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Spent Fuel Pool Criticality Management 

Spreadsheet: A compilation of the means used 

to meet subcriticality requirements for all 

operating domestic spent fuel pools, public 

version 

Technical Letter Report

April Pulvirenti and Matthew Hiser 

Nuclear Regulatory Commission 

Office of Nuclear Regulatory Research 

December 21, 2011 

 
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Contents 

Summary .................................................................................................................................... 1

Background ................................................................................................................................ 2

Spent Fuel Pool Criticality Management Spreadsheet ................................................................ 2

Pools in the Spent Fuel Pool Criticality Management Spreadsheet ............................................. 2

Organization of subcriticality credit data for a Pool ..................................................................... 3

Current means used to meet subcriticality requirements: ........................................................ 3

Reference License Amendment and Confirmation .................................................................. 4

Citations for the references: .................................................................................................... 4

Date Updated: ......................................................................................................................... 5

Decommissioned Reactors ......................................................................................................... 5

Conclusion.................................................................................................................................. 5
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Summary 

In spent fuel in pools, Nuclear Regulatory Commission (NRC) licensees use several means to 

meet subcriticality requirements specified by Title 10 of the Code of Federal Regulations 

Section 50.68 (10 CFR 50.68) or General Design Criteria (GDC) 62. For example, licensees 

may credit soluble boron in the pool water, or boron-based neutron absorber materials 

incorporated into the racks which support the spent fuel assemblies. Because the means used 

to meet subcriticality requirements are not standardized among pools and because these 

means may change during plant operation, it was necessary to collect this information into a 

single updatable reference document that can inform regulatory decisions. Information about 

the means used to meet subcriticality requirements for each spent fuel pool was taken from 

public documents found in the NRC Agencywide Document Access Management System 

(ADAMS) and compiled into two spreadsheets. 

The Spent Fuel Pool Criticality Management spreadsheet lists the means by which each 

operating spent fuel pool in the entire US fleet meets its subcriticality requirement of keff ≤ 0.95, 

along with the original License Amendment reference which approved the current means. All 

information in the spreadsheet is obtained from publicly available documents. 
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Background 

NRC licensees use many means to meet subcriticality requirements in the spent fuel pool 

specified by the regulations 10 CFR 50.68 or GDC 62. Most spent fuel pools now store spent 

fuel assemblies in high-density racks which incorporate neutron absorber materials into the rack 

walls. These neutron absorber materials, especially Boraflex, can degrade enough to lose their 

neutron absorbing capabilities and challenge subcriticality requirements. Due to this 

degradation, many licensees now employ other means to meet subcriticality requirements such 

as spent fuel loading patterns, fuel burnup credit, control rods or other neutron poisons 

contained within spent fuel bundles, soluble boron [B] in the pool water, or some combination of 

all of these. In some cases, a licensee will credit no neutron absorber material and rely entirely 

on other means to meet subcriticality requirements. Since licensees use many different means 

to meet subcriticality requirements, and because licensees may employ new means due to 

neutron absorbing material degradation or spent fuel pool storage needs, it is necessary to 

compile this information into a single updateable reference to inform regulatory decisions. 

Spent Fuel Pool Criticality Management Spreadsheet 

Before describing the spreadsheet, it should be noted that not each reactor is assigned to one 

dedicated spent fuel pool. Two reactors may share a single pool, a single reactor may store fuel 

in more than one pool, or two reactors at a single site may move fuel between both pools. In 

addition, there are three operating pools associated with decommissioned reactors. Therefore, 

entries in all spreadsheet will refer to individual spent fuel pools rather than to reactor licensees. 

To populate the spreadsheet, staff from the Office of Nuclear Regulatory Research (RES) first 

obtained information on the various means used to meet subcriticality requirements for each 

spent fuel pool using the licensees’ Updated Final Safety Analysis Reports (UFSARs). 

UFSARs, while not publicly available, provide a broad description of spent fuel storage, 

including rack design, neutron absorbing materials and other means used to meet subcriticality 

requirements. To confirm the means used to meet subcriticality requirements employed by 

each spent fuel pool as stated in the UFSAR, RES staff searched the ADAMS document 

database the definitive documents which clearly defined these means. 

Pools in the Spent Fuel Pool Criticality Management Spreadsheet 

Spent Fuel Pool Criticality Management spreadsheet is a Microsoft Excel Workbook comprising 

three worksheets. One worksheet lists pressurized water reactor (PWR) pools, a second 

worksheet lists boiling water reactor (BWR) pools, and the third worksheet lists pools at 

decommissioned reactors. In general, each pool is entered on one line. If two reactors at a 

plant site share a pool, the entry is listed as one pool and is indicated as a shared pool, for 

example: “North Anna Units 1+2 (shared pool).” The Calvert Cliffs pool shares one large pool 
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divided by a wall. However, because the two halves employ a different strategy for meeting 

subcriticality requirements, each half of the Calvert Cliffs pool is listed separately. Three Mile 

Island Unit 1 operates two pools: Pools A and B. Each pool employs different means to meet 

subcriticality requirements, and so they are listed separately. Shearon Harris is a one-reactor 

unit, but employs four interconnected pools. To be consistent with the Shearon Harris UFSAR, 

Pools A and B are grouped as one pool and Pools C and D are grouped as a second pool. In 

several plants, an additional spent fuel rack was installed in other sections of the fuel handing 

system such as the cask pool or refueling channel. 

Organization of subcriticality credit data for a Pool 

For each pool, the spreadsheet lists the most current means to meet subcriticality requirements, 

as of December 16, 2011, along with the reference for the License Amendment which describes 

the current means(s). The last column of the spreadsheet is a time stamp to facilitate further 

updates. The information in the spreadsheet was confirmed by both public and non-public 

sources, such as Technical Specifications, Safety Evaluation Reports (SERs), and the pool’s 

most recent UFSAR. Columns are described separately: 

Current means used to meet subcriticality requirements: 

This column lists all means used to meet subcriticality requirements in the spent fuel pool. In 

general, the subcriticality requirements are met by a combination of neutron absorber materials, 

storage patterns/burnup, and, in PWR’s only, soluble boron [B]. The means used to meet 

subcriticality requirements is the most current as of the latest applicable License Amendment 

reference found in ADAMS, and does not include previous rack configurations or previous 

means used to meet subcriticality requirements. 

For the PWR worksheet in the spreadsheet, the Current Means column is divided into 

subcolumns denoting Region 1, designed for freshly discharged fuel, and Region 2, designed 

for older fuel. If a PWR licensee utilizes soluble boron as a means of meeting subcriticality 

requirements, then the licensee will calculate the minimum ppm of soluble boron necessary to 

meet the subcriticality requirement under normal operations. This calculated ppm [B], as stated 

in the reference license amendment, is listed for each region of the pool. Furthermore, some 

licensees explicitly estimate the necessary operating B concentration for each region of the pool 

separately. Therefore, the ppm [B] listed may be different for Region 1 and Region 2. 

The term “storage patterns/burnup” encompasses a variety of means to meet subcriticality 

requirements. These means include low-density or flux-trap racks, storage configurations 

which place a high-burnup assembly adjacent to a low-burnup assembly, storage configurations 

such as checkerboarding or administrative water holes, or storage configurations to place less 

reactive fuel assemblies next to degraded panels where less neutron absorption is needed. 

Fuel burnup credit includes credit for the net reduction in fissile isotopes and net increase in 

neutron absorption in the fuel bundle. Final net depletion in reactivity, for which burnup credit is 

taken, depends on numerous factors such as reactor operating parameters, initial enrichment, 
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or presence of fixed or integrated neutron absorbers. Because the types and combinations of 

storage patterns and burnup are numerous, varied, and often changed, no attempt was made to 

list the specific storage patterns or fuel burnup credit in this spreadsheet. Users are referred to 

the references for details of a specific pool. 

Reference License Amendment and Confirmation 

Each change in assignment of means used to meet subcriticality requirements is reviewed and 

approved under the NRC License Amendment process. The reference for Issuance of License 

Amendment is cited in the Original Reference column by License Amendment number, date, 

and ADAMS Accession (ML) number. The Reference License Amendment refers to the license 

amendment where the current means of meeting subcriticality was approved. Amendments for 

past configurations are not listed in the Management of Spent Fuel Pool Criticality spreadsheet, 

although it is planned to include historical information in a future database. If two or three 

license amendments are issued for a plant site, one for each pool, then the data cells in this 

column will cite the same reference ADAMS Accession ML number, but will denote the 

respective license amendment number. In several cases, a license amendment was issued, but 

not implemented until sometime after issuance. In a few cases, new license amendments are 

pending. These instances are noted in the comments. If no license amendment is found, that is 

also specified. For several pools, no License Amendment was found for the pool’s current 

method to manage subcriticality; for example if the pool adopted a new subcriticality method 

under 10 CFR 50.59. These cases are specified in the Reference License Amendment 

Columns and are described further in the Comment column. 

In order to confirm that the means used to meet subcriticality requirements specified in the 

Reference License Amendment are still valid, RES staff identified a recent confirmatory 

reference, e.g., Technical Specifications, Safety Evaluation Reports, or the pool’s most recent 

UFSAR. Where Technical Specifications or UFSARS did not confirm the information from the 

Reference License Amendment, a recent corroborating reference, such as a Safety Evaluation 

Report (SER), Technical Specification Bases, or a response to a Request for Additional 

Information (RAI) was used for confirmation. 

Citations for the references: 

For most spent fuel pools, the reference License Amendment is cited as the source of each 

pool’s means to meet criticality requirements. Documents are cited by date, and, when 

available, by ADAMS Accession ML number. References may also include a page number; for 

example: “ML052720276 p243.” The page number refers to the page number of the .PDF file 

as entered into ADAMS, not the page number within the actual document. Citing the .PDF page 
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number allows the user to quickly jump to the page where the information can be found. All 

page number references in the Management of Spent Fuel Pool Criticality spreadsheet refer to 

the PDF page number. 

Date Updated: 

Each entry on all worksheets is time-stamped with a “Date Updated.”. Thisdate refers to the last 

date when ADAMS was searched, not the date when the spreadsheet itself was updated. For 

example, in the current version of Spent Fuel Pool Criticality Management, ADAMS was 

searched for documents as late as December 16, 2011. Thus, if a user were to update the 

spreadsheet, the user would search ADAMS for documents dated after December 16, 2011. 

Decommissioned Reactors 

The last worksheet in the Spent Fuel Pool Criticality Management spreadsheet compiles 

information about decommissioned reactors. Reactors are only listed if they possess fuel in wet 

storage. Of the decommissioned reactor sites, three wet pools are still actively storing spent 

fuel: LaCrosse, Millstone 1, and Zion Units 1+2 (shared pool). Only one source is cited for 

means used to meet criticality requirements. Millstone 1 and Zion Units 1+2 cite a 

Decommissioned Safety Analysis Report (DSAR), and LaCrosse cites a document related to a 

recent license amendment. Citations and ADAMS Data are similar to the PWR and BWR 

worksheets. 

Conclusion 

The means used to meet subcriticality requirements for all US domestic active spent fuel pools 

was compiled from publicly available documents for PWR pools, BWR pools, and active pools 

for decommissioned plants. For each pool, the current means used to meet subcriticality 

requirements is described and referenced by the original License Amendment. All information 

was confirmed by a recent definitive source. Relevant comments are also included and 

referenced. The database contains a date stamp to facilitate future searching and updates. 

The spreadsheet gathers the relevant public information concerning means used to meet 

subcriticality requirements for all spent fuel pools into one reference document.