Post-irradiation examination of "JOYO" MK-II control rods; Study on unusual mechanical behavior of absorber pin(2)

Maruyama, Tadashi; Kaito, Takeji; ; ; Unno, Ichiro

PNC TN9410 94-293, 88 Pages, 1994/05

PNC-TN9410-94-293.pdf:5.25MB

Post-irradiation examination (PIE) for "JOYO" MK-II control rods has been carried out with thirteen subassemblies, five of which indicated occurence of absorber pin breach. The present report describes results of the PIE made for the breached pins and studies were made on features on the breached pins and the cause of breach mechanisms. It was observed that the threshold burnup of breached pins was 5710 cap/m above which total 15 Pins breached. The pins with wider gap between B4C pellet and cladding tended to breach at lower burnup. In the breached pins, extensive absorber cladding mechanical interactions (ACMI) due to relocation of BC pellet took place together with accumulation of He in the cladding material to make He embrittlement. It is inferred that these phenomena caused pin breach when they were in the reactor core. However, it is very difficult to clearly specify the time whether the pin breach occured during the reactor operation or at the time when the reactor was shut down. The results of the present investigation did not support the conclusion made in the previous report that the main cause of pin breach was the stress corrosion cracking and pin breach took place when they were removed from the reactor core. It was concluded that reducing the ACMI due to relocation of BC pellet is very important for extension of life of "JOYO" absorber pin.