ORNL-1771 - Oak Ridge National Laboratory

ANP QUARTERLY PROGRESS REPORT
REFLECTOR-MODERATOR REACTOR
D. Scott B. L. Greenstreet
ARE Division
J. J. Lynn
Physics Division
J. S. Crudele J. W. Nooks
Pratt and Whitney Aircraft Division
The first step of the present critical experiment
program was the construction of a small two-region
reflector-moderated reactor to provide experimental
data on a system of simple geometry and materials
for use in checking the present calculational
methods. ' The assembly is approximately spheri-
cal, 41 in. in outside diameter, and has a center, or
core region, that is about 15 in. in diameter.2 The
core consists of alternate sheets of enriched
uranium metal (4 mils thick) and Teflon, (CF,),
and is surrounded by a beryllium reflector. It is
possible to vary the uranium loading, within the
specified dimensions, in order to make the system
critical. The Teflon and the uranium have been cut
to the various shapes necessary to form the sub-
assemblies required to conveniently construct the
core. A summary of the materials in the first criti-
cal assembly is given in Table 4.1. The aluminum,
which is a necessary structural material, is present
in the core as a vertical double sheet at the mid-
plane and in the reflector as fairly well-distributed
sheets and rods. The coating material, which con-
tained 31.8 wt % C, 61.9 wt % F, 5.5 wt % Cr, and
0.83 wt % H, was applied in a thin layer to the
uranium metal to reduce oxidation. The Scotch
cellophane tape was used to hold the core sub-
assemblies together. A view of the mid-plane of
the reactor as initially assembled is shown in Fig.
4.1. The small rectangular holes below and to the
right of the core are the aluminum guide channels
for the control and safety rods.
'D. Scott and B. L. Greenstreet, ANP Quar. Prog.
Rep. Mar. 10. 1954, ORNL-1692, p 45.
,A more complete description is given in Reflector-
Moderated Critical Assembly Experimental Program by
D. Scott and B. L. Greenstreet, ORNL CF-54-4-53
(April 8, 1954).
44
4. CRITICAL EXPERIMENTS
A. D. Callihan
Physics Division
An earlier assembly was loaded with 17.4 Ib of
U235, as prescribed by the multigroup calculations,
and was not critical. It was discovered, however,
that these calculations were in error because of
a numerical error in one of the cross sections and
that the predicted critical mass range should have
been 20.9 to 22.75 Ib of U235. The system was
made critical with 24.35 Ib of U235 and had an
excess reactivity of approximately 7 cents, with
about 20% of the mid-plane aluminum plates omitted.
It has since been shown that the mounting of this
plate resulted in a reduction in reactivity of ap-
proximately 10 cents, and therefore the system as
designed and loaded with 24.35 Ib was slightly
subcritical. Sufficient excess reactivity for sub-
sequent experiments has been gained by the addi-
tion of a cylindrical annulus of beryllium reflector
17i in. long and 2T8 in. wide with its axis perpen-
dicular to the mid-plane. This addition gave an
increase in reactivity of 65 cents. The extra
beryllium at the top of the reactor was removed
during flux measurements.
TABLE 4.1. COMPOSITION OF CORE AND
REFLECTOR OF FIRST REFLECTOR-MODERATED
REACTOR CRITICAL ASSEMBLY
Core
Volume
Average radius
Weight of
Teflon*
Uranium
u235**
Uranium coating material
Scotch tape
Aluminum
Ref lector
Volume
Minimum thickness
Weight of
Beryl I ium
Alurni num
*Density: 1.96 g/cm3 of core.
** Density: 0.372 g/cm3 of core.
1.05 ft3
7.48 in.
58.464 kg (128.62 Ib)
11.878 kg
11.067 kg (24.348 lb)
0.1 12 kg
0.085 kg
0.92 kg (2.02 Ib)
21.24 ft3
12.9 in.
1102.5 kg (2425.7 Ib)
16.8 kg (37.06 Ib)