Two presentations on the regulatory challenges of converting research reactors were given by Panel 2.2 speakers: Alexander Adams (U. S. Nuclear Regulatory Commission) provided a U. S. viewpoint (Adams, 2011), and V. S. Bezzubtsev provided a Russian viewpoint (Bezzubtsev, 2011).

U. S. Viewpoint on Regulatory Challenges

Alexander Adams

The mission of the U. S. Nuclear Regulatory Commission (USNRC) is to ensure that the commercial use of nuclear materials in the United States is conducted safely. The USNRC is responsible for regulating civilian research reactors, including research reactor fuels and conversions, but the agency does not regulate U. S. Department of Energy (DOE) reactors.

Regulation of Research Reactor Fuel

Research reactor fuel development is the responsibility of DOE under the GTRI program. The USNRC does not get involved directly in these fuel development activities, but it does have the responsibility for approving LEU fuels for use in USNRC-licensed reactors.

USNRC approval of new LEU fuels is based on information submitted by DOE, including:

• Results of LEU fuel development and testing.

• Information on LEU fuel fabrication.

• LEU fuel qualification reports.

The USNRC must conclude that an LEU fuel is suitable and acceptable for use before approving it for use in USNRC-licensed reactors. Once an LEU fuel is approved, licensees can reference the USNRC evaluation in their Safety Analysis Reports; licensees do not have to justify the generic aspects of an LEU fuel that has been approved by the USNRC. However, licensees are required to address any facility-specific issues related to use of that fuel.

To date, the USNRC has approved three LEU fuels for use in USNRC- licensed reactors:

• Uranium silicide (U3Si2) fuel;

• U-ZrHx fuel for TRIGA reactors; and

• Special Power Excursion Reactor Test (SPERT) fuel elements.

Richard A. Meserve (U. S. Chair) became the ninth president of the Carnegie Institution for Science in 2003. Dr. Meserve was the chairman of the U. S. Nuclear Regulatory Commission (USNRC) from October 1999 until March 2003. He is currently Senior of Counsel in the Washington, D. C. law firm of Covington & Burling, where he was a partner before joining the USNRC. He devoted his legal practice to technical issues arising in environmental and toxic tort litigation, counseling scientific societies and high-tech com­panies, and nuclear licensing. Dr. Meserve also served as an adviser to the President’s Science and Technology Advisor from 1977-1981, and as a law clerk to Justice Harry A. Blackmun of the United States Supreme Court and Judge Benjamin Kaplan of the Massachusetts Supreme Judicial Court. Among other affiliations, he is a member of the American Philosophical So­ciety and an elected fellow of the American Academy of Arts and Sciences, the American Association for the Advancement of Science (AAAS), and the American Physical Society. He has served as chairman or a member of numerous committees of the National Academies, including the Committee on Science, Technology and Law, the Board on Energy and Environmental Systems, the Board on Radioactive Waste Management, and the Nuclear and Radiation Studies Board. He also was chair of the Committee on Upgrading Russian Capabilities for Controlling Highly Enriched Uranium and Plutonium. He received his bachelor’s degree from Tufts University in 1966, a law degree from Harvard in 1975, and his Ph. D. degree in applied physics from Stanford in 1976. He was elected to the National Academy of Engineering in 2003.

Nikolay P. Laverov (Russian Chair) is vice president of the Russian Acad­emy of Sciences (RAS) and former director of the Institute of Geology of Ore Deposits, Petrology, Mineralogy, and Geochemistry. He has worked in and with the USSR and Russian governments on a range of ecological problems, particularly nuclear waste disposal, and has been a leader in radiogeological studies aimed at using the protective properties of the geo­logical environment to prevent pollution of the ecosphere by radionuclides. In addition to his research activities, Dr. Laverov has held a variety of prominent positions in scientific administration and government, including chief of the Scientific Research Organizations Administration of the USSR Ministry of Geology (1972-1983), pro-rector of the Academy of the Na­tional Economy (1983-1987), president of the Kyrgyzstan Academy of Sci­ences (1987-1989), and USSR deputy prime minister and chairman of the USSR State Committee for Science and Technology (1989-1991). In 1989, Dr. Laverov was elected vice president of the USSR Academy of Sciences, a post to which he was subsequently re-elected in the RAS. In 1992, he was named co-chair of the Earth Science Joint Working Group, which is under the auspices of the U. S.-Russian Space Agreement. He is also a member of the Council on Science and Technology under the President of the Rus­sian Federation. Dr. Laverov graduated from the M. I. Kalinin Nonferrous Metals and Gold Institute in Moscow in 1954 and earned a doctorate in geological-mineralogical sciences in 1958. A full member (academician) of the RAS since 1987, he has authored or co-authored more than 250 publi­cations including 20 books and has served as editor-in-chief of the journal Geology of Ore Deposits since 1989.

Vladmir Asmolov is first deputy director general-director for scientific-tech­nical policy of Energoatom Concern OJSC. Prior to this position, he served as deputy director general-director for science and engineering of FSUE Concern Rosenergoatom. He has also served as director of the Kurchatov Institute, and from 2003-2004, he served as deputy minister of atomic energy of the Russian Federation. In addition, Dr. Asmolov is currently serving as representative of the Russian Federation to the Organization for Economic Cooperation (OECD) Nuclear Energy Agency (NEA) Committee on the Safety of Nuclear Installations (CSNI); as a professor at Moscow Power Engineering Institute (Technical University); as chairman of the Sci­entific and Technical Panel of Rosatom (Federal Agency of Atomic Energy, formerly Minatom); as chairman of the scientific and technical panel of Concern Rosenergoatom; and as a member of the International Nuclear Safety Group (INSAG). He has received a certificate of appreciation from the U. S. Nuclear Regulatory Commission, the Order of Courage from the President of the Russian Federation, and the Order of Honour from the President of the Russian Federation. He received a master’s degree from the Moscow Power-Engineering Institute and a Ph. D. from the Kurchatov Institute.

David J. Diamond is chief scientist in the Nuclear Science and Technology Department at Brookhaven National Laboratory. He is also acting leader of the Nuclear Analysis Group. He has extensive experience in nuclear reactor safety, primarily through his work for the U. S. Nuclear Regulatory Commission (USNRC). He has also worked on safety issues with regula­tory bodies in more than a half dozen countries as well as the International Atomic Energy Agency. His technical contributions are through the applica­tion of neutronics and thermal-hydraulics models, and the combining of de­terministic and statistical analyses. The applications have been to problems in light and heavy water power and non-power reactors. For research and test reactors (RTRs) he has led a team providing support in reactor analysis and other disciplines for the research reactor at the National Institute of Standards and Technology (NIST) Center for Neutron Research. The team also provides support to the USNRC staff responsible for RTR licensing. An example of the latter work has been the review of the safety reports for conversion (HEU to LEU fuel) of the USNRC-licensed university reactors. Dr. Diamond has been asked to chair various international panels address­ing safety issues. Dr. Diamond received his Ph. D. from the Massachusetts Institute of Technology, and he is a fellow of the American Nuclear Society (ANS) and a recipient of the ANS’ Tommy Thompson Award recognizing contributions to nuclear installation safety.

Valentin B. Ivanov is chief research scientist at the RAS Institute of Ore Deposits, Petrography, Mineralogy, and Geochemistry. He graduated from the Samara Technical University with a degree in electrical engineering and received his doctorate of technical sciences in Moscow from Institute of Radiation Techniques in 1991. His sphere of professional interests includes the nuclear fuel cycle and spent nuclear fuel management. From 1963 to 1998, he worked at Research Institute of Atomic Reactors (RIAR), for the last nine of those years serving as its director general. From 1998 to 2002, he served as first deputy minister for atomic energy of the Russian Federa­tion. In 2003, he was elected to the Russian State Duma, where he served as a member of the parliamentary Committee on Energy, Transport, and Communication until 2008.

Boris F. Myasoedov is deputy secretary general for science of the Rus­sian Academy of Sciences (RAS), head of laboratories at both the RAS Vernadsky Institute of Geochemistry and Analytical Chemistry and the RAS Frumkin Institute of Physical Chemistry and Electrochemistry. His scientific activity covers such fields as the fundamental chemistry of actinides, fuel reprocessing, partitioning of radioactive waste, and environmental protec­tion. He has authored more than 500 publications and serves as editor of the journals Problems of Analytical Chemistry and Radiochemistry. Acade­mician Myasoedov graduated from D. I. Mendeleev Chemical-Technology Institute in Moscow in 1954 and earned a Ph. D. in radiochemistry from the Vernadsky Institute in 1965 and his full doctorate in 1975 from the same institute. He was elected to the Russian Academy of Sciences in 1994 and has been awarded two State Prizes for his research on the chemistry of transplutonium elements (1986 and 2001), the Khlopin Prize for his studies of the chemistry of protactinium (1974), and the Ipatiev Prize of the RAS Presidium in 2003.

James L. Snelgrove retired from Argonne National Laboratory (ANL) as senior physicist in February 2007. During his first 10 years, he worked in the areas of fast reactor critical experiments and test reactor analysis and design. He worked on the Reduced Enrichment for Research and Test Reactors (RERTR) program from its inception in 1978 until he retired, mainly in the areas of high-density fuel and Mo-99 target development and testing. He led the fuel development and testing effort from late 1981 until mid-2004 and coordinated the program’s collaboration on fuel develop­ment with the Russian RERTR program from 1996 until his retirement. From 2005 through 2008, he coordinated the International Atomic Energy Agency’s (IAEA) effort to produce a document on “Good Practices for Qualification of High Density LEU Research Reactor Fuels,” which was published as a Nuclear Energy Series document in 2009. Since late 2009, he has been coordinating preparation of another IAEA document on the properties of uranium molybdenum alloy research reactor fuels. Currently he works part time at ANL for the RERTR program as a senior advisor for research reactor fuels, and he occasionally consults with agencies and com­panies around the world in the area of research reactor fuel development and qualification. Dr. Snelgrove received his B. S. in physics from Tennessee Technological University in 1964 and his M. S. in physics in 1966 and Ph. D. in experimental nuclear physics in 1968 from Michigan State University.

Anatoly Zrodnikov is scientific leader of the State Scientific Center of the Russian Federation, the Institute for Physics and Power Engineering (IPPE) in Obninsk (since 2010), IPPE director general (1996-2010), and head of Department of National Research Nuclear University “Moscow Engineering & Physics Institute” (since 2005). He joined the IPPE in 1969 after graduation from the Moscow Power Engineering Institute (Techni­cal University) with an M. S. in applied physics, and he received his Ph. D. in nuclear engineering in 1975 and his D. Sc. in physics and mathematics in 1994. His scientific interests are in the areas of neutronics, thermal

and plasma physics, direct energy conversion, perturbation theory, nuclear power engineering including the space one, fast neutron reactors, and nuclear pumped lasers. Dr. Zrodnikov was the president of the Russian Nuclear Society in 2001-2003, a member of the Government Committee of the Russian Federation on Science and Innovation Policy (2003-2005), chairman of the Obninsk City Scientific and Technical Council (since 1996), and president of the Kaluga Regional Scientific Center. Also he is a mem­ber of editorial boards of scientific journals, including Laser and Particle Beams, Atomic Energy, and Nuclear Power Engineering, and he is author and co-author of more than 300 scientific publications. He holds the title of Honored Scientist of the Russian Federation and was awarded an Order of Honor of the Russian Federation, and Honorary Citizen of Kaluga region.

There were 34 civilian research reactors in operation in the United States in 2011 (Table 1-1)[21] As of June 2011, all but 8 of these reactors had been converted to LEU fuel. Two of these 8 reactors[22] appear to be convert­ible using current-type LEU fuels. DOE is completing studies to confirm the feasibility of converting these reactors using current-type LEU fuels. Additional research will be required to more fully develop the capability to fabricate these LEU fuels.

However, the following six reactors (including one critical assembly[23]) comprise what DOE refers to as “high-performance” reactors that pose many challenges for conversion, as discussed in more detail in Chapters 2 and 3:

• Advanced Test Reactor (ATR) at the Idaho National Laboratory

• The ATRC critical assembly associated with the ATR

• High-Flux Isotope Reactor (HFIR) at the Oak Ridge National Laboratory in Oak Ridge, Tennessee

• Massachusetts Institute of Technology Reactor (MITR) in Cambridge

• Missouri University Research Reactor (MURR) in Columbia

• National Bureau of Standards Reactor (NBSR) at the National Institute of Standards and Technology in Germantown, Maryland

New high-density LEU fuels are now under development to convert these reactors (Roglans, 2011a). These fuel development efforts are described in Chapter 2.

^Currently operating with LEU fuel.

a Armed Forces Radiobiology Research Institute.

b U. S. Geological Survey.

c Oak Ridge National Laboratory.

d National Institute of Standards and Technology.

There were 24 operating research reactors, 30 critical assemblies, and 12 subcritical assemblies in the Russian Federation in 2011 (Bezzubtsev, 2011; see Figure 2-10 in Chapter 2).[24] Basic information on currently operating Russian research reactors is given in Table 1-2. Several civil­ian reactors pose substantive technical challenges to conversion, such as reactors using fuel pins consisting of UO2 dispersed in a copper-beryllium matrix with stainless steel cladding designed to operate at significantly higher fuel temperatures than most research reactors.

Note: This table does not include critical assemblies. a Research Institute for Atomic Reactors. b Joint Institute for Nuclear Research.

c Dollezhal Scientific Research and Design Institute of Energy Technologies. d Moscow Engineering Physics Institute.

SOURCE: IAEA (2011).

The potential for a fission product release under accident conditions was analyzed for a maximum hypothetical accident consisting of cladding failure in the high-power fuel assembly (25 kW) after continuous full-power operation. The accident analysis was carried out with and without an intact water pool and operating ventilation system.[64] Reactivity insertion was also analyzed. Additionally, a loss-of-cooling accident was analyzed to determine the fuel temperature and radiation dose from the exposed core.

The accident analysis used Oak Ridge National Laboratory’s ORIGEN code to calculate the fission product inventory in case of accident. An analy­sis of release fractions used a Gaussian plume model, and radiation doses were calculated using MCNP5.

The accident analysis had an overall positive outcome. LEU conversion required no changes in response to any accident. The reactor remained within regulatory limits under all variations to the maximum hypothetical accident.

This analysis had another positive benefit: The university’s capabilities to analyze core accidents increased significantly; previously, only simple methods and models had been used to analyze such accidents. As a result, a more detailed understanding of the potential radiation dose was gained, including the time-dependent behavior and the spatial distribution of dose.

The Belgian BR2 reactor typically operates at 50-80 MW with a peak thermal flux of about 0.8-1.1 x 1015 n/cm2-s. The fuel consists of curved plates that are swaged together at stiffener joints to form six concentric tubes. The fuel meat is 93 percent enriched uranium containing integrated boron and samarium burnable poisons.

The reactor is planned to be converted using a 19.75 percent enriched UMo dispersion LEU fuel. However, integrating a burnable poison into these fuel plates will be difficult owing to the high-volume fraction of UMo in the dispersion. Consequently, the reactor operator plans to install cad­mium wires in the swage joints between the curved fuel plates to control reactivity, a technique that has been used successfully in some other conver­sions to silicide fuel.

Although Russia has successfully converted many foreign research re­actors from HEU fuel to LEU fuel, it has not historically had a domestic conversion program comparable to that of the United States. To date, no research reactors in the Russian Federation have been converted from HEU to LEU fuel. However, as noted in Chapter 1, in December 2010 the U. S. and Russian governments agreed to initiate feasibility studies to analyze the conversion potential of the following six Russian research reactors that are currently operating with HEU fuel:[72]

1. MIR. M1 (Research Institute of Atomic Reactors [RIAR], Dimitrovgrad);

2. IR-8 (Kurchatov Institute, Moscow);

3. OR (listed as OP-M in Table 1-2 in Chapter 1) (Kurchatov Insti­tute, Moscow);

4. ARGUS (Kurchatov Institute, Moscow);

5. IRT (Moscow Engineering Physics Institute [MEPhI], Moscow);

and

6. IRT-T (Tomsk Polytechnical Institute, Tomsk).

The feasibility studies for these reactors are planned to be completed at the end of 2011.

During the symposium, significant concern was expressed by many members of the Russian delegation regarding the possibility of performance degradation accompanying conversion from HEU to LEU cores. Many members of the U. S. delegation were significantly more optimistic that good design of the replacement LEU core could eliminate concerns about performance degradation. This difference in view may be attributable to the considerably greater U. S. experience with research reactor conversions (see Chapter 2).

The current missions and currently assessed conversion potentials of five of the six reactors listed above were described by Russian presenters during the symposium:

• V. A. Starkov (RIAR) discussed the conversion potential of MIR. M1 (Starkov, 2011).

• V. A. Pavshuk (Kurchatov Institute) discussed the conversion poten­tial of Argus (Pavshuk, 2011).

• V. A. Nasonov (Kurchatov Institute) discussed the conversion po­tential of IR-8 (Nasonov, 2011).

• Yu. A. Tzibulnikov (Tomsk Polytechnic Institute) discussed the con­version potential of IRT-T (Tzibulnikov, 2011).

• E. A. Kryuchkov (MEPhI) discussed the conversion potential of IRT (Kryuchkov, 2011).

Because the feasibility studies of these reactors were at an earlier stage of development than the U. S. studies when the symposium was held, less detail is provided in presentation summaries than was given for the U. S. reactor conversions.

MIR. M1

V. A. Starkov

The MIR. M1 reactor is a 100 MW pool-type research reactor located at RIAR in Dimitrovgrad. It has a maximum thermal neutron flux at the experimental positions of 5 x 1014 n/cm2-s. Its primary mission is to test experimental fuel assemblies and fuel rods under normal, abnormal, and accident conditions.

The core and beryllium reflector blocks are stacked in a hexagonal grid comprising 127 hexagonal blocks 148.5 mm in size, installed at a pitch of 150 mm (see Figure 3-7). Four central rows of beryllium blocks operate as

• • Conirol and safety rod

:Qi Combined hanger consisting of the operating fuel assembly and absorber

a moderator, and two external rows of beryllium blocks act as a neutron reflector. The core also contains 11 loop channels where experiments are placed. Each experimental channel is surrounded by six fuel assemblies to maximally isolate each experiment from neighboring experiments. Each fuel assembly consists of four cylindrical fuel tubes arranged concentrically (see Figure 3-8). Absorbing rods are located along the edges of the blocks. For every channel there are two to three such absorbers for a total of about 30. This core design is very flexible and allows for the simultaneous irradia­tion of multiple experiments in different power regimes.

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.

Robert A. Bari is a senior physicist at the U. S. Department of Energy’s Brookhaven National Laboratory. He has been involved in the design and safety assessments of complex, high-technology facilities since he joined the applied programs at the Laboratory in 1974. He has worked on projects and issues regarding nuclear safety and nonproliferation technologies, nuclear waste management, development of advanced nuclear reactors, and other related technologies. During the 1980s, at the request of the

U. S. Nuclear Regulatory Commission (USNRC), Dr. Bari created and led a team of experts in the area of probabilistic risk assessment (PRA). This team expanded PRA methodologies in areas of importance to safety of nuclear power plants. In addition to his work for the USNRC, Dr. Bari led a four-laboratory team in a year-long evaluation of the impact of fuel enrichment on the performance of the Advanced Neutron Source, formerly planned for operation at Oak Ridge National Laboratory. His current research involves energy resources, national security, and reliability of the national electrical grid. Dr. Bari has lectured internationally on risk as­sessment and nuclear safety and has authored more than 100 papers and key reports in these areas. Dr. Bari earned an A. B. in physics in 1965 from Rutgers University and a Ph. D. from Brandeis University in 1970.

The symposium featured a range of briefings from R. F., U. S., and in­ternational experts on policy, science, and engineering issues relevant to the conversion of research reactors from HEU fuel to LEU fuel. These briefings were organized into several sessions, reflected in the four chapters of this report: [25]

• Chapter 3 addresses the challenges and successes associated with converting eight specific U. S. and Russian reactors; and

• Chapter 4 addresses future research directions and opportunities, including opportunities for further interaction between the U. S. and Russia on research reactor conversion.

Although the overall conversion experience was positive, the converted reactor core behaved somewhat differently than the calculated core. In particular, the converted reactor was substantially less reactive than was calculated. The reason for this difference is still not fully understood. In the near term, the UWNR staff is pursuing a plan to shuffle the core and reduce the number of reflectors. This will cause a slight reduction of the neutron flux in some positions; however, the reshuffling should increase the flux in other positions. This reshuffling would take the core from the “X” configuration of 21 fuel assemblies and 14 reflectors to a “+” configuration of 21 fuel assemblies and 6 reflectors (see Figure 3-2).

Overall, the conversion-related work enabled a widespread upgrade in UWNR staff’s analysis capabilities, and it has also provided opportunities for further analysis. For example, experimental research is ongoing to better understand natural circulation heat transfer in TRIGA-relevant conditions, and the fresh LEU core provides a wide variety of benchmark data for continuing to improve analytical capabilities.