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.