ORNL-4191 - the Molten Salt Energy Technologies Web Site

failure that brought run 12 to an end. Between
March 1 and August 5 there were 11.1 operations,
as follows:
Salt samples 71
Freeze-valve samples of cover gas 6
Exposure of graphite to salt and cover gas
Beryllium additions 6
Uranium additions 27
Of the salt samples, 14 were 50-g samples and 2
were taken in special three-compartment capsules.
The special samples are described in the section
“Reactor Chemistry,” These operations brought
the total, since the sampler-enricher was installed
in March 1965, to 114 uranium enrichments and
279 samples and special exposures.
The uranium additions, the first made with the
reactor critical, provided information on mixing
between salt in the sample enclosure and the
main circulating stream. Response of the ieactivity
(Fig. 1.7) revealed a time constant close
to that for mixing between the pump bowl and the
main stream, indicating rapid melting of the enriching
salt and good circulation through the
sample enclosure.
Near the end of run 11 the manipulator arm and
boot assembly was replaced after a small leak
appeared in one ply of the boot. The arm was
quite contaminated (300 r/hr at 3 in.), but it was
successfully decontaminated and saved for possible
future use (see p. 40). At the start of
run 12, one ply of the boot was ruptured when excessive
differential pressure was inadvertently
applied, and again the assembly V J ~ replaced.
S
This time a slightly different boot was used. The
new boot was thicker and more durable (at the
expense of ease of manipulation), and the outside
was coated with a white plastic spray which effectively
improved viewing in the sampler by decreasing
light absorption.
One of the beryllium addition capsules was
dropped while it was being removed from the 1-C
area with the manipulator. The capsule fell onto
the gate of the operational valve, where it was
retrieved by a inagnet lowered on a cable.
Enriching capsules occasionally jammed iii the
transport container until the difficulty was eliminated
by increasing the length of the cavity in the
disposable portiori to give more room for the long
capsule and attached key.
1
32
The neoprene seals on the 1-C access port began
to show increased leakage, possibly due to
radiation damage. Fission products, mostly the
species found in cover-gas sarnples, produced
radiation levels in area 3A of several hundred
rads per hour. Before run 12, the radiation in this
area was reduced a factor of 10 by using the manip-
ulator to wipe down surfaces with damp sponges.
In run 12, increased leakage from the buffer zone
between the seals on the 1-C access port was
met by increasing the size of the helium supply
flow restrictor so that a satisfactory buffer pressure
could be maintained. Operation of the pneumatic
clamps on the 1-C access door occasionally re-
sulted in gaseous fission products being vented
through the operator vent line. A small charcoal
filter was added to prevent this activity from
reach;-ng the stack.
As described on p. 15, a situation developed on
August 5 that led to a shutdown of the reactor and
replacement of the 1-C assembly. It now appears
that the trouble started when the latch hung at
the maintenance valve. Indications were that both
isolation valves were fully open and that the
capsule key was hanging properly in the latch at
the outset; so the exact cause of the hangup is
not known. But there is convincing evidence that
the drive cable was severed by the operational-
valve gate while the latch was at the maintenance
valve. The length of the latch and remaining drive
cable (Fig. 1.3) equals the distance from the
maintenance valve up to the gate of the operational
valve, and the cable end appeared to have been
cut. It was believed that the operational valve
was practically closed before substantial resist-
ance was felt, but it would have been easy to
mistake the torque and motion involved in shear-
ing the cable and seating the valve for simply
seating the valve. (The tiandwlieel torque re-
quired to shear the cable was subsequently cal-
culated to be less than 100 in.-lb.)
Once before, in December 1965, the latch ap-
parently hung up at one of the isolation valves,
causing the drive cable to coil up in the 1-C area
and in the drive unit. Following that occasion,
the travel of the valve gates on opening was in-
creased slightly, and when the 1-C assembly was
removed after the recent trouble, the valve gates
were observed to completely clear the sample tube.
The valves were also observed to function reli-
ably, so no changes were made.