Two presentations on modifications of research reactor cores to address the technical challenges of conversion were given by Panel 2.1 speakers: John Stevens (Argonne National Laboratory) provided a U. S. viewpoint on core modifications (Stevens, 2011), and I. T. Tetiyakov (NIKIET) provided a Russian viewpoint (Tetiyakov, 2011).

U. S. Viewpoint on Core Modifications

John Stevens

The conversion of a research reactor from HEU to LEU fuel can result in performance penalties in the reactor, primarily arising from the reduced density of uranium-235 and absorption of neutrons by uranium-238. Modi­fications to a reactor core may be required to overcome these penalties. Several core modification strategies have been used to overcome the penal­ties associated with the conversion of U. S.-origin research reactors; these include modifications to the following:

• fuel plate thickness and reflector locations;

• fuel meat thickness;

• uranium and burnable absorber loading; and

• fueled height of the core.

When making modifications to a reactor core one should strive to change as little as possible. Two particularly successful strategies for over­coming performance penalties that entail minimal changes are (1) tuning the burnable absorber to match the fuel composition; and (2) if cost is ac­ceptable, modifying reflector materials and/or geometries.

Of course, the fuel will, by definition, change from HEU to LEU during the conversion process, and the LEU fuel must be “acceptable” for conver­sion. An LEU fuel is considered to be acceptable for conversion when it meets the following criteria:

• Qualified: the fuel assembly has been successfully irradiation tested and is licensable.

• Commercially available: The fuel assembly is available from a com­mercial manufacturer.

• Suitable: The fuel assembly satisfies the criteria for LEU conver­sion of a specific reactor; safety criteria are satisfied; fuel service lifetime is comparable to current HEU fuel; and the performance of experiments is not significantly lower than for HEU fuel.

• The reactor operator and regulator agree to accept fuel assembly for conversion.

Successful conversion requires the involvement of reactor operators to un­derstand their needs and constraints.

The following examples were presented to illustrate some of the core modification options that are available to overcome conversion penalties. Some of the reactors described in these examples have already been con­verted, whereas others have not yet been converted.