[en] Large thermal power reactors operating at sufficiently high flux levels are susceptible to oscillations in the power distribution that are associated with a periodic redistribution of xenon poison. To investigate the xenon-induced spatial instabilities of nuclear cores a method based upon an extended lambda-mode linear analysis has been used and is presented in this report. A detailed description of the lambda-mode mathematical formalism, its incorporation in the computer program MODAX, and some applications to CANDU reactors are reported. (author)