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(Yorkmesh) was % 275°F when the particle trap
was first used and -380'F near the end of run 12.
The temperatures decrease rapidly when the reactor
power is reduced, however, and the zeropower
steady-state temperatures are essentially
unchanged. These effects indicate the accumulation
of some material, presumably organic, on the
filtering media that enhances the retention of
short-lived fission products.
Main Charcoal Beds. - The performance of the
charcoal beds in holding up noble-gas fission
products has continued to be satisfactory. The
gradual development of restrictions at the inlet
ends of the beds has also continued, but this has
not limited the reactor operation in any way, since
effective measures can be taken to reduce the
restriction when necessary.
Run 11 was started in January 1967 with the
charcoal bed sections 1A and 1B in service with
an initial pressure drop of 2.5 psi at normal offgas
flow. The pressure drop increased very slowly,
reaching 7 psi on March 29, two months after the
start of the run, At that time the standby beds,
2A and 2B, were put in service, and the restricted
sections were valved out. The pressure drop
across these sections built up from 2.6 to 9 psi
in only ten days. We then cleared the restrictions
from all four sections by forcing clean helium
through sections 2A and 2B in the normal flow
direction and heating the inlet ends of sections
1A and 1B with previously installed6 electric
heaters. These operations did not require a reactor
shutdown but only a temporary lowering of
the water level in the charcoal bed pit to allow
the heaters to function.
After the restrictions in both sets of beds had
been cleared, sections 1A and 1B were put back
in service. In the ensuing three weeks the pressure
drop increased from 2.4 to 3.5 psi. At that
time, we decided to increase the helium purge
flow by 1 liter/min to see if the xenon poisoning
would be affected by lower concentrations in the
fuel pump gas space. To accommodate the higher
gas flow without an increase in fuel pump pressure,
sections 1A and 1B were valved out and 2A
and 2B were put in service. The next day section
1A had to be reopened to keep the fuel pump pressure
at 5 psig. The normal purge flow was restored
after three days, and, just before the power
shutdown at the end of run 11, the partial restric-
61bid., pp. 30-31.
28
tions in all four beds were again removed by heat-
ing sections 1A and 1B and forward blowing sec-
tions 2A and 2B.
Owing to the success of the heaters in clearing
the restrictions from sections 1A and lB, we in-
stalled similar heaters at the inlets of sections
2A and 2B during the shutdown between runs 11
and 12. The differential pressure transmitter
that senses charcoal bed pressure drop directly
was also replaced. This instrument had failed
earlier, possibly because of the pressure differ-
ences imposed during blowouts of the charcoal
beds. However, all these pressure differences
were within the specified overrange capability of
the instrument.
Power operation in run 12 was started with sec-
tions 1A and 1B in service. The gradual increase
in pressure drop made it necessary to change to
sections 2A and 2B after about three weeks. The
pressure drop across the second sections reached
an unsatisfactory level after only six days. Then
the restrictions were cleared from all four sections
by heating the inlet ends. The remainder of run 12
was completed with sections 1A and 1B in service.
The development of flow restrictions at the char-
coal beds appears to be related to the accumula-
tion of volatile organic matter on the steel wool
packing at the bed inlets. Physical variations in
this packing probably account for the different
times required to plug various individual sections.
The experience in runs 11 and 12 indicates that
the restrictions can be effectively removed by
electrically heating the inlet ends of the beds.
Presumably, this heating drives the volatile matter
off the steel wool packing in the inlets and moves
it farther downstream where the flow areas are
larger. There is no evidence from the charcoal
temperatures that this material has reduced the
fission product retention capability of the charcoal.
Since the heating operations do not affect reactor
performance, there are no plans at present to make
further modifications at the charcoal beds.
Coolant Off-gas System. - Very slow plugging
of the coolant off-gas system at the filter that
precedes the coolant-loop pressure control valve
has been encountered throughout the reactor opera-
tion. The originally installed filter was replaced
in February 1965, during the preoperational check-
out of the system. Subsequent replacements were
made in March and September 1966 and on March
1, 1967. The replacement on March 1, 1967, re-
quired a reactor power reduction for 7 hr to permit