The attenuation of the SMP shielding was measured as part of SMP commissioning (Document-170404, R1, Section 4, Test #8)

The field on the shutter side of TM3 was measured without any shielding as it was removed from a storage silo.  At 0.5m from the edge of the module the maximum field was 5.00mSv.  After installation into the SMP the maximum field outside the SMP shielding at 0.5m from the edge of the module was found to be 27uSv/hr. The resulting attenuation is 185x

Expected attenuation was calculated by Joe Mildenberger assuming that the majority of the activated components in that module are either copper or aluminum.  With 6" of steel shielding (SMP shield vessel plus vacuum vessel), the attenuation factors were found for the following isotopes: 

Na-22 (created in Al): 115

Co-60 (created in steel (Fe) or Cu): 70

Mn-54: (created in steel (Fe) or Cu): 200