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NRC ISFSI PadConstruction Inspection

Taiwan ISFSI WorkshopDecember 9, 2008

Ray Kellar, P. E., RIV Inspector817-860-8164 Ray.Kellar@nrc.gov

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Outline• ISFSI Inspection Procedure 60853• Important-To-Safety (ITS) vs. Not-Important-To-Safety

(NITS)• Design Considerations

– Concrete and Subgrade Stiffness– Seismic Issues– Anchored casks– Coefficient of Friction between cask and pad– Differential settlement– Liquefaction– Strength and Durability

• Concrete Construction– American Concrete Institute Codes (ACI) 318 & (ACI) 349– Additional ACI Codes– ASTM Standards

• NRC Findings

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ISFSI Inspection Procedure 60853

IP 60853 – On-Site Fabrication of Components and Construction of an ISFSI

• Determine if the licensee has applied an appropriate safety classification (ITS vs. NITS)

• The vendor typically specifies the ISFSI pad classification (ITS or NITS)

• For NITS, the licensee may use commercial grade practices but must still conform with design requirements

• The general licensee will have to address whether the ISFSI pad will support the static and dynamic loads from the casks, consider seismic interaction and address soil liquefaction as required by 10 CFR 72.212 (b)(2)(i)(B)

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ITS vs. NITS Classification

• Important-To-Safety (ITS) classification is normally used for high seismic regions in the U.S.

• ITS requirements include:– Design using American Concrete Institute (ACI) 349

Code requirements– Increased Quality Assurance oversight– Dedication of materials, including concrete and

reinforcing bars– Documentation for materials and testing– Strict adherence to design requirements– Construction in accordance with ACI 349 Code

requirements– Strict adherence to ACI Code requirements - changes

have to be documented and approved

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ITS vs. NITS Classification

• Not-Important-To-Safety (NITS) classification normally specified for ISFSI pad design and construction

• NITS requirements include:– Design using ACI 318 Code requirements– Construction using commercial grade

processes– Construction in accordance with ACI 318

Code requirements.– Less documentation

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Design ConsiderationsConcrete and Subgrade

• Transport Evaluation Program established to ensure that design basis drop limits are not exceeded

• Some cask vendors place a upper limit on the concrete strength and the subgrade stiffness to keep the design basis drop impact loading below a specified limit Lower limits on concrete strength and subgrade stiffness are always appropriate to ensure that the ISFSI pad will perform adequately (settlement)

• Subgrade tests are necessary to determine stiffness. Variety of tests are available including plate test and penetration test

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Design ConsiderationsSeismic Issues

• The ISFSI design basis earthquake must be equivalent to or be enveloped by the safe shutdown earthquake of the nuclear plant

• The seismic response must be translated to the base of the ISFSI pad

• The ISFSI pad must be sufficiently imbedded in the ground such that vertical seismic loads will not cause slab uplift

• Seismic analysis may be performed by either dynamic or static methods. Static methods are normally used in low seismic zones

• A Soil-Structure-Interaction (SSI) analysis is required by 10 CFR 72.212(b)(2)(i)(B)

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Design ConsiderationsAnchored Casks

• High seismic zones may require that the ISFSI pad be classified as Important-To-Safety

• If the seismic forces may overturn or cause excessive sliding of free-standing casks then cask anchorage must be used

• If cask anchorage is necessary, the anchor bolts will also be classified as ITS and require adequate material test reports

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Design ConsiderationsCoefficient of Friction

• During a seismic event, the friction between the unanchored cask and the ISFSI concrete is included in the design calculations to ensure that the cask does not slide off the ISFSI pad or overturn

• A layer of ice was discovered between a portion of the cask bottom and the concrete ISFSI pad. During a seismic event, the frictional forces could have been overcome and the cask could slide off the ISFSI pad

• Precautions were implemented by licensees to keep water and ice from underneath the casks

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Design ConsiderationsDifferential Settlement

• The pad must be designed to limit differential settlement

• Many licensees perform calculations to determine the predicted differential settlement based on the sequential placement of the casks

• The analysis determines an optimum cask placement sequence to limit the differential settlement of the ISFSI pad

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Design ConsiderationsLiquefaction

• 10 CFR 72.212 requires that the potential for liquefaction be analyzed for the ISFSI pad

• Loose material, such as sand and silt can cause the ISFSI pad/casks to settle during a seismic event

• Some sites have had to use elaborate methods to support the pad including piles and piers

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Design ConsiderationsStrength and Durability

• The strength and durability of the concrete are important design and installation concerns

• Strength:– Pad must be sufficiently rigid (strength at 28

days)– Affected by mix design and amount of water

added at jobsite• Durability

– Types of aggregates selected and amounts of contaminants present (chlorides, etc.)

– Affected by rebar spacing and concrete placement methods

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Concrete ConstructionACI 318 & ACI 349 Codes

• American Concrete Institute (ACI) 318 provides building code requirements for structural concrete

• ACI 349 provides code requirements for nuclear safety related structures

• The ISFSI and cask concrete are classified as either Important-To-Safety or Not-Important-To-Safety

• The requirements of both ACI Codes are very similar and reference numerous other ACI Codes and American Society for Testing and Materials (ASTM) standards

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Concrete ConstructionACI 318 & ACI 349 Codes

• The major construction requirements contained in ACI 318 and 349 are:

• Testing of materials– Cement– Aggregates– Water– Reinforcing steel– Admixtures

• Concrete durability• Concrete quality, mixing and placing• Concrete formwork• Details of reinforcement

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Concrete ConstructionAdditional ACI Codes

• Additional ACI Codes that are referenced and need to be addressed are:– ACI 305 – Hot Weather Concreting– ACI 306 – Cold Weather Concreting– ACI 308 – Concrete Curing– ACI 309 – Concrete Consolidation

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Concrete ConstructionASTM Standards

• The American Society for Testing and Materials (ASTM) provide standards for testing methods and concrete components

• The major ASTM standards that apply to concrete are:– ASTM C31 – Making & curing concrete samples– ASTM C33 – Aggregate testing– ASTM C39 – Compressive strength tests– ASTM C94 – Ready-mixed concrete– ASTM C150 – Portland cement– ASTM C494 – Chemical admixtures– ASTM A615 – Reinforcing bars

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ISFSI Construction

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Cask Construction

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Cask Construction

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NRC Findings

• Design– Soil Structure Interaction not being completed

• Construction– Concrete compressive strength is either too low or

too high– Adherence to ACI Code and ASTM Standard

requirements – Reinforcement bar tolerances not being achieved– Concrete slump outside specified range on VCC

cask placement