Analytical Study of the Dynamic Behavior of the Self-Controlled Marviken Boiling Heavy-Water Reactor

The quantitative influence of the void and Doppler coefficients on the dynamics of the Boiling Heavy-Water Reactor (BHWR) under different operating conditions has been examined extensively. Owing to the difficulty of calculating these coefficients exactly and their great influence on the dynamic behavior of the self-controlled reactor, these parameters have been studied over a wide range. The studies also improve understanding of the transient behavior of the BHWR concept. It has been found that undamped oscillations can arise with special combinations of the reactivity coefficients. In the nonlinear dynamic model used, the void reactivity contribution comes from the changes of the exit void and the subcooling. The oscillations induced by the reactivity feedbacks have different dampings and frequencies of ≈0.003 or ≈0.03 cps, depending on the sign and the magnitude of the void reactivity coefficients. The transient neutron flux responses to step reactivity perturbations that have the first peak (limited by the fuel Doppler coefficient) as the largest overshoot or that are strongly damped oscillations lie in a well definable area in the exit void and subcooling reactivity coefficient coordinate system. Disturbances introduced by reactivity variation and main steam-valve opening are discussed. The transient responses calculated were those of the neutron flux, reactivity, pressure, moderator temperature, exit void, average void, subcooling, and temperatures of the boiler and superheater fuel.