Study of startup conditions of a pulsed annular reactor

A new concept of reactor, which combines features of pulsed and stationary reactors, was proposed to produce intense neutronic fluxes. Such a reactor, known as Very Intense Continuous High Flux Pulsed Reactor (VICHFPR), consists of a subcritical core with an annular geometry and pulsed by a rotating reflector that acts as a reactivity modulator as it produces a short pulse (approximately equal to 1 ms) of high intensity, leading the region near the pulser to prompt supercritical state.This work tends to analyze the startup conditions of a pulsed annular reactor. The neutron pulse evolution is analyzed since the reactivity modulator is brought upwards according to a helicoidal path from its initial position (distant from the core – where the multiplication factor has a subcritical value) up to the final position (near the core), in which a prompt supercritical state is reached. Part of the analysis is based on the variation of neutron reflection, which is a uniform function of the exit and reflection angles between the core and the modulator.As the initial and final parameters are known, a programming code is worked out to provide the multiplication factor and the flux intensity evolution. According to the obtained results, the conditions under which the modulator must be lifted up during the startup are established. Basically, these conditions are related to the analysis of the raising and the rotating velocities, the reflector saving and the initial distance between the reactor and the modulator.The pulsed annular reactor startup was divided into three stages. Because of its negative reactivity in the first two stages, the neutron multiplication is not large, while the last one, having a positive reactivity, shows an intense multiplication as is usually expected when handling pulsed systems. This last stage is quite sensitive to some reactor parameters such as the modulator rotation velocity and the final reactivity, but has little sensibility to the other reactor parameters.

► A new concept of reactor, combining features of pulsed and stationary reactors, was proposed to produce intense neutron fluxes.
► This reactor consists of a subcritical core with annular geometry and it is pulsed by a reactivity modulator.
► The startup of this reactor was investigated by using the solid angle technique.
► The final pulse intensity depends strongly on final reactivity and modulator rotation velocity.