Self-Assigned Surfrider Foundation Question (2) What are potential impacts on the ISFSI and canisters if directly exposed to saltwater or groundwater?

Research conducted by Katie Day, Staff Scientist, Surfrider Foundation, for the Technical Advisory Committee of Congressman Mike Levin’s, CA-49, SONGS Task Force.

Disclaimer: This review relied heavily upon information provided by the Holtec HI-STORM UMAX Final Safety Evaluation Report (FSAR) and HI-STORM FW FSAR. The FSAR’s provide non-location specific evaluations of the waste storage systems, and often allow alternative “replacement materials” to be utilized for many of the processes described below, which could vary in their corrosion resistance levels. Where available, additional information specific to the processes and structures utilized at the San Onofre Nuclear Generating Station (SONGS) is provided and assessed.

Abstract

Due to the immediate coastal location and subterranean design of the San Onofre Nuclear Generating Station (SONGS) Holtec Independent Spent Fuel Storage Installation (ISFSI), the proximity of this structure to both seawater and groundwater is concerning. The base of the ISFSI already sits in close proximity to the groundwater table, with certain agencies estimating a mere 1.5 ft difference between the base of the ISFSI and groundwater table. Over the next 50 years, coastal hazards, including exacerbated storms, coastal erosion, sea level rise, groundwater level rise and seawater intrusion into groundwater aquifers could cause the Holtec ISFSI to be directly exposed to seawater and/or freshwater. Understanding the impacts and risks to the ISFSI from water exposure is important to determine the current and future structural integrity of the facility. This review summarizes the potential impacts the Holtec ISFSI could experience from seawater or freshwater exposure, based on information provided by Holtec Final Safety Evaluation Reports and supplemental information provided by Southern California Edison (SCE) representatives. Materials reviewed include reinforced concrete, the Vertical Ventilated Module (VVM)’s Cavity Enclosure Container (CEC), and the Multi-Purpose Canister (MPC).

Based on this review, potential impacts to the ISFSI and canisters from direct groundwater or seawater exposure that warrant further analysis include: (1) reduced structural integrity of the concrete “monolith” due to corrosion induced spalling from uncoated rebar in reinforced concrete, (2) corrosion of exposed carbon steel of the CEC divider shell after coating is scratched during canister downloading, (3) lack of an enclosure wall to further avoid groundwater intrusion, (4) chloride induced stress corrosion cracking on the MPC and (5) general corrosion of the MPC due to scratching of the chrome-oxide layer during downloading. Additional information on the ISFSI components and issues listed above should be requested to determine the risk to the Holtec ISFSI from water exposure, including clarification on any coatings or sealants used at SONGS, and the level of corrosivity of sediment adjacent to the SONGS ISFSI.

Introduction

The San Onofre Nuclear Generating Station (SONGS) is in the process of decommissioning, and has 3.6 million pounds of spent fuel onsite. During decommissioning, spent fuel assemblies are transferred from cooling pools to dry storage, called an Independent Spent Fuel Storage Installation (ISFSI), to undergo passive cooling. Waste will remain stored in the ISFSI until federal agencies determine an offsite interim storage location or permanent repository for the waste, or until state agencies refuse to award or extend Coastal Development Permits. There are two ISFSI facilities at SONGS; the Areva ISFSI, previously constructed and loaded; and the Holtec ISFSI, constructed in 2017 and in the process of being loaded with 73 canisters.

Due to the immediate coastal location and subterranean design of the Holtec ISFSI, the proximity of this structure to both seawater and groundwater is concerning. The exact subterranean location of the base of the ISFSI is reported at different elevations, with the NRC reporting a location at 8.5 ft mean lower low water level (MLLW)[footnoteRef:1] and the CCC reporting the ISFSI base at 7.5 ft MLLW.[footnoteRef:2] Regardless, the groundwater table at the site of the ISFSI sits in close proximity at 5.4 ft MLLW and fluctuates as high as 6.1 ft MLLW,[footnoteRef:3] meaning the ISFSI base can already be as close as 1.4 ft above the ground water table. Over the next 50 years, coastal hazards, including exacerbated storms, coastal erosion, sea level rise, groundwater level rise and seawater intrusion into groundwater aquifers could cause the Holtec ISFSI to be directly exposed to seawater, groundwater and heavy rain events. Understanding the impacts and risks to the ISFSI from water exposure is important to determine the current and future structural integrity of the facility. This review summarizes the potential impacts the Holtec ISFSI could experience from seawater or freshwater exposure, based on information provided by the Holtec HI-STORM UMAX Final Safety Evaluation Report (herein referred to as “UMAX FSAR”), Holtec HI-STORM FW FSAR (herein referred to as “FW FSAR”) and information provided by Southern California Edison (SCE) representatives. Materials reviewed include reinforced concrete, the Vertical Ventilated Module (VVM)’s Cavity Enclosure Container (CEC), and the Multi-Purpose Canister (MPC). [1: See US NRC email to Tom Palmisano, dated May 22, 2017. Subject: SAN ONOFRE NUCLEAR GENERATING STATION – NRC INSPECTION REPORT 05000206/2016004, 05000361/2016004, 05000362/2016004, AND 07200041/2016002] [2: See CCC. 2015. Tu14a. Application Number 9-15-0228. Adopted Findings: Regular Permit. pg. 47] [3: See CCC. 2015. Tu14a. Application Number 9-15-0228. Adopted Findings: Regular Permit. pg. 47]

(1) Concrete

Concrete is used for many purposes at the SONGS Holtec ISFSI. To narrow what impacts to consider for this analysis, it was important to first understand the use, structure, and services of concrete at the ISFSI. In general, concrete is used for backfill, a barrier against water seepage, shielding from radiation, structural stability and resiliency against missile impacts and seismic events.

A. Use of Concrete at SONGS Holtec ISFSI

According to the UMAX FSAR, concrete is used as the material for five main structures. Descriptions are provided by excerpts for each of the structures below.[footnoteRef:4] [4: See https://www.nrc.gov/docs/ML1619/ML16193A339.pdf]

i. ISFSI Pad

“ISFSI Pad means the reinforced concrete pad that provides the support surface for the cask handling device… to augment shielding, to provide a sufficiently stiff riding surface for the cask transporter, to act as a barrier against gravity-induced seepage of rain or floodwater around the VVM body as well as to shield against a missile. The ISFSI pad is a monolithic reinforced concrete structure that provides the load bearing surface for the cask transporter”

ii. Support Foundation Pad

“Support Foundation Pad (SFP) means the reinforced concrete pad located underground on which the CECs are situated… The SFP and the under-grade must have sufficient strength to support the weight of all the loaded VVMs during long-term storage and earthquake conditions.”

iii. Self-hardening Engineered Subgrade (SES or CLSM)

“The lateral space between each CEC, the SFP and the ISFSI pad is referred to as the subgrade and is filled with a Controlled Low-Strength Material (CLSM). Alternatively, ‘lean concrete’ may also be used. CLSM is a self-compacted, cementitious material used primarily as a backfill in place of compacted fill… The space below the SFP is referred to as the under-grade. ACI 116R-00 defines lean concrete as a material with low cementitious content. CLSM and lean concrete are also referred to as ‘Self-hardening Engineered Subgrade (SES).”

iv. Closure Lid

“The Closure Lid is a steel structure filled with plain concrete that can withstand the impact of the Design Basis Missiles defined in Chapter 2… To minimize the radiation emitted from the storage cavity, a portion of the Closure Lid extends into the cylindrical space above the MPC. This cylindrical below-surface extension of the Closure Lid is also made of steel filled with shielding concrete to maximize the blockage of skyward radiation issuing from the MPC… Concrete, which serves strictly as a shielding material in the Closure lid, is completely encased in steel.”

v. Optional Enclosure Wall: Not used at SONGS Holtec ISFSI

“The Enclosure Wall is an optional structure which may be utilized to mitigate groundwater intrusion at sites with a high water table. Analyses in Chapter 3 show that the Self-hardening Engineered Subgrade (SES) provides a stable lateral support system to the ISFSI under the Design Basis Earthquake. In the absence of an Enclosure wall, the interface between the SES and the native subgrade defines the radiation protection boundary of the ISFSI.” SCE did not opt to construct an Enclosure Wall at the SONGS Holtec ISFSI.[footnoteRef:5] [5: Information provided during phone conversation with Ron Pontes, Manager SONGS Decommissioning Environmental Strategy]

B. Services of Concrete at SONGS ISFSI

These concrete structures provide three main services to the ISFSI, which include additional radiation shielding, both lateral and vertical; structural stability under various weight loads; and ability to withstand design-based earthquake and missile threats. Explicit reference to these services are provided by excerpts from the FSAR below.

i. Radiation Shielding

“Steel, concrete, and the subgrade are the principal shielding materials in the HI-STORM UMAX. The steel and concrete shielding materials in the Closure lid provide additional gamma and neutron attenuation to reduce dose rates.”

ii. Structural Stability of ISFSI

“The soil lateral to the CECs (termed Space A in this FSAR) is required to be removed and replaced with a Self-hardening Engineered Subgrade (SES) such as CLSM or lean concrete which imparts enhanced structural characteristics to the ISFSI pad support system improving its ability to support the Casktransporter during MPC transfer operations. The minimum average density and the minimum shear wave velocity in the lateral subgrade surrounding the VVMs have been specified in Table 2.3.2”

iii. Ability to Withstand Earthquake and Missile Threats (Design Based)

“The subgrade must continue to maintain its physical integrity under the DBE load combination in Table 2.4.3. Maintaining physical integrity means no structural collapse, instability or cracks that produce an unobstructed direct streaming path in the subgrade for the radiation emanating from the stored fuel, and no constriction of the CEC that may interfere with retrievability of the MPC. b. In the scenario where the adjacent subgrade has been excavated exposing the lateral surface of the subgrade and a Design Basis Missile (see Table 2.3.3) strikes the exposed surface in the most severe orientation, the sub grade must be capable of stopping the missile before it reaches the MPC… To meet increased seismic inertia loads, the strength of the material in the interstitial space between the VVMs (Space A in Figure 2.4.4) is increased by using normal density concrete. The minimum compressive strength of the Space A concrete is provided in Table 2.3.10”. For earthquakes specifically, “[t]he fill material interstitial space between the CECs, referred to as Space A in Figure 2.4.4, is replaced with plain concrete with a minimum compressive strength of 3000 psi”.

C. Impacts to ISFSI Concrete from Water Exposure

While Holtec and Southern California Edison (SCE) frequently refer to the Holtec ISFSI as a “monolithic” structure, concrete is not actually monolithic. Concrete is highly heterogeneous, often made of a mix of gravel, sand, cement, water, air and additives.[footnoteRef:6] Though concrete may look like one solid structure, it is filled with air pockets and pores caused by the evaporation of water from the initial mix.[footnoteRef:7] The size, amount and connectivity of the pores can directly impact the structural integrity of the structure. This is especially the case with reinforced concrete, where pore size and connectivity can contribute to the potential oxidation, or corrosion, of the “reinforcing steel”, and ultimately reduce the stability and integrity of the structure. The pore size distribution, connectivity and total porosity is often determined by the “Water-Cement Ratio” of the concrete.[footnoteRef:8] The ISFSI Pad and Support Foundation Pad at SONGS both utilize reinforced concrete. [6: See https://www.researchgate.net/publication/275350234_Concrete_technology_-_porosity_is_decisive] [7: See https://www.researchgate.net/publication/275350234_Concrete_technology_-_porosity_is_decisive] [8: See https://www.hindawi.com/journals/amse/2014/273460/]

Reinforced concrete means that the concrete is lined with steel bars (often rebar) to add strength to the structure. Unfortunately, rebar is an unstable metal and will chemically recombine with elements to return to its natural, stable state of iron, a process called oxidation, rust and/or corrosion. According to Kepler et al,[footnoteRef:9] concrete pore water solution has a relatively high pH (generally greater than 11) which forms an oxide layer on the contained rebar, making the rebar “passive”. This layer naturally protects the rebar from direct oxygen exposure. If the pH is sufficiently reduced or there is an increase in chloride ions (such as from saltwater), this layer can be broken. In areas with oxygen and water exposure, this could lead to significant corrosion. In conducive environments, again such as saltwater, dissolved chloride can induce an “iron chloride complex” causing enhanced corrosion through electrochemical processes (formation of an electrochemical cell), either between the steel rebar and the concrete pore solution, or between materials within the steel itself, especially if that steel has been scratched. Both instances can lead to tensile stresses on the concrete, causing cracking and eventual spalling, where large chunks of the concrete actually break off. The more interconnected the pores in the concrete, the faster this voltage induced corrosion can occur.[footnoteRef:10] Other risks to reinforced concrete from water exposure include surface scaling, which is a loss of surface mortar often caused by freeze-thaw reactions; delamination[footnoteRef:11]; and brittle fracture in low temperatures. [9: See http://www2.ku.edu/~iri/projects/corrosion/SM58.PDF] [10: See http://www2.ku.edu/~iri/projects/corrosion/SM58.PDF] [11: See https://www.buildingforensicsintl.com/concrete-delamination-damage]

It is unclear if the SES used for the subgrade between each VVM at SONGS is made from lean concrete or CLSM, or if one material is more resistant to porosity than the other. The SES is not a form of reinforced concrete because it does not contain rebar; however, this material fills gaps between the steel VVMs, meaning its porosity may have an impact on the corrosion risk of the interior steel components (CEC’s), just as the porosity of the reinforced ISFSI Pad and Support Foundation Pad could have an impact on the corrosion risk of the contained rebar. Based on the information collected during this review, corrosion of reinforcing metals from water intrusion through porous concrete, and subsequent spalling or cracking of the concrete, is the main threat that water exposure poses to reinforced concrete at the Holtec ISFSI. This is important as concrete provides important services at the ISFSI, including radiation shielding, structural stability and ability to withstand seismic and missile events. It will be important to get specific information on the total porosity and pore connectivity of the SES, ISFSI Pad and Support Foundation Pad to determine water permeability through the “concrete monolith”.

D. Methods to Prevent Spalling and Other Risks to Reinforced Concrete

Galvanic corrosion is one form of protecting reinforcing steel from corrosion, and therefore protecting the integrity of the reinforced concrete structure as a whole. Galvanic corrosion directs corrosive currents to another, more negatively charged metal alloy, thus protecting the more positively charged metal. Generally, when it comes to steel; zinc, aluminum, and magnesium are more negatively charged, while copper and stainless steel are more positively charged. Other options to prevent corrosion induced cracking or spalling of reinforced concrete is by adding a water sealant to the concrete surface, adding a protective coating to the rebar (such as epoxy), or by using an Impressed Current Cathodic Protection System (ICCP), which is flowing a charged current to the metal.

In the Holtec FSAR, there is no discussion on concrete porosity or mention of methods to mitigate corrosion of reinforced concrete in the Support Foundation Pad (below the VVMs) or the ISFSI Pad (above the VVM’s), which both contain rebar. SONGS does not utilize many of the above mentioned corrosion mitigation techniques, including rebar coatings, concrete sealants or ICCP.[footnoteRef:12] [12: Information provided during phone conversation with Ron Pontes, Manager SONGS Decommissioning Environmental Strategy]

It is explicitly mentioned in the UMAX FSAR that “corrosion of structural steel embedded in the concrete structures due to salinity in the environment at coastal sites is not a concern for HI-STORM UMAX VVM because it does not rely on rebars (indeed, it contains no rebars).” This is in reference to the SES/ CLSM fill that surrounds the VVM CEC shells. However, the UMAX FSAR later refers to the SES as a concrete encasement that could be reinforced: “[r]egardless of reinforcement method, the material selected shall be corrosion-resistant or otherwise appropriately coated (e.g., epoxy coated steel wire) for corrosion resistance.” A representative at SCE confirmed that the rebar used in the Support Foundation Pad and ISFSI pad are not coated.[footnoteRef:13] It seems the only corrosion protection of reinforcing steel is from the cathodic protection provided by the high alkalinity of concrete, which could be disturbed by seawater or salty groundwater exposure. [13: Information provided during phone conversation with Ron Pontes, Manager SONGS Decommissioning Environmental Strategy]

Additionally, since the SES encases metal VVM’s, it should be explored if corrosion of VVM’s could cause cracking and scaling of the SES, similar to the impacts from corrosion of reinforcing steel. The UMAX FSAR states that “[a]ll exposed surfaces of the HI-STORM UMAX VVM components are made from stainless steels or ferritic steels that are readily painted. Concrete, which serves strictly as a shielding material in the VVM Closure Lid, is encased in steel. Therefore, the potential of environmental vagaries such as spalling of concrete are ruled out for HI-STORM UMAX VVM.” Getting additional assurance that this statement applies to SONGS is recommended.

The UMAX FSAR table below (p. 8-12) references CLSM performance properties. To note, the pH of the CLSM is said to range as low as 7.5, which may not be high enough to create a passive layer to protect the encased metal alloys (literature ranges based on type of steel and chloride content, but it’s frequently referenced that the pH must be 10.5 or greater for steel passivity to occur)[footnoteRef:14]. In discussion of corrosion protection for the CEC, design standards from ACI 318 are required to be deployed for the SES, yet again this is not explicitly mentioned for the reinforced concrete used for the Support Foundation Pad or ISFSI Pad. [14: See https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5457108/]

Table 1. Additional CLSM Performance Properties. Source: UMAX FSAR, p. 8-12.

In regards to seismic risk from cracked concrete, the LS-DYNA SSI analysis on a design basis earthquake (DBE) “only considers the governing uncracked concrete condition” of concrete. However, the FSAR also states that previous analysis on “cracked scenarios” found that “[t]he DBE analysis results summarized in Table 3.4.3 consistently demonstrate that the key seismic response (i.e., the seismic impact loading on the SFP) under the uncracked condition is bounding”.

(2) Cavity Enclosure Container (CEC)

The Cavity Enclosure Container (CEC) is defined in the UMAX FSAR as a “thick walled cylindrical steel weldment that defines the storage cavity for the MPCs”. More specifically, it is described as:

[A] weldment of the Container Shell, Container Flange, Bottom Plate, Lower MPC Guides, and MPC Bearing Pads. The Closure Lid is a weldment of structural steel encasing plain concrete and arranged to provide an appropriate outlet passage for the heated air issuing from the storage cavity. An insulated Divider Shell with Upper MPC Guides is situated within the CEC and restrained by the Lower MPC Guides at the bottom and by the Container Flange at the top. These individual components are collectively referred to as VVM Components.

Each canister (MPC) is downloaded into a pre-installed CEC in the Holtec ISFSI. The CEC contains a divider shell that bifurcates the area between the canister and CEC as either down-flow or up-flow of air for passive cooling. The divider shell has insulation to help prevent preheating of the cool down-flow air, which is said to be “water and radiation resistant” yet the exact material is not provided. The CEC, along with the closure lid, is referred to as the Vertical Ventilated Module (VVM). The VVM is described as providing “structural protection, cooling, and radiological shielding for the MPC.”

A. Materials and Design of the CEC

The CEC is referenced in the UMAX FSAR as a thick walled weldment, yet the thickness of the CEC containment shell is said to be just 0.75 inches, with a baseplate of 1.5 inches thick (p.5-30). Components of the CEC are said to be made out of carbon steel in the Holtec FSAR. However, an SCE representative confirmed that almost all of the CEC components are made out of stainless steel (type 304) at SONGS; including the Container Shell, Bottom Plate, Lower MPC Guides, Upper MPC Guides, MPC Bearing Pads, Closure Lid, and Container Flange. Stainless steel was used to provide stronger corrosion resistance. Only the divider shells are made out of carbon steel.

The CEC is welded at the bottom, providing a physical barrier preventing water penetration from the bottom or sides; however, the top of the CEC is open when the closure lid is open. Even when the closure lid is on, the lid has air vents to allow for passive cooling, so the CEC is never completely sealed. The UMAX FSAR states that there is sloping on the ISFSI pad to divert water away from the CEC vents and into the storm drain system, but heavy rainstorms or flood events may still result in water entering the CEC air ducts and have direct contact with the MPC. Any water that enters the CEC will remain contained in the CEC unless explicitly pumped out. However, “[t]he cutouts in the Divider Shell are sufficiently tall to ensure that if the cavity were to be filled with water, the bottom region of the MPC would be submerged for several inches. This design feature is important to ensure adequate thermal performance of the system if flood water would stop air flow”.

To note, the UMAX FSAR states that “all HI-STORM MPCs are designed to withstand 125 feet of water submergence (Table 2.4.1). The VVM will clearly withstand this static head of water above the surface of the ISFSI because all structural members are either not subject to any pressure differential from the flood or are backed by the subgrade, which resists the flood water directly.” Regarding pressure, the Holtec FSAR explains that submergence of the CEC to the “Container Flange” is not at risk to hydrostatic pressure so “potential for significant stresses is not a concern”. Buoyancy is not also not a concern due to the dead weight of the system. There is no discussion of increased risk to corrosion of the CEC or VVM from water submergence.

Blockage of air vents caused by debris from flood events is a noted risk, which is why the visual inspection of air vents and/or monitoring of heat loads at air vents is critical. A system pre-established and approved to remove extensive debris from CEC vaults and vents would be a recommended contingency plan. “…[t]he water could enter the inlet ducts and block portion or the entire cooling air flow passageway at the bottom of the cavity, which reduces the air flow ventilating through VVM and causes an elevation of the fuel cladding temperature and system component temperatures”, including “overheating of the concrete in the overpack”.

B. Methods of Corrosion Resistance for CEC Components

Methods to resist corrosion on the VVM CEC vary by component, and according to the UMAX FSAR range from zinc coatings, reliance on pH protection from concrete encasement, or use of stainless steel. Metal types used on the CEC may include “SA516 Gr. 70, SA515, Gr. 70, [and] SA36 or austenitic stainless steel.” Coatings and ICCP are required depending on the level of “environment corrosivity” which is determined by soil testing. It is unclear what the level of soil corrosivity is at SONGS, but it is expected to be highly corrosive due to coastal proximity. Since SCE opted to use SES as infill between VVMs, instead of natural soil, it would be beneficial to confirm if soil corrosivity requirements still apply. Specifics on corrosion mitigation techniques used for various components are provided below, often by relevant excerpts from the UMAX FSAR.

i. Carbon Steel Components (the Divider Shell)

“Except for the CEC exterior surfaces (exterior CEC surface coating requirements discussed separately), all carbon steel surfaces of the VVM are lined and coated with the same or equivalent surface preservative that is used in the aboveground HI-STORM FW and HI-STORM 100 overpacks. The pre-approved surface preservative is a proven zinc-rich inorganic/metallic (may also be an organic zinc rich coating) material that protects galvanically and has self-healing characteristics for added protection. All exposed surfaces interior to the VVM are accessible for the reapplication of surface preservative, if necessary” (UMAX FSAR). Interestingly, the UMAX FSAR states that “[c]hloride corrosion is not a concern since chloride leachables are limited and sufficiently low”, yet this may not apply when exposed to chloride ions in saltwater.

An SCE representative confirmed that there are no coatings added to components made out of stainless steel, except for those stainless steel components that are directly welded to the carbon steel Divider Shell, such as seismic restraints. The Divider Shell and the stainless steel components welded to the Divider Shell are coated with a Carbonic Zinc 11, Sherwin Williams Zinc Clad II HS, or Sherwin Williams Zinc Clad II Plus. The risk from canister to CEC scratching during loading or seismic events should be considered, as its important to ensure that the coating does not get penetrated, and if so, that it is able to sufficiently self-heal.

ii. Foil and Jacketing

“Stress corrosion cracking of the foil or jacketing, whether made from stainless steel or other material, is not an applicable corrosion mechanism due to minimal stresses derived from self-weight. The foil or jacketing and attachment hardware shall either have sufficient corrosion resistance (e.g., stainless steel, aluminum, or galvanized steel) or shall be protected with a suitable surface preservative” (UMAX FSAR).

iii. Bolts and Fasteners

“All bolts and fasteners are made of alloy materials which are not expected to experience any significant corrosion in the operating environment. The ISFSI operation and maintenance program shall call for coating of bolts and fasteners if the ambient environment is aggressive. All threaded surfaces are treated with a preservative to prevent corrosion. The O&M program for the storage system calls for all bolts to be monitored for corrosion damage and replaced, as necessary” (UMAX FSAR). Clarification on both the determination of the ambient environment at SONGS and use of coatings bolts and fasteners is recommended.

iv. CEC Shells

CEC shells are made out of austenitic stainless steel. Stainless steel is a type of steel that’s been “passivated”, where the surface oxidizes with chromium to create a protective layer, referred to as a “chrome-oxide” layer.[footnoteRef:15] While this layer makes stainless steel corrosion-resistant in coastal environments, it is still able to experience corrosion if submerged in saltwater. The UMAX FSAR acknowledges “peeling or perforation of surface preservatives on steel surfaces and corrosion to exposed steel surfaces” as potential degradation modes to the VVM in Table 8.1.12. This clarifies that exposed steel surfaces are also at risk to corrosion. Additionally, “[t]he exterior surfaces of the CEC are in contact with either engineered fill or concrete (concrete encasement or “free-flow “concrete ) and may be subjected to cathodic protection, as applicable.” As such, the plain concrete surrounding the stainless steel VVM CEC shell is considered as a form of corrosion mitigation. [15: See https://www.mgnewell.com/wp-content/uploads/2016/11/Passivation-of-stainless-steel.pdf]

The UMAX FSAR also states that the CEC “is substantially sequestered from the native soil through two engineered features: a. A thick reinforced concrete Enclosure Wall surrounds the VVM array and, along with the Support Foundation pad, provides a physical separation (water intrusion protection) to the CECs. b. The subgrade in contact with the CECs is either a “free flow” concrete or an engineered fill selected to provide a non-aggressive environment around the CECs.” The Enclosure Wall was not constructed at SONGS. The referenced concrete or engineered fill is the SES/CLSM, which is also referred to as the 5 inch-thick “CEC concrete encasement”, protects against corrosion by providing a pH buffering effect. This is thicker than the ACI 318 recommendation to use a minimum of 3-inch thick concrete to protect reinforcing metals in “aggressive environments”. Mechanisms to minimize voids and micro-cracks are said to be required to be deployed, but specifics are not provided, and it would be beneficial to ensure that these mechanisms are utilized at SONGS.

v. CEC Lid

“All exposed surfaces of the HI-STORM UMAX VVM components are made from stainless steels or ferritic steels that are readily painted. Concrete, which serves strictly as a shielding material in the VVM Closure Lid, is encased in steel. Therefore, the potential of environmental vagaries such as spalling of concrete are ruled out for HI-STORM UMAX VVM” (UMAX FSAR).

vi. Impressed Current Cathodic Protection System (ICCPS)

“If the aggressiveness of the subgrade around the CEC is highly aggressive and warrants an ICCPS then the user may choose to either extend an existing ICCPS to protect the installed ISFSI, or to establish an autonomous system. The initial startup of the ICCPS must occur within one year after installation of the VVM to ensure timely corrosion mitigation. In addition, the ICCPS should be maintained operable at all times after initial startup except for system shutdowns due to power outages, repair or preventive maintenance and testing, or system modifications. Because there are a multitude of ISFSI variables that will bear upon the design of the ICCPS for a particular site, the essential criteria for its performance and operational characteristics are set down in this FSAR, which the detailed design work for each ISFSI site must follow” (UMAX FSAR). To note, ICCPS is not a passive management technique, as it provides corrosion protection by flowing an electrical current through the surrounding structure (so in this case, the SES) and on to the metal. An ICCPS is not being utilized at the SONGS Holtec ISFSI.

Tables 8.1.2 to 8.1.4. Stated degradation modes to VVM, including the mechanism and area affected. Source: UMAX FSAR.

(3) Multi-Purpose Canister (MPC)

The MPC is defined as “the sealed canister consisting of a fuel basket for spent nuclear fuel storage, contained in a cylindrical canister shell (the MPC Enclosure Vessel). The MPC is the confinement boundary for storage conditions.” In other words, the MPC is the canister that is able to contain up to 37 spent fuel assemblies, and is the vessel that is lowered into the VVM of the ISFSI. Much of the information regarding degradation modes and mitigation to the MPC is listed in its original host docket (72-1032 for HI-STORM FW), not the UMAX FSAR. For the purpose of this self-assigned question for Congressman Levin’s Task Force, degradation modes from seawater and freshwater exposure are only reviewed for the surface of the canister; degradation modes of internal MPC materials are not considered.

A. Materials of the MPC

The MPC is made out of the austenitic stainless steel 316L. According to Table 8.1.3 of the UMAX FSAR, the external surface of the MPC is exposed to the ambient environment. Austenitic stainless steel 316L is one of the most corrosion-resistant stainless steel available, however, it is still prone to corrosion risk. The FW FSAR states that “[t]he confinement boundary is made of stainless steel material for its superior strength, ductility, and resistance to corrosion and brittle fracture for long term storage. The basket shims used to support the basket are made of a creep resistant aluminum alloy. The two-piece MPC lid is either made entirely of Alloy X or the bottom portion of the lid is made of carbon steel with stainless steel veneer.”

More specifically, “[t]he available austenitic stainless steels are AISI Types 304, 304LN, 316 and 316LN containing a minimum of 16% chromium and 8% nickel, and at least traces of molybdenum. The passive films (formed due to atmospheric exposure) of stainless steels range between 10 to 50 angstroms (lxl0-6 to 5x10-6 mm) thick [8.12.4]. Of all types of stainless steels (i.e., austenitic, ferritic, martensitic, precipitation hardenable and two-phase), "the austenitic stainless alloys are considered the most resistant to industrial atmospheres and acid media" [8.12.4]. The MPC contains no gasketed, threaded, or packed joints for maintaining confinement. The all welded construction of the MPC confinement boundary and the inert backfill gas within ensures that the interior surfaces and the MPC internals (Metamic-HT baskets, shims, etc.) are not subject to corrosion. Exterior MPC surfaces would be exposed to the ambient environment while inside of a HI-STORM FW storage overpack or a HI-TRAC VW transfer cask.”

B. Impacts to MPC from Saltwater or Groundwater Exposure

According to the UMAX FSAR and FW FSAR, the following failure modes that could result from water exposure were identified and details of the determined level of concern are provided by excerpts below. Note that no specific reference to submergence in the UMAX FSAR acknowledges added chloride ions from seawater exposure.

i. Thermal

“Full or partial submergence of the MPC is not a concern from a thermal perspective, as discussed in Chapter 1, because heat removal is enhanced by the floodwater” (UMAX FSAR).

ii. Galling

“Preventing galling of interfacing surfaces is another critical consideration in selecting bolt materials. Use of austenitic stainless bolts on interfacing austenitic stainless steel surfaces is not permitted. All threaded surfaces are treated with a preservative to prevent corrosion. The O&M program for the storage system calls for all bolts to be monitored for corrosion damage and replaced, as necessary” (FW FSAR).

iii. Corrosion and Pitting

Table 8.1.4 states that stress corrosion cracking of austenitic stainless steel is a failure and degradation mechanism of the MPC (UMAX FSAR). The FW FSAR also states that “[p]otential problems from general corrosion, pitting, stress corrosion cracking, or other types of corrosion, should be evaluated for the environmental conditions and dynamic loading effects that are specific to the component.” Based on the literature, corrosion methods of stainless steel include pitting corrosion, crevice corrosion, general corrosion, galvanic corrosion, stress corrosion cracking and intergranular attack.

It is explicitly stated in the FW FSAR that “[c]asks deployed at coastal ISFSI sites that would

be exposed to the harsh marine environment for prolonged periods must not suffer corrosion that

will impair their functionality”. It is also stated that “[e]xtensive data show corrosion rates (pitting) to 0.00 18 (mm/yr) for 304, 304LN, 316 and 316LN in marine environments at ambient temperatures after 26 years [8.12.1]. Using this bounding corrosion rate data, a Holtec Position Paper [8.12.3] estimates the total corrosion of the external surface of the MPC in 100 years of service is about half a millimeter which is significantly smaller than the available design margins in the material thickness. It is to be noted that this upper-bound is estimated for an extreme hypothetical marine environment. As discussed earlier for inland applications the corrosion rates are insignificant. Therefore, corrosion of the MPC in long-term storage is not a credible safety concern.”

The FW FSAR states that “[i]t is also recognized that moisture will not exist on the MPC exterior surfaces for many years since moisture will not condense on hot surfaces... It is estimated that it would take decades for the hottest MPC to approach ambient temperatures and once at ambient temperature, any MPC surfaces will be highly corrosion resistance even when wet.” As such, the FW FSAR states that “MPC surfaces are not coated”. MPC’s are located within the CEC, so cathodic protection from concrete exposure does not apply. There is no additional corrosion prevention methods used for MPC at SONGS, as the 316L steel is considered strongly resistant to corrosion, even in coastal environments. While this may suffice for ambient conditions, a change in corrosion risk from direct exposure to seawater must be considered, and does not seem adequately addressed in the FW FSAR or UMAX FSAR.

a. Concern Regarding Scratching

Consideration of the change in corrosion risk due to scratching must be considered, as the UMAX FSAR states that “[d]eparture from the assumed values of material properties in the safety analyses can, in certain cases, adversely affect the computed safety margins”. In response to concerns regarding unapproved canister scratching from contact with the CEC during downloading, the NRC and SCE conducted an “Analysis of the Effects of Incidental Contact to Multipurpose Canisters During the Downloading Process”. This analysis measured the severity of scratches on eight of the 29 downloaded canisters at SONGS. Due to the use of laser peening during canister manufacturing, the canisters have a “a compressive stress field to a depth of at least 0.080 inches. Stress corrosion cracks cannot initiate or grow on surfaces with residual compressive stress.” Regarding the risk of pitting and general corrosion, “[c]ontact breaks through the chrome-oxide layer that protects stainless steel from pitting and general corrosion. However, any new surfaces exposed by wear marks are quickly (within weeks) covered by a newly formed chrome-oxide layer due the reaction of air with the chrome alloy in stainless steel. As a result, these wear marks will not have a significant effect on pitting and general corrosion rates.”[footnoteRef:16] Assurance that a chrome-oxide layer has re-established on all scratched canisters at SONGS is recommended to ensure that corrosion risk is not significantly increased due to scratching. [16: See https://scng-dash.digitalfirstmedia.com/wp-content/uploads/2019/06/Effects-of-Incidental-Contact-White-Paper-FINAL-060319.pdf]

iv. Brittle Fracture

“Since stainless steel materials do not undergo a ductile-to-brittle transition in the minimum

permissible service temperature range of the HI-STORM FW System, brittle fracture is not a

concern for the MPC components” (FW FSAR).

Conclusion

The main threat to the structural integrity of the ISFSI concrete and VVM structures is contingent upon the porosity of the concrete, as water permeability through the structure and exposure to reinforcing steel or the CEC could cause corrosion and subsequent loss of structural integrity of the rebar, CEC, and concrete structure as hole. This could have impacts on the eventual retrievability of downloaded canisters due to reduced ability for the VVM and/or ISFSI Pad to withhold necessary weight loads, it could also reduce earthquake resilience and missile resilience. As mentioned in the UMAX FSAR “[t]he materials that comprise the dry spent fuel storage should maintain their physical and mechanical properties during all conditions of operations. The spent fuel should be readily retrievable without posing operational safety problems… Dry spent fuel storage protective coatings should remain intact and adherent during all loading and unloading operations within wet or dry spent fuel facilities, and during longterm storage”.

Based on this review, notable potential impacts to the ISFSI and canisters from direct groundwater or seawater exposure include: (1) reduced structural integrity of the concrete “monolith” due to corrosion induced spalling from uncoated rebar in reinforced concrete, (2) corrosion of exposed carbon steel of the CEC divider shell after coating is scratched during canister downloading, (3) lack of an enclosure wall to further avoid groundwater intrusion, (4) chloride induced stress corrosion cracking on the MPC and (5) general corrosion of the MPC due to scratching of the chrome-oxide layer during downloading. Additional information on the ISFSI components and issues listed above should be requested to determine the risk to the Holtec ISFSI from water exposure, including clarification on any coatings or sealants used at SONGS, and the level of corrosivity of sediment adjacent to the SONGS ISFSI.

While the FSARs determine that a 60 year design life and 100 year service life is expected for the Holtec ISFSI, including the VVM and reinforced concrete, the atmospheric and environmental conditions at the plant may warrant a request for more robust inspections of the ISFSI. As stated in the UMAX FSAR “ISFSIs located in areas subject to atmospheric conditions that may degrade the storage cask or canister should be evaluated by the licensee on a site-specific basis to determine the frequency for such inspections to assure longterm performance.”

Suggested Further Analysis

There are many approaches that could be used to identify impacts to the Holtec ISFSI from seawater and groundwater exposure. Due to time limitations, this review focused on consolidating degradation modes identified in relevant NRC Final Safety Analysis Reports and identifying potential issues not adequately addressed. Additional research methods to understand water impacts to the ISFSI could include reviewing case studies of similar structures overtime after water exposure, and conducting a more thorough literature review of research on each of the structural components of the ISFSI.

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July 2019