Patent Number: 
Section: claims

1. A preventive maintenance method of a structural member in a reactor pressure vessel for reducing a tensile residual stress on a surface thereof comprising the steps of: impinging a water jet from a nozzle onto a plane surface of a deflector to thereby change direction of flow of said water jet; and  impinging the water jet after being deflected directly onto the surface of the structural member to be treated. 2. A preventive maintenance method of a structural member in a reactor pressure vessel according to  claim 1 , wherein a distance between the nozzle and said plane surface of the deflector is at most 100 times as large as a hole diameter of the nozzle. claim 1 3. A preventive maintenance method of a structural member in a reactor pressure vessel according to  claim 2 , wherein said distance is at most 50 times as large as the hole diameter of the nozzle. claim 2 4. A preventive maintenance method of a structural member in a reactor pressure vessel according to  claim 1 , wherein an angle formed between a central axis passing through an opening of the nozzle and said plane surface of the deflector is in a range of 10xc2x0 to 90xc2x0. claim 1 5. A preventive maintenance method of a structural member in a reactor pressure vessel according to  claim 4 , wherein said angle is in a range of 40xc2x0 to 90xc2x0. claim 4 6. A preventive maintenance method of a structural member in a reactor pressure vessel according to  claim 5 , wherein said angle is in a range of 60xc2x0 to 90xc2x0. claim 5 7. A preventive maintenance method of a structural member in a reactor pressure vessel for reducing a tensile residual stress on a surface thereof, comprising the steps of: impinging a water jet from a nozzle onto a recess of a deflector to thereby change direction of flow of said water jet; and  impinging the water jet after being deflected directly onto the surface of the structural member to be treated. 8. A preventive maintenance method of a structural member in a reactor pressure vessel according to  claim 7 , wherein said recess is in shape of cone with an apex angle of at least 90xc2x0 in a longitudinal cross section thereof. claim 7 9. A preventive maintenance method of a structural member in a reactor pressure vessel according to  claim 8 , wherein said apex angle is at least 120xc2x0. claim 8 10. A preventive maintenance method of a structural member in a reactor pressure vessel according to  claim 9 , wherein said structural member is a core shroud, an in-core monitor housing or a water-level measuring nozzle. claim 9