The oxidation of structured stainless steels and fuel cladding in the advanced gas-cooled reactors

This paper surveys the oxidation behaviour of the whole range of stainless steels used in the Advanced Gas-cooled Reactors (AGRs). The steels are categorised into: 18Cr varieties containing between 9 and 12Ni which are used for structural items and boiler tubes; 25Cr/20Ni steel used as thermal insulation foils; 20Cr/25Ni alloys used as fuel element cladding. The maximum operating temperatures vary from ∼923K for the structural steels and insulation foils to ∼1143K for fuel cladding. An important feature which determines performance is the metal section lost due to oxidation. To assess this, proper allowance must be made for localised as well as uniform attack and, as a result, much emphasis is placed on the ability of the steel to form healing oxide layers. Under normal operating conditions, no restrictions are anticipated as a result of oxidation. A detailed discussion is provided of the principal postulated faults within the AGR system. The extent of clad oxidation under such circumstances is an important factor in assessing fuel pin integrity. (author)