good question
Strangely the in the masses of nuclear-powered spacecraft documentation available on the internet
are very vague about this
Wat i known:
a unshielded working Nerva engine would kill astronauts on distance of 14 km or 8,69 miles in space
The crew is protected by a "shadow shield" between the NERVA engine and crew Mission Module
can be like water, liquid hydrogen propellant tanks, lithium hydride, paraffin or a hydrogenated polyethylene composite.
Shield builds up after webpage Atomic Rocket
18 centimeters of beryllium (which acts as a neutron reflector), followed by 2 centimeters of tungsten (mainly a gamma-ray shield but also does a good job on neutrons), and finally 5 centimeters of lithium hydroxide (To stop the remaining neutrons. Hydrogen slows down the neutrons and lithium absorbs them.). This attenuates the gamma flux to a value of 0.00105, and neutron flux to 4.0e-9. This has a mass of 3,500 kilograms per square meter of shadow shield
http://www.projectrho.com/rocket/rocket3ah.html
NERVA "shadow shield" had to be Mercury and lithium hydride
to the exhaust
Hydrogen will not much radioactive
but the exhaust gas will be contaminating with piece of reactor core !
the Tory A Nuclear Ramjet engine had that Problem during test
yep they tested Nuclear Aircraft engine and Nuclear rocket engine in USA and USSR
they tested ROVER (KIWI, Phoebus, NERVA, Tory A and others nuclear engine ) in Nevada 1959
in place called "Jackass Flats" west of Nuclear Bomb Test Rage.
first as USAF/AEC study for ICBM Nuclear rocket engine ! (the USSR also)
later under NASA/AEC in 1960 until stop of program in January 1973
the test were made as the wind blow from south, so L.A. and Las Vegas were "Save"
on of the test this happens : KIWI engine at full power and its EXPOLDE !
safety regulation say stay away 100 km from testside after use for some weeks (in wind direction)
Wat indicates that after a NERVA test the area was about radioactive with deathly 500 Milli rem