ORNL-1816 - the Molten Salt Energy Technologies Web Site

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ANP QUARTERLY PROGRESS REPORT
The addition of the swirl pump made it neces-
sary, however, to replace the slinger ring, previ-
ously employed, with a slinger impeller in order
to buck any leakage from the swirl pump. Although
the complete assembly has not yet been tested in
an inverted position, the slinger impeller seems to
be very effective. A similar arrangement is now
being tried as a means of controlling the leakage
from the main pump discharge through the clearance
between the impeller and the casing past the
centrifuge. More conventional sealing arrange-
ments were found to be ineffective because of the
abnormally large radial clearances (0.060 in.)
specified in the design objectives. Initial tests
currently in progress with the centrifuge seal
impeller have been quite promising. Although no
detailed studies of the off-gas system and fuel
pill addition system have been made, the changes
-removal system will make it neces-
off-gas system connection and the
fuel pill addition be made through the swirl
chamber rather than through the pump housing as
i I I ustrated previous I y.
CONTROL ROD DESIGN CONSIDERATIONS
The design of the ART has been predicated on
the belief that no fast-moving control rods will be
required because of the inherent stability of the
reactor that will arise from the strong negative
temperature coefficient of the circulating fuel.
It is further expected that only about 1% Ak/k
will be required to compensate for xenon poisoning,
since it is anticipated that with the bypass ex-
pansion tank and gas-scrubbing system planned,
the xenon will be removed almost as rapidly as it
is formed. It is expected that compensation for
burnup and the accumulation of fission-product
poisons can be effected by adding pills of solid
fuel having a high percentage of U235. Thus the
principal function of the control rod will be to
control the mean temperature level of the reactor
fuel. Since the temperature coefficient will
probably be approximately 5 x Ak/k per OC
and since a temperature level variation of the
order of 15OoC will probably be required, provision
of a Ak/k of 1% in the control rod for temperature
level control should be more than adequate. In
reviewing the various requirements for control, it
thus follows that a control rod having a total keff
of 2% should prove to be adequate. Thisreactivity
24
’lbid., Fig. 3.4, p 40.
can be readily incorporated in a single slow-moving
control rod. It would seem entirely adequate to
move this control rod at a rate of 1% Ak/k per
minute, a slow and conservative value.
The size and stroke of the rod are important
considerations. Two items of experimental data
are available to indicate the size of the rod re-
quired, In the course of the second reflector-
moderated reactor critical experiment (that making
use of powdered fuel placed inside 1 k-in.-square
aluminum tubes), it was found that with an 18-in.-
dia core and a 9-in.-dia island a control rod
effectiveness of about 0.6% Ak/k was obtained
from a 3/,6-in.-OD stainless steel tube filled with
powdered boron to a density of about 50% and
inserted in one-half of the critical experiment
assembly. A second indication of the effective-
ness of control rod material is available in that a
piece of gadolinium oxide approximately the
diameter and thickness of a 25-cent piece was in-
serted at the center of the island in the three-
region octahedron critical assembly (without core
shells). The value of this gadolinium oxide wafer
was found to be approximately 0.2% Ak/k. Some
additional data are also available from multigroup
calculations. All these data seem to indicate that
a rod approximately to ’/2 in. in diameter having
a stroke of approximately 20 in., about 7% in. of
which would be below the equator, should prove
adequate to give a Ak/k of 2%.
The heat generation to be expected in the con-
trol rod will establish the amount of cooling that
will be required and, to a large degree, much of
the detail design of the rod. The heat generation,
in turn, is dependent, in part, on the number of
neutrons absorbed in the rod at full power. Multi-
group calculations indicate that 1.3% of the
neutrons produced will be absorbed in a control
rod having a Ak/k of 1%, if it is assumed that the
average effectiveness of the control rod is half
that of the portion at the center of the island.
Thus a control rod having an effectiveness of 2%
Ak/k would, when in the “full in” position, absorb
2.6% of the neutrons. If a value of 3 Mev per
neutron absorbed is assumed, the energy associated
with neutron absorption in the control rod would
amount to0.039% of the total power generated in the
reactor. This would total 24 kw for a 60-Mw power
level. The control rod cooling requirement would
be a minimum if gadolinium rather than boron were
used as the absorbing material in the rod. This
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