EDF Experience about RCCA Behaviour

The concerns which may affect the RCCA (rod control cluster assembly) component are known today. The cladding wear, due to hydraulic forces in the guide tube, remains the first RCCA life limiting factor on the 900 MWe and the 1300 MWe reactors. The clad swelling induced by the buckling of the SIC (Silver Indium Cadmium) absorber material is also a potential problem but, up to now, no jam occurring inside the core may be attributed to this factor. A lot of monitoring campaigns have been performed since 1988. The inspections have allowed to identify the wear amplitude and the swelling when sufficient to induce cracks. Three actions were initiated, mainly to deal with these problems: the first concerned the immediate requirements and consisted in defining a RCCA management strategy based on mechanical calculations; this strategy includes periodical controls, axial repositioning and specific rejection criteria; the criteria are based on safety considerations like the mechanical integrity of the RCCA, the integrity of the absorber itself and the primary coolant activity. The second is devoted to some design improvements of RCCA; regarding the wear problem, various hardening methods of the clad have been investigated (nitride, chrome carbide, chrome); for the swelling, geometrical modifications should delay the interaction between the absorber and the clad. The third action is a long term work with a more ambitious approach; it consist in a large R and D program concerning the study of the phenomena involved in the wear process. The aim is to build a full set of physical models and to introduce them in a computer code, in order to be able to predict RCCA wear. The present status of these actions is the following. The management strategy is now in current use on all the reactors and allows to operate them in safe conditions, even with standard RCCA. This strategy will require some adjustments when reactor operating conditions are modified. Out of pile tests have showed the potential efficiency of various remedies. These remedies are progressively introduced in the reactors. Now about 1000 new designed RCCA are in reactor and the oldest ones have yet experienced 8 cycles. The results already obtained confirm the better behaviour of these RCCA compared to the standard one. The third action is still in progress. EDF has planned to qualify the code in 1997. Then it will be possible to perform analytical studies to investigate modifications on the design of RCCA or internal guide-tubes