Main Document

Return to the Main Document

1. purpose

This Letter Report presents the results of supplemental evaluations and analyses designed to assess long-term performance of the potential repository at Yucca Mountain. The evaluations were developed in the context of the Nuclear Regulatory Commission (NRC) final public regulation, or rule, 10 CFR Part 63 (66 FR 55732 [DIRS 156671]), which was issued on November 2, 2001. This Letter Report addresses the issues identified in the Department of Energy (DOE) technical direction letter dated October 2, 2001 (Adams 2001 [ DIRS 156708]).

The main objective of this Letter Report is to evaluate performance of the potential Yucca Mountain repository using assumptions consistent with performance-assessment-related provisions of 10 CFR Part 63. The incorporation of the final Environmental Protection Agency (EPA) standard, 40 CFR Part 197 (66 FR 32074 [DIRS 155216]), and the analysis of the effect of the 40 CFR Part 197 EPA final rule on long-term repository performance are presented in the Total System Performance Assessment- Analyses for Disposal of Commercial and DOE Waste Inventories at Yucca Mountain – Input to Final Environmental Impact Statement and Site Suitability Evaluation (BSC 2001 [DIRS 156460]), referred to hereafter as the FEIS/SSE Letter Report.

The Total System Performance Assessment (TSPA) analyses conducted and documented prior to promulgation of the NRC final rule 10 CFR Part 63 (66 FR 55732 [DIRS 156671]), were based on the NRC proposed rule (64 FR 8640 [DIRS 101680]). Slight differences exist between the NRC’s proposed and final rules which were not within the scope of the FEIS/SSE Letter Report (BSC 2001 [DIRS 156460]), the Preliminary Site Suitability Evaluation (PSSE) (DOE 2001 [DIRS 155743]), and supporting documents for these reports. These differences include (1) the possible treatment of "unlikely" features, events and processes (FEPs) in evaluation of both the groundwater protection standard and the human-intrusion scenario of the individual protection standard, and (2) the definition of the water demand of the reasonably maximally exposed individual (RMEI). It is these differences, as well as uncertainty associated with the appropriate approach to implement these differences, that have necessitated the additional analyses presented in this document.

This Letter Report will consider the following three performance-assessment-related topics based on the final NRC rule at 10 CFR Part 63 (66 FR 55732 [DIRS 156671]).

Groundwater Protection Standard and the Consideration of Unlikely Events (i.e., igneous intrusion). This Letter Report evaluates the radionuclide concentrations in groundwater assuming an igneous-intrusion scenario. The report considers this scenario under both the higher-temperature operating mode (HTOM), and the lower-temperature operating mode (LTOM), with respect to total radium concentration, gross-alpha concentration, and dose to critical organs. Even though not expressly required by the NRC final rule, this evaluation is being conducted because the phrase "unlikely features events and processes" is not defined in 10 CFR Part 63.342 (66 FR 55732 [DIRS 156671]). The mean annual probability of an igneous intrusion at the location of the potential repository is 1.6 x 10^-8 (CRWMS M&O 2000 [DIRS 153246] Table 3.10-5, p. 3-203).
Individual Protection Standard for Human Intrusion and the Consideration of Unlikely Events (igneous intrusion followed by human intrusion). This Letter Report discusses the scenario of an igneous intrusion immediately followed by a human intrusion event for both HTOM and LTOM. Even though not expressly required by the NRC final rule, this discussion is being included because the term "unlikely features, events, and processes" is not defined in 10 CFR Part 63. The mean annual probability of an igneous intrusion at the location of the potential repository is 1.6 x 10^-8 (CRWMS M&O 2000 [DIRS 153246] Table 3.10-5, p. 3-203).
Individual Protection Standard and the Effect of 3,000 Acre-Feet Annual Water Demand. This Letter Report discusses the effect of assuming a 3,000 acre-feet annual water demand for the individual protection standard as the water demand for the simulated calculation of dose to the RMEI in accordance with the NRC final rule. This Letter Report contrasts this water demand with the water demand assumed in the TSPA-SR (CRWMS M&O 2000 [DIRS 153246]), and implemented in the Supplemental Science and Performance Analysis (SSPA) Volumes 1 and 2 (BSC 2001 [DIRS 154657]; BSC 2001 [DIRS 154659], Section 4.0), and included in the FEIS/SSE Letter Report (BSC 2001 [DIRS 156460]). The probabilistic calculations in these documents used a range of water demand from 887 to 3,367 acre-ft per year and averaging approximately 2,000 acre-ft per year. This discussion is being included in this Letter Report because 10 CFR 63.312 specifies 3,000 acre-feet per year as the water demand for the dose calculation to the RMEI (66 FR 55732 [DIRS 156671] p. 55814). The EPA did not specify a volume for water demand for the calculation of dose to the RMEI.

This Letter Report is not intended to reflect the updated and new information included in the Technical Update Impact Letter Report because that report was in preparation and not final during final preparation of this Letter Report.

Background The Nuclear Waste Policy Act of 1982, as amended (NWPA, Public Law 97-425 [DIRS 131951]) directs the DOE to evaluate the suitability of the Yucca Mountain site in southern Nevada as a potential site for development of a geologic repository for the disposal of spent nuclear fuel (SNF) and high-level waste (HLW). 10 CFR 63.342 (66 FR 55732 [DIRS 156671] p. 55815) specifies that "unlikely features, events, and processes or sequences of events and processes shall be excluded from the assessments for the human intrusion and groundwater protection standards upon prior Commission approval for the probability limit used for unlikely features, events, and processes." The NRC provided no specific quantitative value for determining when exclusion of unlikely FEPs is appropriate, the final regulations require DOE to exclude unlikely FEPs from the specified analyses upon prior approval of the Commission for the probability limit used for unlikely FEPs. The Commission recognizes that specification of a probability limit for unlikely FEPs, as is done for "very" unlikely FEPs would be a more direct approach. The Commission considers a frequency for unlikely FEPs would fall somewhere between 10^-8 to 10^-4 per year; however, they have not specified a numerical value in the regulations at this time (66 FR 55732 [DIRS 156671] Section II, p. 55734).

Given the absence of precise quantitative frequency that may be applied to the definition of the term "unlikely", but considering that it is expected to be between 10^-8 and 10^-4 per year, the performance assessments of the groundwater protection standard and the human-intrusion scenario for the individual protection standard documented herein consider the possibility of FEPs previously classified as "unlikely" potentially affecting the results of the analyses conducted to date and documented in the TSPA-SR (CRWMS M&O 2000 [DIRS 153246]) and the SSPA Volumes 1 and 2 (BSC 2001 [DIRS 154657]; BSC 2001 [DIRS 154659]). If the quantitative definition of "unlikely" were defined at the most conservative end of the range of frequencies greater than or equal to 10^-8 per year, then such previously "unlikely" FEPs would need to be considered in the human-intrusion and groundwater-protection performance-assessment analyses. Therefore, new analyses have been conducted to assess the potential effects of the one event that was previously excluded from groundwater protection and human intrusion analyses and which has an expected mean annual probability of greater than 10^-8 per year, i.e., an igneous intrusion. All other potential features, events and processes that may reasonably be considered "unlikely" have been either screened out of the performance assessment on the basis of low consequence or low risk in the FEPs screening process defined in the TSPA-SR (CRWMS M&O 2000 [DIRS 153246]) and associated references or have been included in the nominal performance-assessment-scenario class which was used as the basis for the human-intrusion and groundwater-protection analyses in the TSPA-SR. Although it is anticipated that the expedited rulemaking will ultimately specify whether or not such FEPs need to be considered in any licensing proceeding, DOE is considering them at this point as alternative analyses that provide DOE with a conservative estimate of potential dose and radionuclide concentrations.

The NRC proposed rule (64 FR 8640 [DIRS 101680]) allowed for some flexibility in the treatment of the appropriate water demand of the small farming community considered as the receptors in assessing the individual protection standard. In assessments conducted in the TSPA-SR (CRWMS M&O 2000 [DIRS 153246]), the farming community was considered to represent from 15 to 25 small farms, consistent with the proposed NRC rule (64 FR 8640 [DIRS 101680]). The NRC final rule 10 CFR Part 63 (66 FR 55732 [DIRS 156671]) states that the RMEI "uses well water with average concentrations of radionuclides based on an annual water demand of 3,000 acre-ft."

The results of assessments assuming an igneous-intrusion event in the groundwater-protection analysis are presented in Section 6.1. The results of assessments assuming an igneous-intrusion event in the evaluation of the individual protection standard for human intrusion are presented in Section 6.2. Section 6.3 presents the results of assessments using the specified value of 3,000 acre-ft as the annual water demand for purposes of assessing the individual protection standard.

Analyses The simulations and evaluations presented in this Letter Report include estimates of the radionuclide concentrations in groundwater, as specified in 10 CFR 63.331 (66 FR 55732 [DIRS 156671] p. 55814), from the simulated release of radioactive materials from the potential Yucca Mountain repository to the accessible environment. The location for these calculations is the same as the RMEI location in the accessible environment where the groundwater path of the highest concentration of the contaminant plume would cross the southernmost boundary of the controlled area of the potential repository at a Latitude of 36º 40' 13.6661" North as specified in 10 CFR 63.332 (66 FR 55732 [DIRS 156671] p. 55814). This location is approximately 18 km from within the potential repository footprint. This Letter Report considers only the igneous-intrusion aspect of the igneous-activity scenario discussed in the FEIS/SSE Letter Report and does not consider the volcanic-eruption scenario, because the groundwater-protection assessments for estimating the radionuclide concentrations in groundwater are unaffected by the volcanic eruption scenario (CRWMS M&O 2000 [DIRS 153246] Section 4). The volcanic-eruption scenario is not considered because the main biosphere pathway due to a volcanic eruption is the air pathway as a result of volcanic ash released into the atmosphere.

The evaluations included in this Letter Report were developed from the calculations completed and documented in the FEIS/SSE Letter Report (BSC 2001 [DIRS 156460]). Process models included in the model configuration used in the calculations and sensitivity analyses presented in this Letter Report are the same as the process models referred to in the FEIS/SSE Letter Report (BSC 2001 [DIRS 156460]).

This Letter Report is part of the TSPA suite of documentation that entails a number of documents, each with a specific purpose, but utilizing the same information base in assessing the long-term performance of the Yucca Mountain site to support the postclosure suitability evaluation. Table 1-1 identifies the five key documents in the sequence of TSPA modeling, starting with the earliest, and provides a general description of the changes made in each TSPA document relative to succeeding documents.

This Letter Report was developed under Technical Work Plan TWP-MGR-PA-000004 REV 00 (BSC 2001 [DIRS 156459]). This Technical Work Plan and the included activity evaluation specified that this Letter Report is not to be developed per the Yucca Mountain Project (YMP) Quality Assurance (QA) program and thus Quality Assurance Requirements Document (QARD) requirements are not applicable. The Technical Work Plan specifies that the document be subject to the same rigorous checking practices as performed for similar documents prepared under the Yucca Mountain QARD.

The performance-assessment calculations included in this Letter Report were performed for the 70,000-MTHM (metric tons of heavy metal) inventory according to the Nuclear Waste Policy Act of 1982, as amended (NWPA, Public Law 97-425 [DIRS 131951]). The 70,000-MTHM-Inventory Case is comprised of 63,000 MTHM of CSNF (commercial SNF), 2,333 MTHM of DSNF (DOE SNF), and 4,667 MTHM of HLW.

2. METHOD

The analyses in this Letter Report were conducted by the Bechtel SAIC Company (BSC) Total System Performance Assessment Department in support of the DOE Yucca Mountain program. This Letter Report includes evaluations and calculations of various topics pertinent to certain aspects of the NRC final rule 10 CFR Part 63 (66 FR 55732 [DIRS 156671]). The calculations presented in this Letter Report were previously performed using the numerical code GoldSim (BSC 2001, GoldSim V7.17.200 STN:10344-7.17.200-00 [DIRS 155182]). The GoldSim calculations were performed for the evaluation of the EPA final rule 40 CFR Part 197 and documented in the FEIS/SSE Letter Report (BSC 2001 [DIRS 156460], Section 2). The model in the FEIS/SSE Letter Report was based on two previous analyses, the TSPA-SR REV 00, ICN 01 (CRWMS M&O 2000 [DIRS 153246]) that was expanded upon in the SSPA (BSC 2001, Volume 1, [DIRS 154657] and BSC 2001, Volume 2, [DIRS 154659]).

As mentioned previously, this Letter Report addresses provisions of the NRC final rule 10 CFR Part 63 that are beyond what was considered in the FEIS/SSE Letter Report. The FEIS/SSE Letter Report considers radiation protection standards addressed in the final EPA rule 40 CFR Part 197 and adopted with added provisions by the NRC in its final rule 10 CFR Part 63. The areas are, as discussed in Section 1: 1) groundwater protection standard and the consideration of unlikely events (i.e., igneous intrusion); 2) individual protection standard for human intrusion and the consideration of unlikely events (i.e., igneous intrusion followed by human intrusion); and 3) the effect on the calculated dose for the individual protection standard due to the 3,000 acre-ft annual water demand specified in the final NRC rule at 10 CFR 63.312(c) (66 FR 55732 [DIRS 156671] p. 55814). The evaluations and comparisons relative to these radiation protection standards in the NRC final rule are presented in Section 6 below.

The revised supplemental TSPA model used in the FEIS/SSE Letter Report was developed from a previous TSPA model to incorporate the requirements of the final EPA Rule. The analyses in this Letter Report are based on that the supplemental TSPA model discussed in the SSPA. The calculations were performed within a probabilistic framework combining the various component models, processes, and corresponding parameters included in the overall conceptual/process model describing the performance of the repository. GoldSim integrates the sub-system models using a Monte-Carlo simulation-based methodology to create multiple random combinations of the uncertain variables, and computes the probabilistic performance of the entire waste-disposal system in terms of mean annual dose to receptors. The GoldSim software calculates radionuclide release and radiological dose as defined in 10 CFR 63.311 (66 FR 55732 [DIRS 156671] p. 55814) from individual radionuclides and the combined annual dose due to all radionuclides released from the repository from failed waste packages. For these analyses, GoldSim calculates the combined annual dose for multiple realizations (300 realizations for the human-intrusion scenarios and 5,000 realizations for the igneous-activity scenario) of the model configuration using randomly selected values of distributed parameters for each realization.

The performance-assessment methodology used for the calculations presented in this Letter Report draws upon the extensive analyses carried out in support of the TSPA-SR (CRWMS M&O 2000 [DIRS 153246]) and Volumes 1 and 2 of the SSPA (BSC 2001 Volume 1 [DIRS 154657] and BSC 2001 Volume 2 [DIRS 154659]). Those calculations and analyses were based on proposed EPA and NRC rules. The EPA in its final rule (40 CFR 197.31) defined the representative volume for groundwater protection as 3,000 acre-ft/yr of water and did not specify a volume for water demand for the dose calculation to the RMEI for the evaluation against the individual protection standard. Results in FEIS/SSE Letter Report (BSC 2001 [DIRS 156460], Section 6.6) reflect this change for the evaluation of groundwater protection. In its final rule the NRC included 3,000 acre-ft/yr as water demand for the dose calculation to the reasonably maximally exposed individual for the evaluation of individual protection, as well as for the evaluation of groundwater protection. In the FEIS/SSE Letter Report (BSC 2001 [DIRS 156460], Sections 6.2 and 6.3), the annual volume of water used for the evaluation of individual protection used a range from 887 to 3,367 acre-feet consistent with the final EPA rule. Thus, the FEIS/SSE Letter Report considered the effects of the use of a range of groundwater demand volumes as opposed to the use of 3,000 acre-feet annual water demand, as set forth in 10 CFR 63.312(c) (66 FR 55732 [DIRS 156671] p. 55814), for the simulated calculation of dose to the RMEI for evaluation against the individual protection standard. In this Letter Report, the evaluation of groundwater protection considering an unlikely event (i.e. igneous intrusion) uses a representative water volume of 3,000 acre-ft (10 CFR 63.331, 66 FR 55732 [DIRS 156671], Table 1, p. 55814) consistent with both the final EPA rule and the final NRC rule.

This Letter Report utilizes the results of the following scenarios previously calculated for the 70,000-MTHM-inventory case, HTOM and LTOM, and presented in the FEIS/SSE Letter Report or the SSPA, Volume 2. The following description of these scenarios used in the calculations presented in the FEIS/SSE Letter Report is provided for completeness.

The human-intrusion scenario in the FEIS/SSE Letter Report (BSC 2001 [DIRS 156460] Sections 5.2.7 and 6.4; BSC 2001 Volume 1 [DIRS 154657] Appendix A) considers an "intruder" drilling a land-surface borehole using a drilling apparatus (under the common techniques and practices that are currently employed in exploratory drilling for groundwater in the region around Yucca Mountain). The "intruder" drills directly through a degraded waste package, and subsequently into the uppermost aquifer underlying the Yucca Mountain repository. The "intrusion" then causes the subsequent compromise and release to groundwater of the contaminated waste in the penetrated waste package. Two human-intrusion scenarios were simulated for a one-million-year performance period (BSC 2001 [DIRS 156460] Sections 5.2.7 and 6.4): one intrusion at 30,000 years and the other intrusion at 100 years after repository closure in accordance with the proposed NRC rule.
The igneous-intrusion scenario, which was included as part of the more comprehensive igneous-activity scenario for the evaluation of individual protection, considered a dike (or dikes) intersecting emplacement drifts, causing varying degrees of waste-package damage in the affected drift(s) and subsequent release of radioactive material. The release and transport to the unsaturated zone (UZ) and subsequently to the saturated zone (SZ) was evaluated in the FEIS/SSE Letter Report (BSC 2001 [DIRS 156460] Sections 6.3.1 and 6.3.2).

3. ASSUMPTIONS

The TSPA results utilized for the evaluations in this Letter Report have been previously calculated for the FEIS/SSE Letter Report (BSC 2001 [156460]) based on the final EPA rule 40 CFR Part 197 (66 FR 32074 [DIRS 155216]). Some key assumptions used in those calculations and in this Letter Report are listed here for completeness.

3.1 Assumption 1

The 70,000-MTHM-case model configuration for the calculations in this Letter Report is the same as the model configuration discussed in the FEIS/SSE Letter Report (BSC 2001 [DIRS 156460]), which was, in turn, based on the SSPA model configuration (BSC 2001 Volume 1 [DIRS 154657]), and the TSPA-SR model (CRWMS M&O 2000 [DIRS 153246]). The model used for the calculations used in Sections 6.0 of this Letter Report includes the modifications from the SSPA and TSPA-SR models as described in the FEIS/SSE Letter Report (BSC 2001 [DIRS 156460], Section 5).

3.2 Assumption 2

The radionuclide inventories used in the calculations in Section 6.0 are those developed in the Inventory Abstraction (BSC 2001 [DIRS 154841], Table 36, p.38). The per-package inventories for the CSNF, and co-disposal waste packages are the same as those used in the TSPA-SR (CRWMS M&O 2000 [DIRS 153246], Section 3.5.1, p. 3-94 to 3-100). The DSNF inventory includes the U.S. Navy spent nuclear fuel which was modeled using CSNF as a bounding surrogate for naval spent nuclear fuel. Because of the robust design of naval spent nuclear fuel, releases from naval spent nuclear fuel are significantly less than releases from commercial spent fuel (BSC 2001 [DIRS 152059], Section 6.1.1). Therefore, for analysis purposes, releases from naval spent nuclear fuel waste packages are included with releases from CSNF waste packages (DOE 2001 [DIRS 153849], Section 4.2.6.3.9, p. 4-257).

4. USE OF COMPUTER SOFTWARE

The calculations described in this Letter Report were performed using the numerical code GoldSim, Version 7.17.200 (BSC 2001, GoldSim V7.17.200 STN:10344-7.17.200-00 [DIRS 155182]). GoldSim is designed so that probabilistic simulations can be conducted and represented graphically. Although not currently qualified in accordance with AP-SI.1Q, Software Management, GoldSim Version 7.17.200 (STN: 10344-7.17.200-00 [DIRS 155182]) is used for this application and used within a specified range that complies with the applicable range for GoldSim Version 7.17.200.

5. CALCULATION

5.1 BACKGROUND

The scope of the calculations presented in this Letter Report includes evaluation of long-term performance after the eventual closure of a potential Yucca Mountain repository as located on Figure 5-1. In particular, these calculations focus on a potential Yucca Mountain repository for the disposal of 70,000 MTHM of nuclear material allowed by the NWPA. The evaluations utilize calculations from the FEIS/SSE Letter Report (BSC 2001 [DIRS 156460]) which were performed in a manner consistent with the specifications of the final EPA standard regarding a repository at Yucca Mountain found at 40 CFR Part 197 (66 FR 32074 [DIRS 155216], pp. 32132 - 32135) and with the final NRC rule at 10 CFR Part 63 (66 FR 55732 [DIRS 156671] p. 55813 - 55816).

5.2 MODEL CONFIGURATION

This Letter Report utilizes the FEIS/SSE (BSC 2001 [DIRS 156460]) modeling of the 70,000-MTHM-case repository configuration. The analysis is provided only for the release of radioactive material from the Primary Block, the area designated for the 70,000 MTHM inventory HTOM of the potential Yucca Mountain repository as shown in Figure 5-2.

The GoldSim numerical code simulates transport of radionuclides from the repository, through the UZ, and through the SZ to the accessible environment. The different process models included in the GoldSim code are fully described and documented in the TSPA-SR (CRWMS M&O 2000 [DIRS 153246]).

The GoldSim model configuration used for the FEIS/SSE Letter Report conforms to the recently published EPA final rule 40 CFR Part 197 (66 FR 32074 [DIRS 155216], p. 32132 to 32135). The FEIS/SSE model configuration and changes from previous TSPA model configurations found in the TSPA-SR (CRWMS M&O 2000 [DIRS 153246]) and the Volumes 1 and 2 of the SSPA (BSC 2001 [DIRS 154657] and BSC 2001 [DIRS 154659]) are described in the FEIS/SSE Letter Report (BSC 2001 [DIRS 156460] Section 5.2).

5.3 SCENARIOS CONSIDERED IN THIS REPORT

In this Letter Report, the TSPA evaluation considers three cases relative to the impact on protection standards in the final NRC rule 10 CFR Part 63 using the results of calculations presented in the FEIS/SSE Letter Report: (1) the groundwater protection standard considering the unlikely event of an igneous intrusion; (2) the individual protection standard for human intrusion considering the unlikely event of an igneous intrusion followed by a human intrusion; and (3) the effect of the use of a specific water-demand value (i.e., 3,000 acre-ft per year) on the individual protection standard.

5.3.1 Groundwater Protection Standard Evaluation Utilizing an Unlikely Igneous-Intrusion Scenario

The evaluation of the groundwater protection standard utilizing the igneous-intrusion scenario used the results of the same igneous-activity model scenario used for the FEIS/SSE analysis and described in the FEIS/SSE Letter Report (BSC 2001 [DIRS 156460] Section 2, p. 5 and Section 5.2.6, p. 16). However, the results of the calculations based on that scenario were post processed to capture the information relevant to this particular case, i.e., the specific groundwater concentrations and critical organ doses attributed to drinking 2 liters per day of water derived from the representative volume.

5.3.2 Individual Protection Standard for Human Intrusion Considering an Unlikely Igneous Intrusion

The evaluation scenario utilized for this topic considered the igneous-intrusion scenario and the human-intrusion scenario, both of which were described in the FEIS/SSE Letter Report (BSC 2001 [DIRS 156460] Section 5.2.6, p. 16 and 6.3, p. 24 and Sections 5.2.7, p. 17 and 6.4, p. 25). This scenario assumes that a driller would not be able to detect a waste package that may have been previously damaged by a low-probability igneous-intrusion event. As noted in the FEIS/SSE Letter Report, in the nominal scenario, the driller is expected to be able to detect the presence of waste packages until sufficient time has passed to allow enough damage of the waste package to occur, a minimum of 30,000 years (BSC 2001 [DIRS 156460] Section 5.2.7, p.17). The scenario was examined by comparing the results of these two scenarios as calculated for the FEIS/SSE Letter Report (BSC 2001, [DIRS 156460] Sections 6.3.1 and 6.4.2). Note that the results used in the evaluations in this Letter Report were those due to the igneous intrusion only, which were extracted from the combined igneous-activity scenario described in the FEIS/SSE Letter Report (BSC 2001, [DIRS 156460] Table 6-1, p. 23).

5.3.3 Use of 3,000 acre-ft Per Year Water Demand on Individual Protection

The evaluation assessed the effect on individual protection of assuming a 3,000 acre-ft annual water demand as the water demand. The assessment was developed by comparing the simulated dose to the RMEI, as presented in the FEIS/SSE Letter report (BSC 2001 [DIRS 156460] Section 6.6), to the possible effect of applying the 3,000 acre-ft water demand, per 10 CFR 63.312 (66 FR 55732 [DIRS 156671] p. 55814).

6. RESULTS

The GoldSim simulations in support of this Letter Report were presented in the FEIS/SSE Letter report and provide estimates of the groundwater radiation concentrations and dose to critical organs assuming an unlikely event (i.e., an igneous intrusion) (See Section 6.1), dose from a human-intrusion event preceded by an unlikely event (i.e., an igneous intrusion) (See Section 6.2), and the effect of consideration of a 3,000 acre-feet water demand on the calculated dose for comparison to the individual-protection standard (See Section 6.3).

The data input and output files containing all the graphical and tabular results for the calculations presented in this report have been submitted to the Records Processing Center along with a list of the files and instructions on retrieving these data (Saulnier 2001 [DIRS 155948]).

6.1 Groundwater Protection Standard Considering an unlikely event (igneous intrusion)

Figure 6-1 presents the mean activity concentrations of gross alpha activity and total radium (²²⁶Ra plus ²²⁸Ra) in the representative volume of groundwater for the igneous-intrusion scenario and the 70,000 MTHM inventory, HTOM. These mean activity concentration values are calculated in the same manner that the mean dose is calculated for the individual protection standard; that is, they are the concentrations weighted by the probability of occurrence of the igneous intrusion event. The concentrations are calculated for a representative volume of water of 3,000 acre-ft at the accessible environment at the same location as the RMEI as described in 10 CFR 63.312 and 63.331 (66 FR 55732 [DIRS 156671] p. 55814]). Figure 6-2 presents the same information for the LTOM. Both the radionuclide concentrations and dose are presented to 10,000 years; there are no regulatory requirements for groundwater protection analyses beyond 10,000 years.

The results for the evaluation of groundwater protection considering an unlikely event (igneous intrusion) are obtained by combining the results for the nominal scenario with the results for the igneous intrusion scenario. Results presented in the three tables below show the calculated results for both the nominal scenario (BSC 2001 [DIRS 156460], Tables 6-2, 6-3, and 6-4) and the igneous intrusion scenario. Because the nominal scenario results for groundwater protection calculations are negligible compared to the results calculated for an igneous intrusion, the activity concentrations and dose estimates for groundwater protection considering igneous intrusion are approximated as those calculated for an igneous intrusion.

Naturally occurring background radionuclide concentrations are not included on Figures 6-1 and 6-2 but are presented in Tables 6-1 and 6-2. Table 6-1 shows the maximum mean gross alpha activity for both the nominal scenario and igneous intrusion only (with and without natural background) for the 10,000-year performance period. Table 6-2 shows maximum mean total radium concentration for both the nominal scenario and the igneous intrusion only (with and without natural background) for the 10,000-year performance period. For the first 10,000 years after closure, the gross alpha concentrations are about 10 per cent of natural background. The calculated total radium concentrations are orders of magnitude lower than natural background.

Figure 6-3 presents the mean dose to critical organs for ⁹⁹Tc, ¹⁴C, and ¹²⁹I, the prominent beta and photon-emitting radionuclides (CRWMS M&O 2000 [DIRS 153246], Section 4.1.5, p. 4-17) considering an igneous-intrusion scenario and the nominal scenario for the 70,000 MTHM inventory, HTOM, for a 10,000-year performance period. Figure 6-4 presents the same information for the LTOM. Table 6-3 shows the maximum mean dose to critical organs in 10,000 years for ⁹⁹Tc, ¹⁴C, and ¹²⁹I for the nominal scenario and for the igneous-intrusion scenario only.

6.2 Individual protection standard for human intrusion considering a prior unlikely event (Igneous intrusion)

The FEIS/SSE Letter Report presents analyses of a 100-year human intrusion scenario (per proposed NRC regulations (64 FR 8640 [DIRS 101680]) and a 30,000 year human-intrusion scenario, HTOM (BSC 2001 [DIRS 156460] Section 6.4, p.25-26)). The calculated dose for a human intrusion at 30,000 years is consistent with the final EPA and NRC rules and represents the DOE determination that the earliest time after disposal that an individual waste package would degrade sufficiently that a human intrusion could occur without recognition by a driller is 30,000 years, assuming no unlikely event (such as an igneous intrusion) affected the waste package.

An analysis of a possible human intrusion of the potential Yucca Mountain repository, according to the requirements of 10 CFR 63.321, resulted in the determination that an undetected human intrusion would not occur until at least 30,000 years post closure. That determination was based on the following summary analysis excerpted from the TSPA-SR (CRWMS M&O 2000 [DIRS 153246], Section 3.4) and also described in Volume 1 of the SSPA (BSC 2001 [DIRS 154657], Appendix A). The results of the TSPA-SR drip shield and waste package studies indicate long lifetimes for these components (CRWMS M&O 2000 [DIRS 153246], Section 3.4), with the first drip shield failures occurring after about 20,000 years. The first failures of the waste package outer material, Alloy 22, by general corrosion occur after approximately 30,000 years. The duration of general corrosion for this analysis does not consider the 5 cm of stainless steel beneath the Alloy 22 in the standard waste-package configuration. Although, general corrosion occurs gradually over time up to the time of failure, the oxidation process is a surface phenomenon, and the underlying metal retains its integrity and resistance to drilling. Although there is a low probability of early waste package failure by improper weld heat treatment (BSC 2001 [DIRS 154657], Section 7.3.6), these failures are not associated with the degradation of the overall structural integrity of the waste package, and therefore the resistance to drilling is maintained. Considering the nature of the waste-package materials in the current design, the earliest time after disposal at which a driller would not recognize they had penetrated a waste-package and drip-shield would be about 30,000 years.

As discussed in Section 1, the NRC final rule does not define "unlikely," and does not expressly require consideration of a human intrusion preceded by an igneous intrusion. Nonetheless, this event is considered here as an alternative analysis even though the mean annual probability of an igneous intrusion at the location of the potential repository is 1.6 x 10^-8 (CRWMS M&O 2000 [DIRS 153246] Table 3.10-5, p. 3-203). If an igneous-intrusion event were to occur, it could compromise some waste packages such that a driller would not recognize a waste-package penetration. If such an unlikely event were to precede a human intrusion, the consequences of the human intrusion should reasonably be weighted by the probability of the initiating igneous intrusion in order to calculate the mean value results of a human intrusion that could occur before 30,000 years. As discussed above, the results of a human intrusion at 100 years without a prior igneous intrusion were presented in the FEIS/SSE Letter Report (BSC 2001 [DIRS 156460] Table 6-1a, Figures 6-10b and 6-15). However, the calculated dose for the time period of that scenario before 30,000 years would have to be reduced because of the probabilistic weighting due to the two unlikely events occurring in succession (i.e., the igneous-intrusion event followed by a human intrusion). A waste package damaged by an igneous event and then drilled through during a human intrusion would release the inventory of only one waste package versus the results of the waste packages compromised in the repository drift or drifts affected by the igneous intrusion. Thus, because the human intrusion post-dates the igneous intrusion, the dose due to the igneous intrusion bounds the possible dose due to the human intrusion.

To provide a quantitative understanding of the potential dose attributed to a human-intrusion event that follows an unlikely igneous-intrusion event, the following estimate of dose was prepared. The mean annual individual dose due to a human intrusion following an unlikely igneous intrusion can be approximated by multiplying (1) the conditional dose assuming a human-intrusion event occurs times (2) the probability of the initiating igneous-intrusion event times (3) the probability of the driller not detecting the waste package (assumed to be equal to one if the drilling is preceded by an igneous-intrusion event). The conditional human-intrusion dose would be a function of when the initiating igneous intrusion occurs as well as when the succeeding human-intrusion event occurs. However, a worse case would be assuming the human-intrusion event occurs immediately following the loss of institutional controls. Such a calculation has been presented in the FEIS/SSE Letter Report for a human intrusion at 100 years, and the resultant maximum mean dose was 4.8 x 10^-3 mrem/yr (BSC 2001 [DIRS 156460] Table 6-1, p. 23). The probability of the igneous initiating event occurring is a function of time. Considering that we are interested in the possibility of an event occurring prior to the 30,000 years (the time at which the driller would not be expected to detect a waste package due to likely features, events and processes), the probability of an igneous event occurring sometime in 30,000 years is 4.8 x 10^-4 (30,000 years times a mean annual probability of 1.6 x 10^-8 per year (CRWMS M&O 2000 [DIRS 153246] Table 3.10-5, p. 3-203). Therefore the approximate maximum mean dose due to a human intrusion following an igneous intrusion would be the probability times the maximum mean dose or 2.3 x 10^-6 mrem/year. In the FEIS/SSE Letter Report, the maximum mean dose in the first 10,000 years postclosure due to an igneous intrusion is 4.3 x 10^-4 mrem/year (BSC 2001 [DIRS 156460] Table 6-1, p. 23). Therefore, the potential maximum mean dose due to a human intrusion preceded by an igneous intrusion is concluded to be much lower than the maximum mean dose due to the igneous intrusion alone.

6.3 Individual Protection Standard and the effect of 3,000 acre-feet water demand

The TSPA-SR (CRWMS M&O 2000 [DIRS 153246]), Volume 2 of the SSPA (BSC 2001 [DIRS 154659], and the FEIS/SSE Letter Report BSC 2001 [DIRS 156460], Section 4.0) calculated individual-protection-standard dose in a probabilistic manner using a volume of water necessary to operate 15 to 25 farms, representing a range of groundwater volumes from 887 to 3,367 acre-ft, with an average water demand of approximately 2,000 acre-ft/year. The calculations assume that the water volume contains all of the annual radionuclide releases. This range and approximate average demand is consistent with the proposed NRC rule and the final EPA rule. The final NRC rule states that the individual protection standard should be calculated using an average water demand of 3,000 acre-ft (10 CFR 63.312(c)). The use of 3,000 acre-ft /year as the water demand would result in a subsequent decrease in the calculated mean annual dose relative to the individual protection standard. Dose calculated to the RMEI for the assessment of individual protection in the FEIS/SSE Letter Report represents a more conservative (i.e., higher) estimate of dose because the average water demand for the calculations was approximately two thirds of the water demand specified by the NRC in 10 CFR 63.312(c). Thus, increasing the water demand to the water volume specified by NRC at 10 CFR Part 63.312(c) (66 FR 55732 [DIRS 156671] p. 55814) would result in calculated values of dose that would be approximately two thirds of the calculated values of dose for the groundwater-pathway-dominated scenarios shown on Table 6-1 (all scenarios except the Igneous-Activity scenarios, and the Igneous-Activity (Eruptive Only) scenarios) of the FEIS/SSE Letter Report (BSC 2001 [DIRS 156460] p. 23). For example, the peak mean annual dose for the 70,000 MTHM (i.e., nominal) scenario (HTOM) would be reduced from 1.7x10^-5 mrem/year to 1.1x10^-5 mrem/year using the increased water demand. Other groundwater-pathway-dominated scenarios on this table would be similarly reduced.

7. REFERENCES

7.1 DOCUMENTS CITED

152059	BSC (Bechtel SAIC Company) 2001. Performance Assessment of U. S. Department of Energy Spent Fuels in Support of Site Recommendation. CAL-WIS-PA-000002 REV 00. Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20010627.0026.

153246	CRWMS M&O 2000. Total System Performance Assessment for the Site Recommendation. TDR-WIS-PA-000001 REV 00 ICN 01. Las Vegas, Nevada: CRWMS M&O. ACC: MOL.20001220.0045.

153849	DOE (U.S. Department of Energy) 2001. Yucca Mountain Science and Engineering Report. DOE/RW-0539. [Washington, D.C.]: U.S. Department of Energy, Office of Civilian Radioactive Waste Management. ACC: MOL.20010524.0272.

154554	BSC (Bechtel SAIC Company) 2001. Lower-Temperature Subsurface Layout and Ventilation Concepts. ANL-WER-MD-000002 REV 00. Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20010718.0225.

154657	BSC (Bechtel SAIC Company) 2001. FY01 Supplemental Science and Performance Analyses, Volume 1: Scientific Bases and Analyses. TDR-MGR-MD-000007 REV 00. Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20010712.0062.

154659	BSC (Bechtel SAIC Company) 2001. FY01 Supplemental Science and Performance Analyses, Volume 2: Performance Analyses. TDR-MGR-PA-000001 REV 00. Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20010724.0110.

154841	BSC (Bechtel SAIC Company) 2001. Inventory Abstraction. ANL-WIS-MD-000006 REV 00 ICN 02. Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20010416.0088.

155182	BSC (Bechtel SAIC Company) 2001. Software Code: GoldSim. V7.17.200. 10344-7.17.200-00.

155734	DOE (U.S. Department of Energy) 2001. Yucca Mountain Preliminary Site Suitability Evaluation. DOE/RW-0540. Washington, D.C.: U.S. Department of Energy, Office of Civilian Radioactive Waste Management. ACC: MOL.20011101.0082.

155948	Saulnier, G. 2001. "Delivery of Input and Output Files used in the Analyses for Disposal of Commercial and DOE Waste Inventories at Yucca Mountain - Input to Final Environmental Impact Statement and Site Suitability Evaluation." Memorandum from G. Saulnier (BSC) to Jerry McNeish (BSC), September 5, 2001, EIS 9/05/01, with enclosures. ACC: MOL.20010906.0087.
156459	BSC 2001. Technical Work Plan for TSPA Sensitivity Analyses for Final Regulations. TWP-MGR-PA-000004 REV 00. Las Vegas, NV: Bechtel SAIC Company LLC. ACC: MOL.20011105.0324..

156460	BSC 2001. Total System Performance Assessment - Analyses for Disposal of Commercial and DOE Waste Inventories at Yucca Mountain - Input to Final Environmental Impact Statement and Site Suitability Evaluation. Deliverable ID: SL986M3. Las Vegas, Nevada: Bechtel SAIC Company, LLC. ACC: Submit to RPC.

156708	Adams, J.J. 2001. Direction to Assess Postclosure Impacts of Draft 10 Code of Federal Regulations (CFR) Part 63 Final Rule; Contract Number DE-AC08-01RW12101, DOE Letter From JJ Adams (DOE/YMSCO) to K. Hess (BSC) 10/2/01. ACC: MOL.20011030.0394.

7.2 CODES, STANDARDS, AND REGULATIONS

131951	Nuclear Waste Policy Act of 1982, as amended (NWPA, Public Law 97-425). Readily available.

101680	64 FR 8640. Disposal of High-Level Radioactive Wastes in a Proposed Geologic Repository at Yucca Mountain, Nevada. Proposed rule 10 CFR Part 63. Readily available.

155216	66 FR 32074. 40 CFR Part 197, Public Health and Environmental Radiation Protection Standards for Yucca Mountain, NV; Final Rule. Readily available.
156671	66 FR 55732. Disposal of High-Level Radioactive Wastes in a Proposed Geologic Repository at Yucca Mountain, NV. Final Rule 10 CFR Part 63. Readily available.

7.3 PROCEDURES

AP-SI.1Q, Rev. 3, ICN 02, ECN 01. Software Management. Washington, D.C.: U.S. Department of Energy, Office of Civilian Radioactive Waste Management. ACC: MOL.20011030.0598.

8. ATTACHMENTS

Attachments	Title
I	ACRONYMS AND ABBREVIATIONS

Previous Section | Next Section