Precise integral measurement of fast neutron-induced fission product yields for various actinides is of high interest for applied nuclear science. The goal of this effort is to improve uncertainties in fission product yield values of 239Pu. Fission was induced in a pure 239Pu (>99%) target using the Godiva IV critical assembly in burst mode. The irradiated sample was transferred to a high-resolution γ-ray detector within 45 minutes. γ-ray list mode data was collected from 45 minutes to 1 week after the irradiation. γ-ray spectroscopy was performed to analyze the time dependent γ-ray yields using an automated peak search algorithm to identify isotopes by their decay γ-ray energy and half-life. The initial activity for each isotope identified was used to calculate their fission product yield.

The Short-Lived Fission Product Yield (SLFPY) project is one of many efforts in the NA22 F2016 Venture Project. The objective of the SLFPY project is to provide improved measurements of fission product yields for select nuclides. The measured fission product yields are needed for various areas of applied and fundamental nuclear science. Data exists for fission product yields on fissioning of 235U, 238U, and 239Pu with fission spectrum neutrons. This report summarizes the experimental setup, analysis, and results of the 239Pu fission product yields.

Two isotopically-characterized targets of high-purity (99.961±0.002%) 238U metal and a neutron dosimetry package were exposed to a pulsed neutron irradiation using the Godiva IV critical assembly at the National Criticality Experiments Research Center to measure the integral fission product yields produced in a Watt fission neutron-energy spectrum. Following irradiation, the fissioned targets were counted with two broad energy germanium detectors. Estimates of the total fissions for each target were made using measurements of well-known fission products. The total number of fissions in each target were then used to measure the fission yields of 33 fission products, 18 of which are published as new measurements. Twelve of the 18 isotopes were in agreement with ENDF/B-VIII.0 using the zeta test and treating ENDF/B-VIII.0 as the true mean: 84mBr, 93Y, 94Y, 104Tc, 134I, 135I, 138Xe, 141Ba, 142Ba, 142La, 146Ce, and 149Nd. The ENDF/B-VIII.0 fission yield for 139Ba was only slightly outside of 1-σ relative to the uncertainty measured in this work. Uncertainties for 5 fission product yields were markedly improved over those reported in ENDF/B-VIII.0: 128Sn, 129Sb, 130gSb, 131mTe, and 133I. The fission yields measured for 128Sn, 131mTe, and 133I were in good agreement with the ENDF/B-VIII.0 predicted fission yields. However, fission yields measurements of the isotopes 129Sb, and 130gSb deviated from the ENDF/B-VIII.0 value. Both of these isotopes reside near the doubly magic nuclear shell closures of 50 protons and 82 neutrons. These fission yields may be suppressed because of the odd-odd nuclear structure of 130gSb and the odd-even structure of 129Sb and isomer splitting. Comparisons to other recent work and a brief review with citations are included to support this conclusion.

Abstract To validate lead (Pb) nuclear cross sections, a series of integral experiments to measure lead void reactivity worth was conducted systematically in three fast neutron spectra with different fuel compositions on the Comet critical assembly of the National Criticality Experiments Research Center. Previous experiments in high-enriched uranium (HEU)/Pb and low-enriched uranium (LEU)/Pb systems had been performed in 2016 and 2017, respectively. A follow-on experiment in a plutonium (Pu)/Pb system has been completed. The Pu/Pb system was constructed using lead plates and weapons-grade Pu plates that had been used in the Zero Power Physics Reactor (ZPPR) of Argonne National Laboratory until the 1990s. Furthermore, the HEU/Pb system was reexamined on the Comet critical assembly with a newly installed device that can measure the compression of the stack, improving reproducibility. Using the lead void reactivity worth measured in these three cores with different fuel compositions, the latest nuclear data libraries, JENDL-4.0 and ENDF/B-VIII.0, were tested with the Monte Carlo calculation code MCNP® version 6.1. As a result, the calculations by ENDF/B-VIII.0 were confirmed to agree with lead void reactivity worth measured in all the cores. It was furthermore found that the calculations by JENDL-4.0 overestimate by more than 20% for the Pu/Pb core while being in good agreement for the HEU/Pb and LEU/Pb cores.

Abstract To validate lead (Pb) nuclear cross sections, a series of integral experiments to measure lead void reactivity worths was conducted in a high-enriched uranium (HEU)/Pb system and a low-enriched uranium (LEU)/Pb system using the Comet Critical Assembly at the National Criticality Experiments Research Center. There is a follow-on experiment to measure the lead void reactivity worths in a plutonium/Pb system that is currently under investigation. The critical experiments in the two uranium systems were designed to provide complementary data sets having different sensitivities to scattering cross sections of lead. The larger amount of the 238U present in the LEU/Pb core increases the neutron importance above 1 MeV compared with the HEU/Pb core. Since removal of lead from the core shifts the neutron spectrum to the higher energy region, positive lead void reactivity worths were observed in the LEU/Pb core while negative values were observed in the HEU/Pb core. This technical note is a preliminarily report of the experimental analysis results for the lead void reactivity worths with the Monte Carlo calculation code MCNP® version 6.1 together with nuclear data libraries JENDL-4.0 and ENDF/B-VII.1. The calculation values were found to overestimate the negative reactivity worths for the HEU/Pb core while being consistent for the LEU/Pb core.

The KRUSTY critical experiment was a joint collaboration between Los Alamos National Laboratory, NASA, and Y-12. The project designed and tested a prototype reactor designed for space applications as shown in Fig. 1. The reactor was designed to provide fission energy to increase the temperature differential between the “hot” and “cold” ends of the heat pipes which in turn drive Stirling engines. The reactor itself consists of highly enriched uranium (HEU) fuel reflected by beryllium oxide (BeO). The prototype version also has a stainless steel bio-shield which provides additional reflection. The stainless steel shield is expected to reduce leakage, but not greatly impact the spectrum of the system or alter the reactors neutronic properties.

Results from the use of a commercial, off-the-shelf X-ray scanner using storage phosphors to measure neutron activation in 1- and 2-D are presented. The technique consists of irradiating thin foils or wires of various elements, then placing the activated material on the storage phosphors to expose them. The amount of exposure is proportional to the activation obtained. Examples of wires, small foils, and large area foils with asymmetric irradiation using critical assemblies are presented. Combined with isotope-specific gamma counting of the entire foil or wire, the technique offers a simple way to obtain both qualitative and quantitative 2-D activation information.

Radiation Isotope Identification Devices (RIID) are used in homeland security and other applications to identify radioactive materials. RIIDs have trouble automatically identifying radioisotopes accurately and robustly, which leads to both false positive and false negative alarms. Expert spectroscopists are often called to resolve difficult-to-resolve alarms. Spectroscopists are estimated to correctly resolve ~90% of such alarms. A study of the four most commonly used commercial RIID detectors found that the overall “correct” identification value in a laboratory setting was 39%. False positive identifications occurred 44% of the time and false negative identifications occurred 10% of the time. The current study shows that RIID accuracy can be significantly improved by using a spectrum denoising signal processing step based on a wavelet shrinkage method.

The usefulness of traditional alpha detectors for contamination monitoring is limited by the size and sensitivity of the detectors and by the short range of alpha particles in air. The long-range alpha detector (LRAD) detects the ions produced by the alpha particles passing through air, rather than the alpha particles themselves. Thus, LRAD detection is limited by the range of the ions (tens of meters), rather than the range of the alpha particles (a few centimeters). Since it collects all ions simultaneously, an LRAD monitor will be sensitive to all of the sources of contamination contained within it. In addition, the electronic noise within the LRAD can be reduced so that better sensitivity than that of traditional detectors is possible. Both soil surface and object monitors incorporate these advantages in their designs. Field-test results for these monitors are discussed

Radioactive waste containing fissile material is frequently encountered in decontamination and decommissioning activities. For the most part, this waste is placed in containers or drums and stored in storage facilities. The amount of fissile material in each drum is generally small because of criticality safety limits that have been calculated with computer transport codes such as MCNP,1 KENO,2 or ONEDANT.3 To the best of our knowledge, no experimental critical mass data are available to verify the accuracy of these calculations or any calculations for systems containing fissile material (U-235, Pu-239, U-233) in contact with matrix material such as Al2O3, CaO, SiO2, Al, MgO, etc. The experiments presented in this paper establish the critical masses of highly enriched uranium foils diluted to various X/235U ratios with polyethylene and SiO2, polyethylene and aluminum, polyethylene and MgO, polyethylene and Gd, polyethylene and Fe, and moderated and reflected with polyethylene. In addition, these critical mass experimental data will be used to validate cross section data.