Operational Experience with and Postirradiation Examinations on Boiling Water Reactor Control Rods

The control rods of the KRB-I 250-MW(electric) boiling water reactor contain Vipac B₄C powder in Type 304 stainless steel tubes as a neutron-absorbing material Because of an increase in the reactor coolant ³H activity, defective control rods were suspected. The hot cell examination of a highly exposed control rod revealed B4C losses. The mechanism of failure was shown to be B4C swelling and stress corrosion cracking of the absorber tubes, followed by B4C washout. The B4C volume swelling is ΔV(%) = 0.851x + 0.0449x² [x = ¹⁰B decays in 10²¹(n,α)/cm³]. The tube cracking starts at 30 to 35% and the B₄C washout at 50 to 55% local ¹⁰B burnup in the tubes.