Publications

A number of Hanford tanks have leaked high level radioactive wastes (HLW) into the surrounding unconsolidated sediments. The disequilibrium between atmospheric C0{sub 2} or silica-rich soils and the highly caustic (pH > 13) fluids is a driving force for numerous reactions. Hazardous dissolved components such as {sup 133}Cs, {sup 79}Se, {sup 99}Tc may be adsorbed or sequestered by alteration phases, or released in the vadose zone for further transport by surface water. Additionally, it is likely that precipitation and alteration reactions will change the soil permeability and consequently the fluid flow path in the sediments. In order to ascertain the location and mobility/immobility of the radionuclides from leaked solutions within the vadose zone, the authors are currently studying the chemical reactions between: (1) tank simulant solutions and Hanford soil fill minerals; and (2) tank simulant solutions and C0{sub 2}. The authors are investigating soil-solution reactions at: (1) elevated temperatures (60--200 C) to simulate reactions which occur immediately adjacent a radiogenically heated tank; and (2) ambient temperature (25 C) to simulate reactions which take place further from the tanks. The authors studies show that reactions at elevated temperature result in dissolution of silicate minerals and precipitation of zeolitic phases. At 25 C, silicate dissolution is not significant except where smectite clays are involved. However, at this temperature CO{sub 2} uptake by the solution results in precipitation of Al(OH){sub 3} (bayerite). In these studies, radionuclide analogues (Cs, Se and Re--for Tc) were partially removed from the test solutions both during high-temperature fluid-soil interactions and during room temperature bayerite precipitation. Altered soils would permanently retain a fraction of the Cs but essentially all of the Se and Re would be released once the plume was past and normal groundwater came in contact with the contaminated soil. Bayerite, however, will retain significant amounts of all three radionuclides.

Exploratory hydrothermal synthesis in the system Cs{sub 2}O-SiO{sub 2}-TiO{sub 2}-H{sub 2}O has produced a new polymorph of Cs{sub 2}TiSi{sub 6}O{sub 15} (SNL-A) whose structure was determined using a combination of experimental and theoretical techniques ({sup 29}Si and {sup 133}Cs NMR, X-ray Rietveld refinement, and Density Functional Theory). SNL-A crystallizes in the monoclinic space-group Cc with unit cell parameters: a = 12.998(2) {angstrom}, b = 7.5014(3) {angstrom}, c = 15.156(3) {angstrom}, {eta} = 105.80(3) {degree}. The SNL-A framework consists of silicon tetrahedra and titanium octahedra which are linked in 3-, 5-, 6-, 7- and 8-membered rings in three dimensions. SNL-A is distinctive from a previously reported C2/c polymorph of Cs{sub 2}TiSi{sub 6}O{sub 15} by different ring geometries. Similarities and differences between the two structures are discussed. Other characterizations of SNL-A include TGA-DTA, Cs/Si/Ti elemental analyses, and SEM/EDS. Furthermore, the chemical and radiation durability of SNL-A was studied in interest of ceramic waste form applications. These studies show that SNL-A is durable in both radioactive and rigorous chemical environments. Finally, calculated cohesive energies of the two Cs{sub 2}TiSi{sub 6}O{sub 15} polymorphs suggest that the SNL-A phase (synthesized at 200 C) is energetically more favorable than the C2/c polymorph (synthesized at 1,050 C).

Ongoing hydrothermal Cs-Ti-Si-O-H2O phase investigations has produced several new ternary phases including a novel microporous Cs-silicotitanate molecular sieve, SNL-B with the approximate formula of Cs3TiSi3O9.5 · 3H2O SNL-B is only the second molecular sieve, Cs-silicotitanate phase reported to have been synthesized by hydrothermal methods. Crystallites are very small (0.1 x 2 μm2) with a blade-like morphology. SNL-B is confirmed to be a three-dimensional molecular sieve by a variety of characterization techniques (N2 adsorption, ion exchange, water adsorption/desorption, solid state cross polarization-magic angle spinning nuclear magnetic resonance). SNL-B is able to desorb and adsorb water from its pores while retaining its crystal structure and exchanges Cs cations readily. Additional techniques were used to describe fundamental properties (powder X-ray diffraction, FTIR, 29Si and 133Cs MAS NMR, DTA, SEM/EDS, ion selectivity, and radiation stability). The phase relationships of metastable SNL-B to other hydrothermally synthesized Cs-Ti-Si-O-H2O phases are discussed, particularly its relationship to a Cs-silicotitanate analogue of pharmacosiderite, and a novel condensed phase, a polymorph of Cs2TiSi6O15 (SNL-A). (C) 2000 Elsevier Science B.V. All rights reserved. Ongoing hydrothermal Cs-Ti-Si-O-H2O phase investigations has produced several new ternary phases including a novel microporous Cs-silicotitanate molecular sieve, SNL-B with the approximate formula of Cs3TiSi3O9.5·3H2O. SNL-B is only the second molecular sieve, Cs-silicotitanate phase reported to have been synthesized by hydrothermal methods. Crystallites are very small (0.1×2 μm2) with a blade-like morphology. SNL-B is confirmed to be a three-dimensional molecular sieve by a variety of characterization techniques (N2 adsorption, ion exchange, water adsorption/desorption, solid state cross polarization-magic angle spinning nuclear magnetic resonance). SNL-B is able to desorb and adsorb water from its pores while retaining its crystal structure and exchanges Cs cations readily. Additional techniques were used to describe fundamental properties (powder X-ray diffraction, FTIR, 29Si and 133Cs MAS NMR, DTA, SEM/EDS, ion selectivity, and radiation stability). The phase relationships of metastable SNL-B to other hydrothermally synthesized Cs-Ti-Si-O-H2O phases are discussed, particularly its relationship to a Cs-silicotitanate analogue of pharmacosiderite, and a novel condensed phase, a polymorph of Cs2TiSi6O15 (SNL-A).

This report summarizes research on the aging of Class I components in environments representative of nuclear power plants. It discusses Class IE equipment used in nuclear power plants, typical environments encountered by Class IE components, and aging techniques used to qualify this equipment. General discussions of radiation chemistry of polymers and accelerated aging techniques are also included. Based on the inadequacies of present aging techniques for Class IE equipment, a proposal for an experimental program on electrical cables is presented. One of the main purposes of the proposed work is to obtain relevant data in two areas of particular concern--the effect of radiation dose rate on polymer degradation, and the importance of synergism for combined thermal and radiation environments. A new model that allows combined environment accelerated aging to be carried out is introduced, and it is shown how the experimental data to be generated can be used to test this model.