In a nuclear reactor, the fission of 235 U, on an average, is characterized by the release of two to three neutrons which fall under two categories namely prompt and delayed neutrons[24]. The former are released almost instantaneously whereas the latter are released after significant time delays subsequent to a fission. The time delays associated with delayed neutrons vary from a fraction of a second to about a minute. Moreover, phenomena due to fission products like xenon have even much larger time constants, of the order of several hours. Hence, a nuclear reactor system is essentially characterized by a multi-time-scale behavior. Likewise the AHWR also exhibits a three-time-scale behavior with eigenvalues falling in three distinct and widely separated clusters, as described in chapter 4. © 2013, Springer-Verlag Berlin Heidelberg.