Analysis of control rod ejection accidents in large boiling water reactors

Expressions are evaluated for the control rod velocity as the hydraulic pressure inside the control rod guide tube during a rod ejection accident and a rod drop accident. For the rod ejection transients, the effectiveness of the control rod velocity limiter is found to be strongly dependent on the discharge flow from a postulated rupture of the control rod drive thimble. The nuclear transient is analysed by means of the three-dimensional dynamic BWR simulator DANAID, which is based on nodal technique. The kinetics equations are written in modified one-energy group theory. Both Doppler feedback and moderator feedback are incorporated in the program. The Doppler effect is expecially strong, but because of the power directly deposited in the coolant the moderator feedback is also significant and rapid. Removal of 2% Δk/k rod, with a velocity corresponding to the maximum drop velocity of a General Electric control rod, should not result in any significant damage to the reactor system. The result of the rod ejection analysis is a maximum fuel enthalpy below the fracture thresholds. However, the hot channels are voided and the radioactivity from 800-1200 fuel rods is released to the coolant. (author)