Electronic structure, phase stability and chemical bonding in Th2_2Al and Th2_2AlH4_4

We present the results of theoretical investigation on the electronic structure, bonding nature and ground state properties of Th$_2$Al and Th$_2$AlH$_4$ using generalized-gradient-corrected first-principles full-potential density-functional calculations. Th$_2$AlH$_4$ has been reported to violate the "2 \AA rule" of H-H separation in hydrides. From our total energy as well as force-minimization calculations, we found a shortest H-H separation of 1.95 {\AA} in accordance with recent high resolution powder neutron diffraction experiments. When the Th$_2$Al matrix is hydrogenated, the volume expansion is highly anisotropic, which is quite opposite to other hydrides having the same crystal structure. The bonding nature of these materials are analyzed from the density of states, crystal-orbital Hamiltonian population and valence-charge-density analyses. Our calculation predicts different nature of bonding for the H atoms along $a$ and $c$. The strongest bonding in Th$_2$AlH$_4$ is between Th and H along $c$ which form dumb-bell shaped H-Th-H subunits. Due to this strong covalent interaction there is very small amount of electrons present between H atoms along $c$ which makes repulsive interaction between the H atoms smaller and this is the precise reason why the 2 {\AA} rule is violated. The large difference in the interatomic distances between the interstitial region where one can accommodate H in the $ac$ and $ab$ planes along with the strong covalent interaction between Th and H are the main reasons for highly anisotropic volume expansion on hydrogenation of Th$_2$Al.Comment: 14 pages, 9 figure

Verification and update of BNL mixed waste survey

This report briefly describes attempts to verify the results of a previous survey on the amount of mixed wastes generated at various facilities during 1985. The original survey indicated some lack of understanding of current EPA regulations. This telephone survey verification indicated a better understanding of these regulations in recent months. Changes in EPA regulations and the addition of new compounds to the list of hazardous wastes are causing problems for organizations trying to comply. 7 refs. (TEM

Application of EPA regulations to low-level radioactive waste

The survey reported here was conducted with the intent of identifying categories of low-level radioactive wastes which would be classified under EPA regulations 40 CFR Part 261 as hazardous due to the chemical properties of the waste. Three waste types are identified under these criteria as potential radioactive mixed wastes: wastes containing organic liquids; wastes containing lead metal; and wastes containing chromium. The survey also indicated that certain wastes, specific to particular generators, may also be radioactive mixed wastes. Ultimately, the responsibility for determining whether a facility's wastes are mixed wastes rest with the generator. However, the uncertainties as to which regulations are applicable, and the fact that no legal definition of mixed wastes exists, make such a determination difficult. In addition to identifying mixed wastes, appropriate methods for the management of mixed wastes must be defined. In an ongoing study, BNL is evaluating options for the management of mixed wastes. These options will include segregation, substitution, and treatments to reduce or eliminate chemical hazards associated with the wastes listed above. The impacts of the EPA regulations governing hazardous wastes on radioactive mixed waste cannot be assessed in detail until the applicability of these regulations is agreed upon. This issue is still being discussed by EPA and NRC and should be resolved in the near future. Areas of waste management which may affect generators of mixed wastes include: monitoring/tracking of wastes before shipment; chemical testing of wastes; permits for treatment of storage of wastes; and additional packaging requirements. 3 refs., 1 fig., 2 tabs

Biodegradation of ion-exchange media

The purpose of this study was to investigate further the potential for ion-exchange media (resin beads or powdered filter media) to support biological growth. A mixed microbial culture was grown from resin wastes obtained from the BNL HFBR by mixing the resin with a nutrient salt solution containing peptone and yeast extract. Bacterial and fungal growths appeared in the solution and on the resins after 7 to 10 days incubation at 337)degree)C. The mixed microbial cultures were used to inoculate several resin types, both irradiated and unirradiated. 12 refs., 5 tabs

Biodegradation testing of bitumen

An estimate of the rate of biodegradation of bituminous material is necessary to predict the long-term stability of low- and intermediate-level waste solidified using bitumen. Data from a series of scoping experiments have been analyzed to determine the rate of degradation of blown bitumen samples under a variety of conditions. Among the variables investigated were the effect of soil type, moisture, sample surface area and microbial strain. The rate of degradation was measured by monitoring metabolic CO/sub 2/ release. Using this data it was found that, for degradation in soil, a mean rate of 5.5 x 10..mu../sup 4/ cm/y represented all data to within a factor of about two. This mean rate is nearly that for distilled bitumen samples measured by other workers

Some considerations in the evaluation of concrete as a structural material for alternative LLW (low-level radioactive waste) disposal technologies

The objective of this study was to develop information needed to evaluate the long-term performance of concrete and reinforced concrete as a structural material for alternative LLW disposal methods. The capability to carry out such an evaluation is required for licensing a site which employs one of these alternative methods. The basis for achieving the study objective was the review and analysis of the literature on concrete and its properties, particularly its durability. In carrying out this program characteristics of concrete useful in evaluating its performance and factors that can affect its performance were identified. The factors are both intrinsic, i.e., associated with composition of the concrete (and thus controllable), and extrinsic, i.e., due to external environmental forces such as climatic conditions and aggressive chemicals in the soil. The testing of concrete, using both accelerated tests and long-term non-accelerated tests, is discussed with special reference to its application to modeling of long-term performance prediction. On the basis of the study's results, conditions for acceptance are recommended as an aid in the licensing of disposal sites which make use of alternative methods

Identification of radioactive mixed wastes in commercial low-level wastes

A literature review and survey were conducted on behalf of the US NRC Division of Waste Management to determine whether any commercial low-level radioactive wastes (LLW) could be considered hazardous as defined by EPA under 40 CFR Part 261. The purpose of the study was to identify broad categories of LLW which may require special management as radioactive mixed waste, and to help address uncertainties regarding the regulation of such wastes. Of 239 questionnaires sent out to reactor and non-reactor LLW generators, there were 91 responses representing 29% by volume of all low-level wastes disposed of at commercial disposal sites in 1984. The analysis of the survey results indicated that three waste streams generic to commercial LLW may be potential radioactive mixed wastes. These are as follows: (1) wastes containing organic liquids, disposed of by all types of generators and representing approx. =2.3% by volume of all wastes reported; (2) wastes containing lead metal, i.e., discarded shielding and lead containers, representing <0.1% by volume of all wastes reported; and (3) wastes containing chromium, i.e., process wastes from nuclear power plants which use chromates as corrosion inhibitors; these represent 0.6% of the total volume reported in the survey. Certain wastes, specific to particular generators, were identified as potential mixed wastes as well. 4 refs., 5 tabs

Management of radioactive mixed wastes in commercial low-level wastes

Potential mixed wastes in commercial low-level wastes have been identified and management options applicable to these wastes have been evaluated. Both the identification and management evaluation have necessarily been based on review of NRC and EPA regulations and recommendations. The underlying intent of both agencies is protection of man and/or environment, but differences may occur in the means by which intent is achieved. Apparent discrepancies, data gaps and unresolved issues that have surfaced during the course of this work are discussed

Summary of BNL studies regarding commercial mixed waste

Based on BNL's study it was concluded that there are low-level radioactive wastes (LLWs) which contain chemically hazardous components. Scintillation liquids may be considered an EPA listed hazardous waste and are, therefore, potential mixed wastes. Since November 1985, no operating LLW disposal site will accept these wastes for disposal. Unless such wastes contain de minimis quantities of radionuclides, they cannot be disposed of at an EPA permitted site. Currently generators of liquid scintillation wastes can ship de minimis wastes to be burned at commercial facilities. Oil wastes may also eventually be an EPA listed waste and thus will have to be considered a potential radioactive mixed waste unless NRC establishes de minimis levels of radionuclides below which oils can be managed as hazardous wastes. Regarding wastes containing lead metal there is some question as to the extent of the hazard posed by lead disposed in a LLW burial trench. Chromium-containing wastes would have to be tested to determine whether they are potential mixed wastes. There may be other wastes that are mixed wastes; the responsibility for determining this rests with the waste generator. While management options for handling potential mixed wastes are available, there is limited regulatory guidance for generators. BNL has identified and evaluated a variety of treatment options for the management of potential radioactive mixed wastes. The findings of that study showed that application of a management option with the purpose of addressing EPA concerns can, at the same time, address stabilization and volume reduction concerns of NRC. 6 refs., 1 tab