Abstract: An optical fiber includes light-transmitting means comprising at least one optical fiber element, and a heat-resistant and radiation-resistant coating formed around the light-transmitting means. The coating comprises an inner buffer layer formed around the light-transmitting means and an outer layer of polyether ether ketone formed around the inner layer.

Abstract: An apparatus for detecting the location of a failed fuel assembly among a number of in-core fuel assemblies, which utilizes a combination of a tag gas system and a selector valve system. The fuel assemblies are divided into a plurality of groups and all the fuel assemblies in the same group are tagged with the same tag gas. Coolants flowing out from the respective fuel assemblies are individually sampled by sampling pipes and led into intermediate mixers wherein the coolants are mixed. The groups of fuel assemblies are mutually connected via the sampling pipes to the intermediate mixers such that each group of fuel assemblies has the sampling pipes extending therefrom and connected to each of the intermediate mixers. The group to which the failed fuel assembly belongs is identified by the tag gas system, and the selector valve system identifies which mixed coolant from the intermediate mixers contains fission products released from the failed fuel assembly.

Abstract: A double-cylinder selector valve for detecting and locating failed nuclear fuel assemblies including a fixed outer cylinder provided on the circumferential wall surface thereof with a plurality of outer sampling holes arranged in a grid in both the longitudinal and circumferential directions, and a movable inner cylinder disposed rotatably within the fixed outer cylinder and provided on the circumferential wall surface thereof with a plurality of inner sampling holes. Each of coolant sampling pipes extending from the coolant outlets of each of a plurality of nuclear fuel assemblies is connected to one of the outer sampling holes. The inner sampling holes communicate with some of the plurality of outer sampling holes in the fixed outer cylinder at one position of the movable inner cylinder and with others of the outer sampling holes at other positions of the movable inner cylinder.