ORNL-4191 - the Molten Salt Energy Technologies Web Site

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REACTIVITY
POWER
ORNL- DWG 67- 9191
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16 18 20 22 24 26 28 3C 1 3 S 7 9
JULY, 1967 nus, 1967
residual reactivity occurred during the first
several weeks of run 11. Detailed analysis of the
individual terms revealed two sources of error.
One was a gradual downward drift in the temperature
indicated by two of the four thermocouples
used to calculate the average reactor outlet temperature.
These two thermocouples were eliminated
and replaced by one other that had not
drifted. The second error was caused by loss of
significance in the calculation of the 14'Sm concentration.
In the program, only the change in
samarium concentration is computed, and that
change is added to the last value to obtain the
current value. As the I4'Sm concentration approached
85% of its equilibrium value, the incremental
concentration change computed for the 5-
miri time step between routine reactivity balances
was outside the five-decimal-digit precision of the
computer. As a result, these increinents were
lost when the concentration was updated. To
avoid using doubl e-precision arithmetic, the program
was modified to only update the 14'Srn and
the 'Sm concentrations every 4 hr while the
reactor is at steady po~~i. Summary calculations
made off line were used to verify the adequacy
of this change.
Fig. 1.6. Residual Reactivity During MSRE Run 12.
When these corrections were introduced on
March 1'7, the apparent downward drift in reactivity
disappeared. At the same time, minor
changes were made in some of the '35Xe stripping
parameters to make the calculated steady-state
xenon poisoning agree more closely with the observed
value.
Other small reactivity variations were observed
in run 11, for example, from March 29 to April 9.
These changes are directly related to changes in
the helium overpressure on the fuel loop; a 1-psi
pressure increase leads to a reversible reactivity
decrease of slightly less than 0.01% 6k/k. The
mechanism through which pressure and reactivity
are coupled has not yet been established. The
direct reactivity effect of the change in circulating
voids caused by a change in absolute pressure
is at least a factor of 10 smaller than the observed
effect of pressure on reactivity. The time constant
of the pressure-reactivity effect is relatively
long, suggesting a possible connection through the
xenon poisoning.
Fuel additions were made for the first time in
run 11 with the reactor at full power. Nine capsules
containing a total of 761 g of 235U were
added between April 18 and 21. The reactivity-