Two presentations discussing performance and missions of reactors after conversion were given by Panel 2.1 speakers: Jordi Roglans (Argonne National Laboratory) provided a U. S. viewpoint on maintaining perfor­mance and missions (Roglans, 2011), and A. L. Petelin (Research Institute of Atomic Reactors [RIAR]) provided a description of several Russian research reactors at RIAR and their missions (Svyatkin et al., 2011).

U. S. Viewpoint on Maintaining Performance and Missions

Jordi Roglans

The Global Threat Reduction Initiative (GTRI) strives to achieve sev­eral goals when converting research reactors: [36]

2
2 3
2
4
5
1-6 are steps of fuel burnup. R5,R6 are safety rods.
R1-R4,R7,R8 are regulating rods. AR is automatic regulating rod.
FIGURE 2-6 Schematic illustration of the WWR-M reactor core. SOURCE: Tetiyakov (2011).

As noted in John Stevens’ presentation (summarized elsewhere in this chapter), a fuel assembly is considered to be acceptable for use in a conver­sion project when it meets the following criteria and the reactor operator and regulator agree to accept fuel assembly for conversion:

• 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.

When converting from an HEU to LEU fuel, one should strive to make as few changes as possible in the fuel assembly and core geometries. Con­version should also be carried out in a way that has the least possible effect on scientific operations in the facility.

The annual operating costs of a reactor will be affected by the costs of the LEU fuel assemblies compared to the HEU fuel assemblies they are replacing. The new very-high-density UMo fuels will likely cost more to fabricate because there are more manufacturing steps. However, work is under way to minimize those cost differences with the goal of maintaining or even reducing when feasible the number of LEU fuel assemblies that are consumed in a reactor each year compared to HEU fuel assemblies.[37] The number of fuel assemblies consumed per year dominates costs when LEU and HEU fuel assemblies are of similar cost.

Analytical studies are typically needed to determine whether conversion can be accomplished without a significant impact on reactor performance and missions. However, such formal studies may not be required for HEU — fueled reactors that are of a similar type and performance to reactors that have already been converted to LEU.

The analytical studies needed to assess the potential for conversion include:

• Feasibility studies that identify suitable LEU fuel assemblies (either existing qualified fuels or new fuels under development), compare reactor performance with HEU and LEU fuels, and calculate key safety parameters.

• Operational and safety analyses to demonstrate that the transition from HEU to LEU fuel can be done safely and without interrupting normal reactor operations, and also that the converted reactor satisfies all safety requirements.

One also needs to formulate safety requirements and resolve any issues raised by regulators regarding the reactor’s safety documentation. Addi­tionally, economic impact studies may be required to determine the overall impact and acceptability of conversion.

A feasibility study entails many activities. Initially, fuel requirements and experimental performance indicators must be defined. With respect to the latter, it is important to determine what the most important experi­mental positions are in the reactor and what performance characteristics (e. g., flux densities and neutron energy distributions) are required in those positions. Iterative modeling studies are used to determine these character­istics as well as other operating criteria such as shutdown margins. Fuel assembly and reactor core designs are adjusted, and the models are rerun until acceptable performance and other important reactor characteristics are achieved. The final LEU fuel assembly design can be selected once these studies are completed.

Some high-performance reactors may require fuel-design optimization and possibly facility-specific mitigation measures to address any perfor­mance penalties arising from conversion. For U. S. high-performance reac­tors, the anticipated unmitigated decreases in performance resulting from conversion do not preclude any current applications but could affect appli­cation throughputs. The high demand for these reactors is already limiting scientific output and isotope production. Consequently, several mitigation strategies are being pursued to avoid throughput penalties.

For the U. S. high-performance reactors, the following mitigation strate­gies are being pursued:

• HFIR: The anticipated performance penalty of 10-15 percent will be mitigated by increasing reactor power from 85 MW to 100 MW. This could result in small gains in performance.

• MITR: The anticipated performance penalty of 5-10 percent will be mitigated by increasing reactor power from 6 MW to 7 MW.

• MURR: The anticipated performance penalty of 15 percent will be mitigated by changing LEU plate thickness (see the presentation by John Stevens elsewhere in this chapter) and by increasing reactor power from 10 MW to 12 MW.

• NBSR: The anticipated performance penalty of 10 percent will be mitigated by upgrading the cold neutron source.

Power increases in HFIR, MITR, and MURR are possible because their existing cooling systems are adequate to handle the increased heat loads. As a result of these mitigation strategies, no current applications are expected to be precluded by conversion. In ATR, preliminary studies indicate that there could be a 5-10 percent performance penalty after conversion. A strategy to mitigate this penalty has not yet been identified.

The key to successful conversion is collaboration. In the case of high — performance reactors or reactors with unique designs, iterative collabora­tions among facility operators, fuel designers, and conversion analysts are essential to optimize fuel and core design and minimize performance impacts.