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een stripped is more nearly 50% of the production
rate.
With the simplifying assumptions that (1) no
89Kr is lost to the moderator graphite, (2) no
"Kr is lost through burnup, and (3) the fuel
which flows through the pump bowl is stripped
of 89Kr with 100% efficiency, it may be simply
shown that
"Kr stripped/min F
__
89Kr produced/min
~
A 4 F '
where F is that fraction of the fuel volume which
passes the stripper (pump bowl) per minute and h
is the decay constant (0.693/3.2 min) for "Kr.
For MSRE, with the pump bowl flow rate at 4% of
total flow, this expression yields the value 29%
for the "Kr lost to the stripper gas. It is un-
likely that the stripping efficiency in the pump
bowl is 100%; moreover, it is certain that some
fraction of the 89Kr is lost by penetration into
the graphite. This rate of loss of 89Kr to the
moderator - which is probably less than 30% of
the production rate - will in effect introduce a
third term to the denominator of the equation and
lower the fraction lost to the off-gas. From these
data, therefore, it seems possible that the gas
at the sampling station may be more concentrated
in *'Sr than is the average gas in the pump bowl,
but the concentration factor is not large.
The absolute amounts of 99Mo, corrected back
to sampling time or time of previous shutdown,
show rather minor variations with reactor operating
conditions. The lowest value is for run
FPll-42, in which the sample was taken 1.5 hr
after shutdown and 1.2 hr after stopping the fuel
pump. The normal runs showed the higher 99M0
concentrations, although the highest value might
have been expected for FPll-53, in which the offgas
pressure was suddenly lowered just before
sampling to ensure a release of helium bubbles
from the pressurized graphite bars into the circulating
fuel. From the amount of 99M0 found in
these samples (mean value 1.9 x 10" dis/min or
2.2 x 10" dis iiiiii-' cc-' or 1.3 x lOI4 atoms
'Mo/cc), the effective partial pressure of the
volatile species is calculated to be 4 x lop6
atm. The total "Mo lost at 4.2 liters/min of
helium flow is 7.8 x 10'' atoms/day. This is
about 18% of the total inventory of 4.4 x 10"
afoms, or about 70% of the daily production rate.
If it is assumed that the material is lost as MoF,,
118
this corresponds to a loss of 4.6 x lo2' fluorine
atoms/day, or an equivalent of F- per 150 days.
The very considerable quantities of 99Mo found
on the graphite and the Hastelloy surveillance
specimens as well as in the fuel stream do not
seem consistent with such large losses to the
gas system.
The lo3Ku, Io6Ru, I2'Te, and 132Te concentrations
generally showed parallel behavior over
all the runs, with particularly good correspondence
between the Io3Ru and 06Ru values. The ratios
for the two isotopic pairs agreed satisfactorily
with the ratios of fission yields divided by halflives.
The effect of short shutdown and pump
stoppage was minor on this set of isotopes. The
highest values for Io3Ru, lo6Ru, and '29Te were
obtained during the pressure release run, FPll-53.
However, '32Te, like 99iV0, did not show a high
value during this particular test.
The appreciable concentrations of *'Sr found in
samples FPll-42 and FP12-7 are puzzling. In
each case the reactor had been shut down long
enough to permit the complete decay of 89Kr before
the sample was taken. The 9sZr analyses
indicated fuel mist concentrations too low to
account for the 89Sr values. It may be that other
activities interfered with the beta counting required
for 89Sr analysis. If these values are
accepted as real (and further sampling must be
done to confirm them) it would appear that, at
least when the pump is off or the reactor is not
at power, the sampling volume is poorly flushed
by the cover gas system.
The 95Nb values showed a fivefold increase of
magnitude from the first run to the fifth, with a
marked peak at the pressure release run, FP11-
53. Not all of the overall increase could be ascribed
to the increase of "Nb inventory in the
reactor system with continued reactor operation
(cf. moderate increases in '03Ru and Io6Ru).
The analyses for 235U from four of these rum
yielded values higher by at least a factor of 10
than those observed in earlier tests.' These
relatively high values for 235U are, at best, difficult
to explain. If, as noted above, the values
for "Zr are taken to indicate the existence, and
quantity, of salt mist in the sample, then sample
F:Pll.-53 might have been expected to have about
20 pg of 23sU in such mist. However, in no other
case of those shown in Table 9.1 can such an explanation
account for more than 20% of the observed
uranium.