Betsy Forinash and Scott Monroe
U.S. Environmental Protection Agency, Office of Radiation and Indoor Air
401 M Street, SW (6602-J)
Washington, DC 20460
(202) 233-9310

The Waste Isolation Pilot Plant (WIPP) is being constructed by the United States (U.S.) Department of Energy (DOE) near Carlsbad, New Mexico, as a potential repository for the disposal of transuranic (TRU) waste. Among the prerequisites for the facility to commence disposal of TRU waste, the U.S. Environmental Protection Agency (EPA) is required to evaluate DOE's compliance certification application (CCA) and to certify whether the WIPP will comply with Subparts B and C of 40 CFR Part 191, known as the disposal regulations. In October 1997, the EPA issued a proposed certification decision, subject to several conditions, that the WIPP will comply with the disposal regulations. If finalized, this decision could allow the WIPP to begin operation and become the first facility of its kind in the world to accept TRU waste.

In developing its proposed decision, the EPA reviewed over 100,000 pages of documentation submitted by the DOE, conducted numerous on-site inspections of WIPP-related facilities, and performed independent modeling. The EPA's evaluation of the CCA addressed significant technical issues associated with future performance of the WIPP disposal system and the DOE's performance assessment for the WIPP, including:

• the closure system used to reduce brine flow between waste panels;
• the capability of individual generator sites to adequately characterize TRU waste;
• testing and implementation of markers to warn potential future intruders;
• confirmatory calculations to verify modeling results;
• models used to predict the volume of radioactive material that could be released through a drilling intrusion;
• the exclusion from performance assessment of the effects of various resource recovery techniques which currently occur in the vicinity of the WIPP; and
• the potential impacts of karst topography.

This paper discusses the EPA's conclusions regarding the validity of the DOE's approach to these issues, and their impact on the EPA's proposed certification of compliance for the WIPP.

The disposal regulations¹ limit releases of radioactive material from disposal systems for spent nuclear fuel, high-level and TRU radioactive waste, and require implementation of measures to provide additional confidence in the performance of such disposal systems. The regulations also limit radiation doses to members of the public and protect ground-water resources by establishing maximum concentrations for radionuclides in ground water. In order to implement the generally applicable disposal regulations at the WIPP, the EPA issued 40 CFR Part 194-known as the WIPP compliance criteria-in 1996.² These criteria interpreted the disposal regulations specifically for TRU waste at the WIPP and established the requirements by which the EPA will judge whether the WIPP complies with the disposal regulations.

In October 1996, the DOE submitted to the EPA a CCA describing measures DOE has taken to comply with the EPA's disposal regulations. The EPA's evaluation of compliance was conducted by comparing relevant information point-by-point to the WIPP compliance criteria in 40 CFR Part 194.² The EPA reviewed the CCA and supplementary information submitted by the DOE in response to requests from the EPA. The EPA also conducted its own independent modeling and analyses, as well as on-site audits and inspections of WIPP-related facilities. Finally, the relevant information considered by the EPA in developing the proposed certification included over 800 public comments which were submitted in response to an Advance Notice of Proposed Rulemaking.³

This paper discusses technical issues related to the proposed conditions of certification (i.e., those issues which affected the certification decision); the conditions arose primarily from the need to ensure that the EPA's evaluation of the WIPP's compliance is based on the features and conditions that actually will be implemented and will exist at the site. This paper also addresses a select number of other technical issues which are either especially important to performance of the WIPP disposal system or were the subject of numerous public comments.

Discussed below are the four conditions described in the EPA's proposed certification decision. The discussion also addresses the EPA's evaluation of, and conclusions regarding, a select number of technical issues.

An important engineered feature of the disposal system design is the panel closure system. The long-term purpose of the closure system is to block potential flow of brine between waste panels in the underground repository at the WIPP. In the CCA, the DOE provided four design options for panel closures, but did not specify which one would be implemented at the WIPP. The EPA identified Option D in the CCA as the most robust design; this design includes an explosion-isolation wall and a concrete barrier with the disturbed rock zone removed. The EPA used the Option D design as the basis for its evaluation of compliance. The EPA determined that the design for Option D could be expected to perform as described, but concluded that the use of a Salado mass concrete (i.e., salt-saturated to match site conditions) rather than fresh water concrete would be more consistent with the panel permeability assumptions used in the performance assessment (PA).

The EPA found that the Option D design, modified to include Salado mass concrete, would be adequate to achieve the long-term performance modeled in the PA, and therefore proposed to find that the engineered design of the WIPP will comply with the disposal regulations. To ensure that the EPA's compliance determination is based on realistic site conditions, the EPA's proposed certification includes a condition requiring the DOE to implement the most robust panel closure system design-Option D-at the WIPP.

The EPA's WIPP compliance criteria require that the DOE describe the measures used to characterize waste proposed for disposal at the WIPP, as well as quality assurance (QA) programs applicable to waste characterization activities.² The DOE proposed a number of techniques, including non-destructive assay and visual examination, to quantify various waste components. The EPA found that these techniques, if implemented appropriately, would be adequate to characterize waste components important to containment of waste at the WIPP. The EPA believes that appropriate execution of waste characterization measures cannot be determined from the general information provided by the DOE in the CCA, but instead must be verified through inspections of individual waste generator sites. For similar reasons, while the EPA found that the DOE's overall QA program complies with 40 CFR Part 194, EPA must verify the establishment and execution of site-specific QA programs at waste generator sites before allowing waste from such sites to be disposed in the WIPP.

Prior to issuing the proposed certification decision, the EPA conducted a series of inspections at the Los Alamos National Laboratory (LANL) and found that adequate QA programs had been established at the site. During these inspections, the DOE further demonstrated that a system of controls has been implemented at the LANL that is adequate to characterize important waste components in several waste streams. Finally, the DOE demonstrated that the waste tracking system, necessary to ensure that WIPP-specific limits on waste components (such as ferrous metals) are not exceeded, is operational at both the WIPP facility and at the LANL. Based on these inspections, the EPA proposes that the LANL has met the relevant QA requirements. The EPA further proposes that the DOE may ship TRU waste from the LANL for disposal at the WIPP; however, this decision is limited to waste from those waste streams which can be characterized using the systems and controls that were inspected by the EPA and found to be adequately implemented at the LANL. At the time of the proposed rule, the EPA did not find that any other waste generator site had implemented adequate QA and waste characterization programs.

The proposed certification of compliance with 40 CFR Part 191 includes a condition that the DOE may not dispose of waste from other waste streams at the WIPP facility until the EPA determines that the DOE has provided information on how process knowledge and other controls will be used to characterize waste from specific waste streams at a given generator site, and that the DOE has established and executed the required QA programs applicable to waste characterization activities at that site. The proposed process for approving additional waste generator sites and waste streams includes a Federal Register notice announcing the EPA's intent to evaluate a given site or waste stream(s); a public comment period; audits or inspections by the EPA at each site; and written approval by letter from the EPA to the DOE.

Based on the design provided in the CCA, the EPA proposed that the DOE will comply with the requirement to implement passive institutional controls (PICs) that are designed to be as permanent as practicable and are intended to alert potential future intruders to the dangers in the WIPP. In the CCA, the DOE proposed to take most of the steps necessary for implementing the proposed PICs, such as refining marker messages, during the WIPP's operational period. However, the DOE proposed to extend some activities-particularly testing of the markers-over nearly 100 years after closure of the facility before finalizing important aspects of the design.

For the purpose of the compliance evaluation, the EPA reviewed the PICs design proposed in the CCA. Because the CCA design is the basis of the EPA's proposed certification decision, and because the issue of PICs implementation has been of great interest to the public, the EPA concluded that the DOE had not sufficiently justified the need for additional testing of markers after closure of the repository or the need to delay implementation for many years after closure.

The EPA believes that PICs should be implemented as soon as possible after the WIPP facility is closed. To that end, the measures necessary to prepare for implementation should be accomplished during the operational period for the WIPP, so far as is practical. (Any changes to the design made during the operational period will be examined by the EPA and may require a modification to any compliance certification.) Therefore, the EPA's proposed certification includes a condition that the DOE must submit a revised PICs implementation schedule showing how markers will be fabricated and emplaced, and how other measures will be implemented, as soon as possible following closure of the disposal system. The DOE must also describe what changes to the design might be expected to result from future testing of the conceptual testing.

The PA constitutes the primary basis for predicting the behavior of the WIPP disposal system. The results of PA (in the form of complementary cumulative distribution functions [CCDFs]) are used to evaluate whether the facility meets the radioactive release limits which apply over the EPA's 10,000-year regulatory time frame.^{1, 2}

The results of the CCA PA complied with the release limits by more than an order of magnitude. In the process of reviewing the CCA, the EPA and public commenters raised concerns about certain assumptions and specific parameter values (the brine pocket volume, for example) used by the DOE in the CCA PA. Also, the DOE found several coding problems in the PA computer software. Therefore, the EPA required the DOE to conduct a PA verification test (PAVT) to confirm that the cumulative impact of changes in the CCA PA codes and parameters would not be significant enough to necessitate a new PA. The PAVT included corrections to computer codes and modifications to fixed values or probability distributions used for 24 parameters. The additional modeling also excluded any credit for the effectiveness of PICs, which had been included in the CCA PA in the form of a reduced future rate of human intrusion.

With the exception of the changes noted above, the PAVT used methods and computer models identical to those in the CCA PA. Like the CCA PA, the PAVT produced 300 CCDF curves. The results of the PAVT showed somewhat higher cumulative releases from the WIPP than the original CCA PA. However, these higher cumulative releases were still about an order of magnitude lower than the containment requirements at the probability levels described in the disposal regulations. Consequently, the PAVT results indicated that the CCA PA would comply with the containment requirements for the WIPP, even including substantial modifications to some of the significant uncertain parameters used in the CCA PA. Therefore, the EPA proposes that the WIPP will comply with the containment requirements of the disposal regulations and will safely contain radioactive waste for at least 10,000 years after disposal.

Human intrusion scenarios were of great interest to both the EPA and public commenters, since the CCA PA results showed no releases of radioactive material from the WIPP for undisturbed conditions. Drilling intrusions received particular attention since the EPA's regulations require that DOE model the occurrence of drilling intrusions throughout the 10,000-year regulatory time frame.² The issue of spallings, releases of solid material which may be forced up and out of an intrusion borehole by gas pressure in the repository, was of specific interest to the EPA and the public since a peer review panel convened by the DOE initially found the conceptual model for spallings to be inadequate. The panel found that the original CCA PA spallings model did not appropriately model the processes that affect pressure-driven solid releases.

In response to the concerns expressed by the peer review panel, the DOE developed an alternative mechanistically-based model. The results of the alternative model were more than an order of magnitude lower than release volumes predicted using the CCA PA spallings model. The peer review panel concluded that, while the CCA PA model does not accurately represent future states of the disposal system, its inaccuracies were of a conservative nature. The EPA concurred with the conclusions of the peer review panel that the CCA PA spallings model produces reasonable and probably conservative results. Therefore, the EPA proposes that the original spallings model was adequate for use in the CCA PA.

The EPA's WIPP compliance criteria require that DOE model the potential occurrence of excavation mining, deep drilling, and shallow drilling over the regulatory time frame. In examining the effects of drilling events during the 10,000-year time frame, the DOE may consider only the act of drilling a borehole and exclude the effects of any resource recovery techniques. However, the WIPP compliance criteria do require the DOE to examine the potential effects of human activities which are expected to occur in the vicinity of the WIPP prior to or soon after disposal.² In particular, public commenters and the EPA had expressed concern that fluid injection activities could affect the containment capabilities of the WIPP. This concern was prompted by the occurrence of a salt-water blowout (often referred to as the "Hartman scenario") in an oil development well about 85 kilometers east of the WIPP, outside the Delaware Basin. The court ruled that the cause of the blowout (water flow) was injection water from a long-term injection borehole located more than a mile away from the oil well. The DOE identified waterflooding (for enhanced oil recovery) and brine disposal as the only fluid injection activities currently occurring in the vicinity of the WIPP.

In the CCA, the DOE excluded near-term fluid injection scenarios from the PA based on a screening analysis that showed the effects of such events would be of low consequence to the disposal system-specifically, that the amount of brine which might enter the WIPP due to fluid injection activities is low compared to the amount which might be expected to enter the waste area naturally. While the EPA's regulations allow scenario screening based on such criteria, both the EPA and the public raised questions regarding the screening analysis used by the DOE. The EPA required the DOE to do a supplemental screening analysis that accounted for the possibility of multiple oil fields (by assuming fluid injection could occur for a time frame of 150 years rather than 50 years); used modified values for some input parameters; and modeled the disturbed rock zone consistent with assumptions used in the CCA PA. In addition, the EPA required the DOE to submit additional information on the nature and frequency of fluid injection well failures.

The supplemental work performed by the DOE on fluid injection addressed all the issues identified by the EPA. The EPA agreed with the DOE's conclusions that the conditions at the WIPP differ significantly from those in the "Hartman scenario," in terms of both geology and well construction and operating practices. Both the DOE's research and the EPA's own review of fluid injection indicate that the probably of catastrophic well failure (similar to the Hartman scenario) in the vicinity of the WIPP is extremely low. Furthermore, the geology at the WIPP would substantially reduce the amount of any injected brine that could enter the waste panels. After examining the additional analysis received from the DOE, the EPA concluded that near-term fluid injection is a low-probability scenario that can be screened out of the PA based on low consequence.

The WIPP site is located in a near-surface karst region (i.e., topography created by dissolution of rock). The karst topography has been of great concern to the public-and the topic of many public comments-since karst features, if present at the WIPP site, could significantly affect groundwater flow and transport times. The EPA reviewed evidence submitted by the DOE, stakeholders, and public commenters regarding the occurrence and development of karst at the WIPP. The EPA concluded that, while the WIPP site is in a karst region and karst features are found to the west of the site, only limited evidence exists that dissolution-related features occur near the WIPP boundary. Moreover, these features are neither pervasive nor associated with any identified preferential groundwater flow paths or anomalies. WIPP field mapping and site-specific hydrologic information (e.g., well tracer tests) do not indicate that any cavernous or other karst-related flow is present at the WIPP site. The EPA concluded that karst is not currently a problem at the WIPP. Furthermore, geologic evidence of approximately the last 500,000 years indicates that future development of karst at the WIPP is not likely.

The EPA examined a number of other issues related to dissolution, including the possibility of lateral dissolution and deep dissolution of the geologic formation in which the underground waste emplacement rooms are located. The EPA determined that such dissolution processes would occur slowly (on the order of hundreds of thousands of years to reach the WIPP), if at all. Therefore, the EPA concluded that such processes are not expected to affect the containment capabilities of the WIPP during the 10,000-year regulatory time frame.

This paper describes only a small selection of the technical issues examined by the EPA in developing a proposed certification decision for the WIPP. The EPA conducted an exhaustive scientific review of over 100,000 pages of documentation - including the CCA, additional information submitted by the DOE, and detailed records on nearly 1500 parameters used in the PA. More detailed treatment of the issues discussed above, as well as many other technical topics considered by the EPA, can be found in the Federal Register notice for the proposed rule.⁴ Compliance Application Review Documents (CARDs) and Technical Support Documents (TSDs) in the EPA's docket provide further information concerning the EPA's compliance evaluation and independent analyses (Docket A-93-02, Category III-B). Docket locations are described in the proposed rule and on the EPA's WIPP Internet Homepage.

Copies of the Federal Register notice for the proposal can be obtained by calling the EPA's WIPP Information Line at 1-800-331-WIPP. Electronic copies of the Federal Register notice and supporting documents can also be found at the EPA's WIPP Internet Homepage (http://www.epa.gov/radiation/wipp).

1. U.S. ENVIRONMENTAL PROTECTION AGENCY, "Environmental Radiation Protection Standards for the Management and Disposal of Spent Nuclear Fuel, High-Level and Transuranic Radioactive Wastes: Final Rule." Codified at Chapter 40 of the Code of Federal Regulations, Part 191 (40 CFR Part 191). Published in the Federal Register on September 19, 1985 (Vol. 50, No. 182, pp. 38066-38089) and December 20, 1993 (Vol. 58, No. 242, pp. 66398-66416).
2. U.S. ENVIRONMENTAL PROTECTION AGENCY, "Criteria for the Certification and Re-Certification of the Waste Isolation Pilot Plant's Compliance With the 40 CFR Part 191 Disposal Regulations: Final Rule." Codified at Chapter 40 of the Code of Federal Regulations, Part 194 (40 CFR Part 194). Published in the Federal Register on February 9, 1996 (Vol. 61, No. 28, pp. 5224-5245).
3. U.S. ENVIRONMENTAL PROTECTION AGENCY, "Decision to Certify Whether the Waste Isolation Pilot Plant Complies With the 40 CFR Part 191 Disposal Regulations and the 40 CFR Part 194 Compliance Criteria: Advance Notice of Proposed Rulemaking." Published in the Federal Register on November 15, 1996 (Vol. 61, No. 222, pp. 58499-58500).
4. U.S., ENVIRONMENTAL PROTECTION AGENCY, "Criteria for the Certification and Recertification of the Waste Isolation Pilot Plant's Compliance With the 40 CFR Part 191 Disposal Regulations: Certification Decision; Proposed Rule." Published in the Federal Register on October 30, 1997 (Vol. 62, No. 210, pp. 58792-58838).

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