R.A. Van Konynenburg

Some electron microscopy observations of 14 MeV neutron damage in niobium

Abstract Structural damage has been produced in two niobium specimens by irradiating them with 14 MeV neutrons. The first specimen has been irradiated to a fluence of 420 neutrons nm−2 at 300°C and the second to a fluence of 6600 neutrons nn−2 at about 25°C. The structural damage in both cases was similar to that observed in fission neutron-irradiated niobium and consisted of either clusters of interstitials and vacancies or Frank dislocation loops ranging from about 1 to 12 nm in diameter. The size number–density distribution of clusters in the second specimen was compared with those reported for fission neutron-irradiated niobium at comparable fluences and temperatures. It was found that the rate of accumulation of displacement damage during 14 MeV neutron irradiation was substantially greater than that during irradiation with an equivalent flux of fission neutrons (E < 0.1 MeV). Evidence was observed of multiple cluster formation from high-energy displacement cascades.

Effect of Ionizing Radiation on Moist Air Systems

The radiation chemistry of nitrogen/oxygen/water systems is reviewed. General radiolytic effects in dry nitrogen/oxygen systems are relatively well characterized. Irradiation results in the formation of steady state concentrations of ozone, nitrous oxide and nitrogen dioxide. In closed systems, the concentration observed depends on the total dose, temperature and initial gas composition. Only three studies have been published that focus on the radiation chemistry of nitrogen/oxygen/water homogeneous gas systems. Mixed phase work that is relevant to the gaseous system is also summarized. The presence of water vapor results in the formation of nitric acid and significantly changes the chemistry observed in dry air systems. Mechanistic evidence from the studies reviewed are summarized and discussed in relation to characterizing the gas phase during the containment period of a repository in tuff.

Comments on the draft paper “underground supercriticality from plutonium and other fissile material,” written by C.D. Bowman and F. Venneri (LANL)

In response to a request from the Director of the Los Alamos National Laboratory, several members of the staff of the Lawrence Livermore National Laboratory participated in a technical review of a draft paper by C. D. Bowman and F. Venneri dealing with the potential for nuclear criticality in the geologic disposal of fissile materials. This review consisted of a consideration of the technical issues raised in the draft paper, and did not include discussions with the authors.

Defect production and recovery in FCC metals irradiated at 4.2 K

Abstract Four fee metals. Al, Cu, Ni, and Pt, have been irradiated at 4.2 K at the Livermore Rotating Target Neutron Source (RTNS-II) to fluences of 1–2 × 10 21 n/m 2 . The sample resistivities were monitored during the irradiation. Following irradiation, the sample resistivities were monitored after isochronal anneals to 320 K. The initial resistivity damage rates for all four metals could be predicted to within ± 10% from the results of irradiations in a pure fission spectrum by damage energy scaling. The results of isochronal annealing studies in Al and Cu were nearly identica to recovery curves obtained after fission spectra irradiations to comparable damage levels. On the other hand, both Ni and P exhibited less recovery than that observed after fission spectra irradiations, due primarily to less recovery in Stage 1.

Fusion-neutron effects on magnetoresistivity of copper stabilizer materials

Abstract We have measured the changes which occur in the magnetoresistivity of coppers (having various purities and pretreatments) at magnetic fields up to 12 T during the course of sequential fusion neutron irradiations at about 4 K and anneals to room temperature. In conjunction with work in progress by Coltman and Klabunde of Oak Ridge National Laboratory (ORNL), the results should lead to engineering design data for the stabilizers of superconducting magnets in fusion reactors. These magnets are expected to be irradiated during reactor operation and warmed to room temperature periodically during maintenance.

Fusion-neutron damage in superconductors and magnet stabilizers

Two NbTi and two Cu wires were irradiated with 14.8 MeV neutrons at 4.2 K to fluences ot 6–8 × 1020 n/m2, using RTNS-II. Electrical resistances ot Cu were monitored during irradiation. Magnetoresistances were measured in fields up to 12.4 T betore and alter irradiation and after isochronal annealing up to 273 K. Critical currents ol NbTi were measurea after irradiation, in fields up to 10 T. The initial rate ot increase of resistivity of the Cu was found to be 2.23 × 1031 (Ω-m)/(n/m2). Ihis rate could be predicted from fission reactor irradiations using damage energy scaling. Ihe maximum observed change in the NbTi critical current was a decrease ot 3% at 4 T. At 6, 8, and 10 T there were no significant changes.

Fusion neutron disordering of CU3AU

Abstract We have measured the disordering rate of highly-ordered Cu3Au (S = .99) at 4 K during irradiation with 14.8 MeV neutrons at RTNS-II. The rate was measured by monitoring changes in resistivity, which can be related to the degree of order by ρ(s) = (ρ(0) 3- ρ(1) (1-s2) + ρ(1) where S is the long-range order parameter and ρ is the sample resistivity. The disordering rate was found to be 15.7 × 10−20 cm2/n. The analysis of the results and a comparison with fission reactor irradiations indicate that the disordering rate in Cu3Au scales with damage energy. This is in agreement with observations made on longrange ordered A15 superconducting compounds.

Corrosion product identification and relative rates of corrosion of candidate metals in an irradiated air-steam environment

The US Department of Energy, through its Office of Civilian Radioactive Waste Management, is conducting the Yucca Mountain Project. The purpose of this project is to determine the suitability of Yucca Mountain, in southern Nevada, as a location for a high level nuclear waste repository. Previously reported work by others indicates that dicopper trihydroxide nitrate, Cu{sub 2}NO{sub 3}(OH){sub 3}, forms on copper and copper alloys subjected to irradiated moist air near room temperature. The authors have performed experiments over a range of temperature and humidity, and have found that this species is formed at temperatures up to at least 150C if low to intermediate relative humidities are present. At 150C and 100% relative humidity, only Cu{sub 2}O and CuO were observed. The relative general corrosion rates of the copper materials tested in 1-month experiments at dose rates of 0.7 and 2.0 kGy/h were Cu > 70/30 Cu-Ni > Al-bronze. High-nickel alloy 825 showed no observable corrosion.

Behavior of carbon-14 in waste packages for spent fuel in a tuff repository

Abstract Light water reactor spent fuel in the U.S. contains only about 37 MBq of carbon-14 per kg U (1 curie per metric tonne of initial uranium); however, the potential U.S. repository at Yucca Mountain would be located above the water table, where air is present. Temperatures might be intentionally elevated by design in order to drive away water. If waste package containment is breached prior to depletion by radioactive decay, C-14 could be released as 14 CO 2 and transported in the gas phase. Since it appears that transport would be rapid relative to radioactive decay, release to the accessible environment would be possible. This paper reviews the evaluation of the inventory of C-14 and what can be projected about its likely behavior. Uncertainties in the containment lifetimes of waste packages, and costs involved in attempting to assure long-term containment, are discussed. Consequences of total release are placed in perspective. An opinion is offered that these consequences are too small to justify the costs of trying to assure that releases would be below the previously prevailing U.S. regulatory limits. The regulations are currently being reconsidered by a committee of the National Academy of Sciences.

Immobilization in ceramic waste forms of the residues from treatment of mixed wastes

The Environmental Restoration and Waste Management Applied Technology Program at LLNL is developing a Mixed Waste Management Facility to demonstrate treatment technologies that provide an alternative to incineration. As part of that program, we are developing final waste forms using ceramic processing methods for the immobilization of the treatment process residues. The ceramic phase assemblages are based on using Synroc D as a starting point and varying the phase assemblage to accommodate the differences in chemistry between the treatment process residues and the defense waste for which Synroc D was developed. Two basic formulations are used, one for low ash residues resulting from treatment of organic materials contaminated with RCRA metals, and one for high ash residues generated from the treatment of plastics and paper products. Treatment process residues are mixed with ceramic precursor materials, dried, calcined, formed into pellets at room temperature, and sintered at 1150 to 1200{degrees}C to produce the final waste form. This paper discusses the chemical composition of the waste streams and waste forms, the phase assemblages that serve as hosts for inorganic waste elements, and the changes in waste form characteristics as a function of variation in process parameters.