The SCALE transport lattice code, Polaris, has been previously developed to generate few-group homogenized cross sections for whole-core nodal diffusion simulators in which the embedded self-shielding method (ESSM) is used for resonance self-shielding calculations to process cross sections. Although the ESSM capability has been very successful in light-water reactor analysis, it may require enhancements in computational efficiency; treatment of spatially dependent resonance self-shielding effects; and handling of interrelated resonance effects among fuel, cladding, and control rod materials. Therefore, this study focuses on improving computational efficiency by using a Dancoff-based Wigner–Seitz approximation combined with a material-based resonance categorization, through which a spatially dependent ESSM capability is developed to accurately estimate self-shielded cross sections inside the fuel. Benchmark results show that the new capability significantly enhances computational efficiency and accuracy for spatially dependent local zones within the fuel and through depletion.

Ensuring data preservation is a top priority for the Organization for Economic Co-operation and Development (OECD), Nuclear Energy Agency (NEA) Data Bank. Within this context, “preserving data” encompasses activities such as verifying, processing, sharing, improving, and storing the data. The NEA aims to automate these processes to the greatest extent possible, with the goal of providing the JEFF community with reproducible, high-quality data. To achieve this, the Data Bank has chosen to utilize GitLab, a web-based distributed Version Control System that facilitates the collaboration of different users. The author will present the current progress of the NEA pipeline, an ongoing collaborative initiative aimed at standardizing the processing, verification, and validation of nuclear data.

We derive an effective Reich-Moore approximation (RMA) of the Wigner-Eisenbud R-matrix formalism parameterized by complex-valued resonance energies and widths; this RMA exactly reproduces the total eliminated cross section. We show that resonance parameters evaluated for a conventional *** boundary conditions (BCs), Bc = Sc(E),are approximately equal to the R-matrix parameters in Park’s formalism by employing a linear approximation of the shift function therein [T.-S. Park, Phys. Rev. C 106 (2021) 064612]. We outline a method for converting Park’s observed reduced width amplitudes (RWAs) and their covariance matrix into Brune’s alternative R-matrix RWAs and their covariance matrix [C. Brune, Phys. Rev. C 66 (2002) 044611]. We extend the Park’s R-matrix formalism into the complex plane by introducing a complex-valued basis set of eigenfunctions of a complex-symmetric (non-Hermitian) Hamiltonian in the R-matrix interior. We observe that its R-matrix resonance energies and widths are directly related to the poles and residues, respectively, of Hwang’s sum-over-poles representation of cross sections [R.N. Hwang, Nucl. Sci. Eng. 96 (1987) 192].

The NEA Data Bank is an international reference centre for computer codes, nuclear and thermochemical data which has traditionally used simple file servers, and even DVDs, to deliver valuable content to end users across the globe. With the recent implementation of a self hosted GitLab system at the NEA it has enabled the Data Bank to streamline delivery, automate processes and testing, while empowering code owners and developers with a secure platform to collaborate and develop codes. In this paper we present the NEA Git- Lab system with some concrete examples of codes such as Kraken, PHITS and FISPACT-II taking advantage of many of the services and functionalities provided by GitLab. We illustrate methodologies on how to work effectively with third party software in a position of being a custodian of code, rather that a code owner, providing DevSecOps as a service.

Conventional nuclear data evaluation methods using generalized linear least squares make the following assumptions: prior and posterior probability distribution functions (PDFs) of all model parameters and data are normal (Gaussian); the linear approximation is sufficiently accurate to minimize the cost function (even for nonlinear models); the model (e.g., of neutron cross section) and experimental data (including covariance data) are without defect and prior PDFs of parameters and measured data are known perfectly. Neglect of covariance between model parameters and measured data in conventional evaluations contributes to imperfections. These assumptions are inherent to the generalized linear least squares minimization method commonly used for resolved resonance region neutron cross section evaluations but are often not justified due to the presence of non-normal PDFs, nonlinear models (e.g., R-matrix formalism), and inherent imperfections in data and models (e.g. imperfect covariance data). Here, these assumptions are removed in a mathematical framework of Bayes’ theorem, which is implemented using the Metropolis-Hastings Monte Carlo method. Most importantly, new parameters are introduced to parameterize discrepancies between the theoretical model and measured data to quantify judgement about discrepancies or imperfections in a reproducible manner. An evaluation of 233 U in the eV region using the ENDF-B/VIII.0 library and transmission data (Guber et al.) is presented, and posterior parameters are compared to those obtained by conventional evaluation methods. This example illustrates the effects of removing the most harmful assumption: that of model-data perfection.

The R-Matrix code SAMMY [1] is a widely used nuclear data evaluation code focused on the resolved range, which includes corrections for experimental effects. The code is still mostly written in FORTRAN 77 and uses a memory management system suitable for the time of its initial writing in 1984. A modernization effort is underway to update the code to modern software development practices. A continuous-integration testing framework was added to automate the large existing set of test cases. Improvements in memory management were implemented to make the code easier to maintain and enable enhancements. The resonance parameters and covariance information are now stored in C++ objects shared by SAMMY and AMPX [2], which is the processing code that generates nuclear data libraries for SCALE [3]. Further plans include switching to the Evaluated Nuclear Data File (ENDF) reading and writing routines in AMPX because these routines are more robust, easier to maintain, and support more features. Support for the new Generalized Nuclear Database Structure (GNDS) format [4] is also of interest. GNDS will share not only the resonance parameters but also the parameters associated with experimental correction in GNDS. The data are currently available in a binary SAMMY format, and the ability to export them to GNDS would make them more widely available and shareable. The next step will be to use the same resonance processing code at 0K in AMPX and SAMMY as an available formalism. Then, any improvements in the formalism can immediately be tested in SCALE because the reconstruction in AMPX will use the same cross section model. The new data library can then be used for testing using the VALID Benchmark suite [5] or other suitable benchmark suites.

The design of a new subcritical assembly at Oak Ridge National Laboratory (ORNL) has been finalized. This design takes the feasibility study of the subcritical assembly performed in August 2020 to an implementable design. This subcritical assembly will support the Nuclear Criticality Safety Program (NCSP) training and education program. The addition of this subcritical assembly into the NCSP training and education will enhance the program by providing backup capacity for training if nuclear facility operations are disrupted at Sandia National Laboratories or the National Criticality Experiments Research Center; providing a new location that is more accessible to students in the Eastern portion of the United States; providing flexibility to support students from a diverse background (e.g., university students, foreign nationals). The proposed subcritical assembly uses legacy AGN-201M research reactor fuel plates that are available at the Y-12 National Security Complex. The final design of this subcritical assembly contains approximately 617 grams of 235U as UO2 powder distributed homogeneously in polyethylene. The fuel plates will have a graphite neutron reflector to obtain a core multiplication, M, from 10 to 20, corresponding to a $k_{eff}$ of 0.90 to 0.95, respectively. The subcritical assembly will be able to support at least four experiments for the training courses: (1) the addition of fissile material to the core (mass), (2) a core separation experiment (interaction), (3) the addition of moderators to the core (moderation), and (4) the addition of neutron absorbers to the core (poison/absorption). The subcritical assembly will be designed to be inherently safe—subcritical under all normal and abnormal conditions— and will provide the capability to conduct hands-on training to support NCSP and general nuclear criticality safety staff training and qualification goals.

R-matrix formalism is extended beyond compound nuclear (CN) resonant reactions to include parameterization of direct as well as doorway processes. Direct processes in the R-matrix exterior are parameterized by a unitary matrix that introduces mixing among wave function coefficients of the incoming and outgoing wave function components at the R-matrix channel surface. Doorway processes are parameterized by separating the Hilbert space of the interior R-matrix region into its doorway and CN subspaces, from which doorway state eigenenergies, reduced width amplitudes, and the strengths of their coupling to CN levels appear as new R-matrix parameters. Parameterization of generalized as well as the conventional Reich–Moore approximation for eliminated capture channels in the presence of direct, doorway, and CN processes is presented along with a complex-valued scattering length with contributions from direct, doorway, and CN capture processes. Derivation of Brune’s alternative R-matrix parameters is extended to include doorway states. This work suggests how R-matrix formalism could be extended further by adopting the concepts from related reaction formalisms.

work continued to fully support new Evaluated Nuclear Data File (ENDF) formats, including the Generalized Nuclear Database Structure (GNDS) in AMPX.

Sufficient AGN-201M fuel plate material exists at Y-12 to support the development of a new, inherently safe, subcritical assembly for use in NCS training courses at ORNL. Four experiments with the ORNL subcritical assembly are possible and feasible. Future work includes the final design of the core and reflector and split table component design specifications. The ORNL Material Demonstration Facility (MDF) may be used to 3D print ORSA stand/table components and graphite reflector. Researchers will consider fuel coatings for contamination control, rather than an Al can. A visit to Y-12 will be made to determine the location of AGN fuel plates and remaining fuel transportation considerations. Final facility location will be considered

BMC evaluation is a tool to address imperfect data & models, non-linear models, and non-normal PDFs. ENDF-6 format does not allow non-normal parameter PDFs. Storing posterior sets allow for variance, covariance, skewness, etc. To better predict criticality, we could document non-normal parameter PDFs (i.e. asymmetric uncertainty), consider non-linear sensitivity of $\kappa$eff to resonance parameters, and reduce uncertainty in key resonance parameters.

This presentation discusses and announces that AMPX is now available as open source. It additionally provides an overview about ENDF and GNDS reading in AMPX. It also discusses thermal scattering law updates and the CE and MG data libraries for SCALE 6.3.0 and SCALE 7.0.0. In addition, the presentation discusses the photonuclear sub library processing capability of AMPX.