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National Advisory Committee for Aeronautics, Technical Notes - Two Dimensional Diffusion Theory Analysis of Reactivity Effects of a Fuel-Plate-Removal Experiment

Two-dimensional two-group diffusion calculations were performed on
the NACA reactor simulator in order to evaluate the reactivity effects
of fuel plates removed successively from the center experimental fuel
element of a sevens by three-element core loading at the Oak Ridge Bulk
Shielding Facility. The reactivity calculations were performed by two
methods: In the first, the slowing-down properties of the experimental
fuel element were represented by infinite media parameters; and, in the
second, the finite size of the experimental fuel element was recognized,
and the slowing-down properties of the surrounding core were attributed
to this small region. The two calculation methods agreed reasonably
well with the experimental reactivity effects.

The NACA Lewis laboratory is interested in a high-flux research re-
actor with which the components of high power density reactors under de-
velopment for various flight applications can be studied. The reactor
being considered employs an array of aluminum fuel elements similar to
those of the Materials Testing Reactor in Idaho.

The geometric center of the active lattice of a reactor is an at—
tractive location for experiments because of high flux and flux symmetry.
The geometric center of the core, however, is also the most sensitive
reactivity region for accidental compositional changes, and great care
must be exercised in the design of such in—pile experiments. Many re—
activity calculations have been made for both fueled and unfueled exper-
iments in a center test hole in support of the design of the NAGA re-
search reactor. Since reactivity calculations are subject to uncertainty,
an experimental program with the Bulk Shielding Reactor at Oak Ridge
National Laboratory was initiated in order to evaluate the methods of
analysis. In these Oak Ridge experiments, the core configuration of the
NACA research reactor was mocked up within the limits of excess reactiv-
ity and materials available to the Bulk Shielding Reactor.