Mechanistic Investigation of Internal Corrosion in Nuclear Waste Containers Over Extended Time Periods
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Mechanistic Investigation of Internal Corrosion in Nuclear Waste Containers Over Extended Time Periods Elsie Onumonu1 and Dr Nicholas P.C. Stevens1 1
The University of Manchester, Materials Performance Centre, Sackville Street, P.O. Box 88, Manchester M60 1QD, UK.
ABSTRACT Storage of the UK's Intermediate Level Wastes (ILW), which comprises Magnox fuel cladding, uranium and small items of equipment exposed to radiation, is currently achieved via encapsulation within cementitious grout housed in 500 litre 316L stainless steel drums. The cements used display a high pH; in such an environment many metals form surface hydroxides or oxides. Magnox reacts with free water at high pH with the liberation of hydrogen whilst undergoing corrosion to form hydroxide species. Corrosion of Magnox cladding has previously been monitored by measuring the rate of hydrogen evolution and/or weight loss. Recent work by our group has shown impedance techniques may also be useful in monitoring early corrosion behaviour. In this project electrochemical polarisation techniques will be employed to examine the corrosion behaviour of Magnox fuel in situations where it is in electrical contact with other metals, including uranium, and hence determine how galvanic effects influence corrosion behaviour. In this paper we describe the background to such experiments along with some preliminary results. INTRODUCTION A Magnox reactor is a nuclear reactor fuelled by uranium metal. UF4 (uranium tetrafluoride) is reduced to uranium metal, which is then cast and machined into rods. The uranium rod is then sealed into a can of Magnox Al80 (a magnesium alloy with small amounts of aluminium), which is often referred to as the fuel cladding. Magnox is used as fuel cladding because of its low neutron absorption cross-section. After discharge from the nuclear reactor, fuel elements are initially stored in cooling ponds of high alkalinity to allow the decay of short lived radioactive isotopes for a minimum of 90 days. The ponds are dosed with 200 g m-3 of sodium hydroxide to give pH > 11.5 [1]. Such conditions are employed to prevent soluble fission products being released as a result of corrosion of the cladding [2]. Magnesium and its alloys display highly anodic corrosion potentials and corrode easily in water below pH 9. However they quickly passivate in more alkaline conditions due to the formation of a magnesium oxide/hydroxide layer [3]. Mg2+ + H2O Mg(OH)2
(1)
This film has a high stability due to its low solubility and rate of dissolution but can be disrupted by the action of aggressive anions, most notably chlorides, which promote localised corrosion [4]. Aluminium quickly develops a thin, dense and strongly adherent alumina layer on exposure to air which protects the metal from rapid reaction with moisture. However, in alkaline conditions, such as those encountered in cement pore waters, OH- ions attack and dissolve this
passive layer. As a result the protection of the air formed oxide layer that forms on this highly electropositive metal is diminis
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