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for release from a dry cask are lower than those from a spent fuel pool because dry casks store older, lower decay-heat fuel.
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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
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
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.
TABLE 4.1 Dry Casks Used for Spent Fuel Storage in the United States
Cask design used for storage |
License holder |
Type |
Fuel type |
Construction |
Closure system |
Number of casks used to date; sites; and number of casks on order1 |
CASTOR V/21 |
GNSI (General Nuclear Systems. Inc.) |
Bare-fuel, storage-only |
BWR |
Ductile cast iron |
Primary lid (44 bolts), secondary lid (48 bolts) |
25 loaded (Surry); 0 purchased |
CASTOR X/33 |
GNS (Gesellschaft für Nuklear-Service mbH) |
Bare-fuel, storage-only |
PWR |
Ductile cast iron |
Primary lid (44 bolts), secondary lid (70 cup screws) |
1 loaded (Surry); 0 purchased |
NAC S/T |
NAC International |
Bare-fuel, storage-only |
PWR |
Inner and outer stainless steel shells |
Closure lid (24 bolts) |
2 loaded (Surry); 0 purchased |
MC-10 |
Westinghouse |
Bare-fuel, storage-only |
PWR |
Stainless and carbon steel |
One shield lid and two sealing lids, all bolted (number of bolts not available) |
1 loaded (Surry); 0 purchased |
TN-32, TN-40 |
Transnuclear Inc. |
Bare-fuel, storage-only |
PWR |
Carbon steel |
One lid (48 bolts) |
61 loaded (4 sites); 22 purchased |
TN-68 |
Transnuclear Inc. |
Bare-fuel, dual-purpose |
BWR |
Carbon steel |
One lid (48 bolts) |
24 loaded (Peach Bottom); 20 purchased |
Fuel Solution W-150 Storage Cask |
BNFL Fuel Solutions |
Canister-based, dual-purpose |
PWR, BWR |
Reinforced concrete with inner steel shell |
Canister lid, welded cask lid (12 bolts) |
7 loaded (Big Rock Point); 0 purchased |
HI-STORM 100 |
Holtec International |
Canister-based, storage-only module |
PWR, BWR |
Stainless steel shells with unreinforced concrete filler |
Canister lid, welded cask lid (4 bolts) |
58 loaded (7 sites); 177 on order |
HI-STAR 100 |
Holtec International |
Canister-based, dual-purpose |
PWR, BWR |
Carbon steel shells with neutron absorber polymer |
Canister lid, welded cask lid (54 bolts) |
7 loaded (2 sites1); 5 on order |
VSC-24 Ventilated Concrete Cask |
BNFL Fuel Solutions |
Canister-based, storage-only |
PWR |
Reinforced concrete with inner steel shell |
Canister lid, welded cask lid (6 bolts) |
58 loaded (3 sites); 4 purchased2 |
NAC-MPC |
NAC International |
Canister-based, dual-purpose |
PWR |
Metal canister surrounded by storage overpack. Storage overpack consists of an inner steel liner 3.5 in. thick, two rebar cages, and concrete |
Canister lid, welded cask lid over a shield plug (6 high-strength bolts) |
21 loaded (Yankee Rowe and CT Yankee); 59 purchased |
NAC-UMS |
NAC International |
Canister-based, dual-purpose |
PWR, BWR |
Metal canister surrounded by storage overpack. Storage overpack consists of inner steel liner 2.5 in. thick, two rebar cages, and concrete |
Canister lid, welded cask lid over a shield plug (6 high-strength bolts) |
80 loaded (2 sites); 165 purchased |
Holtec MPC 24E/EF |
Holtec International |
Canister based, dual-purpose |
PWR, BWR |
Metal canister surrounded by storage overpack. Storage overpack consists of inner and outer steel liners, a double-rebar cage, and concrete |
Canister lid, welded cask lid, shield plug plus 48 bolts |
34 loaded (Trojan); 0 purchased |
NUHOMS 24P, 52B, 61BT, 24PT1, 24PT2, 32PT |
Transnuclear Inc. |
Canister-based, dual-purpose |
PWR, BWR |
Horizontal reinforced concrete storage module with shielded canister |
Canister lid, welded storage module lid, reinforced concrete |
239 loaded (10 sites); >150 purchased |