Electron Probe Microanalysis and Scanning Electron Microscopy of Deposits on the Inner Surface of Boiling Water Reactor Cladding

Samples of Zircaloy-2 cladding from four UO₂ fuel elements irradiated in the Tarapur Atomic Power Station boiling water reactor up to fuel burnup levels of 9599 to 12 627 MWd/tonne U are examined using scanning electron microscopy and electron probe microanalysis to study the nature of deposits formed on the inner surface of the cladding. The clad samples from one fuel element that was located nearest to the control blade and had a fission gas release of 3.7% have deposits on the surface. Uranium, zirconium, cesium, oxygen, tellurium, barium, tin, and rubidium are present in the deposit. The deposit is composed of two layers: a layer toward the fuel, containing mainly uranium, cesium, and oxygen, and another toward the cladding, containing uranium, zirconium, cesium, and oxygen. The formation of the deposits is explained by a localized high release of volatile fission products from the fuel and their migration to the clad surface.