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alance results during this time show good agree-
ment between the calculated and observed effects
of the additions. The transient effects of the
actual fuel additions were very mild. Figure 1.7
shows an on-line plot of the position of the
regulating control rod made during a typical fuel
addition with the reactor on servo control. Con-
trol rod movement. to compensate for the additional
uranium in the core started about 30 sec after
the fuel capsule reached the pump bowl, and the
entire transient was complete about 2 min later.
'This indicates rapid melting of the enriching salt
and quick, even dispersion in the circulating fuel.
The weights of the emptied fuel c:ilpsules indi-
cated that essentially all their contained znsU
was transferred to the fuel loop.
The reactivity-balance results in run 12 (Fig.
1.6) were essentially the same as those in the
preceding run. Minor variations, associated with
pressure and power changes, were again observed.
Another series of fuel additions at full power
was made in this run between July 19 and 26.
'This series consisted of 18 capsules containing
1527 g of 'jSU. 'The purpose of this large addi-
tion was to provide sufficient excess uranium so
that a large amount of integrated power could be
produced without intermediate fuel additions. We
plan to perform a detailed evaluation of the uranium
isotopic-change effects associated with power
operation, and substantial burnup is rtquired to
make the analyses of isotopic composition useful.
A secondary result of this large fuel addition
(0.5% Sk/k) was a drastic change in the control
rod configuration. At the end of the additions
the separation between the tips of the shim rods
I- ;--
r~~~~ CAPSIJLE IN PUMP BOWL
OHNL-DWG El- 10132
0 4 2 3 4 5 6 7
TlMF (minl
0=1150 hr CPML 20.1'367
Fig. 1.7. Regulating Control Rod Position During
Fuel Addition.
21
and that of the regulating rod was 15.5 in., whereas
the normal separation has been 1 to 8 in. The
variation in apparent residual reactivity as a
function of control rod configuration was reexamined,
and we observed a decrease of 0.02% 8k/k when the
more usual configuration was established. This
was consistent with an earlier evaluation (May
1966) of the accuracy of the imalytic expression
used in the computer to calculate control rod
poisoning as a function of rod configuration.
On August 3 a computer failure occurred which
required recalibration of the analog-signal amplifiers
after service was restored. As a result of
this recalibration, there were small shifts in the
values of several of the variables used in the reactivity
balance. Errors in react or-outlet temperature
and regulating-rod position caused a downward
shift of 0.03% 6k/k in the residual reactivity.
Balances at Zero Power
Figure 1.8 shows the long-term variation in
residual reactivity since the start of power operation
(December 1965). 'rile values shown are
average results at zero power with no xenon
present. Corrections have also been applied for
computer-induced errors such as those at the end
of run 12. The results are plotted t.o show their
relationship to the reactor operating limits at
10.5% 6k/k. The discovery of a 0.5-in. shift in
the absolute position of rod 1 at the end of run 12
(see p. 31) add:; some uncertainty to the last point
in this figure. This shift represents a reactivity
effect of i-0.0275 6k/k, which would have been detected
if it had occurred during a run. However,
the dilution corrections which must be applied
Fig. 1.8. Long-Term Drift in Residual Reactivity of
the MSRE at Zero Power.