Abstract

MATSTAB Table of Content Abstract Conclusions

Abstract

Using frequency-domain methods, sparse matrix techniques and advanced numerical algorithms a new computer program MATSTAB has been developed to predict the core stability characteristics of a boiling water nuclear reactor. The code uses the same thermal-hydraulic model as the transient code RAMONA-3B and the same neutronic model as the online steady-state core simulator POLCA. This includes three-dimensional neutronics and an individual representation of each fuel assembly (no lumping). The very large set of equations is linearized and leads to a generalized eigenvalue problem which is solved iteratively using a combination of Newton. s method and sub-space decomposition. The tailor-made algorithm calculates the first few dominating eigenvalues (decay ratios) and their left and right eigenvectors. MATSTAB not only predicts global, but also regional oscillations. A comparison between the decay ratios of the two oscillation types allows to judge which mode will occur.

Using MATSTAB and the interface to the online steady-state core simulator POLCA, it is possible to predict the stability of the reactor core in its present state. A calculation with full spatial resolution (all fuel assemblies, 25 axial nodes) is performed within a few minutes on a standard personal computer.

The eigenvalues and eigenvectors may not only be used to calculate the decay ratio and oscillation frequency, but also to analyze the stability behavior of the coupled neutronic/thermal-hydraulic system. A new method is introduced which allows to calculate and display the contribution to (in)stability of any part of the reactor model (fuel assembly, neutronics, thermal-hydraulics, riser, pumps, etc.). It is also possible to display the contribution to (in)stability of any physical quantity (power-density distribution, void, pumps etc.). This allows to enter a new territory and possibly to gain new insights into the mechanisms behind instabilities. This new method is not yet explored in depth, but some simple judgments were already used to optimize the core design and control rod sequence with respect to stability during start up procedures in Forsmark.

The results of the code have been successfully validated against numerous stability measurements from the Forsmark, Oskarshamn (both in Sweden) and Leibstadt (Switzerland) plants. The predictions show good agreement with the measured data for all global oscillations in all the plants. The regional oscillations in Cycle seven of Leibstadt were clearly predicted by MATSTAB, but the specific values for the decay ratios were underestimated in a systematic way.

	Abstract
	Using frequency-domain methods, sparse matrix techniques and advanced numerical algorithms a new computer program MATSTAB has been developed to predict the core stability characteristics of a boiling water nuclear reactor. The code uses the same thermal-hydraulic model as the transient code RAMONA-3B and the same neutronic model as the online steady-state core simulator POLCA. This includes three-dimensional neutronics and an individual representation of each fuel assembly (no lumping). The very large set of equations is linearized and leads to a generalized eigenvalue problem which is solved iteratively using a combination of Newton. s method and sub-space decomposition. The tailor-made algorithm calculates the first few dominating eigenvalues (decay ratios) and their left and right eigenvectors. MATSTAB not only predicts global, but also regional oscillations. A comparison between the decay ratios of the two oscillation types allows to judge which mode will occur. Using MATSTAB and the interface to the online steady-state core simulator POLCA, it is possible to predict the stability of the reactor core in its present state. A calculation with full spatial resolution (all fuel assemblies, 25 axial nodes) is performed within a few minutes on a standard personal computer.
The eigenvalues and eigenvectors may not only be used to calculate the decay ratio and oscillation frequency, but also to analyze the stability behavior of the coupled neutronic/thermal-hydraulic system. A new method is introduced which allows to calculate and display the contribution to (in)stability of any part of the reactor model (fuel assembly, neutronics, thermal-hydraulics, riser, pumps, etc.). It is also possible to display the contribution to (in)stability of any physical quantity (power-density distribution, void, pumps etc.). This allows to enter a new territory and possibly to gain new insights into the mechanisms behind instabilities. This new method is not yet explored in depth, but some simple judgments were already used to optimize the core design and control rod sequence with respect to stability during start up procedures in Forsmark. The results of the code have been successfully validated against numerous stability measurements from the Forsmark, Oskarshamn (both in Sweden) and Leibstadt (Switzerland) plants. The predictions show good agreement with the measured data for all global oscillations in all the plants. The regional oscillations in Cycle seven of Leibstadt were clearly predicted by MATSTAB, but the specific values for the decay ratios were underestimated in a systematic way.