A method of irradiating an isotope in a commercial nuclear reactor that has a moveable in-core detector system including detectors that travel in retractable thimbles that extend from a seal table, outside the nuclear reactor, up into a pressure vessel of the nuclear reactor and through instrument thimbles within fuel assemblies supported within a reactor core, the moveable in-core detector system further including a multi-path selector (28, 30), positioned on an upstream side of the seal table, that selects the retractable thimbles through which the detectors travel, the method comprising the steps of: providing an elongated, hollow, target specimen cable sized to travel in one of the retractable thimbles with the target specimen cable being sealed at a lead end and having a removable plug that is configured to fit into a trailing end, the target specimen cable having a length sufficient to extend out of the seal table when the target specimen is fully inserted in a preselected, substantially fully extended retractable thimble; loading one or more target specimens through the trailing end into a forward location (44, 46) in the hollow of the target specimen cable; closing off the trailing end with the removable plug; identifying the preselected retractable thimble that extends into the instrument thimble into which the target specimen cable is to be loaded; inserting the lead end of the target specimen cable into the preselected retractable thimble; driving the target specimen cable through the retractable thimble and into the instrument thimble to an elevation that places the target specimen at a predetermined elevation; irradiating the target specimen at the predetermined elevation for a preselected period of time; and withdrawing the target specimen cable from the instrument thimble after the preselected period of time and out of the preselected retractable thimble to a processing area where it can be loaded into a shielded transportation cask. providing an elongated, hollow, target specimen cable sized to travel in one of the retractable thimbles with the target specimen cable being sealed at a lead end and having a removable plug that is configured to fit into a trailing end, the target specimen cable having a length sufficient to extend out of the seal table when the target specimen is fully inserted in a preselected, substantially fully extended retractable thimble; loading one or more target specimens through the trailing end into a forward location (44, 46) in the hollow of the target specimen cable; closing off the trailing end with the removable plug; identifying the preselected retractable thimble that extends into the instrument thimble into which the target specimen cable is to be loaded; inserting the lead end of the target specimen cable into the preselected retractable thimble; driving the target specimen cable through the retractable thimble and into the instrument thimble to an elevation that places the target specimen at a predetermined elevation; irradiating the target specimen at the predetermined elevation for a preselected period of time; and withdrawing the target specimen cable from the instrument thimble after the preselected period of time and out of the preselected retractable thimble to a processing area where it can be loaded into a shielded transportation cask.