in. before
mung the transition to INOR.
The power density is very nearly in a Jo distribution radially and
a cosine distribution axially, as shown in Fig. 6.2 and 6.3. The radial
distribution corresponds to the mid-plane of the core, and the axialtraverse
was made near the center of the core at r= 17 an. Power densities
n
W
/-
w
z
-
U