In this paper we examine the predicted response of a fission chamber detector in the spent fuel pool at the Atucha-I reactor using the adjoint methodology with the aim of detecting proliferation. Burnup calculations to determine material composition and intrinsic neutron source are performed using the ORIGEN-ARP depletion code. Sub-critical multiplication is modeled using a simplified fission-matrix method. Fission-matrix coefficients are determined using MCNP for several burnups and decay times, which can be interpolated to arbitrary values. This method can quickly and accurately calculate the subcritical multiplication for a pool of any size, assembly burnup and cooling time. Adjoint function calculations for a fission chamber placed in the pool were performed using the PENTRAN Sn code. These results show that the detector field-of-view (FOV) is relatively insensitive to detector position within the pool, assembly burnup and cooling time. The adjoint results are coupled with the source calculations to predict the detector response in the spent fuel pool under both normal and proliferation scenarios. Two spent fuel diversion scenarios are examined, including substitution with dummy assemblies and attempted masking of a dummy assembly using a high-burnup assembly. Both assembly diversion scenarios show a predicted deviation from unperturbed response by at least 20%.