RIS 01-012: Nonconservatism in Pressurized Water Reactor Spent Fuel Storage Pool Reactivity Equivalencing Calculations

May 18, 2001


All holders of operating licenses for pressurized water reactors.


The U.S. Nuclear Regulatory Commission (NRC) is issuing this regulatory issue summary (RIS) to notify pressurized water reactor (PWR) licensees of a possible nonconservatism in the practice of equating the reactivity of spent fuel to the reactivity of fresh fuel in spent fuel storage pool calculations using burnup credit.

Background Information

Since the early 1980s the NRC has accepted the practice of "reactivity equivalencing" in analyses that determine the appropriate placement of fuel assemblies within PWR spent fuel storage pools. "Reactivity equivalencing" equates the reactivity of a fuel assembly that has a particular initial enrichment and burnup combination to the reactivity of a fuel assembly that has a different initial enrichment and zero burnup. This is a fictitious fuel assembly that is used in subsequent analyses.

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Summary of Issue

Recent analyses done for NRC by the Oak Ridge National Laboratory (ORNL) have indicated that this practice is acceptable provided the geometric configuration and the conditions under which the equivalency was determined remain unchanged (NUREG/CR-6683 (ORNL/TM-2000/230), "A Critical Review of the Practice of Equating the Reactivity of Spent Fuel to Fresh Fuel in Burnup Credit Criticality Safety Analyses for PWR Spent Fuel Pool Storage," September 2000 [ADAMS Accession Number ML003751298]). However, the equivalent fresh fuel enrichment is often determined for a reference configuration (e.g., an infinite array of storage rack cells in unborated water) and then used for various similar, but not identical, configurations. For example, analyses of checkerboard-type storage configurations in which the spent fuel is placed in alternating locations with higher reactivity fuel assemblies (e.g., fresh or lower burned assemblies) indicate that equivalencing yields nonconservative results (on the order of a few tenths of one percent).

Likewise, analyses for storage conditions with soluble boron present reveal somewhat more significant nonconservative results. An under-estimation of reactivity of more than 3 percent was observed for an infinite array of equivalent fresh fuel assemblies stored in a soluble boron concentration of 500 parts per million (ppm).

Typical boron concentrations in PWR pools are greater than 2000 ppm and result in a subcriticality margin of about 20 to 25 percent. Therefore, the potential nonconservatisms noted above are not a safety concern. However, licensee calculations for spent fuel pool accident conditions such as the misplacement of a fresh fuel assembly in a storage rack designed for spent fuel, or for pools which take partial credit for soluble boron, may be less conservative than originally thought. Licensees are being made aware of this information so that accuracy in spent fuel pool criticality calculations can be maintained.

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Backfit Discussion

This RIS requests no action or written response and is, therefore, not a backfit under 10 CFR 50.109. Consequently, the staff did not do a backfit analysis.

Federal Register Notification

A notice of opportunity for public comment was not published in the Federal Register because this RIS is informational and requires no action or written response by addressees.

Paperwork Reduction Act Statement

This RIS does not request any information collection.

If there are any questions about this matter, please contact the person listed below, or the appropriate Office of Nuclear Reactor Regulation project manager for a specific nuclear power plant.

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David B. Matthews, Director
Division of Regulatory Improvement Programs
Office of Nuclear Reactor Regulation
Technical Contact: Anthony Ulses, NRR
E-mail:  Anthony.Ulses@nrc.gov
Attachment:   List of Recently Issued NRC Regulatory Issue Summaries

(ADAMS Accession Number ML010990300)

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Page Last Reviewed/Updated Wednesday, March 24, 2021