Patent Number: 
Section: description

Referring now to the drawings, particularly to FIG. 1, there is illustrated a reactor pressure vessel, generally designated 10, having a reactor pressure vessel wall 12 and an inner core shroud 14 defining a generally annular space 16 therebetween. The annular space 16 contains coolant. As in a typical boiling water nuclear reactor, a plurality of jet pumps, one being generally designated 18, are disposed at circumferential spaced positions about the pressure vessel between the pressure vessel wall 12 and the core shroud 14 and in the annular space 16. Each jet pump 18 typically comprises an inlet riser 20, a transition piece 28 adjacent the upper end of the inlet riser 20, a pair of elbows 22, inlet-mixers 23, each including nozzles 24 and mixing sections 25, and diffusers 26. Holddown assemblies adjacent the top of the jet pump 18, together with a number of braces and restraints maintain each jet pump 18 in fixed position in the annular space 16 between the core shroud 14 and pressure vessel wall 12. A thermal sleeve 32 penetrates the pressure vessel wall 12 and is welded at its juncture with an inlet elbow. The opposite end of the inlet elbow is secured to the lower end of the inlet riser 20. It will be appreciated that the foregoing-described jet pump 18 is conventional in construction. Thus, coolant enters the thermal sleeve 32 and flows through the elbow, upwardly in the inlet riser 20, through the inlet elbows 22 through nozzles 24 for flow in a downward direction through the mixing sections 25, the diffusers 26 and into a plenum 40 for upward flow through the reactor core. As conventional, the jet pump nozzles 24 induce a suction flow of coolant from the annular space 16 into the mixing section 25 which mixes with the coolant flow through the jet pump nozzles 23. Referring more particularly to FIG. 2, there is illustrated a portion of a jet pump 18 having an inlet elbow 22 adjacent five nozzles 24. The nozzles 24 are supported above the mixing sections 25 and define therewith a generally annular suction flow passage 29 between the nozzles 24 and an inlet to the mixing section 25. It will be appreciated that the mixing section 25 is a cylindrical pipe which terminates at its lower end in an inlet to the diffuser 26. Consequently, the flow of coolant through the nozzles 24 induces a suction flow of coolant through the annular spacer 16 for flow into the mixing section 25. These combined nozzle and suction flows pass through the mixing section 25 and diffuser 26 and into plenum 40. Referring now to FIG. 3, there is illustrated two of the nozzles 24. It will be appreciated that the interior passages through nozzles 24 are conical in shape with the diameter decreasing along the path of the fluid flow, thereby increasing the flow velocity into mixing section 25. The increased velocity induces additional fluid to flow into the sleeve through the annular opening 29 between the nozzles 23 and the mixer sleeve inlet as indicated by the arrows in FIG. 2. In accordance with a preferred embodiment of the present invention, the inlet-mixer is provided with a coating that inhibits or eliminates xe2x80x9ccrudxe2x80x9d build-up. To accomplish this, the inlet-mixer 23 is placed in a chemical vapor deposition (xe2x80x9cCVDxe2x80x9d) reactor. The reactor is a heated vacuum vessel that is sufficiently large to house these parts. The vessel is then evacuated and the pressure is dropped to approximately 20 mtorr. Heat is applied to raise the temperature of the vessel and the part to a reaction temperature within a range of about 400xc2x0-500xc2x0 C. and preferably about 450xc2x0 C. When the vessel reaches the reaction temperature and pressure, chemical precursors, such as Ti(OC2H5)4 or Ta(OC2H5)5, are vaporized in the reactor chamber as a gas. These precursors impinge on the surface of the heated inlet-mixer part and thermally decompose to form a ceramic oxide coating, comprising, e.g., TiO2 or Ta2O5, and byproduct gases. The coating continues to form and to grow until the gas flow is terminated and the temperature decreased. When a sufficiently thick coating is achieved, e.g., within a range of about 0.5-3 microns and preferably about 1.0 micron, heating is terminated and the vessel cools. The vacuum is then released and the coated jet pump part removed. The coating is indicated 31 in FIGS. 2 and 3 along the interior wall surfaces of the inlet-mixer 23. The coating may comprise any dielectric coating, e.g., tantala (tantalum oxide, Ta2O5), titania (titanium oxide TiO2), and zirconia (ZrO2). However, in the preferred form, the dielectric coating is comprised of a ceramic oxide, preferably TiO2 or Ta2O5. Thus, the application of this ceramic oxide coating reduces the electrical potential between the metal of the inlet-mixers and the charged particles in the water, minimizing or eliminating the build-up of xe2x80x9ccrudxe2x80x9d on the surfaces of the inlet-mixers. That is, the rate of ion movement toward the inlet-mixer surface is significantly reduced or eliminated. Further, as a result of the above, the coating also serves to retard or eliminate stress corrosion cracking. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.