Simulating the Power Rundown Transient from Poison Injection System for a CANDU Reactor
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Abstract
CANDU reactors have two independent, diverse, and physically separated shutdown systems. The second shutdown system is the liquid poison injection system. It consists of tanks (six to eight) containing neutron absorbing gadolinium nitrate dissolved in heavy water. Each tank is connected through pipes to a horizontal, cylindrical, multi-hole injection nozzle extending into the reactor moderator. When a trip signal occurs, high pressure helium drives the gadolinium-heavy water poison solution through the holes of the injection nozzles into the moderator. To simulate reactor shutdowns by the poison injection system, the injection of the poisoned fluid into the moderator is modelled by a hydraulics code, and t he neutronics and reactivity transients are modelled by a series of reactor physics codes. This paper deals with the development and validation of the reactor physics component of this model, using the latest improved methodology. It describes the different steps used in modelling the poison evolution in the moderator and simulating the power rundown. It compares measured power rundown transients from power reactors with the corresponding simulations. Data on poison injection power rundown transients were obtained during low power phase B commissioning of Bruce NGS B. In addition, a test of poison injection system effectiveness was recently performed on unit 1 of Bruce NGS A. These transients were simulated with the model we have developed for comparison with the experimentally obtained results. The comparison between the simulated transients and the experimental results shows a reasonable agreement of the reactivity and the neutronic flux transients.
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