Differences between the oxidation behaviour of A3 fuel element matrix graphites in air and in steam and its relevance on accident progress in HTRs
- Forschungszentrum Juelich GmbH, ISR, D 52425 Juelich (Germany)
The fuel element matrix graphites A3-3 and A3-27 were used in High Temperature Reactor fuel pebbles for many years. However, these materials show as other graphites a limited oxidation resistance in contact to oxidising gases (air and steam), which even decreases with increasing temperatures: In HTRs, having in normal operation a non-oxidising environment, an ingress of air or steam leads to corrosion of graphite with the potential of enhanced fission product release Matrix graphites differ by its coked binder content from standard nuclear graphites (e.g. V483T, ASR-1RG, IG110, H-451), where both filler and binder are completely graphitized. The influence of this coked binder content on the oxidation behaviour will be discussed in this paper. Experimental investigations with A3-3 and A3-27 were done in oxygen (air) at temperatures between 673 - 1023 K and in steam between 1173 - 1253 K. These experiments took place under isothermal conditions in the chemical regime, where the chemical reaction itself is the rate limiting step and a homogeneous oxidation inside of the sample occurs. The experiments reveal different oxidation behaviour as well between binder and filler component as between oxidation behaviour in oxygen (air) and steam. In air at low temperatures two rate maxima are observed: The first maximum attend in all experiments at {approx} 5 % burn off, a second one at higher burn off values (35 - 45 % burn off). These rate maxima can be explained by a selective binder-filler oxidation: The first peak at 5 % burn off is due to the oxidation of the binder, the second peak at higher burn off values is caused by the oxidation of the remaining filler. At higher temperatures in air the filler peak becomes more pronounced and the binder peak vanishes, which is due to the lower activation energy of binder oxidation compared to the filler. In steam this behaviour appears contrary: A maximum at 5 % burn off, which is probably also connected to the binder, is observed, but the filler peak is less pronounced. Moreover, this binder peak increases with increasing temperatures during oxidation in steam, contrary to that in air. Obviously, activation energies for both binder and filler and oxidation in air and in steam are different. Other reasons for the outlined behaviour like porosity distributions and chemical reaction enthalpies are discussed, too. Compared with standard nuclear graphites, the overall reaction rate of these matrix materials is not higher, but for most conditions even smaller (except for oxidation in air at low temperatures and low burn-off, i.e. the selective oxidation range of the binder). The latter holds for regime II (in-pore diffusion together with chemical reaction control); too; respective experiments indicate, that Knudsen diffusion is a relevant transport mechanism in matrix graphites, but not in standard nuclear graphites. In order to define needs for additional experimental work concerning graphite oxidation kinetics and to demonstrate the application of the kinetic data outlined here, some representative accident calculations with massive oxidation by air are presented on example of a small HTR with block type fuel. (authors)
- Research Organization:
- American Nuclear Society, 555 North Kensington Avenue, La Grange Park, IL 60526 (United States)
- OSTI ID:
- 21160566
- Resource Relation:
- Conference: ICAPP'04: 2004 international congress on advances in nuclear power plants, Pittsburgh, PA (United States), 13-17 Jun 2004; Other Information: Country of input: France; 13 refs; Related Information: In: Proceedings of the 2004 international congress on advances in nuclear power plants - ICAPP'04, 2338 pages.
- Country of Publication:
- United States
- Language:
- English
Similar Records
Determining the oxidation behavior of matrix graphite
Effects of microstructure on the oxidation behavior of A3 matrix‐grade graphite