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Development of Space-Time Kinetics Techniquee to Analyze Localized Perturbations in Pressurized Water Reactor
M. Philip Soris Fernando, Ph.D, 03
Supervisor(s): J. B. Doshi

This work concerns with the development of space-time kinetics technique which is accurate and fast to analyze localized perturbations in Pressurized Heavy Water Reactor (PHWR). Indirect solution based on modal synthesis is adapted for space-time kinetics solution. To overcome the shortcomings of the simple modal expansion technique, a local effect correction is introduced to take advantage of faster space-time solution characteristic of modal expansion technique as well as not compromising the accuracy of the solution. The local perturbation is accurately treated by solving the local perturbation by means of finite difference formulation and used to correct the flux predicted by simple modal expansion technique. In order to validate the faster space-time kinetics solution, an accurate but time consuming solution by improved quasistatic (IQS) method has been obtained and used as the benchmark solution. Solutions are also validated using appropriate transient benchmarks available in the literature. The 2-D benchmark problem is a realistic representation of CANDU-PHW reactor. The reactor core is divided into 12 regions and consists of inner and outer fuelled regions surrounded by a heavy water reflector. The transient represents a non-uniform loss-of-coolant-accident (LOCA) and a subsequent asymmetric insertion of reactivity devices. The initiating perturbation is due to coolant voiding in one-half of the reactor core. The 3-dimensional benchmark problem chosen is based on the more realistic representation of CANDU-PHWR. The reactor dimensions are 780 cm in X and Y directions and 600 cm in the Z direction. The reactor core is divided into three regions in the radial direction (X-Y plane) consisting of inner core, outer core and reflector. Having established the accuracy of IQS3D method, the benchmark problems are used to test the accuracy and speed of the model developed, in the present work. The present method is based on modal synthesis method coupled with finite difference solution for local perturbation and the programme developed is called FDLP. Considerable gain in computational time is achieved using modal synthesis methods. Methods using modal synthesis along with local perturbation correction have improved the accuracy considerably with only a little increase in the computational time. 2-D benchmark and 3-D benchmark solutions have been obtained by using the method developed in this work. The benchmark problems taken for solution represent non-uniform LOCAs along with asymmetric insertion of shutdown devices. Modal expansion method has been used for the solution. The modes chosen for the expansion are the lambda modes or static modes which are generated considering the steady state before the start of the transient. The accuracy of the modal expansion method is improved by including local perturbation correction. The local perturbation is accurately treated by solving the local perturbation by means of finite difference formulation. Thus a powerful tool, which is computationally inexpensive, has been developed to accurately analyze the 3-D space-time kinetics problems. This method can be used for strictly localized perturbations as well for transients which involve global and local perturbations, giving it versatility.