Determination of transmutation effects in crystalline waste forms. 1997 annual progress report
D.M. Strachan, E.C. Buck, J.A. Fortner, N.J. Hess
A team from two national laboratories is studying transmutation effects in crystalline waste forms. Analyses are being done with 18 year old samples of 13'Cs-bearing pollucite (CsAlSi20~~0.5 HZO) obtained from a French company. These samples are unique in that the pollucite was made with various amounts of 13'Cs, which was then sealed in welded stainlesssteel capsules to be used as tumor irradiation sources. Over the past 18 years, the 13'Cs has been decaying to stable Ba in the capsules, i.e.,
... in the absence of atmospheric effects. This material serves as an analogue to a crystalline waste form in which such a transmutation occurs to possibly disrupt the integrity of the original waste form. Work this year consisted of determining the construction of the capsule and state of the pollucite in the absence of details about these components from the French company. We have opened one capsule containing nonradioactive pollucite. The information on the construction of the stainless-steel capsule is useful for the work that we are preparing to do on capsules containing radioactive pollucite. Microscopic characterization of the nonradioactive pollucite revealed that there are at least two compounds in addition to pollucite: a Cs-silicate and a Cs-aluminosilicate (CsAlSiO 4 ). These findings may complicate the interpretation of the planned experiments using X-ray absorption spectroscopy. Electron energy loss spectroscopy and energy dispersive X-ray spectroscopy (flourescence) have been used to characterize the nonradioactive pollucite. We have investigated the stability of the nonradioactive pollucite to p radiation damage by use of 200 keV electrons in a transmission electron microscope. The samples were found to become amorphous in less than 10 minutes with loss of Cs. This is equivalent to many more years of p radiation damage than under normal decay of the 13'Cs. In fact, the dose was equivalent to several thousand years of normal radiation damage from the decay of .13'Cs. Of course, there would not be any 13'Cs remaining after that length of time because the half-life of 13'Cs is 30 y. Preparations have been started to study the radioactive pollucite samples at the Stanford Synchrotron Radiation Laboratory. Our calculations show that by thinning the base of the capsules we should be able to obtain about a factor of ten increase in the fluorescence signal. Procedures for thinning capsules containing the radioactive pollucite and examining the samples at the Stanford synchrotron are in place.