Omega West Reactor Research Articles

Fission Product Data Measured at Los Alamos for Fission Spectrum and Thermal Neutrons on 239Pu, 235U, 238U

We describe measurements of fission product data at Los Alamos that are important for determining the number of fissions that have occurred when neutrons are incident on plutonium and uranium isotopes. The fission-spectrum measurements were made using a fission chamber designed by the National Institute for Standards and Technology (NIST) in the BIG TEN critical assembly, as part of the Inter-laboratory Liquid Metal Fast Breeder Reactor (LMFBR) Reaction Rate (ILRR) collaboration. The thermal measurements were made at Los Alamos' Omega West Reactor. A related set of measurements were made of fission-product ratios (so-called R-values) in neutron environments provided by a number of Los Alamos critical assemblies that range from having average energies causing fission of 400–600 keV (BIG TEN and the outer regions of the Flattop-25 assembly) to higher energies (1.4–1.9 MeV) in the Jezebel, and in the central regions of the Flattop-25 and Flattop-Pu, critical assemblies. From these data we determine ratios of fission product yields in different fuel and neutron environments (Q-values) and fission product yields in fission spectrum neutron environments for 99Mo, 95Zr, 137Cs, 140Ba, 141,143Ce, and 147Nd. Modest incident-energy dependence exists for the 147Nd fission product yield; this is discussed in the context of models for fission that include thermal and dynamical effects. The fission product data agree with measurements by Maeck and other authors using mass-spectrometry methods, and with the ILRR collaboration results that used gamma spectroscopy for quantifying fission products. We note that the measurements also contradict earlier 1950s historical Los Alamos estimates by ∼5–7%, most likely owing to self-shielding corrections not made in the early thermal measurements. Our experimental results provide a confirmation of the England-Rider ENDF/B-VI evaluated fission-spectrum fission product yields that were carried over to the ENDF/B-VII.0 library, except for 99Mo where the present results are about 4%-relative higher for neutrons incident on 239Pu and 235U. Additionally, our results illustrate the importance of representing the incident energy dependence of fission product yields over the fast neutron energy range for high-accuracy work, for example the 147Nd from neutron reactions on plutonium. An upgrade to the ENDF library, for ENDF/B-VII.1, based on these and other data, is described in a companion paper to this work.

High-energy and thermal-neutron imaging and modeling with an amorphous silicon flat-panel detector

The Los Alamos Neutron Science Center (LANSCE) operates two spallation neutron sources dedicated to research in materials science, condensed-matter physics, and fundamental and applied nuclear physics. Prior to 1995, all thermal neutron radiography at Los Alamos was done on a beam port attached to the Omega West reactor, a small 8 MW research reactor used primarily for radioisotope production and prompt and delayed neutron activation analysis. After the closure of this facility, two largely independent radiography development efforts were begun at LANSCE using moderated cold and thermal neutrons from the Target-1 source and high-energy neutrons from the Target-4 source. Investigations with cold and thermal neutrons employed a neutron converter and film, a scintillation screen and CCD camera system, and a new high-resolution amorphous silicon (a-Si) flat-panel detector system. Recent work with high-energy neutrons ( E n>1 MeV) has involved storage-phosphor image plates. Some comparison high-energy images were obtained with both image plates and the a-Si panel and showed equivalent image quality for approximately equal exposure times.

Tensile property changes of metals irradiated to low doses with fission, fusion and spallation neutrons

Radiation effects due to low doses of spallation neutrons are compared directly to those produced by fission and fusion neutrons. Yield stress changes of pure Cu, alumina dispersion strengthened Cu, and AISI 316 stainless steel irradiated at 36–55°C in the Los Alamos Spallation Radiation Effects Facility (LASREF) are compared with earlier results of irradiations at 90°C using 14 MeV D-T fusion neutrons at the Rotating Target Neutron Source and fission reactor neutrons in the Omega West Reactor. At doses up to 0.04 displacements per atom (dpa), the yield stress changes due to the three quite different neutron spectra correlate well on the basis of dpa in the stainless steel and the Cu alloy. However, in pure Cu, the measured yield stress changes due to spallation neutrons were anomalously small and should be verified by additional irradiations. With the exception of pure Cu, the low dose, low temperature experiments reveal no fundamental differences in radiation hardening by fission, fusion or spallation neutrons when compared on the basis of dpa.

Correlation of mechanical property changes in neutron-irradiated pressure vessel steels on the basis of spectral effects

Comparisons are made of tensile data on specimens of A212B and A302B pressure vessel steels irradiated at low temperatures (40–90°C) and to low doses ( < 0.1 dpa) with 14 MeV D-T fusion neutrons in the Rotating Target Neutron Source (RTNS-II), with fission reactor neutrons in the Omega West Reactor (OWR) and the Oak Ridge Research Reactor (ORR), and with the highly thermal spectrum at the pressure vessel surveillance positions of the High Flux Isotope Reactor (HFIR). For each neutron spectrum, damage cross sections are determined for several defect production functions derived from atomistic computer simulations of collision cascades. Displacements per atom (dpa) and the numbers of freely migrating defects are tested as damage correlation parameters for the tensile data. The data from RTNS-II, OWR and ORR correlate fairly well when compared on the basis of dpa, but the data from HFIR show that only about one sixth as many dpa are needed to produce the same radiation-induced yield stress changes as in the other neutron spectra. In the HFIR surveillance position a significant fraction of the displacements is produced by recoils resulting from thermal neutron captures. Having energies of about 400 eV, these recoils are much more efficient per unit energy at producing freely migrating defects than the high energy recoils responsible for most of the displacements in the other neutron spectra considered. A significantly better correlation of data from HFIR with those from the other spectra is achieved when the property changes are compared on the basis of the production of freely migrating self-interstitial defects. This parameter may better represent the defects participating in the radiation strengthening process in this temperature and fluence regime.

A distributed data-acquisition system for gamma spectral analysis

The neutron-activation analysis staff at Los Alamos National Laboratory's Omega West Reactor has designed and constructed a distributed data-acquisition system for handling a large volume of gamma-ray spectra. Currently the system consists of five independent data-acquisition stations connected to a number of workstations via an Ethernet network. Each data-acquisition station supports up to eight detectors operating simultaneously and is capable of initiating automatic sample manipulation including, but not limited to, sample loading, irradiation, spectrum recording and spectrum storage. Control of the data-acquisition station can be accomplished locally with a computer terminal directly connected to the data-acquisition station or via a workstation connected to the network.

An automated neutron activation procedure for screening PCB content of oil

An analytical procedure for rapidly screening large numbers of oils for polychlorinated biphenyl (PCB) content has been developed and is now in routine use. Pontential levels of PCBs are inferred from chlorine concentrations as determined using the automated neutron activation analysis (NAA) facility at the Los Alamos National Laboratory Omega West Reactor. The technique is designed to screen up to 200 oil samples per eight hour working day, using a sample volume of approximately 1 milliliter. Because of the automated nature of the analysis, elements in addition to chlorine are determined, when present. These include fluorine, bromine, iodine and sulfur. U.S. National Bureau of Standards (NBS) and U. S. Environmental Protection Agency (EPA) Standard Reference Materials are routinely analyzed for quality assurance purposes. The results of our analyses of these materials for certified elements is discussed as well as results for other non-certified elements observed in the standard materials.

The effects of low doses of 14 MeV neutrons on the tensile properties of three binary copper alloys

Abstract Miniature tensile specimens of high purity copper and copper alloyed respectively with five atom percent of Al, Mn, and Ni were irradiated with D-T fusion neutrons in the RTNS-II to fluences up to 1.3 × 10 22 n / m 2 at 363 K. To compare fission and fusion neutron effects, some specimens were also irradiated at the same temperature to similar damage levels in the Omega West Reactor (OWR). Tensile tests were performed at room temperature, and the radiation-induced changes in tensile properties are compared as functions of displacements per atom (dpa). The irradiation-induced strengthening of Cu-5% Mn is greater than that of Cu-5% Al and Cu-5% Ni, which behave about the same. However, all the alloys sustain less irradiation-induced strengthening by 14 MeV neutrons than pure copper. The effects of fission and fusion neutrons on the yield stress of the copper alloys correlate well on the basis of dpa, in contrast to the behavior of pure copper.

Effects of the neutron spectrum on mechanical property changes in low dose irradiations

Mechanical property measurements from comparative low dose irradiations of metals and alloys are reviewed. The emphasis is on recent experiments involving miniature tensile specimens irradiated at the Rotating Target Neutron Source II (RTNS-II) at Lawrence Livermore National Laboratory and the Omega West Reactor at Los Alamos National Laboratory. Pure metals, model alloys and structural alloys have been irradiated in RTNS-II from room temperature to 723 K to fluences as high as 8 × 10 22 n/m 2. Where temperature effects are apparent, the effect of increasing the irradiation temperature is to delay the onset of irradiation hardening to higher fluences. Companion fission reactor irradiations have been more limited in scope, but comparative information has been obtained on tensile properties at irradiation temperatures of 363 K and 563 K. For most of the materials tested the differences in the neutron energy spectrum can be accounted for by using displacements per atom (dpa) as a correlation parameter. The exceptions are all pure metals: copper, niobium and vanadium. There is also evidence for rate effects in some of the data. The rate effects obscure the spectral effects and prevent examination of dpa as a damage correlation parameter.

Effects of neutron irradiation on Nd-Fe-B magnetic properties

Nd-Fe-B permanent magnets from two different manufacturers were irradiated in the Omega West reactor at Los Alamos National Laboratory, with fast neutrons at temperatures of 426 K (153 degrees C) and 350 K (77 degrees C) to fluences of 5.0*10/sup 16/ N/cm/sup 2/ and 6.1*10/sup 16/ N/cm/sup 2/, respectively. At intervals during the irradiation the samples were removed from the reactor and the remanence measured at room temperature. The initial loss of remanence for irradiation at 426 K was 10% for a fluence of 10/sup 15/ N/cm/sup 2/. At 350 K the initial loss rate was roughly half this value. The loss rates were nearly the same for samples from the two different manufacturers. These losses are due to the irradiation since the remanence does not decay with annealing at 426 K. Remagnetization after irradiation results in full recovery of the remanence and roughly a 20% increase in the coercivity. Evidence from this experiment suggests that the primary mechanism for loss of remanence is nucleation of reverse domains by the collision cascade and subsequent growth into the original domain.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Radiation-induced changes in magnetic properties of Nd-Fe-B permanent magnets (abstract)

The high remanence and intrinsic coercivity of Nd-Fe-B magnets make them desirable for use in accelerators. Use in a radiation environment requires that the sensitivity of these magnets to radiation be determined and the mechanisms responsible be understood. We present results comparing the relative radiation sensitivity of a number of commercially available Nd-Fe-B magnets. Small samples of these magnets were irradiated to examine the decay of remanence. Our results show that the sensitivity to radiation varies widely among magnets from different vendors, indicating that microstructure and possibly composition play key roles in determining the loss of remanence. We examined optically the microstructure of several of the magnet materials and report on the correlation between microstructure and radiation-induced changes in magnetic properties. Samples were neutron irradiated at the Omega West Reactor in a neutron flux of 4×1012 n/cm2 s at a temperature of 350 K (77 °C). At various time intervals during the irradiation, samples were removed from the reactor and the open-circuit remanence measured. We also measured the quadrant II demagnetization curve of large samples before and after neutron irradiation to monitor changes in remanence and intrinsic coercivity. In general, these samples showed a decrease in the remanence and an increase in the intrinsic coercivity. We also present results comparing the relative decay of remanence for the large cylindrical samples having three different length-to-diameter ratios. This illustrates the effect of the demagnetizing field in determining the rate of remanence decay.

A new facility for averaged resonance capture studies at the Omega West reactor

A facility has been developed at the Omega West reactor at Los Alamos to provide neutron beams of 2 and 24 keV average neutron energies. The neutron beam currents are comparable with those obtained at the Brookhaven high flux beam reactor. A three-crystal pair spectrometer has been developed to allow acquisition of high resolution primary neutron capture gamma ray spectra. This will be used for nuclear spectroscopy and structure studies.

Optimization of a He-jet activity transport system to use at LAMPF

As part of an assessment of the feasibility for a He-jet coupled on-line mass separator at LAMPF, we have studied performance characteristics of a gas activity transport system under conditions simulating those expected on the main LAMPF beam line. In experiments utilizing a side beam at LAMPF, we have measured absolute transport efficiencies, transit times, aerosol properties, and dependences on beam intensity. Further experiments with a He-jet system at the Omega West Reactor have indicated an optimum configuration of a target chamber to be placed in the LAMPF main beam. The results of these studies suggest that a He-jet activity transport system should work well at LAMPF in the 800-MeV, 1-mA proton beam that is spread over ∼40 cm 2 near the beam stop.

Radiation Effects in Rare-Earth Permanent Magnets

ABSTRACTNd-Fe-B and Sm-Co permanent magnets have been irradiated with fission neutrons and gamma rays. Irradiated samples were periodically removed for room temperature measurements of the open-circuit remanence. Hysteresis loops were measured before and after irradiation. For neutron irradiation, two Nd-Fe-B magnets showed a rapid loss of remanence, while a third magnet from another manufacturer decayed more slowly, suggesting that the radiation hardness of Nd-Fe-B magnets may depend on microstructural details. Irradiation in the Omega West Reactor at Los Alamos with fast neutrons caused the fast-decay samples to have an Wtial loss of remanence of 1% for irradiation at 350 K to a fluence of 1015 n/cm2. Both SmCo5 and Sm2Co17 magnets showed excellent resistance to radiatifg-induied loss of remanence for neutron irradiation to a fluence of 2.6×1018 n/cm2. Results for gamma radiation showed no loss of remanence for a dose of about 49 Mrad using a 60Co source. Possible mechanisms for radiation-induced loss of magnetic properties are discussed.

Swelling of spinel after low-dose neutron irradiation

Swelling was determined in samples of single-crystal MgAl/sub 2/O/sub 4/ spinel, irradiated to doses as high as 8 x 10/sup 22/ n/m/sup 2/ (E > 0.1 MeV) at approx. =50/sup 0/C in the Omega West Reactor. Swelling effectively saturated at approx. =2 x 10/sup 22/ n/m/sup 2/ which corresponds to a damage level of only approx. =2 x 10/sup -3/ dpa. In addition subsequent measurements after irradiation have revealed that the samples continued swelling for several weeks. These results imply that irradiation defects begin to interact by recombination and aggregation at low damage levels in this material at 50/sup 0/C and perhaps continue to cluster at room temperature after irradiation. Rate equations have been employed to determine defect concentrations at saturation. Results to date show that the observed swelling is consistent with the number of surviving defects if swelling per Frenkel defect pair is taken to be one atomic volume.

The effects of low doses of 14 MeV neutrons on the properties of various commercial copper alloys

Miniature tensile specimens of high purity copper and five copper alloys were irradiated with D-T fusion neutrons in the RTNS-II to fluences up to 2.5 × 10 22 n/m 2 at 90°C and 290°C. The series of alloys includes solution-strengthened, precipitate-strengthened and dispersion-strengthened alloys. To compare fission and fusion neutron effects, some of the alloys were also irradiated at the same temperatures to similar damage levels in the Omega West Reactor. Tensile tests were performed at room temperature, and the radiation-induced changes in tensile properties are examined as functions of fluence and displacements per atom (dpa). All the alloys sustain less irradiation-induced strengthening than pure copper. In contrast to pure copper, the effects of fission and fusion neutrons on the yield stress changes in the copper alloys correlate well on the basis of dpa.

Spectral effects in low-dose fission and fusion neutron irradiated metals and alloys

Flat miniature tensile specimens were irradiated to neutron fluences up to 9 × 10 22 n/m 2 in the RTNS-II and in the Omega West Reactor. Specimen temperatures were the same in both environments, with runs being made at both 90°C and 290°C. The results of tensile tests on AISI 316 stainless steel, A302B pressure vessel steel and pure copper are reported here. The radiation-induced changes in yield strength as a function of neutron dose in each spectrum are compared. The data for 316 stainless steel correlate well on the basis of displacements per atom (dpa) while those for copper and A302B do not. In copper the ratio of fission dpa to 14 MeV neutron dpa for a given yield stress change is about three to one. In A302B pressure vessel steel this ratio is more than three at lower fluences, but the yield stress data for fission and 14 MeV neutron-irradiated A302B steel appears to coalesce or intersect at the higher fluences.

Thermal neutron capture gamma rays from sulfur isotopes: Experiment and theory.

We have carried out a systematic investigation of $\ensuremath{\gamma}$ rays after thermal neutron capture by all stable sulfur isotopes ($^{32}\mathrm{S}$, $^{33}\mathrm{S}$, $^{34}\mathrm{S}$, and $^{36}\mathrm{S}$). The measurements were made at the internal target facility at the Los Alamos Omega West Reactor. We detected a larger number of $\ensuremath{\gamma}$ rays: \ensuremath{\sim}100 in $^{33}\mathrm{S}$, \ensuremath{\sim}270 in $^{34}\mathrm{S}$, \ensuremath{\sim}60 in $^{35}\mathrm{S}$, and \ensuremath{\sim}15 in $^{37}\mathrm{S}$. Before developing detailed level schemes, we culled and then consolidated the existing information on energies and ${J}^{\ensuremath{\pi}}$ values for levels of these nuclides. Based on the current data, we have constructed detailed decay schemes, which imply that there are significant populations of 26 excited states in $^{33}\mathrm{S}$, 70 states in $^{34}\mathrm{S}$, 20 states in $^{35}\mathrm{S}$, and 7 states in $^{37}\mathrm{S}$. By checking the intensity balance for these levels and by comparing the total intensity of primary transitions with the total intensity of secondary $\ensuremath{\gamma}$ rays feeding the ground state, we have demonstrated the relative completeness of these decay schemes. For strongly populated levels, the branching ratios based on the current measurements are generally better than those available from previous measurements. In all four cases, a few primary electric dipole ($E1$) transitions account for a large fraction of the capture cross section for that particular nuclide. To understand and explain these transitions, we have recapitulated and further developed the theory of potential capture. Toward this end, we reviewed the theory relating off-resonance neutron capture to the optical-model capture. We studied a range of model-dependent effects (nature and magnitude of imaginary potential, surface diffuseness, etc.) on the potential capture cross section, and we have shown how experimental data may be analyzed using the expression for channel capture suitably modified by a factor that takes into account the model-dependent effects. The calculations of cross sections for most of the primary transitions in the sulfur isotopes are in good agreement with the data. Some discrepancies for weaker transitions can be explained well by an interfering compound-nucleus contribution to capture. This contribution is of the magnitude expected from statistical surveys of resonance capture data. Estimates of the cross section due to the valencecapture mechanism in $s$-wave resonances show that this cross section should dominate the more complicated compound-nucleus contributions.

An automated activation analysis system

An automated delayed neutron counting and instrumental neutron activation analysis system has been developed at Los Alamos National Laboratory's Omega West Reactor (OWR) to analyze samples for uranium and 31 additional elements with a maximum throughput of 400 samples per day. The system and its mode of operation for a large reconnaissance survey will be described.

A Risk Estimate for an Urban-Sited Reactor

A method for estimating the risk from reactor installations is presented and applied to the Omega West Reactor, an 8-MW(th) research reactor at Los Alamos, New Mexico.The method, which considers bo...

Radioresistance of a Pseudomonas Species Isolated from the Omega West Reactor

ABS>The radioresistance of an unidentified species of Pseudomonas contaminating the Omega West Reactor, Los Alamos Scientific Laboratory, was determined under controlled conditions. The organism was found to be much more sensitive to gamma -radiation than originally reported. It had a D value of only 4.4 to 5.6 krad, while a population of 108 cells/ ml could be inactivated by 45 to 57.5 krad.