Leakage of primary coolant from the canopy seal welds of control rod drive mechanism (CRDM) for PWR has been reported not only in Japan but also all over the world since the 1970s. The canopy seal weld is a circular weld designed to retain the pressure of reactor coolant while the strength of joints is maintained by thread. Since the coolant inside the canopy seal welds makes it easier for dissolved oxygen density to increase when a plant starts up, stress corrosion cracking (SCC) becomes a potential issue. Consequently, an advanced CRDM housing that eliminates all canopy seal welds has been studied, verified, and now applied to Vessel Head Replacement (VHR) plants in Japan. © 2002 Elsevier Science B.V. All rights reserved.
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