For pressurized water reactor fuel, to correlate clad failure characterization and localization methods with primary cooling water gamma measurements, it was necessary to define the set of equations describing fission product (FP) generation and transport. Such a technique has been developed by the French Commissariat à l’Energie Atomique (CEA) and Framatome. It required working out the computer code PROFIP.3. The main features of the code are calculations of the FP source term and release mechanisms from the fuel into the coolant through the gap and failed cladding, with mass balance in the coolant. Framatome and CEA experience over past years provided a large data base from which the main characteristics—gaseous FP ratio and release coefficients—have been determined. Localization at the defective elements is based mainly on the 134Cs-to-137Cs ratio during transient periods correlated to burnup. Measurements taken by a sipping test during the refueling periods and comparison with the last three cycles at the Tihange reactor have shown good confirmation of this approach. A large-scale study of FP activity is an on-going joint effort by Framatome and CEA.