Patent Application
20140241486
The following relates to the nuclear reactor arts, nuclear power generation arts, nuclear reactor fuel arts, and related arts.
In nuclear reactor designs of the integral pressurized water (integral PWR) type, a nuclear reactor core is immersed in primary coolant water at or near the bottom of a pressure vessel. In a typical design, the primary coolant is maintained in a subcooled liquid phase in a cylindrical pressure vessel that is mounted generally upright (that is, with its cylinder axis oriented vertically). A hollow cylindrical central riser is disposed concentrically inside the pressure vessel. Primary coolant flows upward through the reactor core where it is heated and rises through the central riser, discharges from the top of the central riser, and reverses direction to flow downward back toward the reactor core through a downcomer annulus defined between the pressure vessel and the central riser. In the integral PWR design, at least one steam generator is located inside the pressure vessel, typically in the downcomer annulus. Some illustrative integral PWR designs are described in Thome et al., “Integral Helical-Coil Pressurized Water Nuclear Reactor”, U.S. Pub. No. 2010/0316181 A1 published Dec. 16, 2010 which is incorporated by reference in its entirety and in Malloy et al., “Compact Nuclear Reactor”, U.S. Pub. No. 2012/0076254 A1 published Mar. 29, 2012 which is incorporated by reference in its entirety. Other light water nuclear reactor designs such as PWR designs with external steam generators, boiling water reactors (BWRs) or so forth, vary the arrangement of the steam generator and other components, but usually locate the radioactive core at or near the bottom of a cylindrical pressure vessel in order to reduce the likelihood of air exposure of the reactor core in a loss of coolant accident (LOCA).
The nuclear reactor core is typically constructed as an array of fuel assemblies in which each fuel assembly is vertically coextensive with the height of the reactor core and the array of fuel assemblies spans the lateral dimensions of the reactor core. Each fuel assembly comprises a bundle of vertically oriented fuel rods held together by a structural skeleton comprising a set of horizontal spacer grids that are spaced apart along the vertical direction and attached to guide tubes that are interspersed amongst the fuel rods. The guide tubes serve as conduits for control rods and/or in-core instrumentation. Typically, the guide tubes are welded to the grid assemblies. Upper and lower end fittings are installed at the top and bottom of the fuel assembly, and connected to the respective upper and lower ends of the guide tubes, typically by threaded fasteners or the like. The lower end fitting serves as the fluid inlet for flow of primary coolant into the fuel assembly, and the upper end fitting serves as the fluid outlet. The end fittings include flow passages which facilitate primary coolant flow. The end fittings are sometimes referred to as nozzles.
In accordance with one aspect, a fuel assembly includes: a plurality of fuel rods comprising fissile material and arranged mutually in parallel; a plurality of guide tubes arranged in parallel with and interspersed amongst the fuel rods; an upper end fitting connected with upper ends of the guide tubes; and a lower end fitting connected with lower ends of the guide tubes. The top ends of the fuel rods extend into the upper end fitting and/or the bottom ends of the fuel rods extend into the lower end fitting.
In accordance with another aspect, a pressurized water reactor (PWR) is disclosed, including a plurality of fuel assemblies as set forth in the immediately preceding paragraph assembled as a nuclear reactor core of the PWR.
In accordance with another aspect, a fuel assembly includes: a bundle of fuel rods comprising fissile material; a plurality of guide tubes interspersed amongst the fuel rods; a set of spacer grids connected with the guide tubes and holding the fuel rods of the bundle of fuel rods in a spaced-apart arrangement; an upper end fitting connected with upper ends of the guide tubes; and a lower end fitting connected with lower ends of the guide tubes. The fuel rods are longer than the distance between the lower surface of the upper end fitting and the upper surface of the lower end fitting. To accommodate the length of the fuel rods, the fuel rods extend into openings or through-holes of the upper end fitting, and/or the fuel rods extend into openings or through-holes of the lower end fitting.
The fuel rods 12 include tapered bottom ends. This shape reduces the force required to insert the fuel rods 12 through the spacer grids 14, as well as reduces hydraulic drag at the flow channels entrance. The top ends of the fuel rods 12, shown in illustrative
With continuing reference to
It is recognized herein, however, that these gaps DU, DL present unrealized thermal power generation potential. For example, consider fuel rods that emit H watts/unit length thermal power. If the reactor core includes NFA fuel assemblies with Nrods rods per fuel assembly, then the unrealized thermal power generation potential is H×(DL+DU)×NFA×Nrods. In one contemplated small modular reactor (SMR) design (the mPower™ PWR reactor design of Babcock & Wilcox Company, see e.g. http://www.babcock.com/products/modular_nuclear/, last accessed Sep. 6, 2012) includes NFA=69 fuel assemblies each comprising a 17×17 bundle of fuel rods (i.e., Nrods=289 rods). In such a reactor core, the unrealized thermal power generation potential due to the gaps DU, DL is about 20,000(DU+DL)×H. In a percentage formulation, the unrealized thermal power generation potential is of order (DU+DL)/D where D is the fuel rod length. (These are merely estimates, and the actual unrealized thermal power generation potential is influenced by fuel rod axial power shapes). One estimate for the foregoing reactor core design is that obtaining the unrealized thermal power generation potential would add up to about 1.0% to the core thermal power output.
Alternatively, the unrealized thermal power generation potential could be used to allow the reactor core to be run slightly cooler while generating the same thermal output (since the overall thermal output scales approximately with core volume, and the volume would also increase by order (DU+DL)/D since the in-plane dimensions would be unchanged). In this case, the unrealized thermal power generation potential could be used to lengthen the fuel cycle. In the aforementioned example, a four year fuel cycle could potentially be increased by almost two weeks.
Recognizing that these gaps DU, DL present a substantial unrealized thermal power generation potential, it is disclosed herein to increase the fuel rod length by extending the bottom ends of the fuel rods into the lower end fitting, and/or by extending the top ends of the fuel rods into the lower end fitting. However, modification, while favorable from a thermal power output standpoint, introduces additional constraints and consideration requiring new innovations to enable realization of thermal power generation gains. As already noted, the gaps DU, DL accommodate thermal expansion of the fuel rods 12 during reactor operation. Additionally, the gaps DU, DL provide a degree of thermal isolation of the fuel rods 12 from the respective end fittings 18, 20. Further, extending the fuel rods into the upper and/or lower end fittings has the potential to interfere with the end fittings flow openings, which can inhibit their performance as nozzles, i.e. as the fluid inlet and outlet of the fuel assembly.
As disclosed herein, these issues are resolved by providing openings or through-holes in the end fittings to accommodate the extended fuel rod length. In some embodiments, these openings or through-holes are sized such that the fuel rods extend into the end fitting(s), but do not contact the end fitting(s). This provides some thermal isolation between the rod ends and the end fitting, and also facilitates fluid flow through the end fitting. Optionally, the alignment pins or other alignment features can be made partially or totally hollow to provide further fluid flow through the end fitting.
In an alternative approach, the end fittings can serve as end grids of the set of spacer grids. In these embodiments, the alternative end fittings hold the ends of the fuel rods, enabling the omission of conventional end grids. An additional benefit of the alternative approach is a decreased pressure drop across the end fittings in comparison to convention end fittings.
It is also recognized herein that by extending the ends of the fuel rods into the upper and/or lower end fittings, the increase in rod length is actually greater than the gap DU and/or gap DL. For example, extending the bottom ends of the fuel rods into the lower end fitting provides an increase in rod length of DL plus the further extension of the bottom of the fuel rod into the lower end fitting.
With reference to
By extending the fuel rods 112 into the upper end fitting 118 and/or the lower end fitting 120, the length of the fuel rods can be increased. In some embodiments, the extra length may be about 1″ for the upper cladding and about ¾″ for the lower cladding. The additional length(s) allows more fissile material (e.g. pellets) to be loaded into the fuel rods, such that the length of the fuel stack within the fuel rod is increased. In one arrangement, the lengthened fuel rod may include end caps, wherein at least a portion of the end caps do not contain a fissile material and the non fissile portion of end caps extends into the end grid. In another arrangement the end caps extend up to but not into the end grids. Advantageously, plenum volume may also be increased.
In general, the lengths of the fuel rods can be greater than the separation between the bottom surface of the upper end fitting 118 and the upper surface of the lower end fitting 120. This can be achieved by extending the bottom ends of the fuel rods 112 into the lower end fitting 120 by an amount greater than the gap DU at the upper ends (as shown in
In some embodiments, the ends of the fuel rods extend into openings or through-holes of the upper and/or lower end fitting. These openings or through-holes may be made larger than the diameter of the fuel rods, so that although the fuel rods extend into the end fitting they do not contact the end fitting. Such gaps provide thermal isolation of the ends of the fuel rods from the end fitting while maintaining a primary coolant flow channel, having a desired hydraulic pressure drop, through the end fitting.
Alternatively, the ends of the fuel rods extending into the end fitting can contact the end fitting (e.g. via grid springs), and the end fitting can serve as an end grid of the set of spacer grids. Conventionally (e.g., as in
On the other hand, when the bottom ends of the fuel rods 112 extend into the lower end fitting 120 (as in
Extending the fuel rods into the end fitting can potentially increase the pressure drop over the end grid. This pressure drop can be reduced by employing through-holes that receive the ends of the fuel rods, with the through-holes having larger diameter than the fuel rods. In this way, an annular gap is present between the fuel rod and the through-hole, and these annular gaps can serve as flow holes.
In embodiments in which the end fitting serves as an end grid and contacts the ends of the fuel rods, the increased pressure drop across the end fitting may be compensated by elimination of the proximate end grid. The use of retention features analogous to the springs and dimples of a conventional spacer grid to hold the fuel rod ends can also provide gaps between the fuel rod and the opening or through-hole that can serve as flow paths.
The optional elimination of one or both of the end grids (by employing one or both end fittings as end grids) also reduces the force necessary to insert the fuel rods 112 through the spacer grids 114. The force reduction simplifies simultaneous and/or automated insertion of all of the fuel rods in automated (e.g. robotic) or semi-automated fashion leading to increased productivity, faster loading, and better accuracy (e.g. less deviation for controlling insertion parameters). The automated insertion process may also result in a more uniform as-built fuel assembly. The spacer grids 114 may be preheated (e.g. by local inductive heating) prior to insertion of the fuel rods 112.
As another approach for reducing the pressure drop over the end fitting, the upper alignment pins 119 and/or the lower alignment pins 122 may include a hollow passageway extending at least partially therethrough. The hollow passageway allows “smoother” hydraulic pressure drops localized around the alignment pin.
With reference to
With reference to
Embodiments in which the ends of the fuel rods extend into through-holes have the advantage that (assuming the through-hole is of larger diameter than the fuel rod) the annular gap between the fuel rod and the through-hole can serve as a fluid flow path. Additionally, thermal expansion of the fuel rod is accommodated by a through-hole so long as the end fitting is of sufficient thickness. On the other hand, a through-hole does nothing to prevent rod ejection, which is a credible accident scenario in some reactor designs. Conversely, embodiments in which the ends of the fuel rods extend into openings which are plugged provide protection against rod ejection, but do not provide a fluidic flow path. In a contemplated “hybrid” approach, the ends of the fuel rods extend into through-holes which are however constricted at the end opposite to the end at which the fuel rod enters (that is, constricted at the upper end of a through-hole through an upper end fitting, or constricted at the lower end of a through-hole through a lower end fitting). By making the constriction of smaller diameter than the fuel rod diameter, the constriction prevents rod ejection while still permitting some fluid flow.
The preferred embodiments have been illustrated and described. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.
