Diogenic Transmutation Effects in a Crystalline Aluminosilicate Ceramic: a Tem Study

Abstract

We demonstrate the use of transmission electron microscopy (TEM) to study the effects of beta-decay of radioactive 137Cs to 137Ba in crystalline pollucite (CsAlSi2O6). Most prior work on radiation effects in materials has focused on structural damage from alpha radiation. Beta radiation, on the other hand, causes little atomic displacement, but the decay transmutation, that is, the radioactive decay of a radioisotope to an isotope of another element, results in progeny with different the valence and ionic radius. Cesium-137, a fission product of uranium, is a major contaminant at U.S. Department of Energy production facilities. Pollucite is an aluminosilicate ceramic with potential use for long-term storage of 137Cs. We focused on one of several available 137Cs sources originally fabricated in the 1970s and 1980s. These sources were small, sealed, stainless steel capsules containing pollucite in which varying amounts of the natural Cs had been replaced by radioactive 137Cs (t1/2 = 30.13 years). The sample chosen for TEM examination, aged for nearly 20 years, contained the most radiogenic barium and was expected to show the largest radiation effects. Bright field transmission images revealed a homogeneous crystalline matrix, with no evidence of distinct Ba phases or ex-solution phenomena resulting from the 137Cs transmutation. Electron diffraction patterns obtained from several portions of the sample were consistent with literature values for pollucite. These data suggest that little substantial damage was done to the crystal structure of this sample, despite the transmutation of nearly 1.5% of the total cesium to barium over the elapsed 20 years. Although our observations are limited, to our knowledge these are the only available data in which transmutation effects have been isolated from other radiation damage phenomena.

Final storage of radioactive cesium by pollucite hydrothermal synthesis

“…In this case, beta radiation results in transmutation, which changes both the ionic radius and valence of the element undergoing decay. The radiation damage over 20 years has been researched, and the results indicate that the pollucite structure remains remarkably unaffected by this transmutation27. Pollucite contains many Cs ions (more than 40 wt%) in its framework, is quite heavy, with a specific gravity of 2.9418, and is insoluble in water.…”

Chemical evolution via beta decay: a case study in strontium-90

“…In the case of 137 Cs and 90 Sr as relevant to nuclear wasteforms, the average recoil energies are less than 10 eV, except for 90 Y which imparts several tens of eV to the nearby atoms. These values provide a guide to the extent of local damage beta-decay is likely to impart on a wasteform, and with the exception of a small number of studies [80,81,27,82], this point has received little attention.…”

Using “radioparagenesis” to design robust nuclear waste forms

“…The potential importance of chemical transmutation for waste form stability has been predicted previously, 12,13 but the detail required to understand the effects has been limited. For example, Fortner et al 14 conducted a study of ''radiogenetic transmutation effects'' of 137 Cs in 20 year old crystalline pollucite (CsAlSi 2 O 6 ) samples. This transmission electron microscopy (TEM) study did not detect any appreciable change to the pollucite crystal structure due to 137 Ba formation.…”