CESIUM A FISSION BY PRODUCT WAS IN THE "FUEL" AND EXHAUST OF THE SR-71

This document is likely a fabrication. Because exhaust of the Nuclear Powered Blackbird that was used for Soviet Overflight would have been transparent. There would be no flames because nothing is combusting. The exhaust plume would have resembled the plume seen in the NERVA. In the late 1940's it was decided that hydrogen would be the reaction mass used in nuclear aerospace program. This reaction mass would supplement the mass of the atmosphere when planes like the U-2 or Canberra were at high altitude.

SOURCE:
https://www.cia.gov/library/readingroom/docs/DOC_0001458639.pdf