Thermal striping limits for components of sodium cooled fast spectrum reactors

Abstract

With the objective of establishing thermal striping limits for future sodium cooled fast spectrum reactors (SFR), a fracture mechanics-based method employing 'σ-d approach' recommended in RCC-MR: Appendix A16 has been followed. Towards this, an idealized geometry, thermal fluctuations in the form of constant power spectral density and pessimistic material data were considered and temperature and thermal stresses are computed taking in to account frequency-dependent thermal attenuation on the structural wall. The effect of attenuation is found to be significant. The limits are derived at various potential locations in SFRs, which are also subjected to creep-fatigue damage due to major cycles caused by startup, shutdown, power failures and pump trips, etc. The maximum range of temperature fluctuations can be as high as 70 K where there is practically no accumulated creep-fatigue damage and 45 K is acceptable where the creep-fatigue is significant (0.9). These limits are found to be consistent with the broad limits extrapolated from the failure experiences of international SFRs and sodium facilities. Pool hydraulic computations carried out to identify and quantify the thermal striping zones confirmed that the proposed limits can be respected with good margins for SFRs.