Nuclear thermal rockets may have great benefits for Solar system exploration. But one consideration is that the launch of a nuclear engine from Earth's surface, as the payload on a chemical rocket, might fail.
How should the danger of such a scenario be assessed? Would the heavy metal radiactive engine piece fall almost intact on one place? Or would it create a radiactive cloud which dangerously contaminates a wide area? Could a failure for example be compared with one of many hundreds nuclear bomb tests in the atmosphere?
What saftey measures could lower this risk? Maybe an abort system like that used on the crewed Apollo missions, which could separate the payload and land it in a controlled way.
Should they maybe be launched from Western Sahara where their trajectories cover a uninhabited area (4 million people live in Sahara, most of which in the far eastern parts along the Nile river and the rest mostly in a few concentrated oasis) and where the polluted sites could relatively easily be cleaned by scooping up contaminated sand.
For example, assume a 20 ton nuclear thermal rocket engine component was the payload of one of these two recently failed Proton launches, what would the environmental impact be?
9 minutes after launch: http://www.spaceflight101.com/ekspress-am4r---proton-launch-updates.html
A few seconds after launch: http://www.spaceflight101.com/proton-m-block-dm-03-glonass-launch-2013.html