Patent Number: 053923251
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with a first preferred embodiment of the invention suitable for installation in pipe 50 of circular cross section, a catalytic recombiner cartridge 52 (see FIGS. 1A and 1B) comprises a first plurality of concentric thin cylindrical shells 54 supported between spacers 56 and 56' and a second plurality of concentric thin cylindrical shells 54' supported between spacers 56' and 56", for example, by spot welding or brazing. The shells are made of catalytic material. Each spacer comprises a number (e.g., four) of planar fins, each fin being welded at one end to a central rod 58 positioned along the axis of the concentric shells. Rod 58 resists torquing of the spacers relative to each other. The fins may be disposed at equiangular intervals, e.g., 90.degree.. The cylindrical shells 54 are formed from thin sheets (e.g., 10-12 mils thick) made of noble metal-doped alloy (e.g., stainless steel doped with at least about 1 wt. % palladium) and provided with a multiplicity of means for generating turbulence. The spacers and central rod may also be made of catalytic material provided with turbulence generating means. The sheets are rolled into a cylindrical shape and then welded (not shown) along the overlapping edges to form a cylindrical shell. The cylindrical shells are then welded (not shown) to the supporting structure, i.e., spacers 56, 56' and 56" welded to central rod 58, to form a cartridge 52 which is installed inside pipe 50 with the surfaces of shells 54 lying generally parallel to the direction of fluid flow. The spacers function to stiffen the concentric shells against flow-induced vibration and to maintain the shells in concentric relationship with channels 64 therebetween. The spacing between adjacent shells is preferably constant, e.g., about 25 mils. Although the preferred embodiment shown in FIG. 1B comprises first and second pluralities of concentric cylindrical shells, it will be appreciated that the invention also encompasses a single plurality of shells supported between a pair of spacers. In accordance with the method of the invention, H.sub.2 gas is injected into the flow at a point immediately upstream of installed cartridge 52. The fluid flowing through pipe 50 should be H.sub.2 -enriched to provide an adequate supply of H.sub.2 for the catalytic recombination of water at the surfaces of shells 54. As a result of this catalytic recombination, the concentrations of O.sub.2 and H.sub.2 O.sub.2 in the fluid exiting cartridge 52 will be reduced to a level whereat the ECP is below the SCC threshold value, thereby reducing the susceptibility to SCC of components immediately downstream of the cartridge. In accordance with a second preferred embodiment of the invention suitable for installation in piping of circular or elliptical cross section, a catalytic recombiner cartridge 80 (see FIG. 1C) comprises a thin sheet 74 fabricated into a helix and welded at its inner edge to a central rod 58 which is supported at both ends inside the pipe 50 by a pair of support members, only one (76') of which is shown. The helix is wound tightly around the central rod and then released to spring into place inside the pipe. In addition to being provided with turbulence generating means, such as perforations (not shown), the sheet 74 has hemispherical spacer bumps 78 for maintaining successive turns of the helix with predetermined spacing. In accordance with a third preferred embodiment suitable for installation in piping of rectangular, square or irregular cross section, a catalytic recombiner cartridge 66 (see FIG. 2A) can be formed by welding a plurality of hexagonal units into a honeycomb array. As shown in FIGS. 2B and 2C, each hexagonal unit comprises a hexagonal shell 68 having spacers 72, 72' and 72" which support first and second pluralities of thin planar sheets 70 and 70'. The thin planar sheets 70 and 70' are spot welded or brazed to the shell as well as to the spacers. The thin sheets (e.g., 10-12 mils thick) are made of noble metal-doped alloy (e.g., stainless steel doped with at least about 1 wt. % palladium) and are provided with a multiplicity of means for generating turbulence. The spacing between adjacent sheets is preferably constant, e.g., about 25 mils. Each spacer has four planar fins welded in an X shape, with the outer tips of the fins being welded to four of the six vertices of the hexagonal shell 68, as shown in FIG. 2A. The spacers and the hexagonal shell may also be made of catalytic material provided with turbulence generating means. Although the preferred embodiment shown in FIG. 2B comprises first and second pluralities of parallel planar sheets, it will be appreciated that the invention also encompasses a single plurality of sheets supported between a pair of spacers. The turbulence generating means function to repeatedly interrupt the boundary layer in fluid flowing along the sheet surfaces. Such turbulence generating means are disclosed in a concurrently filed and commonly assigned U.S. patent application entitled "Catalytic Reactor Element", the disclosure of which is incorporated by reference herein. In accordance with the preferred embodiment disclosed therein, each element of the catalytic reactor is a perforated thin sheet of catalytic material having a multiplicity of small holes, preferably circular holes of equal diameter. The holes can be formed by punching and deburring. The holes are distributed over the sheet in a pattern of staggered rows extending transverse to the direction of fluid flow. The distance between the staggered rows is greater than the distance between holes in each row. This arrangement of perforations optimizes the performance of the elements of the catalytic recombiner in accordance with the invention by limiting boundary layer growth and, at the same time, facilitating communication and mixing of the reactants between flow channels, without introducing an excessive pressure drop in the bulk fluid. In particular, the perforations repeatedly break up the fluid boundary layers on the surfaces of the catalytic reactor elements, which layers block diffusion of the reactants to the surface of the catalytic material. Second, the perforations provide turbulent paths which cause fluid in one flow channel to mix with fluid in another flow channel, thereby preventing formation of local zones of reactant depletion. However, the turbulence generating means need not be holes. Dimples and protuberances (e.g., in the shape of hemispheres) can also be distributed over the sheet surfaces in a pattern of staggered rows. Protuberances can also perform the function of spacers. Although dimples and protuberances are less desirable than holes because the former do not provide mixing of fluid between flow channels, the benefits of disrupting the boundary layer would be realized. Recombiner design in accordance with the invention is effected using general principles of mass transfer which are known in the art. The design process involves selection of material and dimensions so that outlet concentration ratio and pressure drop are consistent with required criteria. In addition, size, weight, accessibility, etc. are important aspects of the design. The key points will be illustrated for typical dimensions and for recombination of water from hydrogen and oxygen, although the results are similar for other species. The outlet concentration ratio for a catalytic recombiner cartridge in accordance with the invention is determined by a mass balance across the recombiner, which yields: EQU ln[C/C.sub.o ]=[1-n.pi.d.sub.p.sup.2 /4][4L/D.sub.h ][.alpha.--be.sup.-CRe ]Sc.sup.-0.67 Re .sup.-0.54 where: ##EQU1## and .alpha..apprxeq.0.77; b.apprxeq.0.59; c.apprxeq.1/3570; D.sub.O2 and D.sub.H2 are the diffusion coefficients of O.sub.2 and H.sub.2, respectively; D.sub.h is the mass transfer diameter; .nu. is the fluid kinematic viscosity; V.sub.0 is the bulk fluid velocity; Re and Sc are the Reynolds and Schmidt numbers, respectively; d.sub.i and L are the diameter and length of the cartridge, respectively; N.sub.0 is the number of concentric shells in the cartridge; t is the shell thickness; n is the number of perforations per unit area of element; and d.sub.p is the perforation diameter. An important design consideration is the pressure drop between the inlet and outlet of the cartridge: ##EQU2## where .rho. is the density of the fluid; g.sub.0 is the acceleration due to gravity; and f is a friction factor which is determined semi-empirically. For compact recombiners, an appropriate correlation is: ##EQU3## where 16,000.gtoreq.Re.gtoreq.1,000, b.sub.0 =2, b.sub.1 =6.10.sup.-4, b.sub.2 =-3. 10.sup.-8 and b.sub.3 =7.10.sup.-13. The recombiner void fraction, .epsilon., is given by: ##EQU4## and the coefficient in .function.(Re) accounts for flow-blockage effects. The outlet concentration ratio for a recombiner cartridge made in accordance with the first preferred embodiment is shown in FIG. 3. The cartridge in this case has 33 concentric shells/inch, each shell having 30 holes/inch.sup.2 of diameter 79 mils and a thickness of 12 mils. The mean spacing of the shells is 30 mils, the void fraction is 80%, and the linear density of a 9-inch-diameter assembly is about 3 lb.sub.m /inch. This recombiner design reduces the outlet concentration of oxidizing agents by a factor of 1000 for a length L of 18 inches at a rated flow of 1.8 ft/sec. The graph shows that the length must increase as the flow rate increases in order to maintain a constant outlet concentration. Thus, a 24-inch length is required at a rated flow of 4.3 ft/sec to obtain the same outlet concentration ratio. This can be achieved by placing 6-inch-long cartridges in series in order to accommodate different flow specifications in pipes of the same inside diameter. Pressure drop increases with overall length and flow velocity, as shown in FIG. 4. The magnitude of .DELTA.p is not sensitive to the alignment of cartridges or to details of the perforation design. Form drag is the major contributor to pressure drop in a compact design. The catalytic recombiner cartridges in accordance with the preferred embodiments of the invention can be installed in a BWR of the type depicted in FIG. 5. In such BWRs, feedwater is admitted into a reactor pressure vessel (RPV) 10 via a feedwater inlet 12 and a feedwater sparger 14, which is a ring-shaped pipe having suitable apertures for circumferentially distributing the feedwater inside the RPV. A core spray inlet 11 supplies water to a core spray sparger 15 via core spray line 13. The feedwater from feedwater sparger 14 flows downwardly through the downcomer annulus 16, which is an annular region between RPV 10 and core shroud 18. Water flowing through downcomer annulus 16 then flows to the core lower plenum 24 and into core 20, which comprises numerous fuel assemblies 22 (only two 2.times.2 arrays of which are depicted in FIG. 5). Each fuel assembly is supported at the top by top guide 19 and at the bottom by core plate 21. A mixture of water and steam exits the core and enters core upper plenum 26 under shroud head 28. Core upper plenum 26 provides standoff 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. The steam-water mixture flows through standpipes 30 and enters steam separators 32, which are of the axial-flow centrifugal type. The separated liquid water then mixes with feedwater in the mixing plenum 33, which mixture then returns to the core via the downcomer annulus. 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 needed to achieve the required power density. A portion of the water is sucked from the lower end of the downcomer annulus 16 via recirculation water outlet 43 and forced by a centrifugal recirculation pump (not shown) into jet pump assemblies 42 (only one of which is shown) via recirculation water inlets 45. The BWR has two recirculation pumps, each of which provides the driving flow for a plurality of jet pump assemblies. The pressurized driving water is supplied to each jet pump nozzle 44 via an inlet riser 47, an elbow 48 and an inlet mixer 46 in flow sequence. The catalytic recombiner cartridge in accordance with the preferred embodiments of the invention can be installed at key locations in the reactor, e.g., in the inlet mixers 46, in the recirculation water outlets 43 or in the core spray sparger 15. Each of these are locations with high concentrations of dissolved oxygen and hydrogen peroxide. Preferably, hydrogen gas is injected upstream of the cartridges. If cartridges are installed in the inlet mixers, the recirculation water and reactor vessel downcomer water (normally having high concentrations of dissolved oxygen and hydrogen peroxide) will flow over the recombiner cartridge surfaces and react with a small hydrogen gas addition to the feedwater. Thus, the catalytically reacted water entering the core lower plenum 24 via the jet pump nozzle 44 through the jet pump throat will be very low in dissolved oxidizing agents. Consequently, the ECP is reduced below the SCC threshold, preventing SCC from occurring in this difficult and costly to repair area. In the absence of the recombiner cartridge, the lower plenum components, such as vessel bottom-head penetrations and access-hole covers, would be susceptible to SCC. The catalytic recombiner cartridge of the invention can be installed in a cylindrical pipe using shim rings. In the case where the diameter of the pipe is decreasing in the downstream direction, the cartridge can be installed with a shim ring wedged between the upstream end of the cartridge and the pipe wall. Although the preferred embodiments have been disclosed in the specific context of BWRs, in principle the invention is applicable to any system made of austenitic stainless steels subject to corrosion from low concentrations of dissolved chemical species from diverse sources. In addition, persons of ordinary skill in the art of nuclear reactor engineering will recognize that the geometry of the catalytic recombiner cartridge in accordance with the invention will depend on the specific design of the component in which the recombiner cartridge is to be installed. Moreover, the structure disclosed herein can be made from catalytic material other than water recombination catalyst to form catalytic reactors other than water recombiners. The preferred embodiments have been disclosed for the purpose of illustration only. Variations and modifications of those embodiments will be readily apparent to mechanical engineers of ordinary skill. For example, the cylindrical shells or planar sheets could be supported in a concentric or parallel array respectively by rounded protuberances distributed over the surfaces instead of by welded spacers. In accordance with a further alternative, shells of rectangular or square section could be used in place of the hexagonal shells of the third preferred embodiment. This variation would be especially useful in pipes of rectangular or square section. All such variations and modifications are intended to be encompassed by the claims appended hereto.