CHANGES IN THE RECERTIFICATION
PERFORMANCE ASSESSMENT
Performance Assessments (PA) are conducted by DOE to
demonstrate that the WIPP meets EPA's release require-
ments for radionuclides. DOE included a PA in the first
certification application and a revised PA was developed
for the 2004 recertification application. During EPA's
review of the recertification PA, both EPA and DOE inde-
pendently identified several technical changes and cor-
rections that were necessary. Because of these changes,
EPA directed DOE to conduct a new PA to demonstrate
continuing compliance for recertification. DOE's revised
PA is now called the Performance Assessment Baseline
Calculations (PABC) and included all of the changes re-
quired by EPA. The PABC shows that WIPP continues to
meet EPA's limits for the release of radionuclides.
What's New in the 2004 Recertification PA:
• The waste inventory has been updated to include new
information on potential waste streams. For more
information on the waste inventory see Fact Sheet #4,
TRU Waste Inventory.
• The updated waste inventory shows that when filled
to capacity, the repository is projected to have less
radioactive content than was previously expected in
the certification PA.
• Projected increases in the drilling rate near the WIPP
may result in slightly higher releases than those stated
in the certification PA. Even with an increased drill-
ing rate, however, the potential releases are still well
below EPA's release limits.
• Changes were made to PA parameters, to incorporate
new information gained since certification.
• DOE incorporated the characteristics of the panel
closure system that was specified by EPA in 1998.
• Incorporation of a simplified model for the sealed
shafts.
• A new model for predicting the release of solids,
called spallings, released in the event of drilling
through the WIPP repository.
2006 EPA WIPP FACT SHEET No. 3
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What is Performance Assessment?
Performance Assessment (PA) is an investigation of the
likelihood that the WIPP will meet release limit require-
ments for radionuclides. EPA required DOE to conduct a
PA as part of the Certification Application to consider how
both natural and man-made processes may affect the
WIPP disposal system. The PA uses computer models to
evaluate hundreds of combinations of possible events to
determine the impact on repository performance.
The results of the 1998 certification PA showed that the
human intrusion scenario is the most important release
mechanism. No appreciable releases occur in the undis-
turbed scenario.
The PA has changed since the 1998 certification of WIPP
to reflect new data collected by DOE. All changes to the PA
are reviewed by EPA. DOE's recertification PA is now called
the PABC. It takes into account new information and
knowledge learned about the WIPP over its first five years
of operation.
EPA conducted a detailed review of the recertification
PABC. Although the PABC does indicate more potential
releases from a human intrusion event, the possible
releases remain well within the limits established by 40
CFR 191. EPA determined that the PABC is a sufficiently
conservative and current representation of the knowledge
of the WIPP and how it will interact with the surrounding
environment.
United States Environmental Protection Agency I Office of Air and Radiation (6608J) I EPA402-F-06-008 I March 2006
www.epa.gov/radiation/wipp
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Human Intrusion
Of all the features, events and pro-
cesses that are considered for the
performance assessment calculations,
the human intrusion scenarios related
to drilling have been shown to have the
most significant impact on the disposal
system and its ability to contain waste.
Human intrusion refers to the breach-
ing of any portion of the WIPP disposal
facility by human activity.
DOE is required to track human activi-
ties within the Land Withdrawal Bound-
ary (LWB)1 for 100 years after closure of
the facility to ensure that no intrusion
occurs. Analysis shows that drilling is
the most likely human activity to have
an impact on WIPP's ability to contain
radioactive waste.
The risk of breaching the disposal facil-
ity through drilling activity associated
with resource extraction is the major
concern. A drilling borehole that goes
through the repository creates a po-
tential for the release of radionuclides
in the vicinity of the borehole. Drilling
through the repository and into the pres-
surized groundwater below it (if present)
could also create a release risk by forc-
ing the groundwater through parts of the
repository. The figure to the right shows
the probable pathway for the release
of radionuclides if such events were to
occur.
While drilling is not permitted inside
the Land Withdrawal Boundary with
the exception of wells for subsurface
investigations, the total number of wells
drilled outside the LWB and inside
the Delaware Basin has increased by
12% since the certification PA. Even
with this increase, the recertification
PA shows that the WIPP continues to
remain in compliance with radioactive
waste release limits.
Drilling Rig
Land Surface
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SX• groundwater
pathway
Human Intrusion by Drilling.
The figure above shows potential pathways for radionuclide releases in
the event of drilling a borehole through the repository and into a pocket of
pressurized groundwater. Groundwater mobilized by drilling could carry
radionuclides into groundwater or to the surface through the borehole.
Other potential pathways for radionuclide releases due to drilling include
cuttings, cavings and spallings:
Cuttings are solids that are cut from the borehole by the drill and
brought to the surface.
Cavings are solids that fall from the wall of the borehole.
Spallings are solids that are forced into the borehole by
pressurized gas when a hole is drilled into the repository.
All of these sceneries are considered in WIPP performance assessments.
More information on the consideration
of human intrustion for recertification
can be obtained from EPA's website or
docket.
'The Land Withdrawal Boundary (LWB) is a 16 square mile area that
extends outwards from the center of the WIPP facility for about 2 miles
in all directions. The surface area map in Fact Sheet #6, Karst, shows
the LWB in relation to the WIPP facility and nearby natural features.
Fact Sheets in this Series:
Recertification
Public Involvement
Fact Sheet No. 1
Fact Sheet No. 2
Performance Assessment Fact Sheet No. 3
TRU Waste Inventory
Groundwater
Karst
Fact Sheet No. 4
Fact Sheet No. 5
Fact Sheet No. 6
United States Environmental Protection Agency I Office of Air and Radiation (6608J) I EPA402-F-06-008 I March 2006
www.epa.gov/radiation/wipp
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