PART

[001] The present disclosure relates generally to cavitation peening of internal surfaces of hollow parts and more specifically to methods of cavitation peening of internal surfaces of nuclear reactor primary nozzles.

BACKGROUND

[002] Peening is a process of introducing mechanical stress into the surface layer of a part to compress and strengthen it against future fractures and wear. Peening can be performed in a variety of manners, including shot peening, laser peening and cavitation peening. Cavitation peening involves the application of bubbles onto the surface with the part in a liquid environment. The collapsing of the bubbles imparts impactive forces to the part.

[003] A nuclear power plant typically has a nuclear reactor and a reactor coolant system (RCS) for removing heat from the reactor and to generate power. The two most common types of reactors, boiling water reactors (BWRs) and pressurized water reactors (PWRs), are water-based. In a pressurized water reactor (PWR), pressurized, heated water from the reactor coolant system transfers heat to an electricity generator, which includes a secondary coolant stream boiling a coolant to power a turbine. The RCS section downstream of the electricity generators but upstream of the reactor is typically called the cold leg, and downstream of the reactor and upstream of the electricity generators is typically called the hot leg.

[004] PWRs typically have either three hot legs and three cold legs or, more commonly in the United States, four hot legs and four cold legs. A PWR reactor vessel thus typically will have six or eight primary nozzles connecting the hot and cold legs to the reactor vessel. Tubing of the hot or cold leg typically is welded to the nozzle at a primary nozzle weld. [005] WO 2016/085745, WO 2016/085747, EP 0622156 Al and JP 4831807 disclose methods of peening parts of a closure head of the nuclear reactor pressure vessel.

[006] U.S. Publication No. 2013/0074561 and U.S. Publication No. 2013/0233040 disclose Ultra High Pressure (UHP) cavitation peening technology.

SUMMARY OF THE INVENTION

[007] The commercial nuclear industry is required to perform inspections of Inconel 600 nozzles and welds due to the emergence of Primary Water Stress Corrosion Cracking (PWSCC). In multiple cases, repairs of these nozzles and welds have been required as these inspections have revealed small indications or, in some cases, through-wall cracking resulting in leakage of reactor coolant through the pressure boundary.

Numerous repair methods have been instituted in order to repair the Inconel 600 nozzles and attachment, pressure boundary weld.

[008] Residual tensile stresses in nozzle material, weld material, and base metal cladding contribute to and exacerbate PWSCC. Changing the stress state from tensile to compressive can prevent PWSCC issues, mitigating the need for costly and time consuming repairs. Peening provides asset life extension through elimination of the degradation process.

[009] A method for cavitation peening of an internal surface of a hollow metal part including an internal surface is provided. The method includes installing a first seal in sealing contact with the internal surface; installing a second seal axially offset from the first seal, the first seal and the second seal defining a chamber within the hollow part axially therebetween; providing a cavitation peening nozzle of a cavitation peening tool within the chamber; pressurizing the chamber; and cavitation peening the internal surface via the cavitation peening nozzle while actuating the cavitation peening nozzle within the pressurized chamber.

[0010] An apparatus for cavitation peening of an internal surface of a hollow metal part, the hollow part including an internal surface is also provided. The apparatus includes a first seal for contacting the internal surface at a first location, a second seal for contacting the internal surface at a second location spaced axially from the first location such that the first seal and the second seal define a chamber within the hollow part axially therebetween, and a cavitation peening tool including a cavitation peening nozzle positioned axially between the first and second seals configured for cavitation peening the internal surface of the hollow part within the chamber. The cavitation peening tool includes at least one actuator for moving the cavitation peening nozzle at least one of axially, radially and rotationally within the chamber.

[0011]

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The present invention is described below by reference to the following drawings, in which:

[0013] Fig. 1 schematically shows a section of a PWR nuclear power plant;

[0014] Fig. 2 shows a cut-away view of a hollow part including a primary nozzle, and showing a maintenance apparatus in position to install a first seal into the hollow part;

[0015] Fig. 3 shows the hollow part after the first seal and second seal are inserted into and appropriately axially aligned in the hollow part; and

[0016] Fig. 4 shows the hollow part after the first and second seal are radially expanded to a create a sealed chamber within the hollow part.

DETAILED DESCRIPTION

[0017] The present disclosure provides a method by which UHP cavitation peening technology, a surface stress improvement process is applied inside a hollow part. In one preferred embodiment, the hollow part is formed by a primary nozzle of a reactor vessel and a tubing connected to the primary nozzle and a weld region connecting the tubing to the primary nozzle. More specifically, the cavitation peening method is applied for mitigating degradation of the inner diameter surfaces of Primary Water Reactor (PWR) vessel hot and cold leg primary nozzles. Because the primary nozzles are located inside the reactor vessel, which is kept under more than twenty feet of water while open for serving during shutdown, delivery of peening equipment to the target area is difficult. This present method and system allows for that delivery to be performed remotely in a timely manner

[0018] in preferred embodiments, the method includes installing seals at both ends of the primary nozzle, creating a localized chamber which can be pressurized to the desired back pressure, which can be 1 to 100 psi or greater, depending on the specific application. A cavitation peening nozzle is installed between the seals inside the seal chamber. The cavitation peening process initiates and the tool assembly which includes the peening nozzle is driven to rotate around the axis of the primary nozzle so that the entire inner diameter surface of the primary nozzle is peened As the peening head rotates the peening nozzle also actuates radially in and out as needed for the optimal process effectiveness.

[0019] Fig. 1 schematically shows a section of a nuclear power plant 10 including a PWR 12. PWR 12 includes a pressure vessel 14 housed in a containment 16. PWR 12 is shutdown for servicing, with a closure head of the pressure vessel 14 being removed from the remainder thereof and containment 16 being flooded with liquid 17, which in this embodiment is water, such that pressure vessel 14 is submerged in the water 17. Primary nozzles 10 connects the hot or cold leg of a reactor coolant system to pressure vessel 14, with tubing 18 of the hot and cold legs each being connected to pressure vessel 14 by primary nozzles 20. More specifically, primary nozzles 20 are integrally connected to a cylindrical wall 34 of pressure vessel 14. An internal cavity 21 of pressure vessel 14, nozzles 20 and tubing 18 are all flooded with water. For simplicity, the internal components of pressure vessel 14 within internal cavity 21 are omitted. A maintenance apparatus 22, which is schematically shown in Fig. 1 but is described in further detail below, is provided for cavitation peening an internal surface 24. Maintenance apparatus 22 is remotely controlled by a controller 26, which may be in wired or wireless communication with maintenance apparatus 22, positioned outside of containment 16. Controller 26 may include a computing unit 28, a display 30 and a user input device 32.

[0020] Fig. 2 shows a cut-away view of a hollow part 39 including primary nozzle 20 and a maintenance apparatus 22 beginning to install a first seal 46 into hollow part 39. Primary nozzle 20 includes a first end 20a, or a vessel-side end, integrally connected to cylindrical wall 34 of pressure vessel 14 and fluidly connected to internal cavity 21, and a second end 20b physically connected to tubing 18 at a weld region 36 by at least one weld 38 and fluidly connected to tubing 18. Primary nozzle 20, weld region 36 and tubing 18 together formed a hollow part 39. Internal surface 24 of hollow part 39 includes a frustoconical surface 40 of primary nozzle 20, a cylindrical surface 42 of weld region 36 and a portion of a cylindrical surface 44 of tubing adjacent to weld region 36.

[0021] Maintenance apparatus 22 includes a seal support 46 configured for holding a first seal 48 and a second seal 50 in hollow part 39. Seal support 46 includes at least one support arm 52 extending from a first or cavity-facing side 48a of first seal 48 to a first or tubing-facing side 50a of second seal 50. Support arm 52 is configured for moving first seal 48 axially with respect to second seal 50. In the embodiment shown in Fig. 2, seals 48, 50 are radially expandable to contact internal surface 24 of hollow part 39 in a sealing manner. More specifically, seals 48, 50 are inflatable by supplying each of seals 48, 50 with a gas such as air after seals 48, 50 are appropriately axially positioned within hollow part 39, with the gas being provided by an external supply from an umbilical. In this embodiment, first seal 48 may be axially movable relatively to seal 50 by support arm 52 to appropriately position the seals 48, 50 in hollow part, which allows maintenance apparatus 22 to be used with hollows parts having different axial lengths. Additionally, the radial expansion capacity of seals 48, 50 is such that seals 48, 50 are configured for use in different hollow part diameters, or even two different diameters in the same hollow part - as is the case with hollow part 39.

[0022] Maintenance apparatus 22 further includes a floatation 54 for positioning within cavity 21 of pressure vessel 14. Floatation 54 can be pre-defined to allow maintenance apparatus 22 to be naturally drawn in front of the primary nozzle 20 or floatation 54 can include tanks to drawn in air or fluid in the same manner a submarine to dynamically adjust the floatation capacity of floatation 54.

[0023] Connectors 56 are also provided on maintenance apparatus 22 for connecting to a tool such as poles for lowering maintenance apparatus 22 into the cavity 21 of pressure vessel 14 after the closure head has been removed and removing maintenance apparatus 22 from the cavity 21 of pressure vessel 14 after the peening is completed. Connectors 56 are fixed to a base section 58 of seal support 46. Base section 58 connects floatation 54 to second seal 50 and is provided with further tooling for stabilizing maintenance apparatus 22 within cavity 21 of pressure vessel 14 in alignment with hollow part 39. More specifically, between floatation 54 and second seal 50, base section 58 is provided with a chuck cylinder 60 on one side thereof and chuck feet 62 on the other side thereof for contacting internal surface 24 to wedge maintenance apparatus 22 in place for peening. Additionally, base section 58 is provided with thrusters 64, which are positioned on a side 55a of floatation 54 opposite of seals 48, 50 - i.e., a hollow part facing side 55a of base section 58, for thrusting against further internals of the pressure vessel 14 to provide an axial force component to axially stabilize seals 48, 50 in place in hollow part 39 during peening. A counterweight 66 and an adjustable counterweight 68 are additionally provided on base section 58 on a side 55b of floatation 54 opposite of seals 48, 50 - i.e., a cavity side 55b of base section 58 to provide a counter balance to the tooling connected to side 55a of base section 58 such that floatation 54 is aligned without tilting in hollow part 39. Fig. 3 shows hollow part 39 after seals 48, 50 are inserted into hollow part 39, with an outer circumferential surface 48b of seal 48 aligned with a predetermined location of inner circumferential surface 44 of tubing 18 and an outer circumferential surface 50b of seal 50 being aligned with a predetermined location of inner frustoconical surface 40 of primary nozzle 20. Chuck cylinder 60 and chuck feet 62 are in wedging contact with internal surface 24, specifically at frustoconical surface 40. A second or tubing-facing side 48c of seal 48 faces tubing 18 and seal 48 is axially aligned for sealing the interior of primary nozzle 20 from a majority of tubing 18. To properly align seals 48, 50 in the predetermined locations, controller 26 instructs movable arm 52 to increase the axial distance between seals 48, 50. Additionally, a further movable arm 70 connecting seal 50 to floatation 54 can actuate seals 48, 50 axially to properly align seals 48, 50 in the predetermined locations.

[0024] After seals 48, 50 are aligned within hollow part 39 in the predetermined locations, controller 26 instructs the radial expansion of seals 48, 50 such that seals 48, 50 are forced into sealing contact with internal surface 24 of hollow part 39. More specifically, as shown in Fig. 4, outer circumferential surface 48b of seal 48 is forced into sealing contact with the predetermined location of inner circumferential surface 44 of tubing 18 and outer circumferential surface 50b of seal 50 is aligned with the

predetermined location of inner frustoconical surface 40 of primary nozzle 20. First side 52a of seal 52 faces internal cavity 21 and seal 52 seals the interior of primary nozzle 20 from cavity 21.

[0025] Seals 48, 50 seal off hollow part 39 and the space between seals 48, 50 to form a chamber 72 which is then pressurized, upon instruction by remote controller 26, to form a pressurized chamber within hollow part 39. To pressurize chamber 72, low pressure water is pumped into the sealed chamber by a pump 73, which may be for example a well pump, and there is also an outlet for example in the form of a ball valve that allows the regulation of the back pressure inside the sealed chamber. Chamber 72 is delimited axially by seals 48, 50 and circumferentially by internal surface 24, more specifically by frustoconical surface 40, cylindrical surface 42 and cylindrical surface 44. In one preferred embodiment, chamber 72 is pressurized to between 1 and 100 psi.

[0026] Maintenance tool 22 also includes a cavitation peening tool 74 including a cavitation peening nozzle 76 positioned axially between cavity-facing side 48a of first seal 48 and tubing-facing side 50a of second seal 50 within chamber 72. In this embodiment, pressurized liquid, such as water, is ejected from peening nozzle 76, causing cavitation bubbles to form. A nozzle flow 78 is directed at weld internal surface 24, causing the cavitation bubbles to settle thereon. Nozzle flow has a center axis 78 that forms an angle a of between 0 and 90 degrees with internal surface 24. In one preferred embodiment, an optimal angle a is 10 degrees. The collapsing impact of the cavitation bubbles imparts compressive stress in the materials of the internal surface 24, and specifically the surface 42 of at least one weld 38, frustoconical surface 40 and cylindrical surface 44.

[0027] Peening tool 74 includes a multi-dimensional actuator including a linear actuator 75a for moving peening nozzle 76 axially with respect to a center axis of hollow part 39 within chamber 72, a rotational actuator 75b for rotating peening nozzle 76 about the center axis of hollow part 39 within chamber 72, and a further linear actuator 75c for moving peening nozzle 76 radially with respect to the center axis of hollow part 39 within chamber 72.

[0028] The radial actuation allows peening nozzle 76 to be at the proper standoff distance - the distance from the peening nozzle to the point of impact on internal surface 24 - and the proper angle of the internal surface 24 of hollow part 39. In one embodiment, nozzle 76 is rotated around the center axis of hollow part 39 to creates a ring of peened material, then nozzle is actuated axially and nozzle 76 is rotated around the center axis of hollow part 39 to an additional ring of peened material. This process is repeated until an entirety of a predetermined area of internal surface 24 to be peened is peened. In another embodiment, nozzle 76 is actuated in a helical path, with nozzle 76 being simultaneously actuated axially and rotationally. The rotational, radial and axial actuations can be performed simultaneously or in sequence based on a predetermined peeing path pre- established from CAD data calculation and stored in computing unit 28 of controller 26.

[0029] The method according to the exemplary embodiment described with respect to Figs. 1 to 4 thus includes installing first seal 48 in sealing contact with internal surface 24 and installing second seal 50 axially offset from first seal 48 in sealing contact with internal surface 24 such that first seal 48 and second seal 50 define a chamber 72 within hollow part 39 axially therebetween. The method also includes providing cavitation peening nozzle 76 within chamber 72, pressurizing the chamber and then cavitation peening internal surface 24 via cavitation peening nozzle 76 while actuating cavitation peening nozzle 76 within pressurized chamber 72.

[0030] In the preceding specification, the invention has been described with reference to specific exemplary embodiments and examples thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of invention as set forth in the claims that follow. The specification and drawings are accordingly to be regarded in an illustrative manner rather than a restrictive sense.