Patent Number: 052290689
Section: summary

This invention relates to nuclear fuel bundles utilized in boiling water nuclear reactors having part length rods. More particularly, the combination of a fuel bundle having part length rods is disclosed wherein pressure reduction obtained by the introduction of part length rods in the upper two phase region of a fuel assembly is reclaimed by the introduction of spacers causing substantial recapture of the reduced pressure drop. Improved critical power results. For example, spacer pitch can be changed to add spacers to the upper two phase region of the fuel bundle. Alternately, so-called vanes, especially swirl vanes can be added. Other expedients are introduced for causing pressure drop with the spacers including increased spacer height and constructing the spacers of thicker metallic materials. This invention also includes the addition of separation devices overlying the part length rods. BACKGROUND OF THE INVENTION In Dix et al. U.S. Pat. No. 5,112,570 issued May 12, 1992, entitled TWO-PHASE PRESSURE DROP REDUCTION BWR ASSEMBLY DESIGN (formerly U.S. patent application Ser. No. 07/176,975 filed Apr. 4, 1988), a fuel bundle having a plurality of part length rods was illustrated. A summary of that construction and the advantages set forth in this reference can be instructive. Construction of the fuel bundle in Dix et al. is conventional with the exception of the addition of less than full length fuel rods. The conventional portion of the disclosed fuel assembly in the Dix et al. Patent is easy to understand. This assembly includes a channel having vertically extending walls for extending around a fuel bundle assembly volume. The channel is open at the bottom for receipt of water moderator and open at the top for the discharge of water and steam. The fuel bundle includes a matrix of vertically upstanding fuel rods--these rods being sealed tubes containing fissionable materials. The fuel rods are supported on a lower tie plate which permits the entry of the water moderator to the fuel bundle. The fuel rods typically extend to an upper tie plate which maintains the fuel rods in their side by side vertical relation and permits the generated steam and remaining water to escape. The Dix et al. disclosure adds to the conventional fuel assembly, a plurality of less than full length spaced apart so-called "part length (fuel) rods" (PLRs). These fuel rods are supported on the lower tie plate, extend upwardly to and toward the upper tie plate, but terminate short of the upper tie plate. Between the point of part length fuel rod termination and the upper tie plate, the part length fuel rod defines 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 two phase region of the fuel bundle during power producing operation. Numerous advantages result from the part length rod construction. Improved cold shut down margin enables fuel to be designed with reduced amounts of burnable absorbers such as gadolinium. The tendency of the fuel bundle in the reactor to produce plutonium at the top of the bundle from resonance neutron capture in uranium 238 is reduced. The void overlying the part length rod has an increased vapor fraction with the result that the full length rods adjacent the voids have an increased liquid fraction. Further, the pressure drop in the upper two phase region of the fuel bundle is reduced. This being the case, the fuel bundle enjoys increased stability from thermal hydraulic and nuclear instabilities. The fuel bundles are elongate. Further, the fuel rods contained within the fuel bundle are flexible. These fuel rods can flex out of their designed side-by-side spacing--and even into interfering contact with one another--due to flow induced vibration and rod bow. Therefore, spacers are utilized throughout the length of the fuel bundle. Fuel bundle spacers have the function of maintaining the individual fuel rods at given elevations in their designed side-by-side relationship. Such spacers usually define a matrix of individual fuel rod containing cells. These cells fit around each and every fuel rod at their particular elevation in a fuel bundle. The fuel bundle spacers maintain the fuel rods in their designed side-by-side relationship and prevent interfering contact between the individual fuel rods. In the case of the part length rods where the fuel rods do not extend to the upper tie plate, the spacers maintain the fuel rods in their designed upstanding relation. All fuel bundles--including those having part length rods--must be designed to operate within thermal limits. Specifically, that thermal limit in boiling water reactors known as critical power has always been a limitation. Critical power originates from rupture of the coolant liquid film on the exterior surface of the fuel rod in a phenomena known as "transition boiling." In this transition boiling condition a liquid film no longer coats the exterior surface of the fuel rod. The rod on the exterior surface is exposed to coolant vapor only. Heat transfer from the fuel interior of the fuel rod undergoing fission reaction to the coolant is reduced. The fuel rod cladding becomes overheated. Naturally, as any fuel rod within a fuel bundle even approaches such a boiling condition anywhere along its length, power is restricted to avoid violation of this "critical power" limitation. Past experimentation has been directed to the critical power limitation. It is known that by decreasing the spacer pitch in the upper two phase region of the fuel bundle, that critical power can be improved. Unfortunately, the additional spacers caused additional pressure loss. This additional pressure loss causes additional tendencies for instabilities at certain power rates of the reactor. These instabilities include local and core wide thermal hydraulic and nuclear thermal hydraulic instabilities. For these reasons, the experimentally determined improvement of critical power could not be implemented by decreasing the spacer pitch in the upper two phase region of boiling water nuclear reactor fuel bundles. It is also known to incorporate so-called "swirl vanes" to both boiling water nuclear reactors and the spacers in boiling water nuclear reactors. These devices can be simply summarized and easily understood. In summary, so-called swirl vanes are placed interstitially of fuel rods. The vanes themselves comprise pieces of metal twisted in a helical pattern. In the earliest known cases, these so-called swirl vanes were the same length as the fuel rods in the reactor. In a later case, a spacer constructed from such swirl vanes was constructed. See Johansson, U.S. Pat. No. 4,913,895 issued Apr. 3, 1990 entitled SWIRL VANES INTEGRAL WITH SPACER GRID. These swirl vanes when added to reactors had a beneficial effect and a detrimental effect. The beneficial effect was the classification of water from upwardly flowing water and steam. Specifically, upwardly flowing water and steam. Simply stated, and despite the helical pattern of the twisted metal strips, steam tended to upwardly flow about the swirl vanes. Water, however, did not tend to join this upward flow. Instead the heavier water received a horizontal velocity component from the swirl vanes. As the swirl vanes were placed interstitially of the fuel rods, the heavier water when thrown horizontally by the momentum of the swirl vanes has the beneficial effect of impacting the adjacent fuel rods. Consequently, the critical power limit is increased. The detrimental effect of such swirl vanes is increased pressure drop. The swirl vanes themselves raise the pressure drop in the upper two phase region of the boiling water reactor. This increase in pressure drop will increase the possibility of instabilities including thermal hydraulic instabilities and nuclear, thermal hydraulic instabilities at high power/low flow conditions of the boiling water nuclear reactor. This being the case, the swirl vanes have not been in large measure introduced into the boiling water nuclear reactors. Any physical explanation of spacer relative thermal hydraulic performance should depend on the flow regimes that the coolant experiences in flowing up the channel as well as how the flow interacts with the spacer. Single phase water enters the bottom of the fuel assembly and is heated until sub cooled boiling occurs. Bubbles are formed at the surface of the fuel rod but quickly condense as they contact the bulk sub cooled flow. At the 100% power/100% flow condition bundle average bulk boiling will begin somewhere between the bottom spacer and the second spacer from the bottom of the fuel assembly. Now bubbles in the main flow stream will grow and the flow regime will progress from bubble flow to a type of slug or froth flow where individual small bubbles are starting to combine to make larger slugs of vapor. During these processes the vapor is flowing as bubbles or slugs in a continuous liquid medium. Depending on conditions somewhere around the middle of the bundle a flow regime transformation takes place. Now there is so much vapor that it becomes the continuous medium and the liquid is either found as a thin film flowing on all the solid surfaces of the bundle or as droplets entrained in the continuous vapor. This is the annular flow regime which is important because it is where dry out or boiling transition will commonly take place in a BWR. The limiting critical power condition in a BWR has been referred to in the literature alternately as dry out, boiling crisis, critical heat flux, burnout and boiling transition, the term which will be used here. Boiling transition is defined as the first condition of degraded heat transfer in the fuel bundle. This occurs in the annular flow regime as a result of the thin liquid film which covers all the fuel rod surfaces going to zero film thickness. A critical power problem results. DISCOVERY We have discovered that there can be a deficiency in fuel bundles having part length rods. Specifically, such fuel bundles have a tendency to have critical power limitations in the upper two phase region of the fuel bundle. This critical power limitation occurs in the full length rods in the upper two phase region of the fuel bundle. It has been determined by experiment that flow rates around and adjacent the full length rods may be below average. This apparently has the tendency to generate transition boiling and the critical power limitations. The reader will understand that this discovery is not prior art. In so far that discovery can constitute invention, our invention incorporates this discovery. SUMMARY OF THE INVENTION In a fuel bundle for use in the core of a boiling water nuclear reactor, part length rods having a tendency to reduce pressure drop are used in combination with spacers and spacer attached devices tending to restore pressure drop to improve critical power. The fuel bundle includes a preferred 9 by 9 matrix of upstanding vertically disposed fuel rods surrounded by a fuel channel between upper and lower tie plates. The tie plates support the fuel rods and permit the entry of water coolant at the lower tie plate and the exit of water and generated steam at the upper tie plate. Part length rods are distributed in the fuel rod matrix and combined with increased spacer pitch. The addition of the part length rods has the advantage of lowering the pressure drop. Spacer additions (such as the increase in spacer pitch in the upper two phase region of the bundle) or spacer attachments (such as vanes and especially so-called swirl vanes) are utilized to restore the pressure drop removed by the insertion of the part length rods. There results a serendipitous improved critical power performance in the upper two phase region of the fuel assembly. One method of achieving the disclosed result is the increase in total number of spacers in the upper two phase region of the fuel bundle to increase pressure drop. The spacers are distributed in the lower portion of the fuel bundle on about 20 inch centers. The increased number of spacers in the upper two phase region of the fuel bundle includes placing them on a pitch of less than 20" so as to allow for the addition of at least one spacer in the upper two phase region of the fuel bundle. The additional spacer is not required for the traditional purpose of preventing either rod bow or flow induced vibration. Indeed the additional spacer causes the pressure loss in the upper two phase region of the fuel bundle to be in part restored to that pressure loss that would be present if the fuel bundle contained an array having full length fuel rods only. However, the additional spacer causes the critical power of the fuel bundle to be improved. There results a fuel bundle with part length rods having all of the advantages inherent in the part length rod construction plus the added benefit of increased critical power. Alternately, and in addition to the disclosed decrease in spacer pitch, spacers incorporating vanes can be used. By way of example, these vanes can be our preferred partial or complete swirl vane arrays. The vanes are incorporated to the spacers in the interstitial volumes between the fuel rods. Such spacers, although increasing pressure drop, cause improvement in critical power. In the case of the incorporation of vanes to the spacer, increased pitch of spacers is not required. Other expedients of spacer modification for realization of pressure drop are disclosed. Spacers on the same pitch having increased vertical height can be utilized. Further, spacer fabricated from thicker metallic construction can be used. In short, an device--preferably a spacer--in the upper two phase region of the fuel bundle which adds back the pressure drop lost by the use of part length rods is sufficient for the practice of this invention. Separation devices can also be used. Two classes of separation devices are disclosed. A first type of device fits to the end of part length rods and is primarily intended for preventing water passing along the surface of the part length rod adjacent the end of the part length rod from entering the volume overlying the part length fuel rod. A second type of device resides in the volume overlying part length rods. This device serves the purpose of ejecting water entrained into the steam vent volume overlying part length rods. These devices can be extended and interconnected. In either case, improved concentration of steam to the vent volume overlying the part length rods with high liquid fraction residing in the surrounding full length rods results. OTHER OBJECTS, FEATURES AND ADVANTAGES A fundamental difficulty in BWR fuel design results from the large variations in moderator density caused by vapor formation. Current design approaches provide some compensation for this by introducing captive-liquid within the fuel bundle. Examples are the various water-rod and water-cross designs. While these approaches provide for effective neutron moderation, their associated blockage of normal coolant flow area causes entirely adverse thermal hydraulic effects. This is particularly true as the blockages become large. In contrast, the steam-vent approach provides synergistic benefits for both neutron moderation and thermal hydraulics. Diverting significant vapor into a low-resistance flow path will allow the average vapor velocity to increase, and thereby reduce the average void fraction. More importantly, local void fractions around the fuel rods will be reduced even more due to the removal of vapor from that region. In contrast, the flow blockages caused by captive-liquid regions force all of the normal liquid and vapor to flow together around the fuel rods, at even higher velocities. This increases local void fractions around the fuel rods. Thus the neutron moderation benefits with steam-vent designs can easily exceed those achieved using large captive liquid regions. The low resistance flow path for vapor will reduce pressure drop in the two-phase region. Removal of normal spacer structure within the steam-vent path will reduce the pressure drop from each spacer, allowing for more spacers to be added (with associated critical power and rod-bow benefits). Channel stability will be improved both by the reduced two-phase pressure drop, and by the damping effect from a separate high velocity flow path within the fuel bundle. An object of this invention is to disclose a first class of separation devices for inhibiting the entry of water into the steam vent volumes overlying the part length rods. According to this aspect of the invention, the part length rod is provided with an attachment at its upper terminal end. This attachment can be either a flared end, deflecting tabs, or a spirally wound piece of metal, hereinafter referred to as a swirl vane. Steam and water passing along the length of the outside of the part length rod adjacent the rod end impact the attachment. Water--with its higher mass--is deflected. Steam--with its lower mass--continues substantially undeflected upwardly into the vent volume overlying the part length rod. There results a reduction of water introduction into the steam vent volume overlying the part length rod. An additional object of this invention is to place steam separation devices in the region overlying one or more part length rods. These devices can be placed at discrete locations, some distance from the part length rods, or they can extend continuously through the void volume overlying the part length rods. Preferable attachment and suspension of such devices is from spacers overlying the ends of the part length rods, or from the upper tieplate. The suspended devices can include twisted metal strips, hereinafter referred to as swirl vanes, cones, or other steam separation devices. Remaining water introduced into and entrained into the steam vent volume is ejected. An advantage of both the attachment to the end of part length rods and the separation device overlying the end of part length rods is flexibility in placement of part length rods while maintaining more effective steam vent channels side-by-side with surrounding higher liquid fraction about the full length rods. There results improved nuclear reaction, improved heat transfer, improved stability and lower pressure drop. An object of this invention is to disclose a balance between the loss of pressure drop due to judicious use of part length rods and the increase in critical power due spacer attached devices restoring the originally decreased pressure drop. There results an improved critical power. A further object of this invention is to set forth preferable spacer attached devices for the increase of critical power through increased pressure drop. By way of example, either increased spacer pitch or the addition of vanes, such as swirl vanes can be used. In either case, the decrease in critical power due to the presence of the part length rods is considerably less than the increase in critical power due to spacers causing the recaptured pressure drop. As a result, overall critical power is improved. By way of example, and using the combination of actual tests and the spacer pitch of FIGS. 2A and 2B with a 9 by 9 array of fuel rods with eight part length rods distributed in a fuel bundle, pressure drop improves 8% or 1.2 psi. in the upper two phase region of the fuel bundle. Critical power loss due to the presence of the part length rods may be in the range of 2 to 4%. At the same time, and as a result of the decreased spacer pitch, pressure drop increases 0.8 psi. in the upper two phase region of the bundle. At the same time critical power gains over the part length rod array have been measured experimentally to be as much as 12%. Thus the net overall gain in critical power could be as much as 10% with pressure drop remaining substantially unchanged relative to the same fuel rod bundle having full length rods. Regarding decreased spacer pitch, the total number of spacers in the upper two phase region of the fuel bundle is increased. Alternately, the spacers may be increased in vertical height. Further, and as a substitute, the thickness in material from which the spacers are fabricated can be increased. In either event, upon the recapture of the originally obtained pressure drop, improved critical power results. Regarding the vane embodiment of this invention, the reader will understand that vanes incorporated to spacers have two effects. First, they are higher pressure loss devices causing pressure drop and hence improved critical power "downstream" (up above) their particular location in a fuel bundle. Thus, where spacers with vanes are utilized, decreased spacer pitch may not be required. The reader should understand that we do not necessarily identify the specific mechanism causing the beneficial increase in critical power. We do identify that where pressure drop is increased, critical power is likewise increased in the upper two phase region of the fuel bundle. The reader will understand that fuel rods are designed to have reduced power output above the last spacer. This being the case, it will be understood that the top most or last spacer is not required to have an appreciable pressure drop effect on the passing fluid flow. Thus in this last location, the use of an Inconel spacer having minimal critical power effect on the passing fluid flow with corresponding reduced pressure drop can be used.