D.W. Kneff

Helium production in pure elements, isotopes, and alloy steels by 14. 8-MeV neutrons

The results of an extensive series of total helium production cross-section measurements for incident neutrons in the 14- to 15-MeV energy region are presented, and an experimental data base for the prediction of helium generation in candidate fusion reactor materials is provided. The measurements were made by isotope-dilution gas mass spectrometry. They include the pure elements Be, B, C, O, F, Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, Nb, Mo, Ag, Sn, Ta, Pt, Au, and Pb; the separated isotopes of B, Fe, Ni, Cu, and Mo; and the alloy steels Type 316 stainless steel, HT-9, and 9 Cr-1 Mo. The results are in generally good agreement with other total helium production measurements in the literature, but comparisons with the ENDF/B-V nuclear data file indicate that the helium gas production files require revision for the structural elements vanadium, chromium, manganese, cobalt, and nickel. Comparisons with published cross sections for individual reaction channels indicate that reactions other than (n,..cap alpha..) contribute significantly to helium production in several materials.

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.

Experimental helium generation cross sections for fast neutrons

Abstract Total helium generation cross sections of Al, V, Fe, Ni, Cu, Zr, Mo, Au, and the separated isotopes of Fe, Ni, Cu, and Mo have been measured for ∼14.8-MeV T(d,n) neutrons and for a ∼0–32 MeV Be(d,n) neutron field. The results, obtained using high-sensitivity gas mass spectrometry, are presented in this paper along with a summary of our previous T(d,n) helium generation cross section measurements.

Experimental and Theoretical Determination of Helium Production in Copper and Aluminum by 14.8-MeV Neutrons

AbstractFast-neutron-induced total helium production cross sections can be determined from a combination of spectrum-integrated measurements and theoretical calculations. The calculations provide information on the energy-dependent cross-section shape that is generally unavailable from the limited experimental data. The measurements in turn provide a normalization for the calculations. In the present work, total helium production cross sections for copper and aluminum bombarded with ~14.8-MeV neutrons from the T(d,n) reaction have been measured by high-sensitivity gas mass spectrometry, and independently calculated using the Hauser-Feshbach statistical model. The experimental results are 51 ± 3 mb for copper and 143 ± 7 mb for aluminum, with corresponding values of 50 and 139 mb obtained from the theoretical calculations. The agreement demonstrates that this statistical model has the potential to predict total helium production cross sections for fusion energy neutrons. Comparison of the experimental resu...

A new technique for enhancing helium production in ferritic materials

Abstract Analyses of iron samples irradiated up to 1027n/m2 in HFIR found more helium than was expected from fast neutron reactions at high neutron fluences. The helium excess increased systematically with neutron exposure, suggesting a transmutation-driven process. The extra helium may be produced in two different ways, either by fast neutron reactions on the transmuted isotopes of iron or by a thermal neutron reaction with the radioactive isotope 55Fe. Radiometric and mass spectrometric measurements of the iron isotopes composing the irradiated samples have been used to determine limits on the cross sections for each process. Either of these processes can be used to enhance helium production in ferritic materials during irradiations in mixed-spectrum reactors by isotopically enriching the samples. Further work is needed to clarify the reaction mechanisms and helium-production cross sections. Our measurements determined the thermal neutron total absorption cross section of 55Fe to be 13.2 ± 2.1 barn.

Helium accumulation fluence dosimetry for fusion reactor materials irradiations

Abstract Extensive sets of helium accumulation fluence monitors (HAFMs) have recently been incorporated in both accelerator- and fission reactor-produced neutron spectra, and results to date have effectively demonstrated their use for accurately characterizing fusion materials neutron test environments. An experiment with 14.8-MeV neutrons from the T(d,n) reaction has combined HAFM materials with radiometric foil dosimetry to provide a detailed fluence profile for the highest-flux irradiation region. Results from a Be(d,n) irradiation are being used to determine the energy sensitivity of HAFMs in a ~ 0–32 MeV neutron spectrum, characterize the Be(d,n) neutron environment, and measure helium generation cross sections for a number of pure elements. Several sets of HAFM materials have been incorporated in fission reactor irradiations to produce an optimum HAFM dosimetry set for environments containing both thermal and fission-spectrum neutrons.

Helium Production by 9.85-MeV Neutrons in Elemental Iron, Nickel, and Copper and in 56Fe and 58,60,61Ni

Neutron-induced helium production, which can contribute substantially to radiation damage, is an important parameter in the choice of structural materials and other component materials for both fusion and fission reactors. Here, helium production cross sections for the elements iron, nickel, and copper and for the isotopes {sup 56}Fe, {sup 58}Ni, {sup 60}Ni, and {sup 61}Ni for 9.85-MeV neutrons have been measured by irradiation with an intense, quasi-monoenergetic neutron source followed by helium analysis with isotope dilution gas mass spectrometry. The results are in fair agreement with (n, {alpha}) cross sections measured by alpha-particle detection and integration over the alpha-particle energies and angular distributions.

Measurement of the 9Be(n,2n)8Be reaction cross section in the 9Be(d,n) thick-target neutron spectrum

Abstract 4 He production has been measured by an isotopic dilution mass spectrometry method, for the irradiation of Be-metal samples by fast neutrons from the 9 Be(d,n) source reaction, corresponding to 7-MeV deuterons incident on a thick Be-metal target. The 58 Ni(n,p) 58g+m Co dosimetry reaction was employed as a reference standard for measurement of the neutron flux. The integral cross section for the 9 Be(n,2n) 8 Be(2 4 He) reaction ( 8 Be breaks up very quickly into two α-particles) was deduced by applying a correction of 9.4% to account for the 9 Be(n,α) 6 He reaction contribution to the total 4 He yield from neutrons on Be. The experimental integral reaction cross-section ratio of 9 Be(n,2n)2 4 He to 58 Ni(n,p) 58g+m Co in this spectrum was found to be 1.217, with an error of 3.5%. This is in good agreement with the corresponding calculated ratio of 1.161 which is based on an average of results obtained by considering four distinct representations of the 9 Be(d,n) neutron spectrum and ENDF/B-VI values for the reaction cross sections. The scatter of the calculated results amounts to 0.5%. It is small because the shapes of the response functions, R ( E ) = o ( E ) σ ( E ), for these two reactions in this neutron spectrum are very similar. The observed C/E is 0.954. This is very good agreement, considering the various uncertainties involved, including those for the differential cross sections from ENDF/B-VI. This integral measurement is sensitive to the differential reaction cross section in the range 3–6 MeV. The uncertainty attributable to the 58 Ni(n,p) 58g+m Co reaction differential cross section in this range is estimated to be 3.5%. Therefore, the results of the present investigation indicate that ENDF/B-VI represents the 9 Be(n,2n)2 4 He reaction differential cross section quite well over this energy range.

Radiation damage in crystalline phases by (n, α) reactions

Abstract Crystalline materials containing 6 Li or 10 B were irradiated with thermal neutrons and the resultant (n, α) reactions produced structural damage. Unenriched cubic boron nitride was heavily damaged, with a density decrease of 34 ± 2%, by irradiation with 4 × 10 20 n / cm 2 (~16% B burnup). Fluences of 2 and 8 × 10 19 n / cm 2 produced density decreases of about 3 and 9%, respectively. This behavior, together with the effect of annealing, was generally similar to that previously observed in diamond irradiated with fast neutrons. However, the heavily damaged BN reverted to the hexagonal form at a lower annealing temperature than unirradiated material, whereas the opposite is true for heavily damaged diamond. Irradiation of 6 LiNbO 3 and 6 Li 3 NbO 4 , respectively, with a thermal neutron fluence of 4 × 10 18 n / cm 2 (~0.4% Li burnup) produced lattice parameter expansions of ~0.1%. Any lattice expansions in 6 Li 2 Ti 3 O 7 and 6 Li 4 Ti 5 O 12 irradiated to the same burnup were

Helium production in copper by a thermal three-stage reaction

Abstract A three-stage reaction process has been identified in copper that produces significant 4He concentrations at high thermal neutron fluences. Cross sections have been determined for four reactions that allow us to calculate the total 4He production in copper for high-fluence irradiations in HFIR.