L.R. Greenwood

A new calculation of thermal neutron damage and helium production in nickel

Abstract It has long been recognized that thermal neutron irradiations of nickel-bearing materials generate high levels of helium from the sequential 58 Ni(n,γ) 59 Ni(n, α) 56 Fe nuclear reactions. This process is used to simulate fusion reactor helium-to-displacement damage rates in stainless steel during fission reactor irradiations. However, it has not previously been recognized that the 56 Fe recoils will also cause significant displacement damage. At thermal neutron energies the 340 keV 56 Fe recoil event will displace 1762 atoms in nickel. The helium (appm)-to-displacement ratio from this process will be 567, a value that can be used to correct previous displacement calculations in nickel-bearing materials. This effect can nearly double the displacement damage in nickel in HFIR at high neutron fluences.

Determination of the neutron flux and energy spectrum in the low-temperature fast-neutron facility in CP-5, calculations of primary-recoil and damage-energy distributions, and comparisons with experiment☆

Abstract We have determined the absolute differential neutron-energy spectrum for the low-temperature fast-neutron irradiation facility in the CP-5 reactor by means of a 20-foil activation technique. This technique employs the most recent version of the SAND-II computer code, which iteratively unfolds the neutron spectrum by fitting the foil activities. A Monte Carlo routine was also employed to calculate standard-deviation errors in each neutron-energy group. Using this differential neutron spectrum we have calculated, for numerous elements, total recoil cross sections, detailed primary-recoil group distributions, total damage-energy cross sections, damage-energy distributions, and an error analysis based on the uncertainties in the neutron spectrum. The significance of this information with respect to the interpretation of various neutron radiation-damage experiments, including sputtering, disordering of ordered alloys, and changes in critical current of A-15 compound superconductors is discussed. A detailed comparison is made among initial resistivity-damage rates for five widely different (well characterized) neutron sources, fission fragments, and heavy ions.

A comparison of measured and calculated helium production in nickel using newly evaluated neutron cross sections for 59Ni

Fusion reactors will produce high levels of helium in surrounding materials with a helium (appm)-to-displacement ratio of about 10-to-1 in stainless steel. This high ratio can be obtained in mixed-spectrum reactors, which are used for fusion materials testing, due to unusually high thermal neutron cross sections for the sequential reactions 58Ni(n,γ)59Ni(n,α)56Fe. The highenergy (∼340 keV)56Fe recoils also add significantly to the displacement damage at the rate of one DPA per 567 appm helium. Until now, the calculation of helium production in nickel has been done in a semi-empirical manner due to a lack of evaluated cross sections for 59Ni. However, this approach cannot be readily transferred between different reactors since we do not know the contributions from epithermal neutrons in different neutron spectra. A new evaluation of the 59Ni cross sections has recently been completed, permitting us to calculate all of the required reaction rates for any given neutron spectrum. Radiometric dosimetry and helium measurements have recently been completed for several different mixed-spectrum reactors. Precise comparisons of the helium production cross sections and measurements can thus be made in well-characterized neutron spectra. Data are presented for several recent fusion materials irradiations in the Oak Ridge Research Reactor and High Flux Isotope Reactor at Oak Ridge National Laboratory and for the Experimental Breeder Reactor II at Argonne National Laboratory. Procedures are recommended for calculating helium production for nickel-bearing materials in any neutron spectrum.

Neutron-source characterization and radiation-damage calculations for material studies

Abstract In our quest to understand radiation damage in materials, it is vital that we characterize radiation sources in terms of neutron flux and spectra as well as the more fundamental displacement damage, gas production, and transmutation rates. Such data are crucial to correlations of materials property changes in different environments and to predictions of materials performance in inaccessible environments, such as fusion reactors. Dosimetry techniques have been developed to measure the neutron flux and spectra in diverse facilities including thermal, fast, and mixed reactors, T(d, n) and Be(d, n) accelerator sources, and high-energy spallation sources. Displacement damage cross-sections have been calculated for 36 elements spanning the periodic table. All of these exposure parameters can now be routinely measured with 10–15% relative accuracy at all existing radiation effect facilities.

Neutron irradiation facilities at the intense pulsed neutron source

Abstract There are facilities for irradiation down to 4.2K with fast neutrons at Argonnes recently constructed Intense Pulsed Neutron Source (IPNS-I). The large irradiation volume, the neutron spectrum and flux, the ability to transfer samples without warm up, and the dedication of the facilities during the irradiation make this ideally suited for radiation damage studies on components for superconducting fusion magnets. Possible experiments are discussed on cyclic irradiation and annealing of stabilizers in a high magnetic field, mechanical tests on organic insulation irradiated at 4K, and superconductors measured in high fields after irradiation.

Experimental method for investigating helium effects in irradiated vanadium

Abstract Analyses have been performed which indicate that an effective method for experimentally investigating helium effects in neutron irradiated vanadium base alloys can be developed. The experimental procedure involves only modest modifications to existing procedures currently used for irradiation testing of vanadium-based alloys in the FFTF reactor. Helium is generated in the vanadium alloy by decay of tritium which is either preinjected or generated within the test capsule. Calculations indicate that nearly constant He/dpa ratios of desired magnitude can be attained by proper selection of experimental parameters. The proposed method could have a major impact on the development of vanadium base alloys for fusion reactor applications.

Measurements of neutron spectra and fluxes at spallation-neutron sources and their application to radiation effects research

Abstract We have measured the neutron spectra, fluxes, and flux distributions produced by nuclear spaliation resulting from 478 MeV proton bombardment of tantalum and depleted uranium targets surrounded by a thick lead neutron reflector. The configuration was chosen to simulate a radiation effects facility at a spallation-neutron source. The method of multiple foil activation with spectrum unfolding by the STAYSL computer code was used to measure the neutron spectra. The experimental results are compared in detail with the results of computer calculations on the same configuration of targets and reflector. The neutron production and transport codes HETC and VIM were employed in these calculations. The neutron spectra, and characteristic parameters for studies of radiation effects, are compared with those of fission-neutron sources. Based on the above measurements, the predicted performance and capabilities of the Radiation Effects Facility at the Intense Pulses Neutron Source at Argonne National Laboratory are discussed.

Microstructure and tensile properties of T (d,n) and Be (d,n) neutron irradiated nickel, niobium and 316SS

Abstract The radiation damage induced in nickel, niobium and 316SS by T (d,n) and Be (d,n) neutrons was evaluated by microstructural analysis and mechanical property measurements. Also, the neutron flux gradients in the Be (d,n) neutron source were compared to the positional dependence of the yield strength of niobium wires. The microstructure and yield stress increase of T (d,n) and Be (d,n) neutron irradiated nickel were similar while there were differences in niobium.

Recent developments in neutron dosimetry and radiation damage calculations for fusion-materials studies

Abstract This paper is intended as an overview of activities designed to characterize neutron irradiation facilities in terms of neutron flux and energy spectrum and to use these data to calculate atomic displacements, gas production, and transmutation during fusion materials irradiations. A new computerized data file, DOSFILE, has been developed to record dosimetry and damage data from a wide variety of materials test facilities. At present data are included from 20 different irradiations at fast and mixed-spectrum reactors, T(d,n) 14 MeV neutron sources, Be(d,n) broadspectrum sources, and spallation neutron sources. Each file entry includes activation data, adjusted neutron flux and spectral data, and calculated atomic displacements and gas production. Such data will be used by materials experimenters to determine the exposure of their samples during specific irradiations. This data base will play an important role in correlating property changes between different facilities and, eventually, in predicting materials performance in fusion reactors. All known uncertainties and covariances are listed for each data record and explicit references are given to nuclear decay data and cross sections. The computer code package SPECTER has also been revised and updated to permit rapid calculation of displacements, gas production, and total dose for any given neutron spectrum. Recent improvements include new calculations with ENDF/B-V neutron cross sections, inclusion of (n,γ) and β-decay processes, inclusion of kerma cross sections for total dose, and the ENDF/B-V gas-production file. Users need only provide a neutron spectrum, with uncertainties if known, and the code will calculate spectral-averaged values of the above quantities as well as recoil atom distributions. Both the DOSFILE and SPECTER computer codes are readily accessible to the fusion community via the National Magnetic Fusion Energy Computer at Lawrence Livermore Laboratory.

New ideas in dosimetry and damage calculations for fusion materials irradiations

Nuclear data and techniques are being developed to improve our ability to characterize fusion materials irradiations. The production of long-lived isotopes have been measured near 14 MeV, including reactions from AI, Fe, and Mo to 26A1, 53Mn, and 94Nb, respectively. These data can be used for fusion reactor activation, dosimetry, and plasma diagnostics. Dosimetry cross sections can be adjusted using measurements at Be(d,n) sources (Ed = 7–40 MeV) and mono-energetic neutron sources from 1 to 14 MeV. Helium production cross sections are being tested at all fusion irradiation facilities. We have recently discovered a new thermal helium production in copper, similar to the well-known effect in nickel. Recommended cross sections and procedures are included. A new program SPECOMP has been developed to calculate displacement damage for compounds. This program uses the SPECTER recoil atom distributions and integrates over secondary displacement functions for each combination of incident ion and matrix material to determine displacement cross sections for alloys, insulators, and breeder materials.