Patent Application
20060222139
The present invention relates to preventing nuclear criticality, and more particularly, the embodiments relate to methods for assuring that requirements are met to comply with the double contingency principle for preventing nuclear criticality through demonstrating that moderator materials will be excluded under normal and accident conditions.
Dry nuclear spent fuel transport technology is deployed throughout the world to assure the capabilities of nuclear power plants to discharge and ship spent fuel away from the plant, thereby preventing loss of spent fuel storage capacity and preserving the planned operating lives of the power plants. Additionally, other fissile materials and waste forms are also transported worldwide to support and sustain many varied forms of commerce associated with fissionable heavy nuclides having atomic numbers greater than 230. The transport of fissile materials, such as spent nuclear fuel, and the packaging systems which are used to accomplish such transport must comply with rigorous regulatory requirements for design and operation to preclude release of radioactive materials to the environment and to prevent inadvertent nuclear criticality from occurring within the packaging as the result of closure (seal) failures, which could allow the ingress of neutron moderation material, such as water, to enhance the potential for the production of thermalized neutrons and the occurrence of nuclear criticality.
For dry spent nuclear fuel and other fissile material transport, two fundamental classes of technology are used: (i) metal casks with final closure lids (or lid) that are bolted closed at power plants or other facilities after loading of the material, such as spent fuel, into open compartments of a separate structure nested within the cask body (termed the “basket”); this technology when used for spent fuel shipment is termed “bare fuel” transport; and (ii) similar metal casks with bolted final closure lids (or lid) having a metal canister within the cask body which contains the basket structure and which also has a canister final closure lid (or lids) that is welded or bolted closed at the power plants or other facility following spent fuel loading; this technology when used for spent fuel shipment is called “canistered fuel” transport. This canistered material transport technology offers the capability to assure that the normal and hypothetical accident conditions imposed as part of the licensing requirements under the Code of Federal Regulations (CFR) cannot result in credible failures that would allow ingress of neutron moderator materials or egress of radioactive materials, and this technology has applicability to the transport of all types of fissile materials and forms of nuclear waste.
Canistered material transport technology and designs should comply with federal regulatory requirements, among which are requirements pertaining to containment, sealing, inspection, and evaluation or testing. In particular, the Code of Federal Regulations (CFR) provides that under both normal and hypothetical accident conditions (see 10 CFR 71.73), the transport system design should assure that nuclear criticality cannot occur, even if parts of the system are flooded and surrounded by water to the most reactive and reflective credible extent and the spent fuel is in its most reactive credible configuration consistent with the form of the material (see 10 CFR 71.55(b), (d) and (e)). However, the CFR also permits alternative transport system designs to be approved and used if the system incorporates special design features to ensure that the most reactive moderation is not credible, or that “no single packaging error” would permit water (moderator) in-leakage and if appropriate measures are taken before each shipment to ensure that the containment boundary (which contains the spent fuel or fissile material contents) portion of the transport system does not leak (see10 CFR 71.55(c)).
These alternative approaches within the CFR apply a long-standing, time-tested protocol within the discipline of nuclear criticality safety design and control systems. This protocol is called the “double contingency principle”, and it is defined in the CFR to mean that “. . . designs should incorporate sufficient factors of safety to require two unlikely, independent, and concurrent changes in process conditions before a criticality accident is possible.” (see 10 CFR 70.4). Thus, following an initiating event of the CFR-prescribed hypothetical accident conditions (see 10 CFR 71.73), the CFR allows for approval of transport system designs that can reasonably assure no single error or failure would permit water ingress and that have operational provisions prior to shipment that reasonably assure the containment system does not leak.
Thus, a heretofore unaddressed need exists in the industry for apparatus and methods to satisfy the aforementioned codes, standards, and regulatory requirements.
Disclosed are methods for assuring that requirements are met for fissile material transportation to comply with the double contingency principle by excluding moderator in-leakage. In one embodiment, among others, a method includes designing and fabricating a canister to achieve an acceptable application, and verifying pre-shipment operation to assure that a transport system is properly assembled to meet its licensed transport configuration.
Other methods, 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 methods, features, and advantages included within this description, be within the scope of the present invention, and be protected by the accompanying claims.
The disclosed methods can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale.
FIGS. 4A-B are flow diagrams that show details of one embodiment of the step of designing the canister shown in
Under the Code of Federal Regulations (CFR) requirements for the transport of spent fuel and other fissile materials, the metal transport cask provides the containment boundary for the approved contents. Therefore, the metal cask body and its closure system provide one level of fissile material containment that meets the CFR. Disclosed herein is a method to assure that a canister system, enclosed within a metal cask complying with the CFR requirements, provides a total system with the necessary and sufficient design features and other conditions for a second level of fissile material containment, such that the double contingency principle can be achieved as stipulated within the CFR to assure exclusion of neutron moderator following the application of both normal and hypothetical accident conditions to the transport system.
Exemplary canisters and metal transport casks are first discussed with reference to the figures. Although the exemplary canisters and metal transport casks are described in detail, they are provided for purposes of illustration only and various modifications are feasible. After the exemplary canisters and metal transport casks have been described, examples of application of the method are explained to assure that requirements are met for the exclusion of moderator in-leakage.
Referring now in more detail to the figures in which like reference numerals identify corresponding parts,
The canister 2 includes a canister shell 10. In an alternative embodiment, the closure lid 6 can include a port opening 12 that is adapted to receive a port cover 14 and a drain or vent member (not shown). In an alternative embodiment, the canister I can include a basket assembly 4 that is preferably disposed in the canister shell 10 before the canister is sealed. In an alternative embodiment, the canister 2 can further include a seal ring (not shown) that is disposed between the canister shell 10 and the closure lid 2, at the inner circumference of the canister shell 10 and at the outer circumference of the closure lid 6. The seal ring can be a single piece or multiple pieces.
The single closure lid design of the canister 2 can comply with the requirements as stipulated within the CFR to assure exclusion of neutron moderator.
Other canister designs, besides the single closure lid design, can also comply with the CFR requirements if the canister design and the canister fabrication with closure installation achieve acceptable application, as disclosed in
Alternatively, the inner lid 28 may be attached to the canister. The inner lid 28 is attached to the transport cask 16 or to the canister by way of, for example, welding, bolting, or any other attaching means. The outer lid 30 is placed on top of the cask body 26 and engages seating surface 34 of the transport 16. The outer circumference of the outer lid 30 substantially aligns with the outside wall of the cask body 26. As may be the case with the inner lid 28, the outer lid 30 may be attached to the transport cask 16 by way of, for example, welding, bolting, or any other attaching means.
The tie-downs 20 mechanically secure the transport cask 16 to the shipping cradle 22, which transports the canister 2 from a power plant (not shown) to a remote facility (not shown). A verifying pre-shipment operation is described below in
The flow charts described hereafter show the architecture, functionality, and application of a method for assuring that requirements are met to comply with the CFR and achieve moderator exclusion to prevent nuclear criticality. It should also be noted that in some alternative implementations, the functions noted in the blocks may occur out of order. For example, two blocks shown in succession in
FIGS. 5A-B are flow diagrams that show details of one embodiment of the step of designing the canister 2 shown in
In block 37, the canister closure system is designed and fabricated in accordance with approved codes, standards, and regulatory requirements. If the closure is a welded lid system, then the lid structure must be designed for the weld depth and type requirements of the closure design, as well as those of applicable regulations, codes and standards. In block 39, the canister closure system is designed so as to preclude any inelastic deformation following the application of CFR hypothetical accident conditions. In block 41, the canister and its closure lid system is designed to be structurally independent of the transport cask 16, in that the canister system 2 is not part of the metal cask body and there is no structural connection between the canister 2 and transport cask 16. The canister 2 may rely only upon pure contact with the transport cask body.
Referring now to block 43 in
In block 46, if the canister closure lid system is a welded design, the critical flaw size for the closure weld is calculated in accordance with approved methods and requirements. In block 47, if the canister closure lid system is a welded design, the closure weld inspection methods for use at the power plant or other facility are designed such that the minimum detectable weld flaw size can be demonstrated to be less than the critical weld flaw size. In block 48, the canister closure system is designed to be a watertight boundary following the application of the CFR hypothetical accident conditions, which can be shown by approved analysis methodologies and/or physical testing.
In block 73, the installation of the transport cask lid system using bolted or other mechanical closure devices is performed in accordance with proper and approved procedures. In block 75, an additional review and approval of the completed process and its documentation and records become the verification of the transport cask configuration, and are performed to assure the cask lid is properly installed. This provides verification that a second level of independent containment is in place. In block 77, a cask transport accident impact protection system is installed in accordance with proper and approved procedures, and in block 79, an additional review and approval of the completed process and its documentation and records become the verification of the impact protection system configuration, and are performed to assure the impact protection system is properly installed. This provides verification that the impact protection system, which moderates accident loads on both levels of containment, is properly configured. A reduction of the impact protection system effectiveness could result in a common mode failure that may affect both containment systems, and this is precluded by proper performance of such verfications.
The proper performance of the steps 3, 5, and 7, involving the internal canister design, canister fabrication with closure installation, and pre-shipment operational verifications, constitutes the disclosed method 1 to assure that requirements are met to comply with the CFR to preclude moderator ingress into fissile material transport packagings under normal or following hypothetical accident conditions.
It should be emphasized that the above-described embodiments of the present invention, particularly, any “preferred” embodiments, are merely possible examples of implementations, set forth for a clear understanding of the principles of the invention.
Many variations and modifications may be made to the above-described embodiment(s) of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and protected by the following claims.
