DOE 2000. - Waste Isolation Pilot Plant - U.S. Department of Energy

WIPP RH PSAR DOE/WIPP-03-3174 CHAPTER 4
Option B represents the preferred option for panels that will be closed for more than 20 years prior to
final facility closure, and whose entries are reasonably intact at time of closure. These will likely be
Panels 2 through 5. Option C may be desirable for panels whose entries require DRZ removal, and
whose closure precedes final facility closure by less than 20 years. This is the likely configuration of the
closure for Panels 9 and 10. Finally, Option D may be appropriate for panels whose entries require
significant removal of the DRZ, and whose closure will precede final facility closure by more than 20
years. Panel 1 is the most likely candidate for this type of closure.
The 20-year limit in the design selection process is based on what the DOE believes to be conservative
analytical results that indicate methane, being generated by waste degradation at the rate of 0.1 mole per
drum per year, will not reach flammable concentrations for at least 20 years. As part of the decision
making process on design selection, an investigation of the DRZ would precede the selection of the
concrete component and the specification of the amount of excavation that is needed. The investigation
could be done using geophysical methods (such as ground penetrating radar) or drill holes. Drill holes
can be investigated using video cameras or "scratchers." The DOE considers the 20-year criterion is still
appropriate, since the design report shows that it takes 25 years to reach explosive limits. A ten percent
reduction in this time is still beyond 20 years. Furthermore, the chances that methane will be generated
initially are minimized by the fact that the closed panels will be initially oxic and may remain so for a
long time after facility closure.
The DOE believes that design Options A through D will function adequately as panel closures, given the
current state of knowledge about gas generation, the understanding of the DRZ, the expected
characteristics of the waste, and the inability of monitoring techniques to accurately detect extremely
small concentrations of VOCs. However, in the event sufficient information is collected that allows the
DOE to make less conservative assumptions regarding these items, designs A through D may provide
significantly more protection than is actually needed. Consequently, the DOE has retained as a design
concept, Option E, which is simply the explosion wall portion of Options B and D. Option E represents
a significantly simpler panel closure system that the DOE would use if either of the following criteria are
met:
Ç
Gas generation rates are smaller. Current (unreported) work being performed by Sandia National
Laboratories indicates that microbial gas generation rates under humid conditions are close to zero,
and/or
Ç
The average headspace concentrations are less than the averages used in the calculations. As new
wastes are generated, the use of organic solvents is expected to drastically be reduced.
Condition 1 of the Certification Decision Final Rule 4 requires that the DOE implement the Option D
panel closure system at the WIPP.
4.2-29 January 28, 2003