Corrosion models are generally able to provide satisfactory predictions of inreactor performance. Nevertheless, in order to emphasize the theoretical or empirical aspects of the corrosion process and to improve the accuracy of models, the major objectives of the present study are: • To provide a comparison of the published corrosion models (CEA, EPRI, etc.). • To discuss the main assumptions described in the proposed models.
Consequently, a methodology was conducted with the objective of providing the following information: • The main assumptions used to describe the thermal/hydraulic corrosion process in the overall models are presented and modelled in a computer program. • Experimental corrosion data are collected from a great number of rods taken from seven different power reactors, from the CIRENE out-of-pile loop, and from autoclave tests.
Metallographic examinations of SRA Zircaloy-4 tubings constitute the experimental data required for the determination of the accuracy of the corrosion models. Measurements give an idea of the waterside corrosion behavior of Zircaloy-4 for the following encountered operating conditions in French or foreign reactors: • One to four cycles of irradiation in 900 MWe PWRs. • Three cycles of irradiation in a four-loop PWRs characterized by a severe thermal/hydraulic environment. • Four to five cycles of irradiation cycles of 4.5% 235U or 3.1% 235U enriched fuel rods leading to average rod burnups in the range of 48 000 to 55 000 MWd/tU. • 365 effective full power days in the CIRENE out-of-pile loop. • Autoclave tests.
The power history of the analyzed rods and the thermohydraulic design of reactors are used for effective data reduction. The operating parameters as a function of time constitute the unique data base required for the prediction of Zircaloy-4 performance by all the corrosion models to be tested.
The comparison of measured and calculated corrosion data from the in-pile and out-of-pile tests is performed and described.
The differences between corrosion models allow for the assessment of the influence of varied parameters on the corrosion rates of the Zircaloy-4 cladding tubes.
In particular, the assumptions concerning the effect of thermal conditions (heat flux, wall temperature), the irradiation environments, and the chemistry data (lithium content in the coolant) are discussed through the different models.