Review of the Yucca Mountain Igneous Consequences Peer Review Panel's
Interim Report presented on August 23, 2002.

Meghan M. Morrissey, Colorado School of Mines

The YMICPR Panel presented a comprehensive review on work that has been
completed by DOE related to consequences of igneous events that might
impact the Yucca Mountain repository. The Panel consists of a group of
researchers well diverse in studies related to igneous activity however
they did acknowledge their limitations in several critical areas such as
groundwater transport, remobilization of tephra and the interaction
between magma and waste packages. In addition to analyzing previous
work, the Panel conducted their own analyses to support their
recommended tasks in areas that they concluded were weak in DOE studies.

The Panel approached their review by beginning with magma transport and
mechanical properties, followed by dike propagation, dike interaction
with repository, and magma interaction with a drift(s), and ending with
the consequences of the break through of magma at the surface. The
review of magma transport and mechanical properties was thorough
emphasizing that these properties are well constrained from thousands of
data collected from volcanoes throughout the world and from established
theoretical and empirical calculations. They demonstrated how the volume
and general composition of magma that might erupt at Yucca Mountain
Repository are constrained from field studies on volcanism within the
region. Also mentioned was the sensitivity of magma flow behavior to
dissolved and exsolved volatiles such as H~2~O and CO~2~. The presence
of H~2~O enhances the solubility of CO~2~, which will affect the depth
of exsolution and fragmentation. This means that the total volatile
content of the magma will be greater as when it nears the surface.

The consequence of how the magma will interact with the drift if
intersected by a dike, strongly depends on the nature of the magma near
the top of the dike. When magma is within upper 500 m of the crust it
will release or exsolve most of its dissolved H~2~O and CO~2~ which can
change its flow character from a viscous liquid to a bubbly magma to a
gas rich mixture of fragmented magma and gas depending on the volatile
content. Prior to the Panels work, the value for estimated H~2~O
contained in Crater Flat, CF, basalts prior to eruption was 1-3 wt.%
based on a literature search of similar basalts worldwide. The Panel
estimated 2.5-4.0 wt.% H~2~O in CF basalts from thermodynamic
calculations based on the crystallization sequence. This demonstrated
the lack of analysis that has been done and the type of analyses that
need to be done to better constrain the volatile contents of magmas
erupted at CF. The Panel recommended the examination of glass and fluid
inclusions in phenocrysts from the Quaternary potassic trachybasalts to
estimate CO~2~ and to utilize up-to-date thermodynamic solubility models
for CO~2~, S, and H~2~0 to better constrain the depth of exsolution and
fragmentation. This and other recommended analyses mentioned in the
interim report on magma transport and mechanical properties should be
considered in future DOE tasks.

The majority of the Panels presentation focused on dike propagation. The
models presented involved the upward migration of a viscous magma and
the results demonstrated how the crack tip might respond to the stress
regime around the repository over time. The part of their presentation
that appeared to be the weakest was their analysis of the consequences
of the dike intersecting a drift. Existing dike propagation models are
limited to 2-dimensions and viscous, incompressible magma flow. In the
case of a dike intersecting a drift, the model needs to consider a
3-dimensional flow to account for the diversion of magma into the drift
and its effect of the propagation of the crack tip. This model also
needs to consider the range of magma flow types given that basaltic
magma could contain up to 4 wt.% H~2~O. Under these conditions, magma
behind the crack tip will likely be a bubbly magma that will respond
very differently than a liquid magma without many bubbles (with a low
void fraction) when it enters the drift. A viscous magma is an
incompressible fluid and does not change in volume in response to a
change in pressure as would be the conditions upon entering the drift or
near the crack tip. A bubbly fluid is compressible given the presence of
gas bubbles and will respond to a change in pressure by expanding or
compressing. For example, in the case of entering the drift or during
the propagation of the crack tip, a bubbly magma will experience a
decrease in pressure which will cause exsolution of volatiles which in
turn will change the its transport and mechanical properties-- factors
that are not considered in current dike propagation model or drift flow
model.

When modeling the dynamic behavior of the magma inside a drift, the
initial pressure gradient becomes important as demonstrated by the Woods
et al. model. A bubbly magma may expand and freeze or produce a
Strombolian type of eruption inside the drift that would lead to rapid
pressurization of the drift a scenario that has not been considered. No
crack propagation flow model currently exists that consider a
compressible bubbly flow regime along with an appropriate viscosity and
solubility model. It is an area of volcanology that has mostly been
studied by laboratory experiments and overly simplified numerical
models. One approach to developing this type of model would be to couple
a dike propagation model for a bubbly fluid and couple it to an
appropriate flow model for inside the drift using the CFSLIB computer
code by Gaffney. The drift model needs to include the elastic properties
of the wall rock to predict failure conditions and shed new light on
their "dog-leg" scenario. Given the complexity of the flow phenomenon,
laboratory experiments are essential to understand the physics and to
validate the numerical model. These were issues/ideas that were
discussed between myself, Alan Rubin and Anthony Pearson after the
Panel's presentation and should be included in their final report.

The last topic presented was the consequences of magma breaking through
to the surface. Like the other topics, there are many scenarios that
influence the related calculations, in this case it the TSPA
calculations for eruptive doses. The Panel recommended that more studies
be conducted on how the waste packages would respond not only to slow
moving, low void fraction magma but also to the other type of flow
regimes especially a high-speed, high-density gas-ash flow or fast
moving bubbly magma. It was brought to the attention of the Panel that
lava flows can carry objects that are much heavier than those predicted
by theory and that laboratory or field experiments are needed to improve
models on how waste packages may become mobilized. Another concern about
the TSPA calculations was the neglect of secondary transportation of
contaminants by wind after deposition and fragmentation of waste
material from the canisters if exposed during an eruption.

The Panel's review raised many new issues as mentioned above and in the
presentation and interim report. In the final report, the Panel needs to
prioritize these issues and elaborate on how they should be addressed in
future DOE tasks.