When regulatory requirements for conversion became effective there were 26 HEU-fueled civilian research reactors in the United States under the regulatory authority of the USNRC. Most of these reactors were being operated by universities. The current conversion status of these reactors is shown below:

• Sixteen reactors were converted to LEU fuel, and five of those reac­tors were subsequently shut down after conversion.

• The licenses of four reactors were terminated before conversion.

• Decommissioning was approved for two reactors before conversion.

• No suitable fuel has been identified for one reactor (MITR).

• Unique purpose applications (described later) are pending for two reactors.

• Suitable fuel has been identified but no funding is available to con­vert one reactor (NTR General Electric).

The first group of reactor conversions (10 reactors) was completed in 2000. The second group of reactor conversions (6 reactors) began in 2006 and was completed in 2009. In 2007, the USNRC staff turned its attention to conversion of three of the four remaining HEU-fueled reactors that it licenses, which are high-performance reactors: MITR, MURR, and NBSR.[43]

The Commission issued a policy statement in 1982 that fully sup­ported the Reduced Enrichment for Research and Test Reactors (RERTR) program. Initially, many research reactor licensees resisted the call for con­version, informing the USNRC that they preferred instead to implement additional security measures at their facilities. The Commission members and staff engaged licensees through a number of outreach activities, and a Commission-sponsored LEU study group comprising licensed technical experts prepared a report on the technical feasibility of conversion.

The Commission also developed a conversion rule, which was promul­gated in Title 10, Section 50.64 of the Code of Federal Regulations (10 CFR 50.64, Limitations on the Use of Highly Enriched Uranium [HEU] in Domestic Non-power Reactors) in 1986. At about the same time this rule was issued, the Commission initiated steps to reduce the amount of unirra­diated HEU fuel that licensees were authorized to possess at their facilities. Licensees now minimize their onsite inventories.

The regulations in 10 CFR 50.64 prohibit new construction permits for HEU-fueled reactors unless those reactors have a “unique purpose.” It also prohibits acquisition of additional HEU fuel for current reactors if LEU fuel acceptable to the Commission is available, again unless the reactor has a unique purpose. The regulations also require reactor licensees to replace HEU fuel with LEU fuel acceptable to the Commission in accordance with an approved schedule. To be acceptable to the Commission, LEU fuel must (1) meet the operating requirements of the existing license, or (2) based on a safety review and approval by the USNRC, be used in a manner that protects public health and safety and promotes the common defense and security, and (3) limit to the maximum extent possible the use of HEU fuel.

The USNRC defines “unique purpose” as a project, program, or com­mercial activity that cannot be reasonably accomplished without HEU. This includes specific projects, programs, or commercial activities that significantly serve the U. S. national interest; reactor physics or reactor development; re­search based on HEU flux levels or spectra; or reactor cores of special design.

The Commission initially received four unique purpose applications from U. S. licensees. Two of these (for the MITR and the Cintichem Reac­tor[44]) were withdrawn, and the other two (for MURR and NBSR) have been pending for about 20 years. The Commission staff decided to defer decisions on these applications shortly after they were submitted; these deci­sions will continue to be deferred until a fuel acceptable to the Commission is developed for use in these reactors.

The timing of conversion depends on several factors: The availability of government funding; the availability of LEU fuel acceptable to the Commis­sion; the availability of shipping casks to remove HEU fuel from the facility after conversion[45]; and the level of reactor usage.

NUREG 1537 (Guidelines for Preparing and Reviewing Applications for the Licensing of Non-Power Reactors) contains guidance for licensees to submit conversion applications to the USNRC. The conversion application must include an update of the reactor’s Safety Analysis Report relating to issues that are impacted by conversion to LEU. Specific areas of focus in the application include the following:

• Reactor neutronics and thermal hydraulics: Codes and calculations that have been benchmarked against the HEU reactor should be used to analyze the LEU reactor. The licensee should show that margins of safety are maintained in the LEU reactor.

• Reactor accidents: The licensee should reanalyze the HEU Safety Analysis Report accidents using LEU fuel to determine the impacts from conversion. Particular concerns include changes in power per fuel element, fission product inventory, and reactivity. The licensee must also perform a review to determine whether conversion to LEU fuel introduces new acci­dent scenarios. Conversion should not have a significant impact on accident analysis results and normally should not introduce new accident scenarios.

The application also identifies all necessary changes to the license, facil­ity, and operating procedures arising from conversion. The application must be limited to conversion and cannot include other changes or upgrades. Those are handled through the normal license amendment process.

Once the USNRC reviews and accepts an application, it issues an en­forcement order directing the licensee to convert to LEU fuel and make any necessary changes to its license, facility, and procedures. By issuing enforce­ment orders, the USNRC assumes the burden for defending against any legal challenges that arise from conversion, thereby relieving the licensee from this responsibility.

Several lessons have been learned from the civilian research reactor conversions that have been carried out to date in the United States. First, updating the safety analyses and preparing the conversion application take time and effort and can result in the discovery of other technical issues. Sec­ond, the key to successful conversions is to develop an LEU reactor design that can be successfully analyzed and built. Finally, conversion has benefits beyond the elimination of HEU: Most notably, it can result in increased technical expertise among reactor staff and improved knowledge of reactor characteristics and operating behavior. Conversion also provides valuable training opportunities: At university reactor facilities, many students have been involved in the development of conversion analyses.