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9. Fission Product Behavior in the MSRE
S. S. Kirslis
Results of previous tests in-pile and in the
MSRE have been very reassuring regarding most
aspects of the chemical compatibility of graphite
and Hastelloy N with molten fissioning salt. The
aspect of chemical behavior currently causing
some practical concern is the observed tendency
of noble-metal fission products (Mo, Ru, Tc, Te,
and Nb) to deposit on graphite surfaces exposed
to fissioning fuel in the MSRE. Some of the iso-
topes of these elements have neutron cross sec-
tions high enough to affect significantly the
neutron economy of an MSBR after several years
of operation if a large fraction of these isotopes
deposited in the graphite core. Recent work in
the MSRE has been directed mainly toward eluci-
dating the behavior of these noble-metal fission
products.
Information derived from several kinds of tests
is reported in some detail in the following sections.
These include (1) quantitative measurements
of the concentration of fission product
species in samples of the MSRE pump bowl cover
gas captured during a number of diverse reactor
operating conditions, (2) analysis of fuel samples
for fission products, (3) examinations and analyses
of gaphite and Hastelloy N specimens exposed to
molten fissioning fuel salt in the MSRE for 24,000
Mwhr of power operation, and (4) qualitative measurements
of fission product deposition on metal
and on one set of graphite samples exposed to the
cover gas and fuel phases in the MSRE pump bowl
under a variety of reactor operating conditions.
9.1 FISSION PRODUCTS IN MSRE COVER GAS
Sampling of the cover gas in the MSRE pump
bowl has been continued in an effort to define the
nature and the quantity of the gas-borne species.
F. F. Blankenship
116
Five gas samples were obtained and analyzed
during the period covered by this report. In these
studies we attempted to determine how the nature
and amount of gas-borne species are affected by
(1) addition of beryllium to the fuel, (2) stopping
the MSRE fuel pump, (3) a long reactor shutdown,
and (4) increasing the volume of helium bubbles
in the fuel.
All samples were taken in evacuated 20-cc
capsules sealed by fusible plugs of 2LiF. BeF,;
these plugs melted and permitted the capsules to
fill with the cover gas upon insertion into the
heated pump bowl. Samplers, sampling procedures,
and analytical operations were, in each case, very
similar to those previously described.'
Since it was not possible to instrument the
sampling assemblies, no definitive information
concerning the temperature of the gas at time of
sampling is available. Temperatures were cer-
tainly higher than the melting point of the fusible
plug (-450OC). It seems likely that the tempera-
ture is near 6OOOC and that the capsule actually
drew 20 cc of gas at this temperature and 5 psig;
the sampled volume of gas at STP was, therefore,
about 8.5 cc.
The results obtained from these five samples
are shown in Table 9.1. All values (except those
for uranium) are given in disintegrations per
minute for the total sample, and all have been
corrected to correspond to the time of sampling
if the reactor was operating, or to time of shut-
down for the two cases where the reactor was
not at power. The uranium values are in micro-
grams of uranium per sample. Conditions of re-
actor operation and special features of each test
are indicated in Table 9.1.
'S. S. Kirslis, MSR Program Semiann. Progr. Rept.
A@. 31, 1966, ORNL-4037, p. 165.