4
DRY CASK STORAGE AND COMPARATIVE RISKS

This chapter addresses the second and third charges of the committee’s statement of task:

  • The safety and security advantages, if any, of dry cask storage1 versus wet pool storage at reactor sites.

  • Potential safety and security advantages, if any, of dry cask storage using various single-, dual-, or multi-purpose cask designs.

The second charge calls for a comparative analysis of dry cask storage versus pool storage, whereas the third charge focuses exclusively on dry casks. The committee will address the third charge first to provide the basis for the comparative analysis.

By the late 1970s, the need for alternatives to spent fuel pool storage was becoming obvious to both commercial nuclear power plant operators and the Nuclear Regulatory Commission. The U.S. government made a policy decision at that time not to support commercial reprocessing of spent nuclear fuel (see Appendix D). At the same time, efforts to open an underground repository for permanent disposal of commercial spent fuel were proving to be more difficult and time consuming than originally anticipated.2 Commercial nuclear power plant operators had no place to ship their growing inventories of spent fuel and were running out of pool storage space.

Dry cask storage was developed to meet the need for expanded onsite storage of spent fuel at commercial nuclear power plants. The first dry cask storage facility in the United States was opened in 1986 at the Surry Nuclear Power Plant in Virginia. Such facilities are now in operation at 28 operating and decommissioned nuclear power plants. In 2000, the nuclear power industry projected that up to three or four plants per year would run out of needed storage space in their pools without additional interim storage capacity.

This chapter is organized into the following sections:

  • Background on dry cask storage.

  • Evaluation of potential risks of dry cask storage.

  • Potential advantages of dry storage over wet storage.

  • Findings and recommendations.

1  

This storage system is referred to as “dry” because the fuel is stored out of water.

2  

The Nuclear Waste Policy Act of 1982 and the Amendments Act of 1987 laid out a process for identifying a site for a geologic repository. That repository was to be opened and operating by the end of January 1998. The federal government now hopes to open a repository at Yucca Mountain, which is located in southwestern Nevada, by the end of 2010,



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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please use the DRY CASK STORAGE AND COMPARATIVE RISKS 60 4 DRY CASK STORAGE AND COMPARATIVE RISKS This chapter addresses the second and third charges of the committee's statement of task: • The safety and security advantages, if any, of dry cask storage1 versus wet pool storage at reactor sites. • Potential safety and security advantages, if any, of dry cask storage using various single-, dual-, or multi- purpose cask designs. The second charge calls for a comparative analysis of dry cask storage versus pool storage, whereas the third charge focuses exclusively on dry casks. The committee will address the third charge first to provide the basis for the comparative analysis. By the late 1970s, the need for alternatives to spent fuel pool storage was becoming obvious to both commercial nuclear power plant operators and the Nuclear Regulatory Commission. The U.S. government made a policy decision at that time not to support commercial reprocessing of spent nuclear fuel (see Appendix D). At the same time, efforts to open an underground repository for permanent disposal of commercial spent fuel were proving to be more difficult and time consuming than originally anticipated.2 Commercial nuclear power plant operators had no place to ship their growing inventories of spent fuel and were running out of pool storage space. Dry cask storage was developed to meet the need for expanded onsite storage of spent fuel at commercial nuclear power plants. The first dry cask storage facility in the United States was opened in 1986 at the Surry Nuclear Power Plant in Virginia. Such facilities are now in operation at 28 operating and decommissioned nuclear power plants. In 2000, the nuclear power industry projected that up to three or four plants per year would run out of needed storage space in their pools without additional interim storage capacity. This chapter is organized into the following sections: • Background on dry cask storage. • Evaluation of potential risks of dry cask storage. • Potential advantages of dry storage over wet storage. • Findings and recommendations. print version of this publication as the authoritative version for attribution. 1 This storage system is referred to as “dry” because the fuel is stored out of water. 2 The Nuclear Waste Policy Act of 1982 and the Amendments Act of 1987 laid out a process for identifying a site for a geologic repository. That repository was to be opened and operating by the end of January 1998. The federal government now hopes to open a repository at Yucca Mountain, which is located in southwestern Nevada, by the end of 2010,

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please use the DRY CASK STORAGE AND COMPARATIVE RISKS 61 4.1 BACKGROUND ON DRY CASK STORAGE The storage of spent fuel in dry casks has the same three primary objectives as pool storage (Chapter 3): • Cool the fuel to prevent heat-up to high temperatures from radioactive decay. • Shield workers and the public from the radiation emitted by radioactive decay in the spent fuel and provide a barrier for any releases of radioactivity. • Prevent criticality accidents. Dry casks are designed to achieve the first two of these objectives without the use of water or mechanical systems. Fuel cooling is passive: that is, it relies upon a combination of heat conduction through solid materials and natural convection or thermal radiation through air to move decay heat from the spent fuel into the ambient environment. Radiation shielding is provided by the cask materials: Typically, concrete, lead, and steel are used to shield gamma radiation, and polyethylene, concrete, and boron-impregnated metals or resins are used to shield neutrons. Criticality control is provided by a lattice structure, referred to as a basket, which holds the spent fuel assemblies within individual compartments in the cask (FIGURE 4.1). These maintain the fuel in a fixed geometry, and the basket may contain boron-doped metals to absorb neutrons.3 Passive cooling and radiation shielding are possible because these casks are designed to store only older spent fuel. This fuel has much lower decay heat than freshly discharged spent fuel as well as smaller inventories of radionuclides. The industry sometimes refers to these casks using the following terms: • Single-, dual-, and multi-purpose casks. • Bare-fuel and canister-based casks. The terms in the first bullet indicate the application for which the casks are intended to be used. Single-purpose cask systems are licensed4 only to store spent fuel. Dual-purpose casks are licensed for both storage and transportation. Multi-purpose casks are intended for storage, transportation, and disposal in a geologic repository. No true multi-purpose casks exist in the United States (or in any other country for that matter) because specifications for acceptable containers for geologic disposal have yet to be finalized by the Department of Energy. Current plans for Yucca Mountain do not contemplate the use of multi-purpose casks. Nevertheless, some cask vendors still refer to their casks as “multi-purpose.” These are at best dual-purpose casks, however, because they have been licensed only for storage and transport. Because true multi-purpose casks do not now exist and are not likely to exist in the future, the committee did not consider them further in this study. print version of this publication as the authoritative version for attribution. 3 Criticality control is less of an issue in dry casks because there is no water moderator present after the cask is sealed and drained. 4 Authority for licensing dry cask storage rests with the Nuclear Regulatory Commission.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please use the DRY CASK STORAGE AND COMPARATIVE RISKS 62 FIGURE 4.1 Photo of NUHOMS canister showing the internal basket for holding the spent fuel assemblies in a fixed geometry. This canister is shown for illustrative purposes only. SOURCE: Courtesy of Transnuclear, Inc., an Areva Company. The terms in the second bullet indicate how spent fuel is loaded into the casks. In bare-fuel5 casks, spent fuel assemblies are placed directly into a basket that is integrated into the cask itself (see FIGURE 4.3B), The cask has a bolted lid closure for sealing. In canister-based casks, spent fuel assemblies are loaded into baskets integrated into a thin-wall (typically 1/2-inch [1.3-centimeter] thick) steel cylinder, referred to as a canister (see FIGURE 4.1 and 4.3A), The canister is sealed with a welded lid. The canister can be stored or transported if it is placed within a suitable overpack. This overpack is closed with a bolted lid. Bare-fuel and canister-based systems are sometimes referred to as “thick-walled” and “thin-walled” casks, respectively, by some cask vendors. This designation is not strictly correct because the overpacks in canister-based systems have thick walls. The only thin-walled component is the canister, which is designed to be stored or transported within the overpack. The designation of a cask as single- or dual-purpose often has less to do with its design and more to do with licensing decisions. Indeed, bare-fuel and canister-based casks can be licensed for either single or dual purposes. Consequently, one should not expect the performance of a cask in accidents or terrorist attacks to depend on its designation as single- or dual-purpose. Rather, performance will depend on the type of attack and construction of the cask. For the purposes of discussion in this chapter, therefore, the committee uses the designations “bare-fuel” and “canister-based,” rather than single- or dual-purpose, when referring to various cask designs. print version of this publication as the authoritative version for attribution. All bare-fuel casks in use in the United States are designed to be stored vertically. Most canister-based systems also are designed for vertical storage, but one overpack 5 The term bare fuel refers to the entire fuel assembly, including the uranium pellets within the fuel rods.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please use the DRY CASK STORAGE AND COMPARATIVE RISKS 63 system is designed as a horizontal concrete module (FIGURE 4.2).6 The principal characteristics of dry cask storage systems are summarized in TABLE 4.1, which is located at the end of this chapter. Dry casks are designed to hold up to about 10 to 15 metric tons of spent fuel. This is equivalent to about 32 pressurized water nuclear reactor (PWR) spent fuel assemblies or 68 boiling water nuclear reactor (BWR) spent fuel assemblies. Although the dimensions vary among manufacturers, fuel types (i.e., BWR or PWR fuel), and amounts of fuel stored, the casks are typically about 19 feet (6 meters) in height, 8 feet (2.5 meters) in diameter, and weigh 100 tons or more when loaded. The casks (for bare-fuel designs) or canisters (for canister-based designs) are placed directly into the spent fuel pool for loading. After they are loaded, the canisters or casks are drained, vacuum dried, and filled with an inert gas (typically helium). The loaded canisters or casks are then removed from the pool, their outer surfaces are decontaminated,7 and they are moved to the dry storage facility on the property of the reactor site. Loading of a single cask or canister can take up to one week. The vacuum drying process is the longest step in the loading process. In the United States, dry casks are stored on open concrete pads within a protected area of the plant site.8,9 This protected area may be contiguous with the protected area of the plant itself or may be located some distance away in its own protected area (see FIGURE 2.1). According to the information provided to the committee by cask vendors, nuclear power plant operators are currently purchasing mostly dual-purpose casks for spent fuel storage. The horizontal NUHOMS cask design is one of the most-ordered designs at present (TABLE 4.3). The vendors informed the committee that cost is the chief consideration for their customers when making purchasing decisions. Cost considerations are driving the cask industry away from all-metal cask designs and toward concrete designs for storage. print version of this publication as the authoritative version for attribution. 6 In addition, there is one modular concrete vault design in the United States: the Fort St. Vrain, Colorado, Independent Spent Fuel Storage Installation, which stores spent fuel from a high-temperature gas-cooled reactor. This reactor operated until 1989 and is now decommissioned. Because this is a one-of-a-kind facility, and the time available to the committee was short, it was not examined in this study. 7 Small amounts of radioactive contamination are present in the cooling water in the spent fuel pool. Some of this contamination is transferred to the cask or canister surfaces when it is immersed in the pool for loading. 8There may be exceptions in the future. Private Fuel Storage has requested a license from the Nuclear Regulatory Commission to construct a dry cask storage facility in Utah that will store fuel from multiple reactor sites. An underground dry cask storage facility has been proposed at the Humbolt Bay power plant in California to store old, low decay-heat fuel. The underground design is being proposed primarily because the site has very demanding seismic design requirements and is possible only because the fuel to be stored generates little heat. 9 In Germany, dry casks are stored in reinforced concrete buildings. These buildings were originally designed to provide additional radiation shielding (beyond what is provided by the cask itself) to reduce doses at plant site boundaries to background levels. Some of these buildings are sufficiently robust to provide protection against crashes of large aircraft. A subgroup of the committee visited spent fuel storage sites at Ahaus and Lingen during this study. See Appendix C for details.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please use the DRY CASK STORAGE AND COMPARATIVE RISKS 64 FIGURE 4.2 Photo showing a canister being loaded into a NUHOMS horizontal storage module. SOURCE: Courtesy of Transnuclear, Inc., an Areva Company. 4.2 EVALUATION OF POTENTIAL RISKS OF DRY CASK STORAGE Dry casks were designed to ensure safe storage of spent fuel,10 not to resist terrorist attacks. The regulations for these storage systems, which are given in Title 10, Part 72 of the Code of Federal Regulations (i.e., 10 CFR 72), are designed to ensure adequate passive heat removal and radiation shielding during normal operations, off-normal events, and accidents. The latter include, for example, accidental drops or tip-overs during routine cask movements. The robust construction of these casks provides some passive protection against external assaults, but the casks were not explicitly designed with this factor in mind.11 The regulations in 10 CFR 72 require that dry cask storage facilities (formally referred to as Independent Spent Fuel Storage Installations, or ISFSIs) be located within a protected area of the plant site (see FIGURE 2.1). However, print version of this publication as the authoritative version for attribution. the protection requirements for these installations are lower than those for reactors and spent fuel pools. The guard force is required to carry side arms, and its main function is surveillance: to detect and assess threats and to summon reinforcements. If the ISFSI is within the protected area of the plant 10 Dual-purpose casks also were designed for safe transport under the requirements of Title 10, Part 71 of the Code of Federal Regulations. The committee did not examine transport of spent fuel in this study. 11 A recent study by the German organization GRS (Gesellschaft für Anlagen- und Reaktorsicherheit, MBH) examined the vulnerability of CASTOR-type casks to large-aircraft impacts.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please use the DRY CASK STORAGE AND COMPARATIVE RISKS 65 it would come directly under the protection of plant's guard forces. The protected area is surrounded by vehicle barriers to protect against the detonation of a design basis threat vehicle bomb.12 A terrorist attack that breached a dry cask could potentially result in the release of radioactive material from the spent fuel into the environment through one or both of the following two processes: (1) mechanical dispersion of fuel particles or fragments; and (2) dispersion of radioactive aerosols (e.g., cesium-137). As described in Chapter 3, the latter process would have greater offsite radiological consequences. The committee evaluates the potential for both of these processes later in this chapter. In the wake of the September 11, 2001, attacks, additional work has been or is being carried out by government and private entities to assess the security risks to dry casks from terrorist attacks. Sandia National Laboratories is currently analyzing the response of dry casks to a number of potential terrorist attack scenarios at the request of the Nuclear Regulatory Commission. The committee was briefed on these analyses at two of its meetings. Sandia is analyzing the responses of three vertical cask designs and one horizontal design to a variety of terrorist attack scenarios (FIGURE 4.3). These designs are considered to be broadly representative of the dry casks currently licensed for storage in the United States by the Nuclear Regulatory Commission (see TABLE 4.1 at the end of this chapter). The committee received briefings on these studies by Nuclear Regulatory Commission and Sandia staff. Several attack scenarios are being considered in the Sandia analyses. They include large aircraft impacts and assaults with various types and sizes of explosive charges and other energetic devices. Details on the large aircraft impact scenarios are provided in the classified report. Most of this work is still in progress and has not yet resulted in reviewable documents. Consequently, the committee had to rely on discussions with the experts who are carrying out these studies and its own expert judgment in assessing the quality and completeness of this work. 4.2.1 Large Aircraft Impacts Sandia analyzed the impact of an airliner traveling at high speed into the four cask designs shown in FIGURE 4.3. These analyses examined the consequences of impacts of the fuselage and the “hard” components of the aircraft (i.e., the engines and wheel struts) into individual casks and arrays of casks on a storage pad. The latter analysis examined the potential consequences of cask-to-cask interactions resulting from cask sliding or partial tip- over The objectives of the analyses were first to determine whether the casks would fail (i.e., the containment would be breached) and, if so, to estimate the radioactive material releases and their health consequences. print version of this publication as the authoritative version for attribution. 12 As noted in Chapter 2, the committee did not examine surveillance requirements or the placement or effectiveness of vehicle barriers and guard stations at commercial nuclear plants.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please use the DRY CASK STORAGE AND COMPARATIVE RISKS 66 FIGURE 4.3 Four cask systems used in the Sandia analyses described in this chapter: (A) HI-STORM-100, (B) TN-68, (C) VSC-24, (D) NUHOMS-32P. The casks shown in A, C, and D are canister-based casks; the cask shown in B is a bare-fuel cask. SOURCE: Nuclear Regulatory Commission briefing materials (2004). print version of this publication as the authoritative version for attribution. The aircraft was modeled using Sandia-developed Eulerian CTH code (see footnote 15 in Chapter 3). The aircraft manufacturer (Boeing Corp.) was consulted to ensure that the aircraft model used in the analyses was accurate. The casks were modeled with standard finite element codes using the published characteristics of the casks. The casks were assumed to be filled with high-burn-up, 10-year-old spent fuel. The fuel rods were assumed to fail (rupture) if the strains in the cladding exceeded 1 percent, which is a conservative assumption. Sandia evaluated the release of radioactive materials from the spent fuel pellets inside the fuel rods when such cladding failures occurred. Radiological consequences of such releases were calculated for “representative” (with respect to weather and population) site conditions for each cask based on the actual average conditions at the

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please use the DRY CASK STORAGE AND COMPARATIVE RISKS 67 site that currently stores the most spent fuel in that cask type.13 Site conditions differed for each cask. The effects of jet fuel fires also were not considered in the analyses. Based on an analysis of actual aircraft accidents, Sandia determined that jet fuel would likely be dispersed over a large area in a low-angle impact. Consequently, the resulting petroleum fire would likely be of short duration (generally less than 15 minutes according to Sandia researchers). Long-duration fires that could damage the casks or even ignite the cladding of the spent fuel were not seen to be credible for the aircraft impact scenarios considered by Sandia.14 The results of these analyses, which are considered by the Nuclear Regulatory Commission to be classified or safeguards information, are detailed in the classified report. In general, the analyses show that some types of impacts will damage some types of casks. For some scenarios there could be substantial cask-to-cask interactions, including collisions and partial tip-overs. Nevertheless, predicted releases of radioactive material from the casks, mainly noble gases, were relatively small for all of the scenarios considered by Sandia. The analyses show that the releases were governed by design- specific features of the casks Sandia noted that the modeling of such releases is difficult and requires expert judgment for several elements of the calculation. Detailed calculations of the consequences were still in progress when the committee was briefed on these analyses. 4.2.2 Other Assaults Analyses are also being carried out to understand the consequences of other types of assaults on the cask designs shown in FIGURE 4.3. These include assaults using explosives and certain types of high-energy devices. The analyses were still underway when the committee was briefed on these analyses, and the results were characterized by the Nuclear Regulatory Commission as preliminary. Details are provided in the classified report. 4.2.3 Discussion As noted previously, the dry cask vulnerability analyses were still underway when the committee's classified study was completed. Based on the analyses it did receive, the committee judges that no cask provides complete protection against all types of terrorist attacks. The committee judges that releases of radioactive material from dry casks are low for the scenarios it examined with one possible exception as discussed in the classified report. It is not clear to the committee whether it is credible to assume that this “exceptional” scenario could actually be carried out. print version of this publication as the authoritative version for attribution. 13 As noted in Chapter 1, the committee did not concern itself with how radioactive materials would be transported through the environment once they were released from a dry cask. Rather, the committee confined its examination to whether and how much radioactive material might be released from a dry cask in the event of a terrorist attack. 14 The committee subgroup that visited Germany was briefed on a fire test on the Castor cask that involved a fully engulfing one-hour petroleum fire. The cask maintained its integrity during and after this test. See Appendix C. The results of this test do not necessarily translate to casks having other designs.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please use the DRY CASK STORAGE AND COMPARATIVE RISKS 68 In the committee's opinion, there are several relatively simple steps that could be taken to reduce the likelihood of releases of radioactive material from dry casks in the event of a terrorist attack: • Additional surveillance could be added to dry cask storage facilities to detect and thwart ground attacks. 15 • Certain types of cask systems could be protected against aircraft strikes by partial earthen berms. Such berms also would deflect the blasts from vehicle bombs. • Visual barriers could be placed around storage pads to prevent targeting of individual casks by aircraft or standoff weapons,16 These would have to be designed so that they would not trap jet fuel in the event of an aircraft attack. • The spacing of vertical casks on the storage pads can be changed, or spacers (shims) can be placed between the casks, to reduce the likelihood of cask-to-cask interactions in the event of an aircraft attack. • Relatively minor changes in the design of newly manufactured casks could be made to improve their resistance to certain types of attack scenarios. 4.3 POTENTIAL ADVANTAGES OF DRY STORAGE OVER WET STORAGE Based on the analyses presented in Chapter 3 and previously in this chapter, the committee judges that dry cask storage has several potential safety and security advantages over pool storage. These differences can best be illustrated using scenarios for both storage systems based on the Sandia analyses reviewed by the committee. The use of such scenarios should not be taken to imply that the committee believes that these scenarios are likely or even possible at all storage facilities. They are used only for illustrative purposes. The following statements can be made about the comparative advantages of dry-cask storage and pool storage based on the Sandia analyses: Less spent fuel is at risk in an accident or attack on a dry storage cask than on a spent fuel pool. An accident or attack on a dry cask storage facility would likely affect at most a few casks and put a few tens of metric tons of spent fuel at risk. An accident or attack on a spent fuel pool puts the entire inventory of the pool, potentially hundreds of metric tons of spent fuel, at risk. The potential consequences of an accident or terrorist attack on a dry cask storage facility are lower than those for a spent fuel pool. There are several reasons for this difference: (1) There is less fuel in a dry cask than in a spent fuel pool and therefore less radioactive material available for release. (2) Measured on a per-fuel-assembly basis, the inventories of radionuclides available print version of this publication as the authoritative version for attribution. 15 As noted in Chapter 1, the committee did not examine surveillance activities at nuclear power plants and has no basis to judge whether current activities at dry cask storage facilities are adequate. 16 The ISFSI at the Palo Verde Nuclear Power Plant in Arizona, which was visited by a subgroup of committee members, incorporates a berm into its design to provide a visual barrier.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please use the DRY CASK STORAGE AND COMPARATIVE RISKS 69 for release from a dry cask are lower than those from a spent fuel pool because dry casks store older, lower decay- heat fuel. (3) Radioactive material releases from a breach in a dry cask would occur through mechanical dispersion. 17 Such releases would be relatively small. Certain types of attacks on spent fuel pools could result in a much larger dispersal of spent fuel fragments. Radioactive material releases from a spent fuel pool also could occur as the result of a zirconium cladding fire, which would produce radioactive aerosols. Such fires have the potential to release large quantities of radioactive material to the environment. The recovery from an attack on a dry cask would be much easier than the recovery from an attack on a spent fuel pool. Breaches in dry casks could be temporarily plugged with radiation-absorbing materials until permanent fixes or replacements could be made. The most significant contamination would likely be confined largely to areas near the cask storage pad and could be detected and decontaminated. The costs of recovery could be high, however, especially if the cask could not be repaired or the spent fuel could not be removed with equipment available at the plant. A special facility might have to be constructed or brought onto the site to transfer the damaged spent fuel to other casks. Breaches in spent fuel pools could be much harder to plug, especially if high radiation fields or the collapse of the overlying building prevented workers from reaching the pool. Complete cleanup from a zirconium cladding fire would be extraordinarily expensive, and even after cleanup was completed large areas downwind of the site might remain contaminated to levels that prevented reoccupation (see Chapter 3). It is the potential for zirconium cladding fires in spent fuel pools that gives dry cask storage most of its comparative safety and security advantages. This comparative advantage can be reduced by lowering the potential for zirconium cladding fires in loss-of-pool-coolant events. As discussed in Chapter 3, the committee believes that there are at least two steps that can be implemented immediately to lower the potential for such fires. 4.4 FINDINGS AND RECOMMENDATIONS With respect to the committee's task to examine potential safety and security advantages of dry cask storage using various single-, dual-, or multi-purpose cask designs, the committee offers the following findings and recommendations: FINDING 4A: Although there are differences in the robustness of different dry cask designs (e.g., bare- fuel versus canister-based), the differences are not large when measured by the absolute magnitudes of radionuclide releases in the event of a breach. All storage cask designs are vulnerable to some types of terrorist attacks for which radionuclide releases would be possible. The vulnerabilities are related to the specific print version of this publication as the authoritative version for attribution. 17 Since the committee's classified report was published, the committee received an additional briefing from the Nuclear Regulatory Commission suggesting that a radioactive aerosol could be released in one type of terrorist attack. However, the scenario in question does not appear to the committee to be credible.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please use the DRY CASK STORAGE AND COMPARATIVE RISKS 70 design features of the casks, but the committee judges that the quantity of radioactive material releases predicted from such attacks is still relatively small. FINDING 4B: Additional steps can be taken to make dry casks less vulnerable to potential terrorist attacks. Although the vulnerabilities of current cask designs are already small, additional, relatively simple steps can be taken to reduce them. Such steps are listed in Section 4.2.3. RECOMMENDATION: The Nuclear Regulatory Commission should consider using the results of the vulnerability analyses for possible upgrades of requirements in 10 CFR 72 for dry casks, specifically to improve their resistance to terrorist attacks. The committee was told by Nuclear Regulatory Commission staff that such a step is already under consideration. Based on the material presented to the committee, there appear to be minor changes that can be made by plant operators and cask vendors to increase the resistance of existing and new casks to terrorist attacks (see Section 4.2.3). With respect to the committee's task to examine the safety and security advantages of dry cask storage versus wet pool storage at reactor sites, the committee offers the following findings and recommendations: FINDING 4C: Dry cask storage does not eliminate the need for pool storage at operating commercial reactors. Newly discharged fuel from the reactor must be stored in the pool for cooling, as discussed in detail in Chapter 3. Under current U.S. practices, dry cask storage can be used only to store fuel that has been out of the reactor long enough (generally greater than five years under current practices) to be air cooled. The fuel in dry cask storage poses less of a risk in the event of a terrorist attack than newly discharged fuel in pools because there is substantially reduced probability of initiating a cladding fire. FINDING 4D: Dry cask storage for older, cooler spent fuel has two inherent advantages over pool storage: (1) It is a passive system that relies on natural air circulation for cooling; and (2) it divides the inventory of that spent fuel among a large number of discrete, robust containers. These factors make it more difficult to attack a large amount of spent fuel at one time and also reduce the consequences of such attacks. Each storage cask holds no more than about 10 to 15 metric tons of spent fuel, compared to the several hundred metric tons of spent fuel that is commonly stored in reactor pools. The robust construction of these casks prevents large-scale releases of radionuclides in all of the attack scenarios examined by the committee. Some of the attacks could breach the casks, but many of these breaches would be small and could probably be more easily plugged than a perforated spent fuel pool wall because radiation fields would be lower and there would be no escaping water to contend with. Even large breaches of the cask would print version of this publication as the authoritative version for attribution.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please use the DRY CASK STORAGE AND COMPARATIVE RISKS 71 result only in the mechanical dispersal of some of its radionuclide inventory in the immediate vicinity of the cask. FINDING 4E: Depending on the outcome of plant-specific vulnerability analyses described in the committee's classified report, the Nuclear Regulatory Commission might determine that earlier movements of spent fuel from pools Into dry cask storage would be prudent to reduce the potential consequences of terrorist attacks on pools at some commercial nuclear plants. The statement of task directs the committee to examine the risks of spent fuel storage options and alternatives for decision makers, not to recommend whether any spent fuel should be transferred from pool storage to cask storage. In fact, there may be some commercial plants that, because of pool designs or fuel loadings, may require some removal of spent fuel from their pools, If there is a need to remove spent fuel it should become clearer once the vulnerability and consequence analyses described in Chapter 3 are completed. The committee expects that cost- benefit considerations would be a part of these analyses. print version of this publication as the authoritative version for attribution.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as the authoritative version for attribution. TABLE 4.1 Dry Casks Used for Spent Fuel Storage in the United States TABLE 4.1 Dry Casks Used for Spent Fuel Storage in the United States Cask design License holder Type Construction Closure system Number of casks used to Cask design License holder Fuell type Fue type Construction Closure system Number of casks used to Type used for date; sites; and number used for dale: sites; and number storage of casks on order1 storage of casks on order1 CASTOR V/21 Bare-fuel, BWR Ductile cast Iron Primary lId (44 bolts), 25 loaded (Surry):; 0 Bare-fuel, BWR CASTOR V/21 GNSI (Generall GNSI (Genera Ductile cast Iron Primary lid {44 bolts), 25 loaded (Surry); 0 Nuclear Systems, storage-only secondary lid (48 bolts) purchased secondary lid (48 bolts) Nuclear Systems, storage-only purchased inc.) Inc.) CASTOR X/33 Bare-fuel, PWR Ductile cast Iron Primary lid (44 bolts), Bare-fuel, PWR Primary lid (44 bolts), CASTOR X/33 GNS (Gesellschaftt GNS (Gesellschaf Ductile cast iron 1 loaded (Suny);; 0 1 loaded (Surry) 0 fur Nuklear-Service storage-only secondary lid (70 cup purchased storage-only secondary lid (70 cup für Nuklear-Service purchased mbH) screws) screws) mbH) DRY CASK STORAGE AND COMPARATIVE RISKS NAC S/T Bare-fuel, PWR Inner and outer Closure lid (24 bolts) 2 loaded (Surry); 0 Closure lid (24 bolts) Bare-fuel, PWR NAC S/T NAC International NAC International Inner and outer 2 loaded (Surry); 0 storage-only purchased storage-only purchased stainless steell stainless stee shells shells MC-10 We sling house Bare-fuel, PWR Stainless and 1 loaded (Surry); 0 PWR Westinghouse Stainless and One shield ltd and two One shield lid and two 1 loaded (Surry); 0 Bare-fuel, MC-10 storage-only purchased sealing lids, alll bolted sealing lids, al bolted storage-only purchased carbon steell carbon stee (number off bolts not (number o bolts not available) available) TN-32, TN-40 Transnuclear Inc. Bare-fuel, PWR Carbon steel! O ne lid (48 bolts) 61 loaded (4 sites); 22 PWR One lid (48 bolts) 61 loaded (4 sites); 22 Transnuclear Inc. Carbon steel TN-32, TN-40 Bare-fuel, storage-only purchased storage-only purchased TiN-68 Transnuclear Inc. Bare-fuel, BWR One lld (48 bolts) 24 loaded (Peach Bare-fuel, BWR One lid (48 bolts) 24 loaded (Peach Transnuclear Inc. Carbon steell Carbon stee TN-68 dual-purpose Bottom); 20 purchased Bottom): 20 purchased dual-purpose Canister- PWR, Reinforced Canister lid, welded 7 loaded (Big Rock PWR, 7 loaded (Big Rock BNFL Fuell BNFL Fue Reinforced Canister lid, welded Canister- Fuell Solution Fue Solution W-150 Solutions based, dual- BWR concrete with Inner cask lid (12 tolls) Point); 0 purchased BWR concrete with inner Point); 0 purchased Solutions based, dual- W-150 cask lid (12 bolts) Storage Cask purpose steall shelll stee shel purpose Storage Cask HI-STORM Canister- PWR, 58 loaded (7 sites); 177 Canister- PWR, Stainless steell Stainless stee Canister Ilid, welded Canister d, welded Hoitec Internationall Hollec Internationa 58 loaded (7 sites); 177 HI-STORM 100 based, BWR cask lid (4 bolls) on order based, shells with un- shells with un- BWR on order 100 cask ltd (4 bolts) Storage-only reinforced storage-only reinforced module concrete filler module concrete filler HI-STAR 100 Canister- PWR, Canister ilid,, welded Canister d welded Canister- PWR, Carbon steell Carbon stee 7 loaded (2 sites1};; 5 on 7 loaded (2 sites1) 5 on Holtec Internationall Holtec Internationa HI-STAR 100 order based, dual- BWR shells with neutron shells with neutron based, ual- BWR order Cask lid (54 bolls)tolls) cask lid (54 purpose absorber polymer absorber polymer purpose 72

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as the authoritative version for attribution. Canister- PWR Reinforced Canister lid, welded SB loaded [3 sites); 4 VSC-24 BNFL Fuel 2 PWR Reinforced Canister (3 sites); 4 VSC-24 VentilatedBNFLFuelSolutions Canister- based, concrete with inner lid, welded bolts) 58 loadedpurchased cask lid (6 purchased based, storage-only concrete with inner l cask Ild (6 bolte) steel shel VentilatedConcretSolutions e Cask Concrete Cask storage-only steel shell Canister tid, welded 21 loaded (Yankee Rowe Metal canister Canister- PWR NAC-MPC NAG International NAC-MPC NAG International PWR Metal canister 21 loaded (Yankee Rowe cask lid o Canister- based, dual- surrounded Canister lld, weldedver a shield plug and CT Yankee); 59 by based, dual- surrounded by e overpacklid over high-strength bolts) CT Yankee); 59 storag cask . (6 a shield plug and purchased purpose purpose (6 high-strength bolts) purchased storage overpack. overpack Storage Storage overpack of an consists consists of an r steel liner 3.5 inne liner 3.5 inner steelin. thick, two rebar in, thick, fora rebar nd cages, a cages, and concrete concrete DRY CASK STORAGE AND COMPARATIVE RISKS Canister lid, welded 80 loaded (2 sites); 165 PWR, Metal canister Canister- NAC-UMS NAC International NAC-UMS MAC International Metal canister (2 sites); BWR surrounded Canister lid, weldedver a shielO poadedpurchased165 by cask lid o Bd l lug Canister- based,PWR, dual- based, dual- (6 a shield plug purchased surrounded by e overpacklid overhigh-strength bolts) storag cask . purposBWR e purpose (S high-strength bolts) storage overpack, overpack Storage Storage overpack of inner consists consists of Inneriner 2.5 in. steel l 2.5 in. steel liner thick, two rebar rebar thick, two cages, and cages, and concrete concrete Canister lid, welded 34 loaded (Trojan); 0 Metal canister PWR, Holtec MPC Holtec International Canister HoltecMPC Metal canister (Trojan); 0' surrounded Canister lid, welded hield plug plus by cask lid, s 34 loadedpurchased dual- BWR 24E/EF Holtac International Canister based,PWR, 24E/EF based, dual- B cask id, hield plug purchased! surrounded by e overpackl. s48 bolts plus storag purposeWR purpose 48 belts storage overpack. overpack Storage Storage overpack of Inner consists consists of Inner r steel and oute steel and outer liners, a double- liners, a double-cage, and rebar rebar cage, and e concret concrete 239 loaded (10 sites); Canister lid, welded PWR, Horizontal NUHOMS Transnuclear Inc. Canister- Transnuclear Inc. storage m >150 purchase Horizontalreinforced Canister lid, welded odule lid,239 loaded (10 sites); d Canister- based,PWR, dual- BWR NUHOMS24P, 5 28 , based, dual- B > storage reinforce reinforcedconcrete storage module lld,d concrete 150 purchased purposeWR 24P.S2B,61BT, 2 4PT1 , purpose reinforced concrete concrete storage with module 61BT, 2 4PT1, , 32PT 24PT2 24PT2 32PT module with shielded canister shielded canister 73

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as the authoritative version for attribution. 2 1The NOTES: DRY CASK STORAGE AND COMPARATIVE RISKS SOURCES: Data compiled from cask license holders (2004). Some licensees have purchased additional casks that have not yet been loaded, nor are they planned for loading. Humboldt Bay Power Plant is licensing a site-specific variation of the HI-STAR System called HI-STAR HE. 74