2重伝熱管における片側壁破損 水リーク時の健全性確認試験

Integrity confirmation test for duplex-wall heat transfer tubes in case of its inner-wall leak

浜田 広次  ; 宇佐美 正行*; 姫野 嘉昭; 田辺 裕美*

Hamada, Hirotsugu; not registered; Himeno, Yoshiaki; not registered

(目的)2重伝熱管に内管リークが発生した場合の,蒸気による外管損耗の可能性を評価することを主な目的にして,試験を実施した。(試験)加工上の制限から予め外管に人口の欠陥孔を設けた2重管の供試体を制作し,それに実機設計例相当の高温・高圧蒸気を注入する蒸気試験と,比較用データ取得のため窒素ガスを高温で通気するガス試験を行った。注入・通気時間はプラントでのリーク検出時間の当面の目標値である数時間よりも長く,保守的な結果となるように24時間とした。各試験の前後には,Heガスを用いた欠陥孔を含む流路の流動抵抗の測定から欠陥孔の等価断面積(直径)の変化量を調べた。試験後は,伝熱管壁の蒸気損耗の有無を明らかにするため,供試体の金相検査を実施した。(結果)試験後は欠陥孔の等価直径が小さくなり,閉塞に至るケースも一部見られ,蒸気損耗に起因する欠陥孔の拡大は生じていない。金相検査からは,欠陥孔近傍表面に付着物やその状況・状態の軽微な変化が一部の供試体の内外管界面部に見られたが,蒸気の衝突による有為な形状変化や組織・材質的な変化は認められない。(結論)実機設計例条件下での24時間の蒸気噴出では伝熱管壁に損耗が生じないことより,2重管に内管リークが発生してから24時間以内に噴出蒸気による損耗で外管が貫通破損する可能性は無い。

Steam wastage tests of the duplex-wall heat transfer tubes for the steam generators were conducted by placing its emphasis on the investigation of the possible occurrence of a subsequent failure on an outer-wall of the tube in case of its inner-wall leak. Based on the limitation from the test rig, the test tubes, each of which has an artificial crack on its outer-wall instead of on its inner-wall, were manufactured and were subjected to the test. In the test, a super-heated and pressurized steam as conceptual plant design or a nitrogen gas was fed to the tube and was impinged against the inner-wall through its crack for 24 hours which are conservative enough to evaluate test results. The test with a nitrogen gas was to obtain reference data. Before and after the test, equivalent hydraulic diameter of the crack was determined by measuring a pressure drop due to a flowing helium assuming that the crack can be regarded as an orifice. Then, changes in the equivalent diameter of the crack were determined. After the test, the tubes were subjected to the post-test metallurgical examination. Results thus obtained are as follows: (1)In all tubes, equivalent diameter of the cracks decreased after the test. Some cracks were even plugged by stream corrosion products. No enlargements of the crack, therefore, was found. (2)Post-test metallurgical examination showed no evidence of a steam wastage. Only steam corrosion products were found in the gap between the inner and outer walls. In conclusion, within the extent of the present test, failure possibility of the duplex-wall tube following a generation of an initial crack on an inner-wall is negligible.