Failed fuel detection in BWR

Two types of method to detect failed fuel are available during Open Vessel period in BWR. One is in core sipping method. A wet-sample apparatus is used to sip water from the individual fuel assemblies within a control rod cell. The upward water flow is blocked by placing the wet-sample apparatus on the upper grid and displacing some of the water within the apparatus by air pressure, thus forming an air cushion. Soak period is used to permit the concentration of fission products in the stagnated water surrounding the fuel assemblies to be sampled. Samples are sipped and transferred to the hot laboratory for chemical analysis. Each sample is passed through an anion exchange resin to separate the iodine. Radioactivity of the separated iodine resin sample is counted using NaI(Tl) crystal connected to pulse height analyzer. Signal to Background ratio of Iodine determines which fuel assemblies are defective. The other is Out of Core sipping method. Special sipper Can is installed in the spent fuel pool. This can holds one fuel assembly transferred from reactor core. Sipper head is set on the can and isolate the bundle completely from water in the pool. The water in the Can is purged and displaced with demineralized water. The procedure after soaking is same as the incore sipping procedure. (auth.)