Patent Number: 052308588
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a fuel bundle B is illustrated both partially broken away and cut off in length so that representative portions of the fuel bundle can be seen and understood. Fuel bundle B includes a lower tie plate L forming the upper portion of nose piece N which commonly communicates across a core separation plate (not shown) for receiving water from the lower portion of a reactor (also not shown). Upper tie plate U captures the upstanding end of a group of fuel rods R that extend the full length of fuel bundle B. It will be noted that some fuel rods are partial length, terminating at less than the full distance to upper tie plate U. Centrally of fuel bundle B there is water rod W. (In some designs more than one may be used.) Water rod W has upper compartment 14 with upwardly exposed open end 15. It is into this open end 15 that liquid moderator (water) falls during normal operation of the fuel bundle to fill the upper compartment 14. Lower compartment 17 is supplied with lower entrance apertures 18 and upper exit aperture 20. Water from the bottom of fuel bundle B flows through water rod W at lower compartment 17 and exits at exit apertures 20. In FIG. 1, such exit is shown between the first and second spacers S1, S2. Referring to FIG. 2, a schematic of the invention is illustrated with the schematic being compressed in the vertical direction. Fuel bundle B includes lower tie plate L, upper tie plate U with water rod W extending therebetween. Water rod W has upper compartment 14 with upward opening 15. Water rod W further includes lower compartment 17 with entrance apertures 18 and discharge apertures 20. All seven spacers S1-S7 are illustrated with discharge of the discharges apertures occurring in accordance with the preferred embodiment of this invention between spacers S2-S3. There is a practical limitation on the length of upper compartment 14. If this compartment becomes too long, it may become the site of unstable flow or "chugging." In this case upward flowing steam at the tube exit will be too great to permit water flow down into the tube. As the completely voided condition is approached, the steam flow will decrease and liquid flow will begin again. This cycle of volume and refilling will continue endlessly. This flouroscillation would not be desired and would defeat the nuclear characteristics of the water rod W. Therefore, we preferred to limit the length of upper compartment 14 to about 1/4 to 1/3 the total length of the fuel bundle depending on the counter current flow characteristics of the particular water rod design. Referring to FIG. 3, mention has been made that the upper two phase region is a portion of the fuel bundle B where the thickness of the liquid film coating the center of the fuel bundle B narrows as the upwardly flowing moderator passes along the fuel rods R between the spacers S3, S2, and S1. Liquid and vapor moderator passes upwardly in the direction of arrow 40. Thickness of the vapor film overlying the central fuel rods R is schematically shown lines T1, T2, and T3. Some remarks may be made relative to the graphic illustration. First, and relative to line T1, enriched nuclear fuel typically ends near the first spacer. As the line Tl suggests, thickness of the water film over the central rods is generally not a problem in these locations. Secondly, and with respect to lines T2 and T3, thickness of liquid film over the central fuel rods can be a problem between spacers S1 and S2 or S2 and S3. At flow rates up to in the order of 75%, maximum thinning of the film will typically occur just upstream of spacer S1. As flow increases above 75% flow rate, maximum thinning of the film will typically occur just upstream of spacer S2. Since spacer location may be vertically variable, optimal placement of discharge apertures 20 will be a matter of design.