[ RadSafe ] Accelerated weathering of borosilicate glass

[Peter Bossew]

A new article, maybe interesting to some.


Eric M. Pierce, B.P. McGrail, P.F. Martin, J. Marra, B.W. Arey and K.N. 
Geiszler
Accelerated weathering of high-level and plutonium-bearing lanthanide 
borosilicate waste glasses under hydraulically unsaturated conditions
Applied Geochemistry
Volume 22, Issue 9, September 2007, Pages 1841-1859
doi:10.1016/j.apgeochem.2007.03.056

Abstract

The US Department of Energy (DOE) has proposed that a can-in-canister 
waste package design be used for disposal of excess weapons-grade Pu at 
the proposed mined geologic repository at Yucca Mountain, Nevada. This 
configuration consists of a high-level waste (HLW) canister fitted with 
a rack that holds mini-canisters containing a Pu-bearing lanthanide 
borosilicate (LaBS) waste glass and/or titanate-based ceramic (not, 
vert, similar15% of the total canister volume). The remaining volume of 
the HLW canister is then filled with HLW glass (not, vert, similar85% of 
the total canister volume). A 6-a pressurized unsaturated flow (PUF) 
test was conducted to investigate waste form–waste form interactions 
that may occur when water penetrates the canisters and contacts the 
waste forms. The PUF column volumetric water content was observed to 
increase steadily during the test because of water accumulation 
associated with alteration phases formed on the surfaces of the glasses. 
Periodic excursions in effluent pH, electrical conductivity, and 
solution chemistry were monitored and correlated with the formation of a 
clay phase(s) during the test. Geochemical modeling, with the EQ3NR 
code, of select effluent solution samples suggests the dominant 
secondary reaction product for the surrogate HLW glass, SRL-202, is a 
smectite di-octahedral clay phase(s), possibly nontronite [Na0.33 
Fe2(AlSi)4O10(OH)2 · n(H2O)] or beidellite [Na0.33Al2.33Si3.67O10(OH)2]. 
This clay phase was identified in scanning electron microscope (SEM) 
images as discrete spherical particles growing out of a hydrated gel 
layer on reacted SRL-202 glass. Alpha energy analysis (AEA) of aliquots 
of select effluent samples that were filtered through a 1.8 nm filter 
suggest that approximately 80% of the total measurable Pu was in the 
form of a filterable particulate, in comparison to unfiltered aliquots 
of the same sample. These results suggest the filterable particles are 
 >1.8 nm but smaller than the 0.2 ?m average diameter openings of the Ti 
porous plate situated at the base of the column. In this 
advection-dominated system, Pu appeared to be migrating principally as 
or in association with colloids after being released from the LaBS 
glass. Analyses of reacted LaBS glass particles with SEM with energy 
dispersive X-ray spectroscopy suggest that Pu may have segregated into a 
discrete disk-like phase, possibly PuO2. Alteration products that 
contain the neutron absorber Gd have not been positively identified. 
Separation of the Pu and the neutron absorber Gd during glass 
dissolution and transport could be a criticality issue for the proposed 
repository. However, the translation and interpretation of these 
long-term PUF test results to actual disposed waste packages requires 
further analysis.




-----------------------------------------------------
Peter Bossew 

European Commission (EC) 
Joint Research Centre (JRC) 
Institute for Environment and Sustainability (IES) 

TP 441, Via Fermi 1 
21020 Ispra (VA) 
ITALY 
  
Tel. +39 0332 78 9109 
Fax. +39 0332 78 5466 
Email: peter.bossew at jrc.it 

WWW: http://rem.jrc.cec.eu.int 
  
"The views expressed are purely those of the writer and may not in any
circumstances be regarded as stating an official position of the European
Commission."