Methods for protecting fuel-element cladding from corrosion in lead coolant and the heat-transfer sublayer

Abstract

The results of investigations of the corrosion of commercial and experimental steels in lead and the possibilities of corrosion protection are presented. The effect of lead coolant and the lead heat-transfer sublayer on fuel-element cladding are examined. Methods based on thermodynamic calculations and experimental data are proposed for protecting fuel element cladding in a lead-cooled reactor from the corrosive effect of the coolant by creating a new corrosion resistant chromium steel and from the corrosive effect of the heat-transfer sublayer by alloying with the components of steel. The results of this work have been implemented in the experimental fuel elements for the BREST-OD-300 reactor which were irradiated in a BOR-60 reactor.The program for the development of the nuclear power complex in our country provides for implementing the lead-cooled BREST-OD-300 fast reactor design. The fuel elements of such a reactor consist of 9.4, 9.8, and 10.7 mm in diameter steel tubes with a 0.5 mm thick wall. Uranium-plutonium nitride fuel pellets, tightened at the ends by molybdenum spacer bushings, surrounded by a heat-transfer sublayer made of S 000 lead as required by GOST 22861-93 [1, 2]. The fuel-element cladding material consists of 16Kh12MVSFBR-Sh (ÉP-823) 12% chromium ferrite-martensite steel, since steel of this class has operated successfully at 550°C in the fuel elements of reactors with the lead-bismuth coolant [3]. One of the main requirements which the material of fuel-element cladding in a BREST reactor must satisfy under the most stressful operating conditions is that it must be corrosion resistant in the lead coolant and the heat-transfer lead sublayer.The existing data on the corrosion behavior of steel in liquid lead are limited or contradictory [4][5][6]. This makes it necessary to study the corrosion behavior of commercial and experimental steel in lead and find ways to protect it from corrosion.Under the operating conditions of the reactor, the lead coolant in the heat-transfer lead sublayer can have a different content of oxygen, depending on the operating regime. This affects the mechanism of the corrosion of fuel-element cladding: oxidation or dissolution, which proceeds much more intensely [7].Computational Procedure. When lead coolant circulates in the high-temperature zone, the fuel-element cladding material dissolves and the products of its corrosion precipitate in the low-temperature zone. The reactor design provides for removal of corrosion products from the coolant and for regulating the oxygen content.The heat-transfer lead sublayer inside a fuel element, which is a closed system, becomes enriched with the products of corrosion during operation. Consequently, the corrosion of the inner surface of fuel-element cladding will have a transient character. Data on the corrosion resistance of fuel-element cladding materials must be examined for two different non-isothermal systems: dynamical for the outer surface and stationary for the inner surface. This makes it necessary to analyze corrosion ...