MURR is a high-performance research reactor with a very compact core (core volume of only 33 liters with 4.3 liters of fuel meat) with a peak thermal flux of about 6.0 x 1014 n/cm2-s (Figure 2-5). The reactor is

refueled weekly to maintain a greater than 90 percent capacity factor for efficient production of medical isotopes.

Conversion studies for this reactor showed that if the fuel geometry was unchanged, conversion using UMo monolithic LEU fuel would result in a harder neutron spectrum and, thus, increased power in some regions of the reactor. A means to control this higher power density needed to be identified for conversion to become possible.

The reactor fuel plates are curved, and there is no flexibility to rear­range them to reduce power peaking. However, it was determined that by using four distinct thicknesses of fuel meat in the assemblies (ranging from 0.23-0.43 millimeters), peaking factors could be reduced to acceptable levels.