Patent Description:
At commercial nuclear power plants, spent nuclear fuel has been stored in deep reservoirs of water, often called spent fuel pools, within the nuclear power plant. When these spent fuel pools reach their spent fuel capacity limits, or when the nuclear power plant undergoes a complete removal of spent fuel from the spent fuel pool at the end of the life of the facility, the fuel is transferred into metal canisters having final closure lids that are welded closed or sealed with mechanical means at the power plants following the spent fuel or radioactive waste loading. The sealed canister is then placed into a ventilated storage overpack (typically consisting of layers of steel and concrete) which serves as an enclosure that provides mechanical protection, passive heat removal features, and additional radiation shielding for the inner metal canister that contains the radioactive material. The ventilated storage overpack, containing the welded or bolted metal canister within which the radioactive materials are stored, is then placed in the designated secure location outside of the nuclear power plant structure yet on owner controlled property so as to ensure proper controls and monitoring are performed in connection with the ventilated storage overpack containing the metal canister.

These ventilated storage overpacks must meet only the regulatory requirements for storage and not the regulations associated with off-site transportation of the metal canisters. Regulations associated with off-site transportation require the use of a specially designed off-site transportation cask, which is quite different in design and materials from the ventilated storage overpack and licensed for use by the regulatory authorities under different rules and regulations than those used to authorize ventilated storage overpacks.

The ventilated storage overpack is designed to: (<NUM>) limit ionizing radiation; (<NUM>) provide suitable structural protection of the metal canister from external threats; and (<NUM>) provide passive heat removal from the contents stored within the metal canister that is stored within the ventilated storage overpack. To satisfy these basic functional attributes, the ventilated storage overpack has typically been constructed from a combination of steel and concrete, which has required that it have a large diameter. This large diameter presents an issue for the users that have areas that are limited in physical size available for deployment of these types of large diameter containers during both operating and decommissioning status.

As an alternative to the concrete and metal ventilated storage overpack previously described, commercial nuclear power plants may choose to utilize a metal based storage system which is also designed to: (<NUM>) limit ionizing radiation; (<NUM>) provide suitable structural protection of the metal canister from external threats; and (<NUM>) provide passive heat removal from the contents stored within the metal canister that is stored within the metal storage overpack. These dual purpose metal storage overpacks are also used to transport the contents after some period of interim storage and therefore are smaller in diameter. Due to the design of the metal storage overpack, it is not ventilated and therefore is considerably restricted in its ability to passively reject heat from the contents stored within it. Based on this very nature, the fuel contents selected for loading of these systems is limited to lower heat loads when compared to the higher heat load storage capacity afforded by the ventilated storage overpack design.

A storage apparatus for storage of radioactive nuclear waste is known from patent application <CIT>.

Another storage apparatus for storage of radioactive nuclear waste is known from patent application <CIT>.

A flask for the transport and storage of irradiated nuclear fuel is known from patent application <CIT>. The flask comprises a hollow cylindrical container having an axis, cooling fins on the exterior of the container, said cooling fins being axially spaced apart along said axis and extending about the circumference of the container, an outer jacket of neutron shielding material substantially surrounding the fins, and spacers carried by the jacket in embracing contact with but unconnected to the exterior of the container and passing between axially adjacent cooling fins to support the jacket on the container and space the jacket radially from the extremities of the fins to provide a space for through-flow of air about the container and fins for cooling. The flask can be provided with means to support a sleeve which fits over the jacket of neutron absorbing material when the flask is to be immersed in a storage pond.

Neutron-absorbing materials are known from patent <CIT>.

The present invention provides a storage apparatus according to claim <NUM>.

The present disclosure provides various embodiments of a ventilated metal storage overpack (VMSO) designed to minimize (<NUM>) the area required to store a canister having radioactive nuclear waste and (<NUM>) radiation emitted to personnel from the contents stored within, while maximizing the passive heat removal capability of the storage system without reducing the protection of the stored contents from external threats.

One embodiment, among others, is a storage apparatus that comprises a sealed canister containing the radioactive nuclear waste and an outer ventilated metal storage overpack (VMSO). The VMSO has a plurality of vents to enable ambient air flow through the VMSO and around the canister to thereby dissipate heat from the canister. The VMSO has a side wall having an inner metal layer and sets of alternating layers. Each set includes a neutron absorbing layer adjacent to another metal layer so that neutron absorbing and metal layers alternate throughout the side wall. The neutron absorbing layer or layers are designed to absorb neutron radiation radiated from the radioactive nuclear waste and the metal layers are designed to absorb gamma radiation radiated from the radioactive nuclear waste as well as radiated from the neutron absorbing layer or layers that result from absorption of neutron radiation.

The metal layers are carbon steel and the neutron absorbing layer or layers are a polymer material, cementitious material, or combination thereof. Furthermore, in any of the embodiments, the steel layers can be different steel materials, and the neutron absorbing layers can be different neutron absorbing materials.

Another embodiment, among others, is a storage apparatus that comprises a sealed canister containing the radioactive nuclear waste and a VMSO. The WMSO has a plurality of vents to enable ambient air flow through the VMSO and around the canister to thereby dissipate heat from the canister. The VMSO has a side wall with five layers, including a first layer (innermost), a second layer adjacent to the first layer, a third layer adjacent to the second layer, a fourth layer adjacent to the third layer, and a fifth layer (outermost) adjacent to the fourth layer. In this embodiment, the first, third, and fifth layers are made of a metal material and the second and fourth layers are made of a neutron inhibiting material. The neutron absorbing layers are designed to absorb neutron radiation radiated from the radioactive nuclear waste and the metal layers are designed to absorb gamma radiation radiated from the radioactive nuclear waste as well as radiated from the neutron absorbing layer or layers that result from absorption of neutron radiation.

Other embodiments, apparatus, features, and advantages of the present invention will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description.

The ventilated metal storage overpack (VMSO) utilizes a combination of dense neutron radiation absorbing materials layered within steel shells such that the overall diameter of the VMSO is minimized in comparison to the metal-concrete storage overpacks of the prior art, while serving to at least: (<NUM>) provide personnel radiological protection from the contents stored within the metal canister; (<NUM>) protect the radioactive contents stored within the metal canister from external events; (<NUM>) maximize the ability to reject heat from the contents stored within the metal canister while (<NUM>) minimize the physical area required for each storage system. By alternating the use of dense neutron absorbing material together with the physical protection of the steel used in the VMSO, the personnel protection from the radiation being emitted can be maximized, the overall diameter of the system can be minimized, and the heat rejection capability of the system can be maximized without reducing the protection capability of the system from external effects.

The dense neutron attenuating material used within the VMSO (may be metallic, polymer, or cementitious in form coupled with any specified neutron absorbing type material) as selected by the designed based on the specific needs of the application which include the physical space availability (i.e., the maximum diameter of the system and number of systems needed) and the radiation levels on the exterior of the VMSO. The design may include three or more alternating layers of steel and dense neutron absorbing materials to form the VMSO. Further, the density of the neutron absorbing materials may be varied to maximize the effect of the materials when analyzed and constructed within two or more alternating layers of steel so as to reduce any gamma radiation that may be emitted from materials as a result of the neutron attenuation. Because of the strategic placement of the dense neutron absorbing materials within alternating layers of steel, the design of the VMSO can be enhanced specifically to diminish the amount of radiation being emitted from the VMSO, while minimizing the overall diameter of the VMSO thereby optimizing the system design which enhances the VMSO in comparison to the standard ventilated metal and concrete storage overpack and more closely resembles a metal storage overpack from a diametrical comparison.

By ventilating the VMSO, the heat rejection capability of the VMSO closely resembles the heat rejection capability of the typical ventilated metal and concrete storage overpack without the increased diameter associated with the typical ventilated metal and concrete storage overpacks of the prior art.

Furthermore, by strategic design and placement of the dense neutron absorbing material, the neutron and gamma radiation emitted from the VMSO can be minimized using the specific energy levels of the neutron and gamma radiation levels being emitted from the contents within the VMSO. The neutron absorbing material is a polymer doped with Boron or a cementitious material doped with Boron.

Referring now to the figures, <FIG> is a top view of a preferred embodiment of the VMSO, denoted by reference numeral <NUM>, and <FIG> is a partial cross sectional view of the VMSO <NUM>, taken along cross section line A-A of <FIG>. The VMSO <NUM> has a sealed elongated cylindrical canister <NUM> containing the hazardous nuclear material, for example but not limited to, spent nuclear fuel rods, etc., and a elongated cylindrical VMSO <NUM> containing the canister <NUM>.

The canister <NUM> has a mounted removable circular top lid <NUM>, a circular flat bottom <NUM>, and an elongated cylindrical side wall <NUM> extending between the lid <NUM> and the flat bottom <NUM>. The canister <NUM> is shown, as an example, with tubes and disks, but other types of canisters <NUM> may be utilized. Generally, the canister <NUM> can implement any conductive or convective heat transfer scheme and is made from stainless steel parts. Other non-limiting examples of suitable canisters are described in <CIT> and <CIT>.

The VMSO <NUM> has a cylindrical longitudinal body <NUM> extending between a mounted removable circular top lid <NUM> and a circular flat bottom <NUM>. As an example, the top lid <NUM> is shown bolted to the body <NUM> via a plurality of bolts <NUM>. The top lid <NUM> could also be welded to the body <NUM> or otherwise attached.

The top of the longitudinal body <NUM> also has a plurality of bolted lift lugs <NUM> that enable the VMSO <NUM> to be moved with, for example, a conventional crane. As an alternative embodiment, the longitudinal body <NUM> could be equipped with a plurality of trunnions.

The bottom <NUM> is welded to, bolted to, or otherwise attached to the longitudinal body <NUM> of the VMSO <NUM>.

The longitudinal body <NUM> has at least three layers <NUM>: an inside layer, at least one middle layer adjacent to the inside layer, and an outside layer adjacent to the at least one middle layer, with the inside and outside layers being metal, preferably but not limited to carbon steel, and the at least one middle layer comprising a neutron inhibiting material. In this embodiment, neutron radiation pass through the first layer of carbon steel and are sufficiently attenuated and/or captured by the single layer of neutron absorbing material. Moreover, gamma radiation from the canister <NUM> are absorbed and attenuated by the multiple layers of carbon steel, and any additional gamma radiation spawned by absorption by neutron radiation in the neutron absorbing layer are sufficiently attenuated and/or captured in the outer carbon steel layer.

In the preferred embodiment, as shown in <FIG>, the layers <NUM> (or shells) include a first layer 32a, a second layer 32b adjacent to the first layer 32a, a third layer 32c adjacent to the second layer 32b, a fourth layer 32d adjacent to the third layer 32c, and a fifth layer 32e adjacent to the fourth layer 32d. Moreover, the first, third, and fifth layers 32a, 32c, and 32e are made of carbon steel, and the second and fourth layers 32b and 32d are made of a neutron inhibiting material, i. a polymer and/or a cementitious material.

In this preferred embodiment, the three carbon steel layers and two neutron absorbing layers effectively and efficiently assist with attenuation of the neutron and gamma radiation that escape from the canister <NUM>. More specifically, neutron radiation may be at different energy levels. The neutron radiation will pass through the steel layers. Moreover, some will be slowed down but will pass through the first neutron absorbing layer, but will be captured by the second neutron absorbing layer. As the neutron radiation are absorbed, additional gamma radiation may be spawned and emitted, but they are attenuated and absorbed by the multiple carbon steel layers.

The VMSO <NUM> is designed with a plurality of screened vents to enable ambient air flow through the VMSO <NUM> from the bottom end to the top end. For example, the VMSO <NUM> is shown with air inlets <NUM> in the bottom <NUM> at the bottom end and air outlets <NUM> in the top lid <NUM> at the top end so that ambient air enters at or near the bottom end, passes through the VMSO <NUM> along the outside of the canister <NUM> to thereby dissipate canister heat, and then out of the VMSO <NUM> at or near the top end. The vents also enable drainage and evaporation of water to keep the interior of the VMSO <NUM> sufficiently dry.

Claim 1:
A storage apparatus (<NUM>) for dry storage of radioactive nuclear waste, the storage apparatus (<NUM>) comprising:
an elongated cylindrical sealed canister (<NUM>) containing the radioactive nuclear waste;
an elongated cylindrical ventilated metal storage overpack (VMSO) (<NUM>) containing the canister (<NUM>), the VMSO (<NUM>) having a longitudinal body (<NUM>) extending between a top at a top end and a bottom at a bottom end, the longitudinal body (<NUM>) having a sidewall (<NUM>) having an innermost first layer being metal, a second layer made of a neutron absorbing material and a third layer; wherein
the VMSO (<NUM>) comprises a plurality of screened vents to enable ambient air flow through the VMSO (<NUM>) from the bottom of the bottom end to the top end to thereby dissipate heat from the canister (<NUM>); and
the neutron absorbing material is designed to absorb neutron radiation radiated from the radioactive nuclear waste,
characterized in that
the sidewall (<NUM>) has five layers (32a to 32e), including the innermost first layer (32a), the second layer (32b) adjacent to the first layer (32a), the third layer (32c) adjacent to the second layer (32b), a fourth layer (32d) adjacent to the third layer (32c), and an outermost fifth layer (32e) adjacent to the fourth layer (32d);
the second and fourth layers (32b, 32d) are made of the neutron absorbing material, the neutron absorbing material being a polymer doped with Boron and/or a cementitious material doped with Boron;
the first, third, and fifth layers (32a, 32c, 32e) are all metal layers made of carbon steel, and that the metal layers are designed to absorb gamma radiation radiated from the radioactive nuclear waste as well as radiated from the neutron absorbing material resulting from reactions associated with absorption of neutron radiation.