Stellite replacement

The highly beneficial effect of Stellite replacement in the newer Siemens PWRs was confirmed by a comparative study performed by Garbett (1992). In this study it was emphasized that cobalt impurities in the primary circuit materials are only minor contributors to the radiation fields in the areas surrounding the piping, components and systems. 

Nuclear Electric has been monitoring the development of radiation fields on KWU/Siemens PWRs in Germany. The most recent reactors of the KONVOI type have eliminated stellite from the reactor pressure vessel and almost all other areas of the primary circuit. The only appreciable area remaining is in the CRDMs (control rod drive mechanisms). This has resulted in dramatic reductions in radiation fields and man-Sv arisings (Ref. 8). Channel head radiation fields average less than 10 mSv per hour, compared with levels of 50-100 mSv per hour on reactors which have not undergone Stellite replacement. 

The radiation fields on the KONVOI reactors are now dominated by Co rather than Co, the latter has been reduced to very low levels as a result of the Stellite replacement programme. This has implications for decommissioning since the total inventory of °Co at the end of reactor life will be reduced by at least an order of magnitude. 

The liners are fabricated in stainless steel with a cast stellite inlet for improved erosion resistance. All liner components are readily replaceable on an individual basis, and with no moving pans, maintenance should be relatively simple with long periods between subsequent inspections once the erosion resistance for a particular application has been confirmed. 

It is found that from induced activity and dose rate considerations, stellite can be replaced by either Colmonoy-6 or Tribaloy-700. Based on fast reactor experience with colmonoy, colmonoy has been chosen. An approximate estimate of the shield requirement assuming a permissible dose rate of 100 mR/h have been worked out. In the following discussions, as the basic SS component itself becomes radioactive and requires shielding, reduction in shield thickness is with respect to the minimum shielding required for the SS component [5]. 

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