The light water breeder reactor (LWBR) was developed by the Bettis Atomic Power Laboratory under the technical direction of the Office of Naval Reactors, U.S. Department of Energy. The LWBR operated successfully in the Shippingport Atomic Power Station from 1977 to 1982, producing more than 29 000 equivalent full-power hours. Because of the small breeding gain (1.35%) predicted for this selfsustaining breeder, proof of breeding required accurate nondestructive assay of expended fuel from the LWBR. The fact that breeding has been proven in the LWBR means that this reactor provides a vast alternative energy resource using plentiful thorium fuel. A gauge, called the production irradiated fuel assay gauge (PIFAG), was developed to nondestructively assay whole irradiated fuel rods from the LWBR. The gauge uses the method of active interrogation with 252Cf source neutrons and delayed neutron counting. It is remotely operated inside a hot cell and has the capability to interrogate fuel rods with two different neutron energy spectra, one with primarily low-energy (<0.625-eV) neutrons and the second with the lowenergy neutrons removed. In addition to delayed neutron counting, the PIFAG has provision for counting 252Cf source neutrons close to the surface of a fuel rod during interrogation, and for gamma scanning the fuel rod. These data are used to calculate a selfshielding correction to the delayed neutron data. A description is given of the PIFAG, its calibration, and its application to the assay of irradiated LWBR fuel rods. A comparison of the PIFAG results with destructive assay results for 17 irradiated LWBR fuel rods shows that the two methods are in excellent agreement, differing by 0.069 and 0.162% for the lowand high-energy PIFAG interrogation spectra, respectively.