The adjacent ends of the inner shells form a disconnected joint and the secondary inner shell is free to move relative to the primary inner shell upon {HEAD ~ thermal expansion} of {TAIL ~ the tubes} 
The adjacent ends of the inner shells form a disconnected joint and the secondary inner shell is free to move relative to the primary inner shell upon { HEAD ~ thermal expansion } of { TAIL ~ the tubes }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL