[en] An analytical/experiment method was developed to monitor the subcritical reactivity and unfold the k/sub infinity/ distribution of a degraded reactor core. The method uses several fixed neutron detectors and a Cf-252 neutron source placed sequentially in multiple positions in the core. Therefore, it is called the asymmetric multiple position neutron source (AMPNS) method. The AMPNS method employs the nucleonic codes to analyze in two dimensions the neutron multiplication of a Cf-252 neutron source. The method uses the tilt independent asymmetric count rate ratios of two neutron detectors positioned at opposite sides of the core to determine the core k/sub eff/. An optimization program, GPM, has been utilized to unfold the K/sub infinity/ distribution of the degraded core, in which the desired performance measure minimizes the error between the calculated and the measured count rates of the degraded reactor core. The analytical/experimental approach was validated by performing experiments using the Penn State Breazeale TRIGA Reactor (PSBR). The experiments involved supercritical, critical and subcritical cores. A comparison of the experimental data with the analytical results showed good agreement, indicating the analytical model of the core used is valid