June 19, 2001

MEMORANDUM

To: Participants in International Workshop on Long-Term Extrapolation of
Passive

> Behavior

From: Carl Di Bella

Subject: Background Information on Waste Package Environment

Below please find a brief background statement for the workshop. The
primary purpose of the statement is to define the possible range of
environments that waste packages might be subject to after being
emplaced in a proposed repository in Yucca Mountain.[^1] This statement
complements a separate statement (which we hope to send Friday)
containing background information on corrosion and the two questions
that will be addressed at the workshop.

**Background --- Repository**

The United States, through the U.S. Department of Energy (DOE), is
characterizing a site at Yucca Mountain, in the state of Nevada, for its
suitability as the site of a repository for high-level nuclear waste. If
the site is determined to be suitable and the U.S. Nuclear Regulatory
Commission licenses subsequent construction and operation plans, a
repository would be built at Yucca Mountain and would begin operating in
approximately 2010. Yucca Mountain is about 150 km northwest of Las
Vegas ([Figure_1](#Figure_1)).

As currently conceptualized by DOE, the repository would consist of an
underground area of approximately 500 hectares containing approximately
60 km of 5.5m-diameter, near-horizontal tunnels ([Figure_2](#Figure_2)).
The tunnels all would be essentially on the same plane, approximately
1,100 m above sea level. The distance from the tunnels to the surface
would vary because of variations in surface topography but would be at
least 200 m.

The waste would be emplaced horizontally in the tunnels in sealed
cylindrical waste packages. Although the dimensions of the waste
packages would vary somewhat, depending on the waste in them, they would
be approximately 1½-2 m in diameter and 3½-6 m long
([Figure_3](#Figure_3)). The repository would contain more than 10,000
such packages. The outer shell of each waste package would be
20-mm-thick Alloy 22 (UNS No. N06022; nominal composition: Ni59, Cr20.4,
Mo14.1, W3.2, Fe2.3). A 50-mm-thick inner shell of type 316NG stainless
steel is included in the design for mechanical strength.

Nuclear waste generates heat by radioactive decay. After emplacement of
the waste packages and before closure of the repository, most of the
heat would be removed by ventilation with outside air
([Figure_4](#Figure_4)). After the last waste package is emplaced, there
would be a period of ventilation that is yet to be determined (at least
25 years and as much as 300 years or more). At the end of the
ventilation period, a decision would be made to close the repository,
titanium (grade 7) drip shields would be placed over the waste packages
(See [Figure_5](#Figure_5).), and all penetrations (e.g., ventilation
shafts, entrances, exits) from the surface to the repository would be
sealed permanently.

# Waste Package Environment

Yucca Mountain is in a desert environment. Average yearly precipitation
is about 170 mm. Runoff and evaporation rates are high, and only a
fraction of the precipitation infiltrates into the mountain. The water
table is approximately 300 m below the tunnels of the conceptual
repository. Thus, the repository would be located in what is known as
the "unsaturated (vadose) zone." The pores in the rock in the
unsaturated zone contain water and air, and the relative humidity at the
level of the repository under undisturbed conditions (i.e., before any
tunneling operations take place) is higher than 90%. Gas pressure in the
vadose zone is essentially atmospheric. The climate at Yucca Mountain
has been wetter in the past, and it is likely to become wetter at some
time in the future. However, although extended periods of higher average
yearly precipitation at Yucca Mountain are likely in the future, such
higher precipitation is unlikely to cause the water table to rise
sufficiently to flood the repository.

+------+------------+------------------+-------------+---------------+
| M    | Nominal    |                  |             |               |
| ajor | C          |                  |             |               |
|      | omposition |                  |             |               |
| Spe  | (mg/L)     |                  |             |               |
| cies |            |                  |             |               |
+------+------------+------------------+-------------+---------------+
|      | Water      | Brine formed by  | Water in    | Brine formed  |
|      | below the  | evaporating      | rock pores  | by            |
|      | water      | water from below | at          | evaporating   |
|      | table      | the water table  | repository  | water from    |
|      |            | to near dryness  | level       | rock pores to |
|      | (based on  |                  |             | near dryness  |
|      | many       |                  | (based on   |               |
|      | samples)   |                  | few         |               |
|      |            |                  | samples)    |               |
+------+------------+------------------+-------------+---------------+
| N    | 45         | 850              | 60          | 500           |
| a^+^ |            |                  |             |               |
+------+------------+------------------+-------------+---------------+
| K^+^ | 5          | 700              | 8           | 500           |
+------+------------+------------------+-------------+---------------+
| Ca   | 10         | 1                | 100         | 200           |
| ^++^ |            |                  |             |               |
+------+------------+------------------+-------------+---------------+
| Mg   | 2          | \<.1             | 17          | 300           |
| ^++^ |            |                  |             |               |
+------+------------+------------------+-------------+---------------+
| Si   | 60         | 760              | 70          | 60            |
| O~2~ |            |                  |             |               |
+------+------------+------------------+-------------+---------------+
| C    | 7          | 850              | 120         | 500           |
| l^-^ |            |                  |             |               |
+------+------------+------------------+-------------+---------------+
| SO   | 18         | 850              | 115         | 15            |
| ~4~^ |            |                  |             |               |
| \--^ |            |                  |             |               |
+------+------------+------------------+-------------+---------------+
| H    | 130        | 300              | 220         | \<3           |
| CO~3 |            |                  |             |               |
| ~^-^ |            |                  |             |               |
+------+------------+------------------+-------------+---------------+
| NO~3 | 9          | 860              | 8           | 500           |
| ~^-^ |            |                  |             |               |
+------+------------+------------------+-------------+---------------+
| F^-^ | 2          | 700              | 1           | 100?          |
+------+------------+------------------+-------------+---------------+

The rocks of Yucca Mountain, from the surface down to and somewhat below
the water table, are varieties of tuff. Tuff is a high-silica rock
deposited as a fine ash from volcanic eruptions and then welded together
to varying degrees by heat and weight. The rock is fractured, and many
of the fractures can transmit water. The water below the water table has
been well characterized. It is a slightly oxidizing, slightly basic,
bicarbonate-type water. (See table below, which shows major species in
water below the water table before and after evaporation to a
concentrated brine and water in rock pores at the repository level
before and after evaporation to a concentrated brine. The many minor
species in these two waters are not shown.).

Evaporating a sample of water from below the water table to concentrated
brine yields a high-pH brine rich in Na and K but relatively depleted in
other cations. The water in the pores at the level of the conceptual
repository has not been well characterized; although it seems to be
slightly oxidizing and near-neutral in pH, it also appears to have
higher relative concentrations of Mg, Ca, and Cl. (See the table.)
Evaporating porewater to a strong brine seems to result in a
near-neutral brine of a different composition (rich in Na, K, Ca, and
Mg) than would be obtained by evaporating water below the water table to
a strong brine.[^2] Of interest from a localized corrosion standpoint,
there seems to be on evaporation of both waters appreciable enrichment
of not only aggressive anions (e.g., Cl^-^, F^-^) but also beneficial
anions (e.g., NO~3~^-^).

During the preclosure period (i.e., during ventilation), the relative
humidity of the air in the emplacement tunnels would be very low and the
waste package surface temperatures would be less than approximately
90ºC. Corrosion occurs under these conditions but at such an
infinitesimally slow rate that it is not considered an issue in
comparison to aqueous corrosion.

After the preclosure period, there are essentially two possible
scenarios, depending largely on the duration (and efficiency) of
ventilation during the preclosure period. One scenario, known as the
\"above-boiling\" case or sometimes simply the \"hot\" case, would occur
if the ventilation period were comparatively short (say, 25-50 years
after emplacement of the last waste package). In this case,
representative temperatures at the waste package surface would rise to
above the boiling point of water (pure water boils at approximately 96ºC
at the repository elevation) within a few weeks after closure, peak at
approximately 160ºC in a few years, and remain above boiling for
500-1,000 years. *Note:* The postclosure temperatures of a waste package
depend principally on the thermal power of the waste in the package and
of the waste in its immediate neighbors, the distance of the package
from its nearest neighbors, and the position of the package in the
repository. (For example, packages close to the edge of the repository
where ventilating air enters during the preclosure period would be
cooler.)

The other scenario, known as the \"below-boiling\" case or sometimes
simply the \"cold\" case, would occur if the ventilation period were
comparatively long (say, 200 years or more after emplacement of the last
waste package) and the distance between waste packages were somewhat
longer. In this case, waste package temperatures would rise after
closure but would peak at no more than 85-90ºC. Time-temperature curves
for a representative waste package for both a hot repository design and
a cold one are depicted in [Figure_6](#_1053514219).

There is a diversity of opinions on whether a hot or a cold repository
would be better. Advocates of a hot repository argue, among other
things, that the heat would drive away as vapor substantially all of the
water in the pores of the rock at least several meters into the tunnel
walls and that the water would condense far away from the waste packages
and drain between the tunnels. (The tunnels would be far enough apart
that some space between the tunnels would always be below boiling.)
Furthermore, they argue, because liquid water would be driven away
during the above boiling period, there would be no liquid water on the
waste package surfaces, and therefore corrosion rates would be
negligible during that period.

Advocates of a cold repository argue that the heterogeneity of the
fractures in the rock renders futile any attempts to predict local water
movement near the tunnels or between the tunnels, other than in a gross
sense. Further complicating the ability to predict water movement, they
say, are the inevitable collapse of parts of some tunnels at some time
after closure and the difficulty of characterizing fractures between
tunnels in other than a gross sense. Cold repository advocates point out
that thermomechanical and thermochemical effects could alter the
transmissivity of the fractures in ways that are difficult to predict
locally. They also argue that in a hot repository the combined effects
of boiling-point elevation (due to dissolved salts), deliquescence,
capillarity, and superheating indicate the possible presence of liquid
water on waste package surfaces at temperatures at least as high as
160ºC and that data on the corrosion behavior of Alloy 22 under such
conditions at Yucca Mountain are, at best, limited (see later). Finally,
these advocates argue, any hot repository will eventually cool down and
become a cold one. Thus, they claim, a hot repository would have not
only whatever uncertainties might be associated with a cold one but also
the additional uncertainties that might be associated with above-boiling
repository temperatures.

The Board\'s position on the hot-versus-cold repository issue is that
DOE should perform a technically defensible and quantitative evaluation
and comparison of hot and cold repository designs to clarify advantages
and disadvantages for performance and uncertainties about that
performance.

From a corrosion perspective, it is important to know whether there is
*liquid* water on the waste package surface and, if so, the temperature
of the water and the chemical species in it. Also important are the
solid materials that may be in contact with waste packages, because
these solids could influence the composition of the water and could form
crevices and local occluded zones. For example, during the preclosure
period, dust would be deposited on the waste packages. The composition
of the dust is unknown and is a matter being studied by DOE. (Although
suggestions have been made to wash emplaced waste packages just before
repository closure or to scrub or filter inlet air, or both, no such
steps are in the current design.)

If the dust contains deliquescent salts and the relative humidity is
above the deliquescence point of the dust, deliquescence could cause
liquid water to form on the waste package at temperatures well above
96ºC. For example, if the dust had a composition similar to the brine
that would result by evaporating water from below the water table, then
liquid water could form on the waste package at temperatures as high as
120ºC (a temperature corresponding closely to the deliquescence point of
sodium nitrate). If the dust had a same composition similar to the brine
resulting from evaporating water found in rock pores at the repository
level, then liquid water could form at temperatures as high as 160ºC (a
temperature that may correspond to the deliquescence points of calcium
chloride or magnesium chloride).

Besides dust, other solid materials close to the waste package would
include carbon steel (used to support the walls of the tunnels),
hydrated cement (used in grout for rock bolts), stainless steel (used in
the pallet that supports the waste package), titanium (used in the drip
shield), and tuff. (See [Figure_5](#Figure_5), which portrays all
constructed elements that would be in the emplacement tunnels.
Collectively, these elements are known as the "engineered barrier
system.") Over time, as degradation of the engineered barrier system
progresses, all of these materials are likely to come into contact with
some parts of some waste packages for extended periods. A particular
concern is very high pH (e.g., 12-13+) deriving from water in the pores
of cementitious grouts. Another concern is low pH, oxidizing
environments caused by iron corrosion products. (The buffering nature of
and reactivity of carbon dioxide in the air would bring both high and
low pH's closer to neutral if there is chemical equilibrium.)

As mentioned above, titanium drip shields would be placed over the waste
packages shortly before closure of the repository. The primary purpose
of the drip shields is to deflect water that may drip from the roof of
the emplacement tunnels. However, there are at least two reasons that
drip shields would not necessarily ensure that liquid water and waste
package surfaces would not be in contact. One reason is that any
deliquescent salts in the dust on waste package surfaces could form
brines if the relative humidity is higher than the deliquescence point
of the salt. The other reason is that very thin layers of liquid water
can form on clean metal surfaces in less than 100% relative humidity if
the relative humidity is high enough.

For the purposes of the workshop, assume that waste package temperatures
follow the upper (red) curve of [Figure_6](#_1053514219). Further assume
that some parts of the surfaces of some waste packages may be in contact
with liquid water at temperatures up to 160ºC and that the composition
of the water may range from pure condensate to concentrated brine.
Contact between liquid water and waste package surfaces may be episodic,
particularly at temperatures above 96ºC. Anions in the water would
include those in the table on page 2 but not necessarily in the same
ratios.

For purposes of the workshop, assume that the bulk gas phase composition
in the tunnels after closure would be as follows: When temperatures are
at or below 96ºC, the composition of the gas phase would be essentially
that of atmospheric air that has a relative humidity in the range of
90-100% and slightly elevated carbon dioxide content. Above 96ºC,
equilibrium considerations would indicate that the gas phase is
essentially entirely steam, with only traces of air. However, the
mountain is permeable, and convective forces due to heating could lead
to gases (air) from another part of the mountain flowing to the
repository area. Another complicating factor is that repository tunnels,
although sealed from the surface, would remain connected to each other.
The differences in temperatures in and between tunnels would cause
convective flow in and between tunnels. (Backfilling the tunnels just
before closure would significantly reduce convective flow, but the
insulating effect from the backfill would lead to unacceptably high
waste package temperatures, so no backfill is planned for the
repository.)

# Additional Information

The term "barrier" refers to any repository element that helps to
protect human health and safety by reducing or retarding the flow of
radionuclides from the repository to the human environment. There would
be many natural and engineered barriers in a repository at Yucca
Mountain. Besides the Alloy 22 outer shell of the waste package, other
engineered barriers that depend on the phenomenon of passivity include
the titanium drip shield, the 316NG stainless steel inner shell of the
waste package, and the zirconium cladding that encases spent-fuel
pellets. Because of the short duration of the Board's workshop, only the
long-term extrapolation of the corrosion resistance of the Alloy 22
outer shell will be addressed. The expectation, however, is that many of
the observations at the workshop will apply in a generic sense to any
metals that depend on passivity for corrosion resistance.

Recently, DOE commissioned a peer review of the technical basis for
predicting long-term behavior of the waste package. The peer review is
being conducted by a panel of seven materials scientists. The panel
began its work in May and plans to issue an interim report in September
and a final report in February 2002. The DOE's peer review and the
Board's workshop are completely independent (but we hope complementary)
efforts. The Board's focus for this workshop is on a critical but narrow
issue: the extrapolation of the body of knowledge consisting of
short-term data (i.e., data from experiments with durations of a few
years or less) and slightly longer-term experience (i.e., experience of
relatively few decades with Alloy 22 or similar high-nickel alloys) over
very long time periods (i.e., ten thousand years and longer).

The DOE peer review panel's charge is much broader than the focus of the
Board's workshop. For example, the DOE panel will examine stress
corrosion cracking issues. Such issues are not in the scope of the
Board's workshop. Presumably the DOE panel also will review the data
from the LLNL test facility and from other DOE-sponsored
corrosion-testing activities. The Board's workshop, however, will accept
the data as given. Finally, the DOE panel will examine corrosion issues
for the titanium drip shield; the Board's workshop will not.

The background information on repository and waste package design and on
waste package environment that is in this document, together with
background information on corrosion that that is in a separate document
along with two questions to be addressed at the workshop, should be
enough preparatory material for the purposes of the workshop. At least,
that is our intent. We realize that some may wish additional information
about the DOE program. To them we recommend a look at the Yucca Mountain
Project Web site, <http://www.ymp.gov/>, where the documents listed in
the next two paragraphs, and many more, can be found.

Figures 1-5 were taken from "Reference Design Description for a Geologic
Repository" which is at the following DOE Web site:
<http://www.ymp.gov/documents/seda06m3_b/index.htm>.

More information on the environment of the waste package is in the
February 2000 document *Environment on the Surfaces of the Drip Shield
and Waste Package Outer Barrier*, which can be found at the following
Web site: <http://www.ymp.gov/documents/amr/21844/21844.pdf>.

[]{#Figure_1 .anchor}

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[]{#Figure_4 .anchor}![](media/image4.jpeg){width="6.69375in"
height="4.647222222222222in"}

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[]{#Figure_5 .anchor}![](media/image2.jpeg){width="6.78125in"
height="4.9006944444444445in"}

[^1]: This document is an effort to simplify some very complex topics
    and does not necessarily represent the views of the U.S. Nuclear
    Waste Technical Review Board (Board).

[^2]: The brines referred to in this and the previous paragraph and in
    the table were produced by slowly evaporating synthesized water
    samples from below the water table or from the rock pores at
    approximately 80ºC in an open system until more than 99% of the
    water was evaporated.