The OH/IR stars evolving on the Asymptotic Giant Branch are large-amplitude variable stars with periods in the range of ~400 to 2500 days, significantly longer than those of the related Mira variables. We use preliminary results from a monitoring program of the 1612-MHz OH maser variations of a sample of > 70 OH/IR stars to study a possible extension of Mira Period-Luminosity Relations to longer periods. The period distribution of the sample is split around P ~ 1100 days. Using WISE W3 absolute magnitudes as proxies for the luminosity and the best available distances, we found no convincing relation between periods and absolute magnitudes. A cause could be rapid evolution of the ‘extreme OH/IR stars’ (the group with P >1100 days) close to the tip of the AGB, where no increase of luminosity is expected on the short time-scales involved.

We present ALMA band 7 data of the extreme OH/IR star, OH 26.5+0.6. In addition to lines of CO and its isotopologues, the circumstellar envelope also exhibits a number of emission lines due to metal-containing molecules, e.g., NaCl and KCl. A lack of C18O is expected, but a non-detection of C17O is puzzling given the strengths of H217O in Herschel spectra of the star. However, a line associated with Si17O is detected. We also report a tentative detection of a gas-phase emission line of MgS. The ALMA spectrum of this object reveals intriguing features which may be used to investigate chemical processes and dust formation during a high mass-loss phase.

The ROSAT All Sky Survey (RASS), performed between July 1990 and February 1991, provided about 60,000 X-ray sources in a soft X-ray band (0.1–2.4keV) with a flux limit of approximately 5 × 10–13 ergs cm–2 s–1 for exposure times of 400 sec (a typical value in low ecliptic latitudes). A wealth of information can be extracted from the RASS source content and many objects deserve (or have already deserved) extensive follow-up studies. However, this possibility is limited by the fact that the nature of most of the RASS sources is unknown i.e., they are unclassified. A correlation with the SIMBAD data base yields only identifications for about one third of the RASS sources.

A general, integrated performance assessment code, AREST-CT, was used toanalyze the influence of various factors on the release rates ofradionuclides from a proposed facility for disposal of low-activity tankwastes. The code couples various process models together based on theframework of reaction-transport theory. The disposal facility was modeled asa 1-D column surrounded by soil. A borosilicate waste glass, LD6–5412 wasthe waste form considered in the analysis. Included in the simulations were38 aqueous species, 14 minerals, 21 equilibrium reactions, and 16 kineticreactions. Dissolution rate of the glass and the release rates of Te, Pu, U,Np, I, Se under different conditions were calculated for 50,000 years. Thesimulations revealed that 1) open exchange between the atmosphere andpore-water within the vault significantly improves the performance; 2) anion-exchange reaction between the glass and aqueous phase increases therelease rates significantly; and 3) at the hydrogeologie conditions underconsideration, variation of the pore-water velocity has little effect on therelease rate of radionuclides. These results provide a scientific basis forformulation of waste forms and engineering design of the disposal facility.Reaction-transport modeling can provide information on the long-termperformance of disposal systems that are not obtainable from laboratoryexperiments alone or by conventional decoupled process models.

A new computer code, Analysis of RadionuclidESource-Term with Chemical Transport (AREST-CT), is described in this paper. The code is being designed to support performance assessment analyses of engineered systems for subsurface isolation of hazardous and radioactive wastes. Radionuclide releases from an engineered system are modeled by solving governing equations describing conservation of water mass, air mass, thermal energy, and chemical species mass. As such, the AREST-CT code will be capable of simulating radionuclide release and transport in a non-isothermal, unsaturated-saturated setting. Constituitive equations are implemented that describe corrosion of iron-based container materials, glass, and spent fuel waste forms. The governing equations are solved in a two-dimensional domain using an integrated finite-volume method. A third-order total variation diminishing (TVD) numerical scheme is evaluated to minimize numerical oscillations and dissipation of steep concentration gradients in advection-dominated transport problems.

The interaction of borosilicate waste glasses with water has been studied extensively, and reasonably good models are available that describe the reaction kinetics and solution chemical effects. Unfortunately, these models have not been utilized in performance assessment analyses, except in estimating radionuclide solubilities at the waste form surface. A geochemical model has been incorporated in the AREST code to examine the coupled processes of glass dissolution and transport within the engineered barrier system. Our calculations show that the typical performance assessment analyses using fixed solubilities or constant reaction rate at the waste form surface are not always conservative or realistic predictions of radionuclide release. Varying the transport properties of the waste package materials is shown to produce counterintuitive effects on the release rates of some radionuclides. The use of noncoupled performance assessment models could lead a repository designer to an erroneous conclusion about the advantages of one waste package design or host rock setting over another.

A mechanistic model describing a dynamic mass balance between the production and consumption of silicic acid was coupled to a near-field mass transport model to predict the dissolution kinetics of a high-level waste glass in a deep geologic repository. The effects of interactions between an iron overpack and the glass are described by a time-dependent precipitation reaction for a ferrous silicate mineral. The kinetic model is used to transform radionuclide concentration-versus-reaction progress values, predicted from a geochemical reaction path computer code, to concentration-versus-time values that are used to calculate the rate of radionuclide release by diffusive mass transfer to the surrounding host rock. The model provides for both solubility-limited and kinetically limited release; the rate-controlling mechanism is dependent on the predicted glass/groundwater chemistry.

A series of calculations of radionuclide release was performed with the AREST and SYVAC-Vault models (SVM) in order to assess concurrance. Specifically, the effects of precipitation and decay chain in-growth on the predicted release of nuclides from waste packages containing spent nuclear fuel were compared between each code. The results for maximum release rates generally agreed within a factor of 10. The differences in results can be explained based on the differences in geometry and boundary conditions between the two codes. Both codes showed nearly identical enhancement factors in release rates of uranium-series nuclides (U-238, U-234, Th-230, Ra-226) arising from the effect of decay-chain in-growth. Calculated enhancement factors in release rates for precipitation of a new uranium-bearing solid within the waste package were also in good agreement between AREST and SVM.