The Yucca Mountain review plan3 July 2002
The US secretary of energy has finally recommended the Yucca Mountain site for development as a nuclear waste repository. The Yucca Mountain review plan is to ensure the quality, uniformity and consistency of NRC reviews of the licence application and any requested amendments.
The Yucca Mountain Review Plan embodies guidance for the review of any licence application from the US Department of Energy (DoE) for a geologic repository for disposal of high-level radioactive waste. The US Nuclear Regulatory Commission (NRC) has directed its staff to carry out risk-informed and performance-based regulatory programmes, based on the risk of health effects to the reasonably maximally exposed individual. It also sets out to ensure the protection of ground water and assess repository performance under conditions of human intrusion.
The principal purpose of the Yucca Mountain Review Plan is to ensure the quality and uniformity of licensing reviews. An acceptance review is the first screening of the DoE licence application. The application must provide enough information to demonstrate compliance with the regulations. The acceptance review does not determine the technical adequacy of the submitted information.
The following three principles are important in implementing the NRC's regulatory mission:
• The NRC does not select sites or designs, or participate with licensees or applicants in selecting proposed sites or designs.
• The Commission's role is not to monitor all licensee activities, but to oversee and audit them.
• The three outcomes available to the NRC at the conclusion of a licensing review are: grant the licence; grant the licence subject to conditions agreed by the licensee; or deny the licence. Other than rejecting an applicant or licensee proposal, the NRC has no power to compel a licensee to come forward with or prepare a different proposal.
Areas of review
The review plan includes:
• A description of the location and facilities of structures, systems and components of the geologic repository operations area, both surface and subsurface.
• A discussion of the proposed geologic repository operations area operations and activities.
• The delineation of the statutory and regulatory basis for proposed geologic repository operations.
Physical protection plan
The review determines whether or not the planned repository provides a high assurance that activities involving high-level radioactive waste present an unreasonable risk to public health and safety. The physical protection system should be designed to protect against a loss of control of the geologic repository operations area that could cause excessive radiation exposure.
Geologic repository operations
The NRC has to verify that the DoE has specified the geologic repository operations area location, the facilities there, the nature of the wastes to be disposed of, the geologic repository operations area layout, the surrounding area, and the surrounding terrain. The DoE has to provide detailed maps indicating the controlled area, the location of all buildings, physical protection systems, subsystems and major components, the protected area, and all entry/exit pointsand control points, alarm stations and security posts.
General performance objectives
The review plan requires the NRC to check that the physical protection plan will:
• Provide assurance that operations involving radioactive waste do not present an unreasonable risk to public health and safety.
• Identify and adequately describe those portions of the physical protection system for which redundant and diverse components and subsystems ensure adequate performance. In general terms, the DoE should describe the subsystems and components to be used to provide redundancy and diversity, and the ways in which they are redundant and diverse.
• Verify that the physical protection system is designed, tested and maintained to ensure its continual effectiveness, reliability and availability. This verification should be conducted on-site.
The DoE must establish a physical protection strategy that denies unauthorised access to the site. This must maintain and update the physical protection plan to reflect any changes that are necessary.
The DoE must have an adequate security organisation to manage, control and implement the physical protection system. The security organisation will be acceptable if it meets the following criteria:
• If the security organisation is to be a contractor to the DoE, there must be adequate written agreements to ensure that the security force will be effective.
• The DoE has to provide an adequate structure and management for the security organisation. This should include both uniformed security personnel and other people responsible for security-related functions.
• The physical protection programme must be reviewed at least once every 24 months, by independent individuals who have no direct responsibility for implementation of the physical protection programme. The review shall evaluate the effectiveness of the physical protection system, and of the liaison established with the designated response force.
• There must be an approved Guard Force Training Plan. This should train, equip and qualify all members of the security organisation to properly perform their duties.
• Records must be retained and maintained, and the DoE has to specify and adequately describe how they will be retained and maintained.
A performance objective of physical barriers is to define areas within which authorised activities and conditions are permitted. Other barrier performance objectives are to channel people, vehicles and material to or from entry/exit control points; to delay or deny unauthorised penetration attempts; to delay attempts to cause loss of control of the site; to assist detection and assessment and to permit a timely response by the security force.
The physical protection plan must be consistent with the following criteria:
• High-level waste must be stored only within a protected area. Access to material in the protected area shall require passage or penetration through two physical barriers, one barrier at the perimeter of the protected area, and one barrier offering substantial penetration resistance. The protected area barrier fence must be such that it cannot be lifted to allow an intruder to crawl under it. Barriers designed to protect against the malevolent use of a vehicle are not required at the site.
• A 7m-wide isolation zone must be maintained clear of obstacles or structures on either side of the physical barriers, at the perimeter of the protected area.
• The lighting system must be adequate to ensure illumination for monitoring, observation and assessment activities for exterior areas within the protected area. The illumination must also be sufficient to assess unauthorised penetrations of, or activities within, the protected area. There must be adequate emergency power for protected area lighting and security assessment if normal power is lost.
The performance objectives of access authorisation controls and procedures are to verify the identity of persons, vehicles and materials, and to initiate timely response to deny unauthorised entries. The subsystems must fulfil the following criteria:
• A personnel identification system to limit access only to authorised personnel must be in place. This system should provide unique identification of individuals granted access to the protected area.
• There must be adequate procedures for controlling points of personnel access into the protected area. These should include a discussion of methods used to identify people and verify individual authorisation. Procedures should also describe techniques for conducting visual searches of individuals, vehicles and hand-carried packages for explosives before entry into the protected area.
• There must be a controlled lock system, to limit access to authorised people.
• The following documentation must be retained for three years after the record is made or until termination of the licence: a log of individuals granted access to the site; screening records of members of the security organisation; a log of all patrols; a record of each alarm received, identifying the type of alarm, location, date and time when received; and the physical protection programme review reports.
Detection, surveillance and alarm
The performance objectives of these subsystems and procedures are to detect, assess and communicate any unauthorised access or penetrations. These subsystems and procedures must be consistent with the following criteria:
• There must be an adequate intrusion detection system in the isolation zone between the two barriers.
• All required alarms must display in a continuously-manned central alarm station located within the protected area and in at least one other continuously manned independent on-site station. It must be confirmed that access to alarm stations will be controlled on a need-to-know basis, and that the central alarm station will not contain any activities that would interfere with the alarm response.
• Detection systems and supporting subsystems must be tamper-indicating with line supervision. These systems and the surveillance/assessment and illumination systems must be maintained in operable condition.
• The protected area must be monitored with daily random patrols. The frequency of the patrols should be based on their remoteness from the geologic repository operations area, the nature of activities adjacent to the site, and the size of the geological repository operations area. A minimum of two patrols per security duty shift should be conducted, unless the facility is in a remote area, where more patrols may be necessary.
The performance objective of communication subsystems is to notify of an attempted unauthorised intrusion, so that response can prevent loss of control of the site. The communication subsystems will be acceptable if they meet the following criteria:
• The individual in each continuously manned alarm station should be able to call for assistance from other guards.
• Redundant and diverse systems should be used to ensure communications with the local law enforcement authority.
The performance objective of test and maintenance procedures is to provide confidence that security equipment will be available and reliable to perform when needed. It is imperative that the DoE has adequate test and maintenance programmes for the site.
The performance objective for contingency response plans and procedures is to provide predetermined response to safeguard contingency events, so the adversary will be engaged and impeded until off-site assistance arrives.
The contingency plans must meet the following criteria:
• The commitment to develop a safeguards contingency plan for unauthorised penetrations of, or activities within, the protected area.
• The DoE must have adequate documented response arrangements with a designated response force or local law enforcement agency.
The DoE must report all safeguards events to the NRC.
Recommendations for future assessments
To provide better support to the next decision point, a number of improvements are required in the DoE's approach to assess the performance of the repository system. The most important recommendations are as follows:
Features events and processes
Some spot checks have been carried out on the features events and processes (FEP) identification and screening processes. These have identified two additional potentially important FEPs.
FEPs have been screened out on the basis of demonstrating compliance up to 10,000 years, and the assessment is less reliable at longer times. In future, the screening of FEPs should be made in two stages. The first stage should retain all FEPs required for a full understanding of repository performance, while the second stage should include regulatory compliance considerations in the screening criteria.
A comprehensive and systematic methodology for identifying and treating all types of uncertainty should be formulated and implemented. This should include the classification of uncertainties as to whether they are due to intrinsic variability or to lack of knowledge, since the latter can lead to non-conservative results when incorporated into a probabilistic framework. This is termed 'risk dilution'. A study should be carried out of the quantitative importance of risk dilution for the expectation value of dose. The reduction of uncertainty should be a major goal.
Given the regulatory requirements in the USA, it is appropriate to make use of a probabilistic systems analysis framework for the repository. However, some particular aspects of the methodology require further consideration. The key concern is the potential problem of risk dilution. This arises because the parameter distributions used represent the combined effects of stochastic variability and subjective probability due to incomplete understanding of the system. Under some situations, the inclusion of subjective uncertainty can lead to non-conservative estimates of the expectation value of dose. When this occurs, it means that increased ignorance leads to lower expected doses, which does not appear to be a sensible basis for decision making, and requires further scrutiny.
Sensitivity analysis should be developed further into a tool to build an integrated and comprehensive understanding of the relative importance and role of different barriers and processes.
Aa a higher level document, a safety case should articulate a strategy to achieve safety as distinct from the strategy for demonstrating compliance, with an emphasis on obtaining and communicating understanding and facilitating dialogue with the relevant stakeholders. A safety case is the integration of relevant arguments supporting the long-term safety of the repository. A statement of confidence should be included, to elucidate the means that were adopted to achieve sufficient confidence, and to acknowledge remaining issues, together with a strategy for resolving those issues.
Most attention has been given to demonstrating quantitative compliance with regulatory criteria. Relatively little emphasis has been placed on the understanding of system behaviour, which is required if decisions are to be based on the full body of evidence. Two approaches are needed.
• The first is to present a realistic analysis of the likely performance of the repository using model assumptions and data. This could draw on evidence from natural and archaeological/historical analogues and should aim to communicate the likely evolution of the repository and its surroundings to a range of stakeholders and give an indication of the safety margins inherent in the analysis.
• The second approach is an analysis for compliance, where conservative assumptions and parameters are used to make the case more defensible. Specific assumptions and models are needed for this and should be identified separately from the less conservative analysis.
There have been major changes in repository design, but no clear rationale for these changes was discernible. In a future safety case, it would be helpful to describe the evolution of the disposal concept. In addition to indicating how design changes have responded to safety concerns, this would provide continuity and would enhance confidence by demonstrating that the project is maturing and developing in a logical and systematic manner.
Engineered barrier materials
The selection of materials for the waste package outer barrier (alloy-22) and drip shield (titanium Grade 7) are in line with international best practice. However, in order to build further confidence in the performance of these materials over thousands of years in their anticipated environment, it is recommended that long-term corrosion tests using multiple specimens are carried out. These should investigate the effects of gamma radiation field, salt deposits, microbes and ageing. A key challenge is to improve confidence in the extrapolation of corrosion measurements to long times. In order to accomplish this, it is recommended that efforts be made to help improve the scientific understanding of the kinetics of pitting and crevice corrosion, and of stress corrosion cracking.
The procedure used for screening the radionuclide inventory may have resulted in some potentially important radionuclides (36Cl, 135Cs) being omitted from detailed analysis. This should be reviewed and amended as appropriate. Fuel cladding remains a significant barrier up to 100,000 years and beyond. One process, the effects of the corrosion of basket components, was found not to have been taken account of and which might compromise the performance of the cladding.
Some of the solubility limits for elements (especially Np, Th and Ra) are simplifications made in the absence of reliable data. More experimental data should be obtained to validate thermodynamic modelling, especially with regard to the complex interactions between the degrading waste form and components of the waste package.
Transport within the engineered barrier system
The proposed mechanism of radionuclide diffusion through stress-corrosion cracks, which is assumed to be dominant for many millennia after the waste package is breached, appears to be overly conservative and complex, and possibly not credible. The model requires a continuous film of water to allow diffusion that extends all the way from the waste form to the cracks in the degrading waste package and to the bottom of the invert. The model assumes diffusion occurs even when there is no dripping in the location and the drip shield is intact.
A key issue concerning the near-field repository environment is whether water is likely to exist in and around waste packages. Very little water should be able to reach the drifts because of the repository design, causing diversion around the emplacement drifts, or by nature due to limited precipitation, infiltration and seepage. At the same time, the evaporation potential of water due to heat output from the waste packages is substantial. Thus, except in areas where seepage is very high, waste packages may remain dry due to evaporation. Design modifications, such as capillary barrier backfill, could be considered in areas of high seepage.
Confidence in the modelling of flow and radionuclide transport in the unsaturated zone should be increased, and the influence of temperature on capillary suction accounted for.
The natural dripping of groundwater from fractures or pores in the matrix has never been clearly observed, primarily as it is affected by drift ventilation, and yet it plays an important role in the analysis. In view of its crucial role in the assessment, the postulated dripping process should be better understood and quantified.
There is concern about the level of knowledge available to assess the role of the saturated zone, both at the regional scale and at the site scale. Further hydrogeological and hydrogeochemical data are required. Moreover, the treatment of this information to construct and calibrate a regional groundwater flow model is considered not to be state of the art. It is therefore recommended that a significant effort be made to improve the regional saturated zone flow model by collecting new data and improving the calibration. This effort should be closely integrated with the improvement of the site flow model, in order that these two models are made consistent with one another.
The Yucca Mountain biosphere modelling programme has been the subject of an international review. A realistic understanding of the long-term fate of radionuclides in the Yucca Mountain basin should be developed.
The DoE should carry out further work at the Peña Blanca uranium deposit in northern Mexico as a natural analogue for Yucca Mountain and use its characteristics to increase the confidence of both the public and the scientific community in the system performance over very long times. Also, investigations of naturally-occurring uranium and its radioactive progeny in the tuffs at Yucca Mountain should be continued to improve understanding of their mobility within the flow systems of the mountain.