Patent Application
20160099083
1. Field
The present disclosure relates to neutron shielding in a nuclear reactor.
2. Description of Related Art
Conventionally, fast neutrons are produced by fission reactions in a nuclear reactor. A fast neutron is a free neutron with a kinetic energy level of about 1 MeV or more. Over time, various structures within a nuclear reactor may be degraded by fast neutron irradiation, thereby shortening the life of those components and requiring mitigating action to continue operation of the nuclear power plant. To extend the life of a nuclear power plant, the fast neutrons which cause the most significant irradiation damage may be converted to lower-energy thermal neutrons via a process called thermalization. In a conventional nuclear power plant, water has been used as a neutron moderator to slow down (thermalize) the fast neutrons.
A boiling water reactor core according to an example embodiment may include a core shroud; a plurality of fuel bundles within the core shroud; and a plurality of shielding bundles between the plurality of fuel bundles and the core shroud. The plurality of shielding bundles include a metal hydride as a neutron moderator.
Example embodiments also relate to a method of reducing fast neutron fluence at a core shroud of a boiling water reactor. The method may include surrounding a plurality of fuel bundles within the core shroud with a plurality of shielding bundles. The plurality of shielding bundles include a metal hydride as a neutron moderator.
The various features and advantages of the non-limiting embodiments herein may become more apparent upon review of the detailed description in conjunction with the accompanying drawings. The accompanying drawings are merely provided for illustrative purposes and should not be interpreted to limit the scope of the claims. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. For purposes of clarity, various dimensions of the drawings may have been exaggerated.
It should be understood that when an element or layer is referred to as being “on,” “connected to,” “coupled to,” or “covering” another element or layer, it may be directly on, connected to, coupled to, or covering the other element or layer or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly connected to,” or “directly coupled to” another element or layer, there are no intervening elements or layers present. Like numbers refer to like elements throughout the specification. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.
It should be understood that, although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers, and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer, or section from another region, layer, or section. Thus, a first element, component, region, layer, or section discussed below could be termed a second element, component, region, layer, or section without departing from the teachings of example embodiments.
Spatially relative terms (e.g., “beneath,” “below,” “lower,” “above,” “upper,” and the like) may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. It should be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the term “below” may encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.
The terminology used herein is for the purpose of describing various embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “includes,” “including,” “comprises,” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
Example embodiments are described herein with reference to cross-sectional illustrations that are schematic illustrations of idealized embodiments (and intermediate structures) of example embodiments. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, example embodiments should not be construed as limited to the shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, an implanted region illustrated as a rectangle will, typically, have rounded or curved features and/or a gradient of implant concentration at its edges rather than a binary change from implanted to non-implanted region. Likewise, a buried region formed by implantation may result in some implantation in the region between the buried region and the surface through which the implantation takes place. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to limit the scope of example embodiments.
Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, including those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
Each of the plurality of fuel bundles 106 may have a standard configuration that includes a channel that houses a plurality of fuel rods therein. The plurality of fuel rods within the channel may be secured at their upper ends by an upper tie plate, secured at their lower ends by a lower tie plate, and/or supported in their middle regions by a spacer. When arranged within the core shroud 102, the plurality of fuel bundles 106 may be supported from below by a fuel support piece and secured from above by a top guide.
The plurality of fuel bundles 106 and the plurality of shielding bundles 104 are arranged in an array within the core shroud 102. The array may be a regular arrangement of the structures into orderly rows and columns. As shown in
In an example embodiment, the plurality of shielding bundles 104 are peripheral constituents of the array of bundles within the core shroud 102. For instance, the peripheral constituents may be outermost components of the array that are closest to the core shroud 102. Although
To obtain the configuration of
Alternatively, the plurality of shielding bundles 104 may be in the form of other containers that house the neutron moderator (e.g., metal hydride). Such containers may have a variety of shapes and sizes. The plurality of shielding bundles 104 may include (or be in the form of) a water-proof sheath that surrounds the neutron moderator (e.g., metal hydride). To implement, the plurality of shielding bundles 104 may be inserted to replace the original outermost fuel bundles within the core shroud 102. However, it should be understood that if space permits within the core, the plurality of shielding bundles 104 may be inserted to surround (without displacing or replacing) the plurality of fuel bundles 106 already present within the core shroud 102. The alternative shapes and/or sizes of the shielding containers may also be structured and/or augmented such that the plurality of shielding bundles 104 may be supported from below by a core plate and secured from above by a top guide.
In
As discussed above, a core may be modified in such a way so as to reduce the fast neutron fluence at a core shroud of a boiling water reactor. The method of reducing fast neutron fluence may include surrounding a plurality of fuel bundles within the core shroud with a plurality of shielding bundles. The plurality of shielding bundles include a metal hydride as a neutron moderator. The metal hydride may be zirconium hydride, although example embodiments are not limited thereto. The surrounding step may include inserting the plurality of shielding bundles between the core shroud and the plurality of fuel bundles. In particular, the plurality of fuel bundles may be arranged in an array within the core shroud, and the surrounding step may include replacing the peripheral bundles of the plurality of fuel bundles with the plurality of shielding bundles. The plurality of shielding bundles may be in a variety of forms, shapes, and sizes, provided that it can adequately contain the neutron moderator therein. Alternatively, the surrounding step may include just replacing (partially or completely) the nuclear fuel in the peripheral bundles of the plurality of fuel bundles with the metal hydride so as to convert the peripheral structures into shielding bundles. The plurality of shielding bundles may be positioned as outermost constituents of the array. As a result, the plurality of shielding bundles may be closer to the core shroud than any of the plurality of fuel bundles. In an example embodiment, the plurality of shielding bundles also do not contact the core shroud. In view of the implementation of the plurality of shielding bundles, the core shroud is shielded from fast neutrons originating from the plurality of fuel bundles. Notably, the neutron moderator within the plurality of shielding bundles helps to convert the fast neutrons into lower-energy thermal neutrons, thereby protecting structures such as the core shroud from degradation and premature failure.
While a number of example embodiments have been disclosed herein, it should be understood that other variations may be possible. Such variations are not to be regarded as a departure from the spirit and scope of the present disclosure, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.
