Used Fuel and High-Level Waste Management January 2016

IN USE: USED FUEL STORAGE, TRANSPORTATION, AND DISPOSAL CRITICALITY CONTROL

ISSUE STATEMENT

Maintaining subcriticality is a key safety requirement for used fuel during wet storage, loading operations in prepara-tion for dry storage, transportation, and disposal. Criticality control is achieved using a combination of several methods: soluble neutron absorbers in spent fuel pools; solid neutron absorber materials in pools and dry storage, transportation, and disposal casks; reactivity control via geometric arrange-ment; exclusion of water during dry storage and transporta-tion (“moderator exclusion”); and depletion of fissile mate-rial and in-growth of neutron absorbers due to in-core irradiation (“burnup credit”). Demonstration of subcritical-ity requires use of models that are well benchmarked against available data, taking into account uncertainties in the data and biases in the criticality analysis methods. Significant amounts of data are required to support these models and satisfy regulatory requirements and practices. Solid neutron absorber materials must be durable, maintaining their design geometry and ability to withstand storage, transportation, and disposal environments without significant loss of neu-tron absorption capability throughout the period of perfor-mance. Degradation issues with several neutron absorber materials occurring over the past 25 years have led to costly assessment campaigns, loss of neutron absorber credit, and the need for replacement of degraded materials.

DRIVERS

Need for Alternative Criticality Data and Models Regulators require detailed data on used fuel properties to ensure subcriticality can be maintained with a high degree of confidence. If sufficient high-quality data are not avail-able, the resultant uncertainties to be used in the criticality analysis can be very large. This results in need for additional absorber requirements and/or penalizing administrative controls that limit the flexibility of plant operators in man-aging their increasing used fuel inventories in a cost-effective manner.

Loss of Storage, Transportation, and Disposal Capacity The availability and capacity of wet storage, dry storage, transportation, and disposal systems can be affected by the criticality models and assumptions acceptable to the regula-tors. Additional criticality control measures to compensate for excessive uncertainties or neutron absorber material deg-radation for example, may involve a reduction in spent fuel capacity. Given that existing storage capacity is already con-strained, further loss of storage capacity negatively impacts plant operations and economics.

Need to Develop a Neutron Absorber Degradation Management Program Neutron absorber degradation management plans are required; however, assessment of neutron absorber material performance via regular in-situ measurements is operation-ally burdensome and costly. A proposed alternative approach is the development of an industry-wide coupon management program that would allow using the performance data col-lected from in-pool neutron absorber coupons that are still available at a limited number of plants to benefit a broader set of spent fuel pools including those that are running out of or lacking coupons. Special “blackness (degree of neutron absorption) testing” tools must be developed and validated, followed by analyses to evaluate the continued ability of the system to maintain subcriticality. If required, replacement of neutron absorbers is costly and can result in increased worker dose and additional operational activities.

RESULTS IMPLEMENTATION

Research conducted in this area should aid the development of:• Reactivity decrement benchmarks and their uncertain-

ties, PWR and BWR nuclear fuel designs as a function of burnup

• Data and criticality assessment methodologies that repre-sent additions or cost-effective alternatives to the existing state of the art

• Criticality sensitivity analyses that support criticality assessment methodologies

• Tools to assess and analyze the degree of degradation of existing neutron absorber materials

• Guidance on the production, qualification testing, and use of new neutron absorber materials

EPRI | Nuclear Sector Roadmaps January 2016

PROJECT PLAN

EPRI work on used fuel storage, transportation, and dis-posal subcriticality falls into several categories:• Develop the use of alternative criticality assessment

methodologies • Perform criticality sensitivity analysis that will focus on

eliminating concerns over parameters and/or assumptions that have no impact on results or developed methodology

• Inform regulatory consideration of alternative assessment methodologies

• Inform regulatory consideration of fission product “burnup credit”

• Work with nuclear plant owners, neutron absorber vendors, and others to identify emerging neutron absorber degradation issues

• Engage in both in-situ and laboratory research to identify degradation and aging mechanism(s) of neutron absorber materials

• Perform tests that will allow evaluation of current monitoring techniques (coupon and in-situ) compared to samples from spent fuel pool neutron absorber panels

• Perform analysis on neutron absorber panels from pool to determine the amount of degradation, if any, and its potential impact

• Support the implementation of aging management plans for neutron absorber materials through the development of new “blackness testing” tools and analyses to assess the degree of degradation in-situ. Validate these methods to allow more efficient use of the limited number of neutron absorber coupons

• Provide guidance on the development, testing, and use of neutron absorbers by conducting annual workshops, and by publishing periodic updates of the Handbook of Neutron Absorber Materials for Spent Nuclear Fuel Transportation and Storage Applications

• Conduct annual Neutron Absorber Users Group meetings

RISKS

• Lack of regulatory review, and thereby acceptance of EPRI products can be a deterrent to their implementation by licensees

• Continuing delays in making progress toward establish-ing away-from-reactor spent-fuel storage, resulting in a lack of motivation for resolving regulatory transportation issues

RECORD OF REVISION

This record of revision will provide a high level summary of the major changes in the document and identify the Road-map Owner.

revision description of change

0 Original Issue: August 2011 Roadmap Owner: Albert Machiels

1 Revision Issued: August 2012 Roadmap Owner: Keith Waldrop

Changes: Roadmap Title changed from Neutron Absorber Materials and major text revisions to expand from just neutron absorber material degradation to all issues related to used fuel criticality control during storage, transportation, and disposal.

2 Revision Issued: December 2012 Roadmap Owner: Albert Machiels

Changes: Roadmap updated to reflect additional R&D activities.

3 Revision Issued: August 2013 Roadmap Owner: Hatice Akkurt

Changes: Roadmap updated to reflect changes in ongoing and proposed activities.

4 Revision Issued: January 2014 Roadmap Owner: Hatice Akkurt

Changes: Roadmap updated to reflect changes in ongoing and proposed activities.

5 Revision Issued: August 2014 Roadmap Owner: Hatice Akkurt

Changes: Roadmap updated to reflect changes in ongoing and proposed activities.

6 Revision Issued: December 2015 Roadmap Owner: Hatice Akkurt

Changes: Roadmap updated to reflect changes in schedule for review of EPRI benchmarks and NEI 12-16 Criticality Gudiance Document.

Used Fuel and High-Level Waste Management January 2016