When converting a research reactor from HEU to LEU fuel it is impor­tant to avoid degradation of the following:

• Consumer characteristics: neutron flux level, thermal power, neu­tron spectrum, and adequacy of safety systems.

• Safety characteristics: reactivity margins, effectiveness of control rods, and peak power density.

• Performance characteristics: fuel cycle duration, number of planned reactor shutdowns, and reactor serviceability.

• Technical and economic indices: mass of uranium loading, volume of spent fuel to be reprocessed, and financial expenditure for fuel purchase and reprocessing of spent fuel.

There are two potential paths for converting a research reactor while maintaining these characteristics. One path is to design a new core that can fit into the existing reactor. The other path is to maintain the geometric configuration of the current core but change the design and arrangement of fuel and/or reflector elements.

Conversion to LEU fuel may result in decreased uranium-235 content and will result in increased uranium-238 content in the reactor core. This can change the neutronic characteristics of the core, which in turn can change its reactivity, the effectiveness of control rods, and the dynamics of fuel burnup. All of these changes can affect reactor safety. Consequently, safety analyses must be carried out to demonstrate that conversion will preserve reactor safety at required levels, including neutron-physical analy­sis, thermal-hydraulic analysis, and an analysis of transient and emergency operations.

As illustrated by the following three examples, for some Russian re­search reactors there are no developed LEU fuel elements that would en­able conversion with acceptable consumer characteristics. Moreover, some Russian research reactors are approaching the ends of their operating lives, and there is a need to consider whether to shut down these reactors or modernize them.

IRT (Moscow Engineering and Physics Institute)

The IRT is a medium-flux, 2.5 MW pool-type reactor with a square core containing 16 IRT-3M fuel elements enriched in uranium-235 to 90 percent. Initial studies have been carried out to examine the feasibility of converting this reactor to 19.75 percent enriched fuel elements of an IRT — 4M design containing a UO2-aluminum dispersion fuel meat.

These studies indicate that conversion would result in some consumer and economic penalties compared to HEU fuel: neutron flux densities in the fuel and reflector regions would decrease by 20-30 percent and 10-20 percent, respectively, and the number of fuel elements in the core would increase by 2-4 elements.[34] The economics of conversion will depend on the cost of LEU fuel elements and their reprocessing compared to the costs for HEU fuel elements.[35] However, there would be no unacceptable changes in safety characteristics, and fuel burnups would not change.