ORNL-1816 - the Molten Salt Energy Technologies Web Site

1. CIRCULATING-FUEL AIRCRAFT REACTOR EXPERIMENT
OPERATION OF THE AIRCRAFT REACTOR
EXPERIMENT
The Aircraft Reactor Exper iment was success-
fully operated during the quarter. Uranium in the
form of molten Na,UF, was added to the barren
carrier, NaZrF,, with which the fuel system was
initially filled, to make the reactor critical. The
fuel composition at initial criticality was 52.8-
41.5-5.7 mole 5% (RaF-ZrF,-UF,), which has a
melting point of 990°F, whereas the final fuel
mixture (which included excess uranium) had a
composition of 53.2-40.5-6.3 mole % (NaF-ZrF,-
UF,) and a melting point of 1000°F.
It was initially intended to remotely add the
concentrate to the fuel system from a large tank
which contained all the concentrate, after first
passing. it through an intermediate transfer tank.
This system was discarded when temperature-
control and continuous-weight-measuring instru-
mentation on the transfer tank proved to be un-
satisfactory. Instead, a less elaborate, but more
direct, method of concentrate addition was em-
ployed. This enrichment operation involved the
successive connection of numerous smal I concen-
trate containers to an intermediate transfer pot,
which was, in turn, connected to the fuel system
by a line which injected the concentrate in the
pump above the liquid level. Each of the concen-
trate containers was weighed before and after a
transfer in order to determine the amount of
uranium injected into the system. The concentrate
was supplied in batches in cans containing from
about 0.25 Ib of Na,UF, (for rod calibration) up
to about 33 Ib (as was used during the first
subcritical loading). In the enrichment operation
the pump bowl served as a mixing chamber and
uniformly distributed the concentrate into the
circulating stream. (For details of the loading
operation see -foliowing subsection on “Loading
of the ARE.”)
The first concentrate addition was made on
October 30, but the reactor did not become critical
until three days later (3:45 PM, November 3).
Most of the intervening time was spent in clearing
the end of the transfer line at the pump, which,
because of limitations inherent to only this par-
E. S. Bettis J. L. Meem
Aircraft Reactor Engineering Division
ticular design, was difficult to heat and even more
difficult to service.
The approach to criticality was carefully charted
after each fuel addition. The resultant curve of
reactivity [I - (l/multiplication constant)] as a
function of the addition of fuel (in terms of pounds
of U235 per cubic foot) is presented in Fig. 1.1.
The data from three different ionization chambers
are presented; meters Nos. 1 and 2 were fission
chambers located in the reflector, and the BF,
counter was located external to the reactor at the
mid-plane of the cylindrical side. The unique
shape of these curves (which, when first (extrapo-
lated, suggested a much lower critical mass than
was actually required) is believed to be due to
the particular radial flux distribution of the reactor
at the location of the chambers and the change
in this distribution as criticality was approached.
The calculated volume of the carrier in the fuel
system (before concentrate addition) was 4.82 ft3.
(The only significant check on this value was
1.0
0.9
0.8
z 0.7
0
+
0
2 0.6
-
+
-I 3
I
1 0.5
-.
0.4
0.3
02
0 4 8 i2 16 20 24 28
U235 CONCENTRATION (lb/ft3)
Fig. 1.1. Approach to Criticality.