The LIFE@PROTEUS program at the Paul Scherrer Institut is being undertaken to characterize the interfaces between burnt and fresh fuel assemblies in modern LWRs. Techniques are being developed to measure fission rates in burnt fuel, following re-irradiation in the zero-power PROTEUS research reactor. In the presented approach, the fission rates are estimated by measuring delayed neutrons emitted by re-irradiated fuel. To demonstrate the feasibility of this technique, fresh and burnt fuel samples (with burnup varying from 36 to 64 GWd/MTU) were irradiated in the PROTEUS reactor, and their neutron outputs were recorded shortly after irradiation. Relative fission rates between different core lattice positions were derived for a fresh sample as well as for the three burnt samples. The measured fission rate ratios have 1-σ uncertainties between 2% and 3.5%, with the larger uncertainties corresponding to the more highly burnt fuel. Results obtained by Monte Carlo simulations agree with the experimentally determined values within these limits. With further development of the technique, the experimental uncertainties can be further reduced. Continuing effort is being directed towards accurate comparison of fission rates between fuel samples of different burn-up.