Abstract
A new time-dependent homogenization approach that accounts for inter-assembly leakage has recently been proposed. The new technique extends Generalized Equivalence Theory (GET) to transient simulations through the use of time-dependent, leakage-corrected discontinuity factors that are calculated at each time step by means of a global-local iterative approach to account for the effect of neighbouring nodes so that highly heterogeneous cores are more accurately modelled than when employing single-node, zero-node-boundary-current Assembly Discontinuity Factors (ADFs). The technique has been previously tested for a one-dimensional, two-energy-group, BWR-like benchmark. The present work expands the analysis to a one-dimensional, two-energy-group, Pressurized Heavy-Water Reactor (PHWR) configuration. The PHWR configuration consists of 22 fuel nodes bounded on either side by two nodes of heavy-water (D2O) reflector. Each fuel node spans 28.575 cm and is a one-dimensional stylized representation of a 37-element, natural uranium fuel bundle with D2O coolant residing in a pressure tube that in turn resides in a calandria tube surrounded by D2O moderator. A simple transient induced by instantaneous half-core voiding of the D2O coolant is studied. Three types of calculations are performed: A reference, heterogeneous-node, fine-mesh calculation, a standardly-homogenized-node calculation and a GET-homogenized-node (using ADFs) calculation. The root-mean-square percent errors introduced by standard homogenization and ADF-based homogenization for kinetics calculations in PHWR cores are found to be 4% and 5%, respectively, after 0.5 s. This suggests that the use of a time-dependent homogenization method is desirable, and its use is shown to reduce the RMS errors to a maximum of 0.003% over the course of the transient. The conclusion is that although PHWR cores are not extremely heterogeneous, the accuracy of transient modelling for PHWRs is improved when using time-dependent homogenization over conventional ADFs and that the newly-developed time-dependent homogenization method promises to offer substantial improvements in accuracy for transient results with particular relevance to safety analyses.
Highlights
A new technique was recently presented [1] that extends Generalized Equivalency Theory (GET) [2] to time-dependent problems by determining homogenized nuclear parameters and discontinuity factors at each time step using a global-local iterative scheme
A distinguishing characteristic of the pressurized heavy-water reactor (PHWR) model is that each fuel node consists of a central fuel region of natural uranium surrounded by a significant amount heavy-water (D2O) moderator whereas for the boiling-water reactor (BWR) model, the fuel region consists of enriched uranium that occupies the majority of the node such that the moderator region, comprised of light water, is much smaller
The fuel node of the PHWR model is comprised of a 37-element, natural uranium fuel bundle with D2O coolant residing in a pressure tube that in turn resides within a calandria tube surrounded by D2O moderator, as shown in the Detailed Geometry of Fig 1
Summary
A new technique was recently presented [1] that extends Generalized Equivalency Theory (GET) [2] to time-dependent problems by determining homogenized nuclear parameters and discontinuity factors at each time step using a global-local iterative scheme. The BWR model is a seven-assembly configuration that uses reflective boundaries at the either end of the reactor model whereas the PHWR model is a 22-fuel-node configuration that has two additional reflector nodes of D2O at either end of the reactor model where vacuum boundaries are applied. A transient is introduced in the PHWR model by voiding 11 of the 22 fuel nodes in one half of the core resulting in a power rise that is followed for 0.5 s using a time-step of 0.0005 s. The node-averaged results of the transient from a full-core heterogeneous (reference) solution are compared to the homogeneous-node results of the global-local iterative scheme as well as to those of a standardly-homogenized-node calculation and a GEThomogenized-node calculation using assembly discontinuity factors
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