Patent Number: 052873927
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The fluid flow in a boiling water reactor will be generally described with reference to FIG. 1. Feedwater is admitted into reactor pressure vessel (RPV) 10 via an inlet 12. Inlet 12 is connected to feedwater sparger 14, which is a ring-shaped pipe having suitable apertures through which the feedwater is distributed inside the RPV. The feedwater from sparger 14 flows downwardly through the downcomer annulus 16, which is an annular region between the RPV and core shroud 18. Core shroud 18 is a stainless steel cylinder which surrounds the core 20 (only one fuel assembly 22 of which is depicted in FIG. 1) and separates the upward flow of coolant through the core from the downward flow in downcomer annulus 16. The water flowing through downcomer annulus 16 then flows to the core lower plenum 24. The water subsequently enters the fuel assemblies 22 disposed within core 20, wherein a boiling boundary layer (not shown) is established, thus causing a lower non-boiling region and an upper boiling region within the fuel assemblies. Next, a mixture of water and steam enters core upper plenum 26 which is formed within shroud head 28 and disposed atop core 20. Core upper plenum 26 provides stand-off between the steam-water mixture exiting core 20 and entering vertical standpipes 30, the latter being disposed atop shroud head 28 and in fluid communication with core upper plenum 26. Each standpipe 30 is in fluid communication with a steam separator 32 mounted thereon. The steam-water mixture flowing through standpipes 30 enters steam separators 32, which are of the axial-flow centrifugal type. These separators separate the liquid water from the steam by employing a swirling motion to drive the water droplets to the outer wall of the separator. The separated liquid water then mixes with feedwater in the mixing plenum 33, which mixture then flows to the core via the downcomer annulus. The liquid water elevation or level established within the RPV during normal operation of the BWR is designated by numeral 50 in FIG. 4. The steam passes through steam dryers 34 and enters steam dome 36. The steam is withdrawn from the RPV via steam outlet 38. The BWR also includes a coolant recirculation system which provides the forced convection flow through the core necessary to achieve the required power density. In some, but not all BWRs, a portion of the water is sucked from the lower end of the downcomer annulus 16 via outlet 43 and forced by a centrifugal recirculation pump 40 (see FIG. 4) into jet pump assemblies 42 via inlet 45. This type of BWR also has two recirculation pumps, each of which provides the driving flow for a plurality of jet pump assemblies. As best seen in FIG. 1, the pressurized driving water is supplied to a jet pump nozzle 44 by an inlet riser 46 via elbow 48. In accordance with the invention, the passive recombiner must be located in a hydrogen-rich region. Hydrogen injection is via the feedwater spargers. Thus, the recombiner must be located between the steam separators and the jet pumps, preferably immediately downstream of the steam water separator assembly of the BWR shown in FIG. 1. Two locations were studied. The first location is attached to the shroud head, so that it is removed when the shroud head is removed. The second location is attached to the shroud above the jet pumps, which would require the ability to periodically remove the recombiner for access to the jet pumps. It is estimated that access to the jet pump annulus is required approximately every second refueling outage. In accordance with a first preferred embodiment of the invention, the recombiner, generally designated by 48 in FIG. 4, has a generally annular configuration and is mounted on the shroud head 28. In accordance with a second preferred embodiment, the recombiner, generally designated by 48' in FIG. 6, is mounted on shroud 28 and located in the downcomer annulus 16 (above the jet pumps for BWRs which have them). FIGS. 4 and 6 respectively depict a cross section of such generally annular recombiners. The honeycombed hatching is intended to symbolize an arrangement in which catalytic material is packed into and held in place by a stiffened metal mesh housing. The catalytic recombiner material packed inside the housing should have a high surface area-to-volume ratio and could take the form of tangled wire or foil strips, crimped ribbon, porous sintered metal composite, a honeycombed structure or any other structure having a high surface area-to-volume ratio. Other geometries would be suitable. The catalytic material could, for example, be platinum or palladium deposited on a stainless steel substrate; a noble metal-doped alloy of stainless steel (or other proven reactor structural material doped with noble metal); or a commercially available noble metal catalytic material. The catalytic material may be formed as a coating on a substrate, or as a solute in an alloy formed into the substrate, the coating or solute being sufficient to catalyze the recombination of oxidizing and reducing species at the surface of the substrate. The preferred catalytic materials are platinum, palladium, osmium, ruthenium, iridium, rhodium, and mixtures thereof, whereas the preferred substrate is stainless steel. The formation of a catalytic layer of a noble metal on an alloy catalyzes the recombination of reducing species, such as hydrogen, with oxidizing species, such as oxygen or hydrogen peroxide, that are present in the water of a BWR. The surface of the recombiner structure also inherently catalyzes the decomposition of hydrogen peroxide via heterogeneous decomposition. Such catalytic action at the surface of the alloy can lower the corrosion potential of the alloy below the critical o corrosion potential where SCC is minimized. As a result, the efficacy of hydrogen additions to high-temperature water in lowering the electrochemical potential of components made from the alloy and exposed to the injected water is increased manyfold. The key requirement of the catalytic material is that it must perform at reactor operating temperatures of 288.degree. C. in the water phase. Current laboratory data suggests that catalytic recombination of H.sub.2 and O.sub.2 or of H.sub.2 and H.sub.2 O.sub.2 only occurs effectively when there is a stoichiometric excess of hydrogen. The H.sub.2 O.sub.2 produced in the core is generally nonvolatile. While the H.sub.2 and O.sub.2 partition in the steam separators to go into the steam, the H.sub.2 O.sub.2 stays in the liquid and gets recirculated. Because of the differences of Henry's Law for H.sub.2 and H.sub.2 O.sub.2, the water exiting the steam separator may be sub-stoichiometric for the molar ratio of H.sub.2 to (O.sub.2 +H.sub.2 O.sub.2). Because of this effect, it may be necessary to have some hydrogen-containing feedwater mix with the water exiting the separation assembly before it enters the recombiner. This can be accomplished by placement of the recombiner downstream of the location where feedwater enters the vessel, as in the embodiment of FIG. 6, or by injecting the feedwater over, around and through the recombiner. The typical residence time for water passing through the recombiner will be a few seconds or less. A calculation of the effectiveness of the passive catalytic recombiner using a radiolysis model is shown in FIG. 5. The calculation predicts that conditions for preventing SCC (i.e., O.sub.2 +H.sub.2 O.sub.2 &lt;2 ppb) can be achieved with feedwater hydrogen injection rates (i.e., about 0.4 ppm) below rates which cause significant increases in the main steam line radiation level. The major advantages of shroud head attachment are that the recombiner can be installed in the separator pool and that the recombiner is removed with the shroud head during refuelings. Also, attachment to the shroud is disadvantageous because it requires many individual pieces to fill the cross-sectional area of the downcomer annulus. Therefore attachment to the shroud head is preferred for most BWRs. However, differences in geometry and design may dictate that attachment to the shroud is preferred for some BWRs. The minimum size of a piece of the recombiner material should be a strip 0.012 inch thick by 12.0 inches long. The minimum thickness of any component currently used in the RPV is 0.012 inches as part of the fuel spacers. Therefore this thickness was chosen from the recombiner strips. The concern for small thicknesses is that pieces could break off and become lodged in the fuel assemblies where they could cause local hot spots. The minimum length of 12 inches was selected to prevent a piece from traversing the path from the control rod guide tube to the fuel rods. The width requirements are more subjective. A reasonable minimum width of 0.25 inch was selected based on fabrication concerns. A piece of this size can be formed into any required shape to facilitate packing. The total weight of a recombiner attached to the shroud head will be approximately 12,000 to 20,000 pounds. The weight of a 251-inch RPV shroud head is approximately 125,000 pounds. The weight of a recombiner was estimated by assuming that 90% of the recombiner volume would be open and 10% would be solid metal. Thus, the recombiner weighs approximately 50 pounds per cubic foot of recombiner volume plus the weight of the support structure. The support structure is expected to add approximately 5000 pounds. A specific design for a recombiner mounted on the shroud head is shown in FIGS. 7 through 9. The inside height of the recombiner 48 will be approximately 72.9 inches. The recombiner structure is supported by a ring 52 which rests on the top of the shroud head flange 54. The ring is attached to the shroud head flange with brackets and bolts. There will be a small amount of leakage past the recombiner at the bottom inside edge. Proper design will cause the feedwater to force all the flow from the steam dryer drain channels 56 into the separator array and then through the recombiner. The recombiner includes a stainless steel flow-through housing packed with catalytic recombiner material, which could take the form of tangled wire or foil strips, crimped ribbon, porous sintered metal composite, a honeycombed structure or any other structure having a high surface area-to-volume ratio. As best seen in FIG. 8, the recombiner is generally annular in shape and has inner and outer circumferential walls of complex configuration. These walls have small holes which allow water to flow-through. FIG. 8 shows how the recombiner 48 fits outside of the separator standpipes 30 and around the shroud head bolts 60. The minimum flow path through the recombiner varies around the circumference from approximately 6 inches to approximately 13 inches. The flow is prevented from taking a shorter flow path by eliminating outlet holes in selected portions of the outer circumferential wall. The recombiner shown in FIG. 8 would be installed in four major pieces, which would bolt together behind the four shroud head lifting rods 62. No flow would go through these four regions. The top of the recombiner would be attached to the shroud head lower bolt ring 58 with brackets and bolts (not shown). There is a minimum of 2 inches of clearance between the recombiner and any part of the reactor assembly that is stationary. The inside volume of the recombiner shown in FIG. 8 is approximately 206 ft.sup.3. The flow area through the recombiner is approximately 320 ft.sup.2. The average residence time of the water in the recombiner is approximately one-quarter of a second, assuming a 6-inch flow path. FIG. 9 shows the geometric configuration of the recombiner housing 64 in an isometric view. All planar wall panel of the inner circumferential wall of housing 64 have a plurality of small holes, as shown for panel 64a, to allow flow-through of the liquid from the steam separation assembly. Although only shown incompletely, the outer flow-through panels are also provided with similar holes. The holes are sufficiently small to prevent escape of the catalytic material packed inside the housing. As previously described, the housing may take the form of a stiffened metal mesh with catalytic recombiner material packed inside the housing. Preferably, the catalytic recombiner material is tangled wire plated with catalytic material, or crimped ribbons or tangled strips made of alloy doped with catalytic material. The preferred embodiments of the hydrogen peroxide decomposer of the invention will have the same structure as is depicted in FIGS. 4 and 6-9. The only difference is that the high surface area-to-volume structure will not be doped or coated with a water recombination catalyst. The preferred catalytic decomposer material is stainless steel because of its predictable performance in a BWR environment. However, other solid materials which cause heterogeneous decomposition and which have structural strength and corrosion resistance suitable for the BWR environment can be used. The key requirement of the catalytic decomposer material is that it must perform at reactor operating temperatures of 288.degree. C, in the water phase. The H.sub.2 O.sub.2 produced in the core is generally nonvolatile. While the H.sub.2 and O.sub.2 partition in the steam separators to go into the steam, the H.sub.2 O.sub.2 stays in the liquid and recirculates through the decomposer. The typical residence time for water passing through the decomposer will be a few seconds or less. Upon passage of this recirculated water through the catalytic decomposer of the invention, hydrogen peroxide is decomposed. The resulting reactor water entering the vessel downcomer annulus will be very low in H.sub.2 O.sub.2 as compared to the level when a decomposer is not used. The net effect of this reduction in the H.sub.2 O.sub.2 concentration will be a decrease in the amount of hydrogen which must be added to the feedwater to establish the low levels of (O.sub.2 +H.sub.2 O.sub.2) which result in corrosion potentials below the critical potential and thus protect against SCC. The specific embodiment shown in FIGS. 7 through 9 has been described in detail for the purpose of illustration only. Practitioners of ordinary skill in the art of nuclear reactor engineering will recognize that the geometry and location of the catalytic device in accordance with the invention will depend on the specific design of the BWR in which the device is to be installed. In accordance with the invention, however, the recombiner/decomposer catalytic device for any given type of BWR must be designed to ensure that virtually all water phase exiting the steam/water separator device flows over the surface of the catalytic material.