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: fixing a preventive maintenance apparatus between a shroud upper body and the reactor pressure vessel;  positioning a nozzle by telescoping means at the position of a target portion of a lower water-level measuring nozzle;  impinging a water jet from the 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 target portion of the lower water-level measuring nozzle. 2. A preventive maintenance method of a structural member in a reactor pressure vessel according to  claim 1 , wherein a nozzle head driver extendable and contractible in the axial direction of the lower water-level measuring nozzle is used for positioning the nozzle at the position of the target portion. claim 1 3. A preventive maintenance method of a structural member in a reactor pressure vessel according to  claim 1 , wherein a fixing apparatus extendable and contractible in the axial direction of the nozzle is used for fixing the preventive maintenance apparatus. claim 1 4. A preventive maintenance method of a structural member in a reactor pressure vessel according to  claim 1 , wherein a distance between the nozzle and the plane surface of the deflector is at most 100 times as large as a hole diameter of the nozzle. claim 1 5. 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: fixing a preventive maintenance apparatus between a shroud upper body and the reactor pressure vessel;  positioning a nozzle telescoping means at the position of a target portion of a lower water-level measuring nozzle;  impinging a water jet from the 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 target portion of the lower water-level measuring nozzle.