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Attendees from Los Alamos National Laboratory (LANL) traveled from the US to Slovenia to attend the meetings in-person at the Jožef Stefan Institute. They were invited to serve on the TRG to review submitted benchmark evaluations prior to their approval for publication in the ICSBEP Handbook.
A new covariance data library based on ENDF/B-VII.1 was recently processed for the SCALE nuclear analysis code system. The multigroup covariance data are discussed here, along with testing and application results for critical benchmark experiments. The cross section covariance library, along with covariances for fission product yields and decay data, is used to compute uncertainties in the decay heat produced by a burned reactor fuel assembly.
We report improved 95 Mo neutron resonance parameters and reaction rates are important for nuclear astrophysics, testing nuclear models, and nuclear criticality safety. However, despite many previous neutron-capture and total cross-section measurements on this nuclide, there still is much room for improvement as well as several discrepancies. For example, there are very few firm resonance spin and parity assignments; average resonance parameters are available only for each parity, the currently recommended astrophysical reaction rate results in disagreements between stellar models and meteoric isotopic anomalies, and there are substantial disagreements in the neutron-capture cross section at low energies important for nuclear criticality safety. To obtain an improved set of neutron resonance parameters and astrophysical reaction rates for 95 Mo. High-resolution neutron-capture and transmission data were measured at the Oak Ridge Electron Linear Accelerator (ORELA) using highly isotopically enriched 95 Mo samples. The neutron-capture apparatus, data reduction, and analysis were improved so that information contained in the γ-ray cascade following neutron capture were used to assign resonance J π values. Following this, simultaneous analysis of the new neutron-capture and transmission data was used to obtain resonance energies, gamma widths, and neutron widths and their uncertainties to a maximum energy of 10 keV. Accurate neutron-capture cross sections also were obtained for the unresolved resonance region to a maximum energy of 500 keV and, together with the new resonance parameters, used to calculate the astrophysical reaction rates in the temperature range from 5 to 30 keV. A vastly improved set of 95Mo neutron resonance parameters and an astrophysical reaction rate accurate to about 3% were obtained. Firm J π assignments were determined for 261 of the 314 observed resonances. This is a very large improvement over the previously published 32 firm J π assignments for 108 resonances. Also, the number of resonances having both firm J π assignments and Γ γ values was increased by almost a factor of 24—from 11 to 261. Neutron- and total-radiation-width distributions and average resonance spacings, average total radiation widths, and neutron strength functions were obtained for the six different s- and p-wave possibilities. Parameters for the lowest s-wave resonance, which is most important for criticality benchmarks, were obtained with high accuracy. Simple modification of the neutron-capture apparatus and expansion and improvement of data analysis techniques led to a large increase in firm J π assignments for 95 Mo neutron resonances. The resulting astrophysical reaction rate is 20%–30% larger than the currently recommended rate at s-process temperatures, which should lead to better agreement between stellar models and meteoric isotopic anomalies. The neutron-capture cross section at low energies is substantially larger than recommended in the latest evaluation, which is problematical for criticality benchmarks. The average resonance spacing as a function of spin and parity is significantly different from current models. The total-radiation-width distributions are significantly broader than predicted by theory and show significant departures from the expected Gaussian shapes.
Thermal neutron scattering laws are important nuclear data for many nuclear science and engineering applications. Validation helps to ensure that a thermal neutron scattering law has a high quality and often employs critical benchmarks as integral experiments. Recently, pulsed-neutron die-away benchmarks have been used as an experiment to validate thermal neutron scattering laws. Herein, we evidence how this alternative integral experiment has a high sensitivity to these nuclear data by performing an uncertainty quantification analysis. The analysis randomly sampled the nuclear model parameters associated with hydrogen bound in light water thermal neutron scattering law and sampled other nuclear data that influenced the experiment’s integral parameter (e.g., elastic scattering, absorption in hydrogen and oxygen) from their respective covariance matrices. The thermal neutron scattering law caused an uncertainty in the integral parameter that reached 2.67%, which exceeds by an order of magnitude the uncertainties induced in commonly used thermal solution critical benchmarks. The validation performed here, although limited due to a poor description of the historical experiment, indicated that the ENDF/B-VIII.0 thermal neutron scattering law well predicted the integral parameter. These results motivate further benchmark and validation efforts using pulsed-neutron die-away experiments.
The Los Alamos Critical Experiments Facility (LACEF) and subsequently its descendant, the National Criticality Experiments Research Center (NCERC) located at the Nevada National Security Site, have been performing critical experiments for over 75 years. The work done continues to build upon lessons learned from the past. It also builds upon knowledge gained from past experiments. The LACEF and NCERC experiments “provide nuclear criticality benchmark data for a wide variety of fissile and non-fissile materials in all neutron energy spectra systems”. In 2014, a subcritical benchmark with plutonium surrounded by nickel was issued in the International Criticality Safety Benchmark Evaluation Project (ICSBEP). Two years later, a new benchmark experiment was performed, commonly known as Subcritical Copper Reflected $\mathcal{a}$-Phase Plutonium SCR$\mathcal{a}$P. The uncertainties in the SCR$\mathcal{a}$P experiment were significantly reduced from the Pu-Ni experiment and the methods were refined. Additionally, a Pu-lead critical experiment was performed in 2017 which followed different methods and with different objectives. Now, an experiment is proposed that builds on both the subcritical nickel experiment and the critical lead experiment. The proposed experiment has interleaved nickel and plutonium, in a thermal arrangement. This particular experiment would support criticality safety at the Hanford Tank Farms, whose tanks contain a significant amount of nickel credited as a neutron absorber in the criticality safety evaluation.
This presentation finds that as it was designed, Cerberus is extremely sensitive to copper nuclear data. Just changing copper nuclear data to another library can swing simulated keff by >1000 pcm. ENDF/B-VIII.1 and JENDL-5.0 both show large downward trends in keff as copper interstitial thickness is increased
Interview for Scientist 2 position in XCP-7: Radiation Transport Applications
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In the frame of its missions, IRSN is widely involved in design, evaluation and analyses of critical experiments in order to contribute to computer codes and nuclear data validation. The work described in this paper was carried out in the framework of the PRINCESS IRSN research project and is part of the IRSN/LLNL collaboration on the TEX program in the frame of the NCSP. The need for epithermal and intermediate energy range critical benchmarks is an established international criticality safety data need, underpinned by the subsequent use of data assimilation methods like GLLSM to reduce Nuclear Data uncertainties. The goal of the TEX program is to address this need by executing experiments with NCSP fissile assets, which can be used to create critical assemblies that span a wide range of fission energy spectra, from the thermal energy range (below 0.625 eV), through the intermediate energy range (0.625 eV to 100 keV), to the fast one (above 100 keV). The TEX program involves different materials of interest, such as TEX-Ta experiments using plutonium ZPPR plates proposed by LLNL. This paper presents an overview of the studies performed at IRSN using numerical optimization algorithms for the design of critical experiments involving Mixed U and Pu fuel plates, referred to as TEX-MOX program. This program ultimately aims to perform critical experiments to validate the MOX fuel configurations encountered in the MOX fuel fabrication process. (authors)
Sandia National Laboratories (SNL) and Oak Ridge National Laboratory (ORNL) have collaborated to develop a capability to test the epithermal/intermediate cross sections of materials at the SNL critical experiments facility using the Seven Percent Critical Experiment (7uPCX) fuel. The Sandia Critical Experiments Program provides a specialized facility for performing water moderated and reflected critical experiments with UO 2 fuel rod arrays. The facility offers the ability to modify the core configuration and reactor tank to evaluate various reactor cores for pitch, moderator characteristics, and other criteria. A history of safe operations and flexibility in reactor core configuration has resulted in the completion of nine sets of critical benchmark experiments that have been documented in the International Criticality Safety Benchmark Evaluation Project (ICSBEP) Handbook. The experiment described here is expected to be evaluated for inclusion in the 2024 edition of the ICSBEP Handbook.
Sandia National Laboratories (SNL) and Oak Ridge National Laboratory (ORNL) are collaborating to develop a capability to test the epithermal/intermediate cross sections of tantalum (Ta) at the SNL critical experiments facility using the Seven Percent Critical Experiment (7uPCX) fuel. The Sandia Critical Experiments (SCX) Program provides a specialized facility for performing water moderated and reflected critical experiments with UO 2 fuel rod arrays. A history of safe reactor operations and flexibility in reactor core configuration has resulted in the completion of several benchmark critical experiment evaluations that are published in the International Criticality Safety Benchmark Evaluation Project (ICSBEP) Handbook. The critical experiments described here are scheduled to be performed at SCX in fiscal year 2023. Currently, fabrication details for the needed new equipment are being finalized for completion of the procurement process. The experiments will then be executed and documented for publication consideration by the ICSBEP.
Many of the advanced reactors that are being designed will use high-assay low-enriched uranium (HALEU) as the reactor fuel. HALEU is fuel that is enriched between 5% – 20% uranium-235. With the change to higher enriched material, the industry will have new challenges with the development and regulatory approval of enrichment and fuel fabrication facilities and suitable transportation packages to support the economic use of HALEU materials. One area of concern relates to ensure sub-criticality of the material during transportation as identified by the Nuclear Energy Institute (NEI). To evaluate the relevant work, experts, and industry perspectives on HALEU, a workshop was organized to share relevant experience and insights into HALEU transportation, handling, and management. At the August 30 and 31, 2018 workshop, NEI and industry provided the following recommendations to DOE and the national lab complex. 1. DOE and the lab complex should communicate and educate the Nuclear Regulatory Commission (NRC) on criticality issues related to HALEU. 2. Idaho National Laboratory (INL) should support work needed to certify package design for the transportation of HALEU. a. Suggest amending the Certificate of Compliance of an existing package used for the shipment of commercial quantities. b. Suggest DOE provide funding to package designer(s) for analysis and engineering work for a package to be submitted to NRC for approval. 3. INL should provide specific, or a range, of the expected impurities that will be present in recycled naval fuel. 4. In the longer term, DOE and the lab complex should increase the availability of criticality benchmark data to further reduce conservatism in package design. In addition, a couple key takeaways were identified, including: 1. Although the labs can provide additional criticality experiments, industry has enough data to license facilities, overpacks, and cylinders. Validation from additional critical experiments to establish less uncertainty in the benchmarks will be helpful. 2. A collective effort from industry is needed to express consistency on how much information exists or is needed related to criticality. Based on interactions with industry, DOE, and national laboratories, it appears that large-volume transportation of fresh HALEU appears feasible from a criticality perspective. Specifically, an initial review of applicable criticality benchmark experiments identified numerous applicable experiments. However, a more thorough review of a realistic transportation package design is suggested. Therefore, the following next steps are proposed: 1. Evaluate a large volume package with uranium dioxide enriched to 20%. Specifically, the GNF-A NPC package was proposed as a potentially viable option. 2. Based on the results of (1), determine if additional package designs and fuel material should be evaluated. 3. Based on the results of (1), determine if additional criticality experiments would be beneficial to improve the margins for criticality due to uncertainties. 4. Determine DOE transportation needs (packages sizes, shielding requirements, handling/operational requirements, and timing of availability) related to HALEU. 5. Continue to interface with NEI and interested industry companies to determine the appropriate time to engage the NRC.
This presentation discusses final designs for Plutonium Thermal Epithermal eXperiments (TEX) configurations. The goal of the experiments is to produce Pu critical benchmarks to address the nuclear data and validations needs for criticality safety.
The Seven Percent Critical Experiment (7uPCX) at Sandia National Laboratories was designed to provide benchmark criticality and reactor physics data for water-moderated pin-fueled nuclear reactor cores in the 5 to 10 percent enrichment range. Approach-to-critical experiments were performed on nineteen partially-reflected arrays of pure water-moderated and -reflected fuel rods with a fuel-to-water volume ratio of 0.67. Those configurations are described and the results of the measurements are reported in this paper. (authors)
This presentation gives a look to new cross section measurements and evaluations of Zirconium (Zr) isotopes. This lecture includes highlights of accomplishments made in 2022. Additionally, it explains the motivation for investigation, new measurements, and re-evaluation of the Zr isotopes. This presentation concludes with a preliminary evaluation result from RPI 90 Zr evaluation and preliminary results from Eigenvalue sensitivity study on Zr sensitive critical benchmark experiments.
The Chlorine Worth Study (CWS) experiments consist of layers of Zero Power Physics Reactor plates moderated by high density polyethylene, polyvinyl chloride, and chlorinated polyvinyl chloride with the goal to produce a thermal critical experiment. A thick high density polyethylene reflector surrounds the experiments. Aluminum fuel trays and frames were used to conduct the intrinsic heat generation of the plutonium plates away from the core and out to the top plate and the platen, both acting as heat sinks. The goal of the CWS experiments was to provide benchmark critical experiments that were sensitive to chlorine and matched the sensitivity of criticality safety applications for aqueous chloride operations at PF-4 at Los Alamos National Laboratory. The specific aqueous chloride PF-4 applications were solutions of 30 g/L plutonium, 300 g/L plutonium, and 600 g/L plutonium. The CWS experiments were designed using prototype methods developed by the ARCHIMEDES and EUCLID projects at Los Alamos National Laboratory. The CWS experiments were performed in December of 2021 at the National Criticality Experiments Research Center at the Nevada Nuclear Security Site on the Planet critical assembly. It is intended that the CWS experiments will be evaluated and submitted to the ICSBEP in the future as criticality safety benchmarks.
The CRItical Simulation Pipeline (CRISP) is a Python package for automating validation of reactor criticality benchmarks. CRISP supplies COG—a multi-particle radiation transport code maintained by the Nuclear Criticality Safety Division—with a pipeline to calculate k eff performance for 400+ benchmark experiments with 3,400+ configurations from the International Criticality Safety Benchmark Evaluation Project (ICSBEP). The pipeline includes four stages: materials configuration, input card templating, cluster submission, and results analysis. CRISP includes a command-line interface to facilitate user interaction.