Abstract
Decay heat (DH) is the heat produced through a radioactive decay of fission products during or after a reactor operation. It is known as the second largest source of power in the core after fission. Being such a strong contributor to reactor power, it should be accurately determined at any time of reactor operation. Currently, there are two main approaches for DH estimation used in reactor simulation codes. One approach is based on careful inventorying of all produced target nuclides and their individual contributions to total power. Alternatively, the other popular approach is based on collapsing all target fission products into a small number of groups similar to delayed neutron estimation techniques. However, the last (multigroup) method currently has limitations when used in some transient scenarios such as transients occurred in fresh fuel. In this study, the multigroup method was further developed for reducing limitations while retaining the advantage in computation speed. Then, it was implemented into Reactor Analysis code for Steady state and Transient (RAST-K) and tested against other codes. As a result, the improved method was found capable of determining DH power at all tested stages of reactor operation under any tested operation scenario. In particular, the test simulations using the improved method showed better results in those scenarios that were under accuracy limitations of the original multigroup method. Overall, the quality of transient calculations in RAST-K was improved when using the newly implemented DH module.
Highlights
During a reactor operation, the dominant source of power is energy of fission released from fuel nuclides
For a preliminary testing of the decay heat (DH) model implemented in RAST-K, two transient scenarios were chosen
Values of Critical Boron Concentration (CBC) for all cases are determined in RAST-K and manually input in Purdue Advanced Reactor Core Simulator (PARCS)
Summary
The dominant source of power is energy of fission released from fuel nuclides. The most frequently studied case in this area is reactor shutdown as discussed by Schrock [2] In this scenario, nuclear fuel could contain large quantities of radioactive nuclides collected during the whole cycle operation, and must be cooled for preventing undesired outcomes such as fuel melting. As stated by Johnson et al, for the case of a detailed inventorying method, 1119 FP and 30 HM nuclide concentrations should be determined and updated over time. As for the other method, the total number is reduced to only 23 condensed nuclide group concentrations Based on such a noticeable difference, Johnson concludes that the detailed inventorying method requires much more computation time and resources compared to the multi-group condensation method. Considering given preconditions, and since the main advantage of modern nodal diffusion codes is the speed of computation, the multigroup method seems to be a better match for being used in such a code
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