CHAPTER 01 final.docx

cHAPTER 1 – tABLE OF CONTENTS

Chapter 1 – Record of Revisions

1 NIST Center for Neutron Research & NBSR

1.1 Introduction

The NIST Center for Neutron Research (NCNR) is a reactor-laboratory complex providing the National Institute of Standards and Technology (NIST) and the nation with a world-class facility for the performance of neutron-based research. The heart of this facility is the National Bureau of Standards Reactor (NBSR). The facility is located on the 575 acre NIST campus in upper Montgomery County, Maryland, approximately twenty miles northwest of the District of Columbia. NIST is a federal agency of the U.S. Commerce Department.

The NCNR is a national resource used by nearly 2000 engineers and scientists for some part of their research every year. The major research areas include materials science, non-destructive evaluation, chemistry, biology, trace analysis, neutron standards and dosimetry, nuclear physics, and quantum metrology. A large cold neutron source and eleven neutron guides provide the United States with world-class capabilities in cold-neutron research. Up to 28 cold and thermal neutron instruments provide internationally competitive neutron scattering capabilities. The operating schedule as described in section 1.5 allows for the operation of a robust user program, which solicits proposals for experiments twice a year. The NBSR is the key to the success of the NCNR, which is critical to national goals, and is at the forefront internationally. Further information may be found in the NCNR annual report.

The NBSR is a tank type reactor designed to operate at 20 MW of power. It is a custom designed variation of the Argonne CP-5 class reactor; it differs from the CP-5 in its power rating, core configuration and cold neutron source, but retains the proven technology. The three most notable modifications to this basic design are: a 7-inch (18-cm) gap between the upper and lower fuel regions in each fuel element to reduce the fast neutron background in the neutron beams; a double plenum at the bottom of the vessel to provide optimized cooling to the core; and the method for remote handling of fuel elements during refueling.

The design of the NBSR includes many inherent, passive safety features. The prompt neutron lifetime is relatively long as a result of heavy water moderation. The reactivity coefficients of void and temperature are negative. The reactor operates in a low temperature, unpressurized condition and has no large stored energy content. Two inner structures within the reactor vessel retain heavy water in the event of a loss of water from the vessel. In the event of a loss of cooling water, one of these structures immediately supplies emergency coolant flow to the elements without any operator intervention, while the other maintains water around the lower half of the core. An overhead reserve tank can supply heavy water for emergency cooling either to the top or to the bottom of the elements for extended periods of time.

1.2 Summary and Conclusions on Principal Safety Considerations

This section summarizes safety criteria, the principal safety considerations and the resulting conclusions of the Safety Analysis Report (SAR). Detailed information is given in the subsequent chapters of this report.

1.2.1 Consequences from the Operation and Use of the NBSR

The principal conclusion of the safety analysis contained in this report is that the continued operation and use of the NBSR will result in considerable benefit without any significant cost to the public health or to the environment. Specific conclusions are:

a. Continued operation and use of the NBSR will provide significant national benefits in research and education

b. Continued operation and use of the NBSR will be conducted without endangering the health and safety of the public. No significant environmental impact will result.

c. The purpose of the NBSR is within the scope of Section 104c, Research and Development, of the Atomic Energy Act of 1954, as amended. It will continue to be operated in conformity with this Act.

The basis for these conclusions is the detailed review of the NBSR design and operational safety that is documented in this SAR and associated NIST documents referenced in the SAR. Areas of review include site characteristics, design bases for facility structures, reactor core and the coolant systems, engineered safety features, instrumentation and control systems, electrical power and auxiliary support systems, and experimental facilities. Administrative elements of design and operation such as radiological safety programs, operating procedures, accident scenarios, technical specifications, and financial qualifications were also reviewed as part of the process that led to the above conclusions.

1.2.2 Safety Considerations on Choice of Site, Type of Reactor and Fuel, and Power

The NBSR initiated neutron research in 1969. The genesis of the project a decade earlier was the recognition that a reactor at the National Bureau of Standards (NBS) would serve the standards and measurements mission of the Bureau, and would act as a regional and national resource to serve other U.S. government agencies, universities and industry. The mission and the measurement needs of this multidisciplinary community encompassed a varied range of interests in materials, chemical analysis, radiation standards, and other areas. The development of the cold neutron source and the construction of the Cold Neutron Guide Hall provided the United States with world-class capabilities in cold-neutron research.

Since almost all of the work at NCNR initially involved the use of thermal neutrons, the reactor type chosen was one that generates a well-thermalized neutron spectrum. The need for thermal neutrons, combined with the requirement that the flux be competitive at a reasonable power, dictated the choice of an enriched fuel, and heavy water moderated and cooled reactor. It consists of an aluminum vessel filled with heavy water, which also contains the core of enriched plate-type fuel elements.

The design of the reactor core includes three unique features. The first is a “split-core,” with uranium fuel placed above and below the mid-plane in the heavy-water moderator tank leaving a 7-inch (18-cm) gap in which the thermal neutron flux reaches a peak and other radiation emanating from fission processes is reduced. The insertion of nine large 5-inch to 6-inch (13-cm to 16-cm) radial beam tubes into this gap allows high intensity beams, with low background noise from unwanted fast neutrons and gamma rays, to be extracted for thermal neutron scattering research. A second feature is the large volume in the core that provides very flexible capabilities for thermal neutron irradiation. Finally, the insertion of the cold neutron source provided the first internationally competitive facility for cold neutron research on materials in the nation (Cappelletti, 2001).

1.2.3 Inherent Safety Features

There are no unusual safety problems connected with the NBSR, either by virtue of its basic type or by virtue of its particular design. The basic nuclear reaction in heavy water is slow; that is, the prompt neutron lifetime is relatively long and reactivity coefficients of temperature and void are negative. The reactor operates in a low temperature unpressurized condition and has no large stored energy content.

The NBSR design includes a number of inherent (or passive) safety features:

a. The reactor core is designed so that the temperature coefficient of reactivity is negative. This mitigates the consequence of any reactivity excursion and also promotes self-regulation of the reactor.

b. The reactor core is designed so that the void coefficient of reactivity is negative. This also mitigates the consequence of any reactivity excursion.

c. There is a passive gravity drain of approximately 800 gallons (3,000 liters) of D2O from the holdup volume of the Inner Reserve Tank (IRT) within the Reactor Vessel into the reactor core.

1.2.4 Design Features for Safe Operation and Shutdown

The NBSR license and the technical specifications define the limits of safe reactor operation and, where applicable, the limits of facility operation. The design features and design bases of the systems and components associated with the NBSR that promote safe operation and shutdown include:

a. Reactor Control System: Four shim safety arms and one regulating rod control the reactor. The shim safety arms are of the semaphore type and use cadmium as the poison. They are located just below the upper grid plate and provide intermittent, coarse control of reactor power. The regulating rod consists of a solid aluminum cylinder and provides continuous fine control of reactor power.

b. Reactor Safety System (RSS): The reactor safety system consists of the nuclear instrumentation and the process instrumentation. The RSS monitors parameters that are important to safety including reactor power and period, coolant flow and vessel level. Actuation of the RSS generates a reactor scram, driving the shim arms in and making reactor subcritical. In emergencies that have a defined release of radioactive material, actuation of the system generates a major scram, initiating closure of the Confinement Building in addition to generating a reactor scram signal.

c. Emergency Shutdown: The primary emergency shutdown mechanism is the shim safety arm system. This is backed up by the moderator dump, which quickly drains moderator from the reactor vessel volume above the core and makes the reactor subcritical.

d. Emergency Cooling: This system provides cooling for the reactor core in the event of a loss of normal coolant through a pipe rupture. The system consists of a D2O emergency cooling tank located external to the reactor vessel and a passive inner emergency cooling tank. Upon a rupture, cooling water drains into the core to remove decay heat. Domestic light water can be added to the emergency cooling system if the drained heavy water cannot be pumped back to the reactor vessel.

e. Confinement Building: The Confinement Building houses the reactor and its primary systems and components. It is designed as a robust structure for containing or mitigating conditions that could arise from an event that affects the safety of the reactor. If an event caused the release of radioactive contaminants to the building atmosphere, the building design would limit leakage of the building atmosphere, under the worse hypothetical conditions, to a rate that would result in a radiation dose to the public of between zero and the regulatory limits for the public. As an additional preventative measure, the building is normally maintained at a small negative pressure relative to the outside of the building, thus eliminating routine air leakage out of the building.

f. Ventilation System: All effluent air that is exhausted from the confinement building is monitored for radioactivity. In the event that high radiation levels are detected, the normal ventilation system is shutdown, all building closure devices operate to seal the building and the emergency ventilation system is activated. The emergency exhaust system is designed to draw air at such a rate from the building that a pressure differential is established across the building structure to assure that any leakage of air is into the building rather than out of it regardless of the outside pressure. All air exhausted is filtered to remove particulate and gaseous effluent such as iodine. All recirculated air is also filtered to remove particulate and gaseous activity.

g. Neutron Lifetime: The D2O moderated NBSR reactor has a relatively long neutron lifetime. This provides a distinct advantage from the point of view of reactor control and safety. A long lifetime not only simplifies reactor control but leads to greater reactor periods for specific reactivity insertions. For a given reactivity insertion that is greater than the delayed neutron fraction, more time is available for heat transfer from the fuel elements and for bubble formation within the moderator.

h. Fuel Handling System: The NBSR fuel handling system provides for defueling, refueling, and re-arranging the fuel entirely within the shielded area around the reactor or under water, which adds a layer of protection in the event of a mishap with irradiated fuel.

1.2.5 Potential Accidents

The maximum hypothetical accident (MHA) for the NBSR is one in which an object blocks all of the flow through a single fuel element while the reactor is operating at full power. This is highly unlikely because the NBSR is a closed system with upward flow. Nevertheless, if the flow in an element is blocked during full power operation, it is possible that some melting of the cladding would occur with a resultant release of some fission products into the primary water. To be conservative, however, it is assumed that the entire blocked element cladding melts and releases fission products into the primary water. Analysis of this accident is given in Chapter 13 of this report and the analysis concludes that the public is not at risk from the MHA.

The MHA bounds other reactor accidents that are evaluated in Chapter 13 of this SAR. Therefore, the consequences of the other accidents do not place the public at risk.

1.3 Shared Facilities and Equipment

The Confinement Building, which is constructed of reinforced concrete and situated partially below grade, is attached to a laboratory complex dedicated primarily to nuclear-science-related research and other reactor support functions. Utilities such as municipal water and sewage, natural gas, and electricity are provided to the complex by the local utilities.

1.4 Comparison with Similar Facilities

The NBSR has a large number of experimental beam lines. This type of reactor (using materials-testing-reactor (MTR) type plate fuel and heavy water cooled) is also being used at the Massachusetts Institute of Technology (MIT), and was used at Brookhaven National Laboratory and at the Savannah River Plant, before they ceased operation. Heavy water (D2O) is used to obtain high flux values not otherwise achievable with that physical size of reactor. The design is a variation of the Argonne CP-5 class reactor.

1.5 Summary of Operations

Through 2012, the NBSR had accumulated in excess of 3.4 million MW-hours of operation. The highly automated experimental facilities associated with the reactor allow it to be operated and utilized 24 hours a day, 7 days a week. Routine shutdowns are scheduled about every five-and-one-half weeks for partial refueling with an additional shutdown at the end of each calendar year. The reactor is normally on line approximately 250 days per year, and is fully utilized for experiments during that time.

1.6 Compliance with the Nuclear Waste Policy Act of 1982

The U.S. Atomic Energy Commission (AEC) originally provided fuel for the operation of the NBSR. The U.S. Department of Energy (DOE) is the successor agency to the AEC. The DOE retains title to the uranium fuel used by NBSR. In a letter dated May 3, 1983, the DOE (R. L. Morgan) informed the NRC (H. Denton) of the title arrangement for the fuel. Specifically, the NBSR and the DOE have a contractual arrangement whereby DOE retains title to the fuel and is obligated to take the spent fuel and/or high-level waste for storage or reprocessing. All of the spent NBSR fuel has been returned to the DOE pursuant to this arrangement. A copy of the contract between NBSR and the DOE for fuel assistance is available.

1.7 Facility Modifications and History

The initial planning for the facility began in March 1958 when an application for a construction permit (CP) was submitted. Construction began in 1963 when the Atomic Energy Commission (AEC) issued the CP. This permit was converted to Operating License TR-5. The reactor achieved initial criticality on December 7, 1967 and began full-power operation at 10 MW on February 9, 1969. On December 2, 1980, the NBSR requested a power increase from 10 MW to 20 MW and a 20-year extension of the existing operating license was issued on May 16, 1984. Another license renewal for 20 years of operation was granted on July 2, 2009.

The principal nuclear and process systems were originally designed and installed for eventual 20 MW operations. All significant modifications made since 1984 are described in this report.

1.9 References

Cappelletti, R.L., et al, "Materials Research With Neutrons at NIST," J. Res. Natl. Inst. Stand. Technol. 106 (2001) 127-230.

Office of Science and Technology Policy (OSTP) (June, 2002). Report on the Status and Needs of Major neutron Scattering Facilities and Instruments in the United States.

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