[ RadSafe ] Accelerated weathering of borosilicate glass

[Dan W McCarn]

Hi Mike:

Just to advance a few points:

The geochemical experiments were conducted under pressurized conditions to
accelerate weathering effects in order to determine the likely outcome of
canister / borosilicate glass weathering. This is a common technique for
investigating potential outcomes for mineral phases - to expose them to
leaching solution under various thermodynamic conditions and observe the
outcome. The leachate and mineral phases present (clays & borosilicate
glass) are then modeled with a geochemical code with an appropriate
thermodynamic database.  If possible, the kinetics of the reaction are
estimated to better understand the ultimate outcome of source term release.

Ultimately, the effectiveness of the repository lies in the geology,
geochemistry and hydrology, not in "man-made" barriers.  That is why the
"base case" for the repository assumes "no credit" from the engineered
barriers.

1) Stability of man-made tunnels (e.g. mine adits)
Collapse of tunnel systems, regardless of engineering, will occur over time,
and perhaps 100-300 years would be sufficient for collapse. I've seen adits
that are/were 100 years old and they are at least 75% collapsed.  Much
depends on the strength of the rock, stress caused by the removal of rock
for the tunnel, stress from thermal effects from increased local
temperature.  The "shields" that drain water away from the canisters
(condensed from the thermal effects of the fuel on water-bearing minerals)
then are no longer effective.  Thus "no credit" is a good assumption.

2) Water of Mineralization
Aside from the water that is naturally present from the hydrogeologic
setting, minerals frequently contain some water of mineralization, that when
heated, give off this water initiating thermal decomposition of the mineral.
These minerals include authigenic clays formed naturally by decomposition of
the aluminosilicates in the host rock.  Water of mineralization is usually
yielded-up at reasonably low temperatures.  Given the significant heating by
the assembled waste canisters, this water will condense and cause the zone
to become quite "wet", thus encouraging early corrosion in the canisters.
This has been demonstrated at Yucca Mountain.  The point is, the zone around
the canisters will be wet, perhaps not "saturated", but certainly dripping
wet.

3) Chemistry of water
Water naturally reacts with CO2 and the minerals present to form a solution
in equilibrium with the mineral phases present as well as the gas phases
such as oxygen. This solution causes corrosion to occur in the canisters.
I'm not sure how much "credit" is given to the canisters themselves, but I
imagine that they would be fairly well corroded after only a few hundred
years, directly exposing the fuels themselves.  Thus over tens of thousands
of years to be modeled, "no credit" is a good assumption to make for the
canisters.

4) Smectite Characteristics
Decomposition of the borosilicate glass forms a smectite mineral.  This is a
general category of clays that are montmorillonites.  These are generally
(Na,Ca)0,3(Al,Mg)2Si4O10(OH)2.n(H2O) in composition with lots of room for
additions of some cations such as bivalent iron.  Features of smectite
chemistry are that 1) they tend to be authigenic or formed from mineral
precursors that are undergoing weathering and 2) they can alternately swell
and shrink on exposure to moisture and can therefore rupture containers as
they swell.  They also adsorb and desorb materials.  I would imagine that
from alpha recoil and radiation damage, the borosilicate glasses would tend
to decompose faster than the modeled equivalent.  The accelerated
decomposition by placing 6 atm pressure was probably just that: An
accelerated test to establish the weathered products. But I would expect
that following corrosion of the canister, the glass, having sustained
significant radiation damage would weather to smectite fairly readily.
Adsorption / desorption of Pu and other R/Ns would follow in the leachate
water.

5) Poor management, poor engineering, bad QA, etc.
I don't quite understand your point, but it seemed more of a rant than
reasoned.  The facility would be, at some point, allowed to settle into the
natural system, meaning collapse of tunnels, etc.  Yucca Mountain is only
"retrievable" storage up to a point in time.

6) Source Term
The flux of R/Ns out of the canisters and into the natural system provides
input into the other models.

7) MOX Fuels
At last something that we can agree on!  I would certainly hope that MOX
reactor fuels will used to dispose of weapons-grade material, thus
converting them to energy and making the material useless for weapons
following discharge from the reactor.  Although the MOX fuels from weapons
don't make that much dent in the reactor-related requirements (perhaps 5%
for 20 years?), it seems the right thing to do.

Dan ii

Dan W McCarn, Geologist
Houston and Albuquerque

-----Original Message-----
From: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] On Behalf
Of Brennan, Mike (DOH)
Sent: Monday, August 27, 2007 11:51
To: radsafe at radlab.nl
Subject: RE: [ RadSafe ] Accelerated weathering of borosilicate glass

First of all, I have to say that I am not in favor of disposal of
weapons grade Pu: I believe it should be used as fuel in reactors, and
be a resource rather than a waste.  Be that as it may, and acknowledging
that I don't know more about this experiment than what is mentioned in
this abstract, I do know a modest amount about the way they plan to
dispose of things at Yucca Mountain, and as an ex-submariner, I have a
fair understanding of the behavior of water under pressure.

So, as I understand it, the glass containing the waste is inside an can,
which is inside a canister, which is inside a chamber, which is inside
the Mountain.  The outside of the Mountain (and any man-made tunnels and
spaces within it) is presumably at one atmosphere pressure.  With the
possible exception of the glass and the can, there is presumably a gap
of some size between the exterior of one layer of containment and the
interior of the one that contains it (the canisters, for example, are
not fit into the rock with no space around them.  If any of these
assumptions are wrong or unreasonable, please let me know.

Now, while I do not agree that the protection of the canister and the
can should be discounted, any more than I agree that the
geologic/climatic changes needed to have pressurized water coursing
through the heart of the mountain should be assumed, I will leave those
for the moment.  I will not concede, however, my long held belief that
pressurized water flows from a point of higher pressure to a point of
lower pressure via the path of least resistance.  Water may flow into
dead end spaces while the pressure in that space is less than the
pressure of the flow, but when it equalizes the water pools and the flow
goes elsewhere.  When the flow reaches a space at a lower pressure the
stream drops to that pressure until it can flood it enough to build up
the pressure.  If the space is open to the atmosphere that never
happens, and the movement of the water is dictated by gravity.

Given that, I have to say that I can't envision any set of
circumstances, up to and including an earthquake that collapses the
mountain, in which the path of least resistance for a 6 atmosphere
stream of water is into the chamber, into the canister, into the can,
across the glass, out of the can, out of the canister, and out of the
chamber.  

So this experiment seems to require poor management, poor engineering,
bad QA, seismic and climatic changes of epic proportions, and selective
suspension of the laws of physics.  

And the ironic thing is that even given all of that, they still can't
build a scenario in which someone is harmed, because the amount of water
needed to weather the glass would dilute the material to infinitesimal
levels by the time it could get to where someone could drink it.

 

-----Original Message-----
From: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] On
Behalf Of Peter Bossew
Sent: Monday, August 27, 2007 3:41 AM
To: radsafe at radlab.nl
Subject: [ RadSafe ] Accelerated weathering of borosilicate glass

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."


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