Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Plural fuel segments or elements
Reexamination Certificate
1997-11-21
2004-07-20
Behrend, Harvey E. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Plural fuel segments or elements
C376S439000, C376S443000, C376S444000
Reexamination Certificate
active
06765979
ABSTRACT:
TECHNICAL FIELD
The present invention relates to fluid separation devices for use in vent volumes within a nuclear fuel bundle and particularly to devices for flowing liquid from a path through the vent volume laterally outwardly into the interstices between and onto adjacent surrounding fuel rods with minimum pressure drop.
BACKGROUND
A typical boiling water nuclear reactor has a reactor core comprised of a plurality of fuel bundles in side-by-side relation to one another. Coolant/moderator flows upwardly within the fuel bundles and about the fuel rods within the fuel bundles where the liquid is converted to steam to produce power.
In U.S. Pat. No. 5,112,570 there is illustrated a fuel bundle having a plurality of part-length fuel rods (PLR). These PLR's are supported on the lower tie plate of each bundle and extend upwardly toward the upper tie plate. The rods, however, terminate short of the upper tie plate and typically between a pair of spacers along the fuel bundle. Between the upper end of each PLR and the upper tie plate, there is defined in the upper two-phase region of the fuel bundle a vent volume. This vent volume preferentially receives vapor from the liquid vapor two phase mixture in the upper region of the fuel bundle during power producing operations. There are many advantages associated with the use of PLR's including the increased vapor fraction within the vent volume and the pressure drop reduction in the upper two phase region of the bundle. This results in increased stability from thermal hydraulic and nuclear instabilities.
It will be appreciated that the mechanical hardware associated with fuel rod spacers causes local reduction in the flow area available for the vapor and liquid flowing through the fuel bundle. This causes significant pressure drops to occur as the flow passes each spacer. By using PLR's, the associated flow blockage effects of one or more of the full-length fuel rods extending through these spacers above the PLR is substantially eliminated. That is, because of the absence of a fuel rod at a lattice location above one or more PLR's, more flow area through the spacer is obtained with consequent reduction in pressure drop across such spacer. Further, significant flow diversion occurs into the lower pressure drop paths or vent volumes above the upper ends of the PLR's. Thus, increased vapor and liquid are pumped from surrounding flow passages, i.e., the interstitial regions around the adjacent fuel rods, into these vent volumes.
The creation of vent volumes, e.g., above PLR's, and flow diversions resulting therefrom, however, can cause some reduction in critical power performance in the fuel bundle. Additional water may accumulate in the vent volume region above the PLR and thus be shunted out of the vent volume without heat generating contact with the remainder of the full-length fuel rods. Separation devices have been utilized to drive the dense liquid or water out of the vent volumes in a generally lateral direction onto the surfaces and into the interstitial regions between the full-length fuel rods to improve heat transfer performance. Such separation devices have generally taken the form of swirlers disposed in the vent volume. These swirlers create a helical flow pattern causing the dense liquid to be driven laterally outwardly of the vent volume by centrifugal force. Such separation devices have been located within the spacers and have extended therefrom above or below the spacers or to both sides of the spacers. By locating the separation devices within the spacers, however, the devices increase the pressure drop across the spacers substantially to the same extent as if full-length fuel rods occupied those lattice positions in the vent volumes. Thus, the value of introducing PLR's in reducing the pressure drop along the length of the fuel bundle is minimized or eliminated by using conventional separation devices in the spacers above the upper ends of the PLR's which restore in part or in whole the pressure drop achieved by the use of PLR's.
DISCLOSURE OF THE INVENTION
According to the present invention, there is provided a fuel bundle for a nuclear reactor having a vent volume wherein one or more separation devices are used to direct liquid laterally onto the surfaces and into the interstices of the full-length fuel rods but without a substantial increase in the pressure drop across the spacers. By placing the separation device(s) above a spacer and leaving the opening(s) through the spacer at that lattice position(s) of the vent volume void of fuel rods, e.g., substantially unobstructed, maximization of the flow through the spacer at a minimum pressure loss while simultaneously directing additional liquid for deposition onto the surfaces of the adjacent full-length fuel rods can be achieved. The location of each separation device above a spacer thus maximizes flow diversion without substantial increase in pressure drop. That is, maximum benefits from depositing the liquid onto adjacent full-length fuel rods with minimum pressure drop are achieved by locating the separation devices just above the spacers. In addition, by streamlining the spacers and separation devices, the pressure drop can be minimized along the length of the vent volume.
In a first preferred form of the present invention, the separation device is located on top of a spacer within a vent volume and may comprise a swirler. It will be appreciated that the separation device in a broad sense need only deflect or divert the liquid flowing upwardly in the vent volume laterally outwardly onto the surfaces and into the interstices of laterally adjacent fuel rods. For that purpose, flow directing devices such as tabs, vanes and the like may be used. Thus, the separation device may have a lower end just above an opening through the spacer (the opening being in a lattice position which would otherwise have supported a fuel or moderator rod) and extend a short distance or an extended distance toward the next adjacent upper spacer.
Where the separation devices comprise the preferred swirlers, each swirler may consist of a single strip of material twisted to form a helical flow path in the vent volume sufficient to direct the heavier liquid laterally outwardly by centrifugal force onto the surfaces and into the interstices of laterally adjacent fuel rods. A more complex configuration may be provided with two or more twisted strips joined along their axes. For example, two flat strips may be slotted at opposite ends, joined along their axes and twisted with the strips maintained perpendicular along their length establishing a cruciform cross-section at any axial location. This can be characterized as a four-blade swirl device. If three strips are joined and twisted with 60° angles maintained along their length, a six-blade swirl device is established. It will be appreciated that the minimum length requirement for a swirler decreases as the number of blades are added to the swirl device. However, the surface area for friction increases with the number of blades and hence a swirl device with minimum length to minimize pressure drop is desirable. Hence, the minimum length for effective separation is that which results in a projected area covering a full 360° which in turn is a function of the number of strips and the angle through which the strips are twisted. For a single twisted strip, this requires a length equal to 180° of rotation. With a double strip configuration of a four-blade swirl device, the minimum length required is equal to 90° rotation, while a six-blade swirl device requires a length equal to 60° of rotation. In general, the minimum length required for any multi-blade swirl device is that which produces blade rotation equal to 360° divided by the number of blades thereby providing a swirl device length which minimizes pressure drop.
Additionally, the separation device may comprise an auger configuration to cause helical flow in the vent volume. Thus, one or more strips of material may be wound on its edge around a cent
Dix Gary E.
Matzner Bruce
Behrend Harvey E.
Nixon & Vanderhye
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