RRFM 2009 Transactions - European Nuclear Society

EXPERIMENTAL SIMULATION OF AL FUEL PLATE OXIDATION
A. BURKART, R. HADDAD, C. LAFONT
Departamento Materiales, Comisión Nacional de Energía Atómica
Av. General Paz 1499, B650KNA San Martín (Bs. As.) Argentina
R. AMORUSO,
Departamento Combustibles, Comisión Nacional de Energía Atómica
Av. General Paz 1499, B650KNA San Martín (Bs. As.) Argentina
ABSTRACT
MTR fuel aluminium plates undergo oxidation in heat transfer conditions during
operation in Research or Production Reactors. The aluminium oxide is a poor
thermal conductor; hence, the fuel plate’s temperature tends to increase with time,
as the oxide grows, up to a point in which it may become dangerous for the fuel
integrity. In order to evaluate this effect, an experimental facility which simulates
heat transfer in a fuel channel was used to measure the amount of oxidation in
different operating conditions. A parabolic growth as a function of time has been
verified, which can be adequately predicted by available published correlations.
Oxidation in a slightly acid environment (pH=5.2) is one order of magnitude lower
than in near neutral water. Pre-filming of the aluminium plates does not seem to
influence the ulterior oxidation behaviour.
1. Introduction
Aluminium clad MTR fuel undergoes oxidation during operation in Research Reactors.
Aluminium oxides are poor heat conductors; hence, heat transfer between fuel and coolant
could be disturbed by the growth of a thick layer. If this should happen, fuel plate
temperature will increase, even at constant coolant temperature, due to the temperature drop
across the oxide layer, thus accelerating the oxidation process, generating a vicious circle.
Aluminium and its alloys may suffer corrosion in water at temperatures above 150-200 °C
(range which could conceivable be attained in the fuel plate if the oxide film exceeds certain
thickness), with penetrating intergranular attack, blistering and oxide exfoliation. As oxide
growth is a function of plate temperature, water condition, coolant flow and other reactor
parameters, the heat transfer situation will vary from reactor to reactor, depending on thermal
power and other specific characteristics. A reliable performance prediction may then be
relevant to fuel design. Several empirical regressions are available, which are just valid for
the conditions of the experiments used to develop them; it can be mentioned, among them,
those by Griess [1], Kritz [2] and the so called “Correlation II” [3]. More recently, a predictive
model was published [4] taking into account various different variables, which is so far the
most comprehensive attempt to describe the influence of all relevant parameters. However,
as can be seen in Figure 1, slight variations in pH (from 7 to 5) give place to a two orders of
magnitude change in the predicted oxide thickness.
An experimental approach is presented, which intends to supply a way of directly measure
oxide growth in different conditions, in order to foresee fuel performance in a variety of
situations. These may include variations in inlet and outlet coolant temperature, heat transfer,
coolant flow and/or speed, coolant chemistry conditions, plate metallurgical conditions, etc
2. Experimental method
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