NISTIR 7352 NIST Liquid Hydrogen Cold Source P. Kopetka R. E. Williams J. M. Rowe NISTIR 7352 NIST Liquid Hydrogen Cold Source P. Kopetka R. E. Williams J. M. Rowe Materials Science and Engineering Laboratory National Institute of Standards and Technology Gaithersburg, MD 20899-8560 September 2006 U.S. Department of Commerce Carlos M. Gutierrez, Secretary Technology Administration Robert Cresanti, Under Secretary of Commerce for Technology National Institute of Standards and Technology William Jeffrey, Director Disclaimer Certain commercial entities, equipment, or materials may be identified in this document in order to describe an experimental procedure or concept adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology (NIST), nor is it intended to imply that the entities, materials, or equipment are necessarily the best available for the purpose. ABSTRACT Nearly two-thirds of the experiments performed at the NIST Center for Neutron Research (NCNR) utilize cold neutrons with wavelengths greater than 4 Angstroms. This report documents the development of the liquid hydrogen cold neutron source in the NIST research reactor. The source was designed to optimize the flux of cold neutrons transported to the scattering instruments in the guide hall. It was also designed to be passively safe, and operate simply and reliably. All hydrogen system components are surrounded with monitored helium containments to ensure that there are at least two barriers between the hydrogen and the atmosphere. Monte Carlo simulations were used to calculate the cold source performance and estimate the nuclear heat load at full reactor power. Thermal-hydraulic tests in a full-scale mockup at NIST Boulder confirmed that a naturally circulating thermosiphon driven by the 2 meter height of the condenser could easily supply the moderator vessel with liquid hydrogen while removing over 2000 watts. The cryostat assembly was designed to withstand any high pressure generated in a credible accident. It was fabricated to rigorous quality assurance standards, resulting in over 10 years of leak-free operation. Contents 1.0 Introduction 2.0 Design Basis 2.1 Requirements of 10 CFR 50.59 2.2 Extraction of Beams 2.3 Simple, Reliable Operation 2.4 Maximize Intensity of Neutrons Below 5 meV 3.0 Cold Neutron Performance and Nuclear Heat Load Calculations 3.1 NBSR Modeling with MCNP 3.2 Neutron Performance Calculations 3.3 Nuclear Heat Load Calculations 3.4 Evolution of the Liquid Hydrogen Sources 3.5 Source Performance 4.0 Thermal Hydraulic Design 4.1 Design Criteria 4.2 Initial Design 4.3 Thermal Hydraulic Tests at NIST Boulder 4.4 Other Components (Condenser, Ballast Tank, Hydride Storage) 4.5 Helium Refrigerator 5.0 Mechanical Design 5.1 Hydrogen Cryostat Assembly 5.2 Outer Plug Assembly 5.3 Cold Hydrogen Transfer Line Assembly 5.4 Hydrogen Condenser Assembly 5.5 Hydrogen Gas Management Assembly 5.6 Insulating Vacuum Pump Assembly 6.0 Instrumentation and Control 6.1 Programmable Logic Controller (PLC) 6.2 Control Program 6.3 User Interface 6.4 Rundowns and Trouble Alarms 7.0 Startup and Operation of the Liquid Hydrogen Cold Sources 7.1 Authorization 7.2 Startup and Operation of Unit 1 7.3 The Advanced Liquid Hydrogen Cold Source, Unit 2 7.4 Heat Load Measurements 8.0 Accident Analysis 8.1 Reactor Power Change 8.2 Refrigerator Failure 8.3 Sudden Loss of Electrical Power 8.4 Loss of Cooling Water Flow to Cryostat and Plug Assembly 8.5 Loss of Cryogenic Insulating Vacuum 8.6 Sudden Release of Liquid Hydrogen to Vacuum Space 8.7 Slow leak of Air into Vacuum Vessel 8.8 Hydrogen Release to Confinement Building 8.9 Stoichiometric Mixture of Hydrogen and Air in Moderator Chamber 8.10 Maximum Hypothetical Accident 9.0 Acknowledgements 9.1 References 9.2 Appendix 1.0 Introduction design, as discussed in NBSR-92 and NBSR-133, was a block of D2O ice, cooled to approximately 25 K The neutrons that are used for neutron scattering and by circulating helium gas. A water cooled lead and other research applications at research reactors are bismuth gamma ray shield was included to reduce those that have been slowed down from the range of heating of the ice. The source was installed in the MeV, at which they are created, to tens of meV by reactor in 1987. Its successful operation was a key to scattering in the moderator and reflector. In most the development of the NIST Cold Neutron Research reactors, the neutron energy spectrum is essentially Facility, a large new experimental area constructed to Maxwellian, with a characteristic temperature take advantage of the longer wavelength neutrons somewhat higher than the actual temperature of the produced by this source. The success of this facility, moderator (usually approximately room temperature). and the high national demand for cold neutrons, For such a spectrum, the mean neutron wavelength, led to a design effort to develop a more intense cold energy and speed are 1.8 Å, 25 meV and 2200 ms-1, neutron source, based on liquid hydrogen. The first values well suited to study the properties of condensed version of this source was installed in the NBSR in matter at the atomic scale, where interatomic spacings 1994, and resulted in a gain in cold neutron intensity are approximately 1 Å, and characteristic vibrational of a factor of seven. Further refinements of the design energies are of order meV. resulted in an additional gain of a factor of two for the unit installed in 2002. Over the past two-three decades, structures with characteristic lengths of 100 Å and correspondingly This report documents the design and analysis of the smaller vibrational energies, have become increasingly latest version of the NIST liquid hydrogen source. important for both science and technology. For example, polymers are being developed with totally new properties as substitutes for other more traditional metal materials for weight savings, nano- structures are being developed with novel properties for processing drug delivery plus other applications and biotechnology applications are increasing in importance. All of these structures have characteristic dimensions that are best matched by neutrons of longer wavelength, which also allow better resolution in energy to match the slower relaxation and vibration of the more massive objects. However, the fraction of neutrons with energies less than 5 meV in a normal moderator spectrum is less than 2 % of the total, which makes their use impractical, except in special cases. In order to address this problem, several groups around the world1 have developed systems that place cryogenically cooled moderators called “cold sources” in the reflector of neutron sources. These cold sources shift the spectrum down in energy to a temperature somewhat above the moderator temperature, providing large gains in intensity for low energy neutrons. The NIST research reactor, NBSR (National Bureau of Standards Reactor), was designed from the beginning with a provision to add a cryogenically cooled moderator system, or cold neutron source. The initial 9 2.0 Design Basis the heat (generated by the reactor) by boiling of the liquid and return of the vapor to the condenser. The The design basis for the liquid hydrogen cold source hydrogen-containing components of this system have is driven by the following requirements, listed in no moving parts, and the entire hydrogen inventory priority order: is contained in a closed system with no automatic or • The finished cold source must satisfy the safety requirements of remotely operated valves or rupture discs. Hydrogen 10 CFR 50.59. vapor returning from the moderator chamber is • The source must allow extraction of beams from all existing cold recondensed in a heat exchanger cooled by helium gas source beam ports (CT-E and CT-W), as well as from the insertion at 15 K, and then flows back down into the vessel port, with full guide illumination to 10 Å. under gravity. The hydrogen supply and return lines • Operation should be simple and reliable, with minimal impact on reactor operation. are sized to provide for two phase (liquid and vapor) • The intensity of neutrons with energies less than 5 meV should be flow in the return from the moderator chamber. This maximized. provides a very stable operating condition under Each of these requirements has several implications changing conditions of heating, as shown for a source for the design of the liquid hydrogen cold source, designed for the High Flux Reactor (HFR) at the 4 which will be addressed in turn. Institut Laue Langevin (ILL), and verified for the NIST geometry in a series of mockup tests5 carried 2.1 Requirements of 10 CFR 50.59 out at NIST-Boulder. A schematic view of this system is shown in Figure 2.1. At the time of cold source design and construction, the requirements of 10 CFR 50.59 were: Note that all hydrogen-containing components, or components that could conceivably contain hydrogen, • (a)(1) The holder of a license authorizing operation of a production facility may (i) make changes in the facility as are surrounded by helium, including the ballast tank described in the safety analysis report, (ii) make changes in the procedures as described in the safety analysis report, and (iii) ,IQUID(YDROGEN4HERMOSIPHON conduct tests or experiments not described in the safety analysis report, without prior Commission approval, unless the proposed change, test or experiment involves a change in the technical specifications incorporated in the license or an unreviewed safety question. (E/UT (E)N "ALLAST4ANK • (2) A proposed change, test, or experiment shall be deemed to involve an unreviewed safety question (i) if the probability of occurrence or the consequences of an accident or malfunction ,ITERS of equipment important to safety previously evaluated in the safety analysis report may be increased; or (ii) if a possibility for (YDROGEN an accident or malfunction of a different type than any evaluated #ONDENSER previously in the safety analysis report may be created; or (iii) 6ACUUM if the margin of safety as defined in the basis for any technical 0UMP specification is reduced.