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All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Space Nuclear Power Autonomous Control Algorithm and Control Element Test Bed

Nuclear thermal rockets are currently NASA’s preferred option for use in a manned mission to Mars in the 2040s. The communication delay between an Earth ground station and a spacecraft heading toward Mars can be up to 20 min. Therefore, controlling the nuclear rocket engine would require either a full-time reactor operator on the mission or an autonomous control system for the reactor. The latter idea of making space nuclear reactors fully autonomous has drawn more interest from stakeholders, but such an autonomous control system must be rigorously tested and validated before it is certified for human use. The cost of a full ground test for a space nuclear reactor is tremendous, so a nonnuclear mock reactor test bed was created to test and validate control elements and control algorithms for space nuclear reactors. The test bed consists of control element hardware that inputs physical measurement data into a reactor emulator to produce the reactor’s performance under steady-state, transient, and fault conditions. The control element hardware consists of six full-sized control drums equipped with servo drives and motors and is instrumented with optical encoders, resolvers, and torque sensors for drum movement characterization. In addition to the drums, a two-phase flow loop was designed and built to mimic the valves and turbomachinery associated with the propellant flow through a nuclear thermal rocket engine; components such as pressure sensors, flow meters, thermocouples, and tachometers are instrumented throughout the loop to characterize the fluid flow, valve, and turbomachinery behavior of the system. The data from the physical hardware (e.g., drum position, propellant flow rates) are input to a nuclear reactor simulator to determine the actual nuclear reactor parameters, and the data are sent back to a control algorithm to complete the control loop. The ability to conduct numerous tests of the control systems and autonomous algorithms can help validate the instrumentation and control aspects for a space nuclear reactor for every possible fault situation.

Wilson, Brandon↗
DOE OSTI2016

Depletion Analysis of Additively Manufactured Control Elements in HFIR

The High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory is a flux-trap type research reactor which provides one of the highest steady-state neutron sources in the world. HFIR neutrons are used for a range of research applications, including scattering experiments, materials irradiation, and isotope production. The HFIR core operates at 85 MW and is light water cooled and moderated. The core consists of several concentric cylindrical regions, as depicted in Fig. 1. At the center of the core is the flux trap, where the thermal flux reaches a maximum on the order of 10{sup 15} neutrons/cm{sup 2}-s. Immediately surrounding the flux trap are two annular fuel elements, an inner fuel element and an outer fuel element, each loaded with hundreds of thin fuel plates curved into an involute shape. Surrounding the outer fuel element are two thin control elements (CEs), an inner CE (ICE) and an outer CE (OCE). The CEs are axially zoned into three regions according to absorption strength: the white regions contain only structural aluminum and do not significantly contribute to absorption; the gray regions contain tantalum (Ta) as the primary absorber with a concentration of 30% by volume; and the black regions contain europium in the form of Eu{sub 2}O{sub 3} with a concentration of 33% by volume. A thick beryllium reflector is the final core component surrounding the CEs. A particular challenge to efficient maintenance and continued operation of HFIR is fabrication of the CEs, the current methods for which are tedious, outdated, and expensive. In an effort to improve the operational economics of HFIR and to demonstrate the application of advanced manufacturing techniques to the production of nuclear reactor components, the ultrasonic additive manufacturing (UAM) process is being investigated for fabrication of the HFIR CEs. This application of UAM involves directly embedding small cylindrical compacts (∼5.6 mm diameter, ∼1.7 mm thick) containing 60% neutron poison by volume into an aluminum matrix, using energy from ultrasonic vibrations to bond the compacts to the aluminum. The CEs contain discrete regions of absorbing material, in contrast to the standard design with homogeneously distributed absorbers. This novel design necessitates an investigation into how the HFIR core neutronic behavior differs from the case with homogeneous CEs so as to ascertain the feasibility of using additively manufactured CEs in HFIR. From an operations perspective, it is desirable for the discrete CEs to match the performance of the homogeneous CEs as closely as possible so as to avoid reconsideration of established safety margins. Earlier analyses of HFIR under beginning of cycle (BOC) conditions have indicated that although absorption properties on a local scale differ significantly between CE designs, the macroscopic behavior (neutron multiplication and flux and power distributions) of the HFIR core is not significantly altered. This study now addresses CE performance throughout depletion of the HFIR core during a reactor cycle. These analyses help to construct a more complete picture of the feasibility of employing additively manufactured CEs in HFIR and will reveal any adjustments which must be made to the design in order to maintain acceptable performance. (authors)

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2016

A Comparison of Modeling Strategies for Additively Manufactured HFIR Control Elements

The High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory is a flux-trap type research reactor that is used for a range of research applications, including neutron scattering experiments, materials irradiation tests, and isotope production. The HFIR core comprises several concentric cylindrical regions; starting from the core centerline and moving radially outwards, these are the flux trap target (FTT) region, the inner and outer fuel elements (IFE and OFE), the inner and outer control elements (ICE and OCE), and the reflector region. The control elements (CEs) are axially zoned into regions of differing absorption strength: the white regions contain only structural aluminum and do not significantly contribute to reactivity control; the gray regions contain tantalum as the primary absorber with a concentration of 38% by volume, and the black regions contain europia (Eu{sub 2}O{sub 3}) as the primary absorber with a concentration of 31% by volume. During operation, the CEs are withdrawn symmetrically in opposite directions from the core to remove the neutron absorbers from the core and maintain criticality. The current approach to fabrication of the HFIR CEs is tedious, outdated, and expensive. In an effort to improve the operational economics of HFIR and to demonstrate the feasibility of advanced manufacturing techniques for production of nuclear reactor components, novel methods for production of neutron-poison bearing aluminum structures have been initiated at ORNL. For this application, the ultrasonic additive manufacturing (UAM) process involves the use of 3D printing technology to construct small cylindrical compacts (0.559 cm in diameter and 0.171 cm in height) of aluminum mixed with neutron-absorbing material. The compacts contain 60% poison (Ta or Eu{sub 2}O{sub 3}) by volume. These compacts are then embedded into an aluminum matrix before formation into the cylindrical shape appropriate for use in HFIR. The CEs produced in this process therefore contain discrete regions of absorbing material, in contrast to the traditional design with homogeneously distributed absorbers. It is therefore of interest to investigate how the HFIR core behavior with discrete CEs differs from the case with homogeneous CEs with respect to some key reactor physics parameters. The primary objective of this study is to compare modeling strategies for the discrete CE design and examine differences in the core physics between the discrete and homogeneous CE designs as predicted by each model. Lessons learned from this investigation may then be used to inform the design process in future endeavors. (authors)

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2016

Additive Manufacturing of Research Reactor Control Elements and Subsequent Neutron Irradiation

Additive manufacturing techniques have made major strides in recent years and are now capable of creating complex components in near-final form. The nuclear industry has many applications with complicated and even unique components that are time consuming, expensive, and more difficult to make using traditional fabrication routes. Modern fabrication techniques possess the potential to reduce fabrication lead-time and costs while increasing quality and efficiency. However, these methods have not been adopted due to the need for strict validation for nuclear applications. Unlike structural components that largely consist of a uniform material, for nuclear reactor core components incorporation of fissile or neutron poison materials inside a cladding becomes necessary. This in turn requires a more complex and flexible manufacturing method to produce these hybrid components. In this study, demonstration of ultrasonic additive manufacturing (UAM) process to produce components resembling research reactor control elements was carried out]. Specifically, the control elements for ORNL's High Flux Isotope Reactor (HFIR) were targeted. These control elements (consisting of an inner cylinder and four outer plates) are placed on the periphery of the circular HFIR core. The control elements consist of Al6061-T6 alloy with a 13-cm-long grey and a 56-cm-long black region, corresponding to 40 wt% of Ta and 30 wt% Eu{sub 2}O{sub 3} (europia) are incorporated along their length. In this study various fabrication parameters for production of aluminum blocks with and without embedded Eu{sub 2}O{sub 3} and Ta bearing compacts were explored. Select optimized specimens we then irradiated inside HFIR for one reactor cycle. The UAM process exploits ultrasonic energy to facilitate solid-state welding of metal foils in contact under pressure. This additive process is carried out in successive layers to build up the material and when coupled with CNC milling, it yields complex 3D components. The milling and subsequent additive steps may be used to embed foreign material into the material undergoing buildup. In this manner fissile or neutron poison elements may be incorporated into the complex structure, though in an inhomogeneous fashion. Multiple Al-6061 blocks were produced by UAM using 25 mm wide by 0.15 mm thick Al-6061-H18 tapes. The mechanical properties of these blocks in the as-fabricated condition was characterized in detail and it was shown that a great degree of anisotropy in the mechanical behavior exists between the direction in the tape plane and the direction perpendicular to the tapes (buildup direction) The latter exhibited reduced modulus and significantly smaller strength. Post production heat treatment appeared to greatly enhance the strength in the buildup direction. Multiple tensile specimens machined per SS-J3 geometry were prepared for irradiation testing. The tensile specimen matrix was set up to enable post irradiation examination of mechanical properties in various directions as well as after heat treatment. A small block with embedded Ta- and Eu{sub 2}O{sub 3}-bearing compacts, three of each kind, was also produced, as demonstrated previously. These compacts were 5.7 mm in diameter and 1.6 mm in height and contained 60 vol Ta or Eu{sub 2}O{sub 3}. The poison-bearing Al-6061 block alongside the tensile bars was loaded into a perforated rabbit (irradiation capsule) for irradiation in HFTK. The perforated rabbit allows penetration of HFIR water coolant into the Interior of the capsule in direct contact with the test specimens. The rabbit was loaded into the flux trap region at the uppermost position in the core. Irradiation temperature was 68 deg. C and fast neutron (E>0.1 MeV) fluence was 10{sup 25} neutrons/m{sup 2}. Currently PIE activities are ongoing that are focused on mechanical testing of irradiated tensile bars and metallographic examination of neutron poison bearing block.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Trace element control in binary Ni-25Cr and ternary Ni-30Co-30Cr master alloy castings

Electro-slag remelting (ESR) is used for control of unwanted elements in commercial alloys. This study focuses on master alloys of Ni-25Cr and Ni-30Co-30Cr, processed through a combination of vacuum induction melting (VIM) and electro-slag remelting (ESR). Minor additions were made to control tramp element levels and modify the melting characteristics. Nitrogen and sulfur levels below 10 ppm and oxygen levels below 100 ppm were obtained in the final products. The role of the alloy additions in lowering the tramp element content, the resulting residual inclusions and the melting characteristics were determined computationally and confirmed experimentally. Additions of titanium were beneficial to the control of oxygen levels during VIM and nitrogen levels during ESR. Aluminum additions helped to control oxygen levels during remelting, however, aluminum pickup occurred when excess titanium was present during ESR. The usefulness of these master alloys for use as experimental remelt stock will also be discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Evaluations of Silica Aerogel-Based Flexible Blanket as Passive Thermal Control Element for Spacecraft Applications

The feasibility of utilizing commercially available silica aerogel-based flexible composite blankets as passive thermal control element in applications such as extraterrestrial environments is investigated. Differential scanning calorimetry showed that aerogel blanket was thermally stable over – 150 to 126 °C. The outgassing behavior, e.g., total mass loss, collected volatile condensable materials, water vapor regained and recovered mass loss, was within acceptable range recommended for the space applications. ASTM tension and tear tests confirmed the material’s mechanical integrity. The thermo-optical properties remained nearly unaltered in simulated space environmental tests such as relative humidity, thermal cycling and thermo-vacuum tests and confirmed the space worthiness of the aerogel. Aluminized Kapton stitched or anchored to the blanket could be used to control the optical transparency of the aerogel. These outcomes highlight the potential of commercial aerogel composite blankets as passive thermal control element in spacecraft. Structural and chemical characterization of the material was also done using scanning electron microscopy, Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Controlling excited-state contamination in nucleon matrix elements

We present a detailed analysis of methods to reduce statistical errors and excited-state contamination in the calculation of matrix elements of quark bilinear operators in nucleon states. All the calculations were done on a 2+1-flavor ensemble with lattices of size 32 3 × 64 generated using the rational hybrid Monte Carlo algorithm at 𝑎 = 0.081 fm and with 𝑀 𝜋 = 312 MeV. The statistical precision of the data is improved using the all-mode-averaging method. We compare two methods for reducing excited-state contamination: a variational analysis and a 2-state fit to data at multiple values of the source-sink separation 𝑡 sep . We show that both methods can be tuned to significantly reduce excited-state contamination and discuss their relative advantages and cost effectiveness. Here a detailed analysis of the size of source smearing used in the calculation of quark propagators and the range of values of 𝑡 sep needed to demonstrate convergence of the isovector charges of the nucleon to the 𝑡 sep → ∞ estimates is presented.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Reactor physics phenomena in additively manufactured control elements for the High Flux Isotope Reactor

Additive manufacturing is under investigation as a novel method of fabricating the control elements (CEs) of the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory with greater simplicity, eliminating numerous highly complex fabrication steps and thereby offering potential for significant savings in cost, time, and effort. This process yields a unique CE design with lumped absorbers, a departure from traditionally manufactured CEs with uniformly distributed absorbing material. Here, this study undertakes a neutronics analysis of the impact of additively manufactured CEs on the HFIR core physics, seeking preliminary assessment of the feasibility of their practical use. The results of the MCNP transport simulations reveal changes in the HFIR reactor physics arising from geometric and nuclear effects. Absorber lumping in the discrete CEs yields a large volume of unpoisoned material that is not present in the homogeneous design, in turn yielding increases in free thermal flux in the CE absorbing regions and their immediate vicinity. The availability of additional free thermal neutrons in the core yields an increase in fission rate density in the fuel closest to the CEs and a corresponding increase in neutron multiplication on the order of 100 pcm. The absorption behavior exhibited by the discrete CEs is markedly different from the homogeneous CEs due to several competing effects. Self-shielding arising from absorber lumping acts to reduce the effective absorption cross section of the discrete CEs, but this effect is offset by geometric and spectral effects. The operational performance of the discrete CEs is found to be comparable to the homogeneous CEs, with only limited deficiencies in reactivity worth that are expected to be operationally recoverable via limited adjustment of the CE positions and withdrawal rate. On the whole, these results indicate that the discrete CEs perform reasonably similarly to the homogeneous CEs and appear feasible for application in HFIR. In conclusion, the physical phenomena identified in this study provide valuable background for follow-up design studies.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2022

Examination of Genetic Control Elements in the Phototrophic Firmicute Heliomicrobium modesticaldum

Heliomicrobium modesticaldum has been used as a model organism for the Heliobacteria, the only phototrophic family in the Firmicutes. It is a moderately thermophilic anoxygenic phototrophic bacterium that is capable of fermentative growth in the dark. The genetic manipulation of H. modesticaldum is still in its infancy. Methods to introduce genes through the use of exogenous plasmids and to delete genes from the chromosome through the use of the native CRISPR/Cas system have been developed in the last several years. To expand our genetic toolkit, it was necessary to control gene expression. In this study, we analyzed constitutive and inducible promoters developed for clostridia for their use in H. modesticaldum and further tested two reporters, adhB and lacZ, as indicators of promoter strength. Alcohol dehydrogenase (AdhB) was unsuitable as a reporter in this species due to high endogenous activity and/or low activity of the reporter, but a thermostable LacZ worked well as a reporter. A set of constitutive promoters previously reported to work in Clostridium thermocellum was found to be reliable for controlling the expression of the lacZ reporter gene in H. modesticaldum at a range of activities spanning an order of magnitude. An anhydrotetracycline-inducible promoter was created by inserting tetO operators into a strong constitutive promoter, but it was not fully repressible. The implementation of a xylose-inducible promoter resulted in complete repression of β-gal in the absence of xylose, and reliable expression tunable through the concentration of xylose added to the culture.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2019

Characterization of the Properties of Barium–Strontium Titanate Films and Controlled Elements Based on Them in the Frequency Range of 1–60 GHz

The electrical properties of barium–strontium titanate films and tunable capacitors based on them are studied in a wide frequency range. The dielectric losses in ferroelectric films at frequencies up to 30 GHz are estimated based on the parameters of capacitors measured using various resonance techniques. Direct measurements of the properties of the films were carried out at a frequency of 60 GHz using the open-resonator technique. Based on the comparison with semiconductor analogues, the prospects of using thin-film ferroelectric elements in the millimeter-wavelength range are shown.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Trace Element Control in Master Alloys and Impact of Feedstock Purity on a Ta-containing Steel during Electroslag Remelting

Advanced materials design often leads to new complex compositions that can bring challenges to manufacturing processes. Melt processing of a novel alloy with a tight chemistry range revealed a 25% Ta loss after manufacturing. Vacuum induction melting (VIM) and electroslag remelting (ESR) were performed using industry practices at a laboratory scale and the loss occurred from the VIM electrode to the ESR ingot. Several tools were used for characterization, including combustion analysis, scanning electron microscopy, and electron probe microanalysis for observation of the precipitate and inclusion phases. Additionally, computational modeling of the ESR process was performed to predict macrosegregation and inclusion travel. It was found that a significant amount of Ta2O5 inclusions formed during VIM and were transferred to the slag during ESR. This led to a 95% decrease in density of inclusions explaining to Ta loss. Pathways to better control advanced alloy chemistries during melt processing will be discussed.

Detrois, Martin↗