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At least 19 records

Data from Rossi-alpha and pulsed neutron prompt neutron time decay measurements at the Oak Ridge Critical Experiments Facility

This report briefly describes the Rossi-alpha and pulsed neutron prompt neutron time decay measurements performed at the Oak Ridge Critical Experiments Facility. This description includes the measurement equipment, type of data, and how to access the actual data from Records Management Services at Oak Ridge National Laboratory and the Nuclear Criticality Safety Benchmark Program (NCSBEP) of the Idaho National Laboratory. Most of the data were acquired between 1960 and 1975 on weapons-grade enriched uranium metal assemblies. Also included are measurements of the same type performed at Los Alamos National Laboratory by Oak Ridge National Laboratory with the JEZEBEL and FLATTOP criticality assemblies and other plutonium metal parts. The purpose of this report is to allow future mining of the data for publication of additional information that can be used to verify calculational methods and nuclear cross sections. These data can be used to create reactor physics benchmarks for the Nuclear Energy Agency’s International Handbook of Evaluated Reactor Physics Experiments. Present calculational methods can be used to directly calculate the measured data. The prompt neutron decay is more neutron spectrum sensitive than other measured quantities, particularly for interacting fissile metal systems where the flight time of neutrons between interacting sections is a large part of the neutron lifetime.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Data from Rossi-α and Pulsed Neutron Prompt Neutron Time Decay Measurements at the Oak Ridge Critical Experiments Facility

This report briefly describes the prompt neutron decay measurements by the Rossi-α and pulsed neutron methods performed at the Oak Ridge Critical Experiments Facility. It briefly describes the measurement equipment, the type of data, and how to access the actual data from the Laboratory Records Department at Oak Ridge National Laboratory and the International Nuclear Criticality Safety Benchmark Evaluation Program at Idaho National Laboratory. Most of the data was acquired between 1960 and 1975 with weapons-grade enriched uranium metal assemblies. It also includes measurement of these types performed at Los Alamos National Laboratory by Oak Ridge National Laboratory with the JEZEBEL and FLATTOP criticality assemblies and other plutonium metal parts. The purpose of this report is to support future mining of the data for publication of additional information that can be used to verify calculational methods and nuclear cross sections. Present calculational methods can be used to directly calculate the measured data.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Sensitivity Coefficients Calculated for the Prompt Neutron Decay Constant At or Near Delayed Critical

The derivation of a non-invasive prompt neutron decay constant sensitivity coefficient is provided in this work. The computation of the sensitivity coefficient derived in this work does not require modification of Monte Carlo source code and is based on capabilities available in Monte Carlo N-Particle R© Code Version 6.2. The prompt neutron decay constant sensitivity coefficients are calculated for 44-group and 252-group energy structures for specific nuclide-reaction pairs in the Jezebel benchmark experiment. The nuclide-reaction pairs investigated in this work include Pu- 239(n,f), Pu-240(n,f), and Pu-241(n,f). Physical explanations of the sensitivity profiles exhibited by the 252-group energy structure are investigated for the prompt neutron multiplication factor, mean neutron lifetime, and prompt neutron decay constant. The prompt neutron decay constant sensitivity coefficients calculated for the 44-group and 252-group energy structure of Pu-239(n,f) are compared. Lastly, the 44-group energy structure sensitivity coefficients calculated are used for nuclear-data induced uncertainty quantification of the neutron multiplication factor. This work shows that a reduction in the nuclear data-induced uncertainty of the neutron multiplication factor is possible for all nuclide-reaction pairs investigated when prompt neutron decay constant sensitivity coefficients are utilized. This is important for new critical experiment design optimization studies of measurement configurations. This work provides a basis for more detailed sensitivity analysis and uncertainty quantification of nuclide, reaction, and energy-specific cross section data.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

Sensitivity Coefficients Calculated for the Prompt Neutron Decay Constant at or Near Delayed Critical [Abstract]

Experimenters at Los Alamos National Laboratory (LANL) measure the prompt neutron decay constant for many experiments at the National Criticality Experiments Research Center (NCERC) to infer reactivity and the effective neutron multiplication factor. These quantities are very important for nuclear criticality safety and validating nuclear data. Uncertainty in measures of criticality of an experimental configuration can be determined prior to physically performing the experiment by applying first order perturbation theory to Monte Carlo codes, such as MCNP®. The first order perturbation theory produces first derivatives of some nuclear parameter to nuclear data (e.g., cross section data). This first derivative is commonly referred to as a sensitivity coefficient. Currently, the MCNP® software has the capability of computing effective neutron multiplication factor sensitivity coefficients to cross section data. This work builds off of this MCNP® capability and the first order perturbation theory to provide a method of calculating sensitivity coefficients for the prompt neutron decay constant at or near delayed critical to cross section data. The prompt neutron decay constant sensitivity coefficient calculated in this work does not depend on any modification of the MCNP® source code. Prompt neutron decay constant sensitivity coefficient calculations can be used to infer reactivity and effective neutron multiplication factor sensitivity coefficient values as well. By investigating the trends of prompt neutron decay constant sensitivity coefficients for nuclide-reaction pairs across energy spectra, experiments can be designed to maximize or minimize the uncertainty in the prompt neutron decay constant in a particular energy region, which can lead to further optimization studies. Prompt neutron decay constant sensitivity coefficients will be calculated for the Jezebel benchmark. Subsequently, these sensitivity coefficients will be used in a data assimilation process to determine if there is or are optimal experiments that can be performed to provide insight into adjustments of uncertain/inaccurate cross section data. Specifically, the effect of the prompt neutron decay constant sensitivity coefficients on the nuclear data-induced uncertainty in the effective neutron multiplication factor will be examined

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

Using Prompt Neutron Decay Constant Measurements to Obtain Additional Kinetics Information

Prompt neutron decay constant measurements are typically used to infer the prompt neutron lifetime in a chain-reacting system, but these measurements include much more information about the kinetics of the system. This work focuses specifically on the Rossi-α method of measuring the prompt neutron decay constant. The Rossi-α system of equations consists of five unknown parameters. With five unknowns and three equations, the system of equations solved by the Rossi-α measurement is under-determined. It has been postulated that a well posed problem can be formulated using independent measurement configurations on a single system. The external source strength, the effective delayed neutron fractions, and Diven’s parameter do not typically change for different configurations of a single experiment. If good detector geometry is chosen, the detector efficiency does not change for multiple measurements of a single experiment using Rossi-α measurements. By making these assumptions, there are only two unique variables. This knowledge can be used to solve for most of the unique parameters using two independent, unique Rossi-α measurements.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

Prompt neutron emission in the neutron-induced fission of 239 Pu and the spontaneous fission of 252 Cf: an unfolding story

Prompt neutron emission is one of the major observables in fission and critical to a sustained chain reaction. The prompt neutron emission spectrum and its precision are essential for adequately modeling the chain reaction for any given application. Within this context, a joint program, called Chi-Nu, was initiated by LANL [1] with the objective to improve the precision of experimental measurements of prompt neutron emission for neutron-induced fission of 235 U and 239 Pu as well as the spontaneous fission of 252 Cf. The measurements were carried out at the Weapons Neutron Research Facility at Los Alamos Neutron Science Center using two scintillator arrays for neutron detection and a PPAC (parallel-plate avalanche counter) for fission-fragment detection.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

Identifying Inconsistencies in Fission Product Yield Evaluations with Prompt Neutron Emission

Here, we present a self-consistency analysis of fission product yield evaluations. Anomalous yields are determined using a series of simple conservation checks and comparing charge distributions with common parameterizations. The summed average prompt neutron multiplicity for both products as a function of the heavy product mass is derived directly from the independent fission product yields with a procedure utilizing average charge conservation. This procedure is validated with Monte Carlo simulations of the de-excitation of the fission fragments in a Hauser-Feshbach statistical decay framework. The derived is compared with experimental data, when available, and then used to determine the prompt neutron multiplicity for the various evaluations. The propagated errors on from the average charge conservation method are significantly lower than the simple summation rules, which reveals that some evaluations are inconsistent with prompt neutron data. We propose possible solutions to remedy the observed inconsistencies and identify sources of the observed differences in between the various evaluation libraries.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

Prompt neutron multiplicity distributions inferred from γ -ray and fission fragment energy measurements

We propose a novel method to extract the prompt neutron multiplicity distribution, P(ν), in fission reactions based on correlations between prompt neutrons, γ rays, and fragment kinetic energy arising from energy conservation. In this approach, only event-by-event measurements of the total γ-ray energy released as a function of the total kinetic energy (TKE) of the fission fragments are performed, and no neutron detection is required. Using the cgmf fission event generator, we illustrate the method and explore the accuracy of extracting the neutron multiplicity distribution when taking into account the energy resolution and calibration of the energy measurements. In conclusion, we find that a TKE resolution of under 2 MeV produces reasonably accurate results, independent of typical γ-ray energy measurement resolution.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Producing ENDF/B-quality Evaluations of 239 Pu(n,f) and 235 U(n,f) Average Prompt Neutron Multiplicities using the CGMF Model

This report documents evaluations of the neutron-induced 239 Pu and 235 U average prompt neutron multiplicities, $\overline{v}_p$, using the CGMF model developed at LANL. These evaluations are not updates of existing ENDF/B-VIII.0 nuclear data, but were both re-done from "scratch". That is, all experimental data were extracted from EXFOR, re-normalized to the newest nuclear data representing monitor observables, and detailed uncertainties were estimated from information in EXFOR as well as from templates of expected measurement uncertainties. We also included experimental data that were not yet available for ENDF/B-VIII.0, for instance, the data of Marini et al. for 239 Pu(n,f) $\overline{v}_p$ or the data of Khoklov et al. for 235 U(n,f) $\overline{v}_p$. Finally, we include the CGMF model; it computes $\overline{v}_p$ based on model parameters that at the same time calculate fission fragments as a function of mass, Y (A), the average total kinetic energy as a function of incident-neutron energy, $\langle TKE \rangle$(E inc ), etc. Such a detailed fission model that ties together many fission quantities has not been used to date for any $\overline{v}_p$ evaluation in ENDF/B. Here, we show that we can get evaluated 235 U(n,f) and 239 Pu $\overline{v}_p$ that not only correspond well to experimental $\overline{v}_p$, but that the associated evaluated model parameters also yield parameterizations of Y (A), $\langle TKE \rangle$(E inc ), etc., that correspond well to their respective experimental data. The new evaluated 239 Pu(n,f) $\overline{v}_p$ was also combined into a 239 Pu test file with the newest nuclear data for the 239 Pu prompt-fission neutron spectrum and fission cross sections. This new 239 Pu file performed reasonably well in predicting PU-MET-FAST assemblies, reaction rates in Jezebel and Flattop and three LLNL pulsed spheres. Due to that, the new 239 Pu(n,f) $\overline{v}_p$ evaluation presented here is being considered for ENDF/B-VIII.1. Hence, we can show here that CGMF is able to produce ENDF/B quality $\overline{v}_p$ nuclear data.

239Pu, 235U, Average Prompt-fission Neutron Multip↗

Rossi and Feynman-alpha formulas including prompt neutron decay and detector die away time constants - 25396

A simple Rossi-alpha formula and the corresponding Feynman-alpha formula have been derived that take into account two time constants; one characterizing the prompt neutron decay in the fissile source domain and the other the detector die-away time. The derivation of the formula is an extension of the classic heuristic method. A test problem is defined and simulated using the Monte Carlo MCNPX code in order to illustrate the applicability of the formulas. (authors)

61 RADIATION PROTECTION AND DOSIMETRY↗

Prompt Neutron Decay Constant Measurements on a Copper Reflected Intermediate Enrichment Uranium System with Lead Intersitial

This work examines a set of prompt neutron decay constant measurements performed on the Zeus critical experiment. The Zeus experiment was designed to measure the lead void reactivity coefficient associated with an intermediate enrichment uranium system. The initial goal was to perform the measurements on a low enriched uranium system, but weight limitations on the Comet critical assembly only allowed for a system with 21%-22% effective uranium enrichment to be built. The Zeus critical experiment was performed on the Comet critical assembly machine located at the National Criticality Experiments Research Center (NCERC). The Comet critical assembly machine is a vertical lift assembly capable of supporting general large size critical experiments. This critical experiment consists of a copper reflector set most notably used in other configurations of the Zeus experiment series. The fuel used during the Zeus experiments consists of cylindrical highly enriched uranium (HEU) plates with a nominal 1/8-in. (0.318-cm) thickness. These plates were collocated with natural uranium plates to reduce the effective enrichment of the system to 21%-22%. Lead plates are interleaved between each fuel set.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

Covariance generation for the prompt neutron multiplicity of 239 Pu including the (n, γ f) process

Fission cross section of 239 Pu can be seen as a sum of the “immediate" fission and “two-step" (n,γf) reactions. In the Resolved Resonance Range of the reaction cross sections, the contribution of the (n,γf) process has an impact on the determination of the partial widths magnitude involved in the Reich-Moore approximation of the R-matrix theory. The present work aims to investigate this impact by using the CONRAD code and the partial width Γ γf for the (n,γf) reaction calculated by Lynn et al. A special attention will be paid to the covariance matrix obtained on ν p .

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Anisotropy in fission fragment and prompt neutron angular distributions

Several physics mechanisms can lead to the deviation from an isotropic angular distribution for both fission fragments and the neutrons that are emitted during the fission event. Two of these effects have recently been implemented into CGMF, the Monte Carlo fission event generator developed at Los Alamos National Laboratory: angular distribution sampling for fission fragments and pre-equilibrium neutrons (those emitted before the compound nucleus forms). Using these new developments, we show that the anisotropy of the neutrons reflects the anisotropy of the fission fragments, in particular as the outgoing energy of neutrons increases. Correlations between the fission fragment and neutron anisotropies could be used to extract the fission fragment anisotropy from the neutron angular distributions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

On the Definition of the Prompt Neutron Lifetime

There are myriad definitions for the mean neutron lifetime, neutron generation time, and other related quantities to characterize a neutron and its progeny’s propagation through an assembly. In this document, we consider lifetime definitions that are associated with eigenvalue forms of the neutron transport equation. Specifically, we focus on the two most widely used eigenvalues: the $\kappa$ eigenvalue and the $\alpha$ eigenvalue. These eigenvalues are used more often than other eigenvalues due to the physical phenomenon they capture. The $\kappa$ eigenvalue allows for the ability to determine if the system can sustain a chain reaction, while the $\alpha$ eigenvalue captures the asymptotic time dependent behavior of the system. An advantage of using these eigenvalues resides in the biorthogonality of the solutions with their adjoint counterpart. This feature allows us to simplify the expressions and to obtain appropriately weighted definitions that highlight important physics and regions of an assembly.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

Modeling and evaluating 239 Pu and 235 U PFNS and average prompt-neutron multiplicity [Slides]

The following are currently in VIII.1 LANL and IAEA test files: 239 Pu nu-bar including CGMF modeling and CEA data, 239 Pu PFNS including Chi-Nu & CEA data, 239 Pu(n,f) cross section including fission TPC data. The following are currently being tested: 235 U nu-bar including CGMF modeling and 235 U PFNS including Chi-Nu data. Upcoming tasks include the correction of 235 U Chi-Nu PFNS at higher E inc , benchmarking 235 U PFNS and nu-bar evaluations, and getting 238 U nu-bar.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗