Hydrogen Gas in CANDU Fuel Elements

Abstract

One of the requirements in the Technical Specification for 37-element fie1 bundles for a CANDU 6 reactor asks for "less than 1 mg of hydrogen gas in a fuel element". This is an important requirement. High contents of hydrogen gas may lead to cracking and to primary defects of fuel-element sheaths. In recent years, there have been some instances of this type of he1 defect. In the paper, the definition of "hydrogen gas" is reviewed and the contributions from the usual sources of hydrogen gas in a fuel element (filling gas, pellets and the CANLUB layer) to the total hydrogen gas are assessed. The paper discusses a model of sheath failure that is due to hydrogen gas and the principal processes related to it: ingress of hydrogen into the sheath, re-distribution of hydrogen in the sheath volume, formation of hydrides and changes in specific volume, and cracking. Factors affecting the risk of defects are discussed: quantity of hydrogen gas as a function of fuel-element geometry, thickness of the CANLUB layer and the conditions of its curing; localized ingress and re-distribution of hydrogen; fuel-element uniformity in axial and circumferential directions. The determination of hydrogen gas as a part of he1 inspection is reviewed. The numerical value of the limit is discussed. The specification limit is separated from the defect threshold by a sufficiently large design margin. At the same time, product manufacturability requires a separation of the specification limit from the distribution of achieved values, i.e., satisfaction of a sufficiently low AQL (acceptable quality level).