ORNL-4191 - the Molten Salt Energy Technologies Web Site

Nuclear heating of the pump tank and the support
structure within the pump tank is removed by
circulating a portion of the fuel salt from the main
salt stream over the heated surfaces. To remove
the nuclear heat from the shaft, a small amount of
salt is bled up the center of the shaft and fed into
an annulus between the shaft and a cooling tube
that extends the length of the pump tank. A
labyrinth seal at the lower end of the tube forces
most of the salt to flow to the upper end of the
tube, where it spills over into the pump tank. An
added benefit is the increased damping and
stiffness provided to the shaft by the salt in the
annulus.
Analyses are being made of the nuclear heating
in that portion of the pump casings and shaft for
which no cooling is provided. if a problem is
found, we can provide cooling or shielding and
thermal insulation where needed to reduce the
heat generation in the pump structure to an acceptable
level.
The seal arrangement at the upper end of the
shaft is similar to that used in the MSRE salt
pumps. It consists of a face-type seal (Craphitar
against tool steel) with the lubricating oil on one
side and the helium in the shaft annulus on the
other. Helium is brought into the annulus to serve
as a split purge between the salt and gaseous
fission products at the lower end of the shaft and
my lubricating oil that leaks through the face seal
into a leak-off line. Part of the helium passes
down the shaft through a close-fitting labyrinth,
where the increased gas velocity reduces the
upward diffusion of molten-salt vapor and gaseous
fission products. Concurrently, that portion of the
helium passing upward through the labyrinth seal
prevents the downward movement of lubricating oil
vapors and also serves to scavenge oil leakage
and vapors overboard from the pump.
Coolant Salt Pumps
Two preliminary layouts of the XSBR coolant
salt pump have been prepared. One layout utilizes
a pump with a short overhung shaft mounted on two
oil-lubricated irolling element bearings, and the
other is a long shaft with an oil-lubricated bearing
at the top end of the shaft and a molten-salt
bearing located just above the impeller. One
criterion for the pump requires variable-speed op-
eration over the range 300 to 1200 rpm. The dif-
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ficulty with the short-shaft pump is that to have
the pump operate below the first critical speed,
the shaft diameter would have to be greater than
8 in., which would present a formidable seal dc-
sign and development problem. If it were designed
to operate above the first critical and below the
second critical speed, the shaft diameter would be
approximately .3 in., which is inadequate to trans-
mit the torque. For the long-shaft pump configu-
ration, however, a shaft with a diameter selected
on the basis of torque requirements would have a
first critical speed well above the maximum oper-
ating speed. The long-shaft pump would also use
the same upper bearing and seal configuration
planned for the fuel and blanket salt pumps. Hence
the long-shaft concept appears to be preferable for
the coolant pumps.
The coolant salt pump will have the impeller and
volute mounted in a pump tank of sufficient volume
to accommodate the thermal expansion of thc cool-
ant salt for the most adverse thermal condition that
might arise during reactor operation. A double vo-
lute pump casing has been selected to reduce ra-
dial loads on the impeller and the resultant loads
on the molten-salt bearing, particularly when op-
erating at off-design conditions, and to reduce the
diameter of the bridge tube, which provides a
flexible connection from the volute to the pump
tank nozzle.
We believe that the coolant pump drive motor,
although having a greater horsepower, can be de-
signed to fit the same containment vessel as that
for the fuel pump drive motor.
Water Pump Test Facility
Preliminary layouts have been prepared of a
facility for testing the fuel pump with water. The
configuration does not incorporate the long shaft
of the high-temperature pump but only mocks up
those portions which affect the fluid flow. The
layout also includes a moclrup of the inlet to the
heat exchanger tube sheet with sufficient instrumentation
to monitor the flow distribution in the
heat exchanger tubes. The distribution of the gas
injected to remove the xenon will be monitored
also.
The configuration has been designed to permit
water testing of the blanket pump in the same
facility. The purpose of the water test facility in
the pump development program is (1) to determine