Using NERVA in space won’t irradiate Earth, dude. Hell, the Earth is bombarded by more radiation from the sun every day than a thousand NERVAs could put out in a century. And, honestly, unless the engine has been damaged, it’s not spewing out radioactive exhaust; if it’s working correctly, it isn’t shooting out fissile particles in its exhaust. All that NERVA does is use the heat of a nuclear reactor to super-heat a propellant gas (usually hydrogen) via a heat exchanger. Heating a gas causes it to expand, but rather than combustion as the source of that heat, they use the heat of the reactor. It thus simultaneously produces electrical power for the spacecraft and produces heat for the engine. What’s better is that you could theoretically use any gas for it, not even needing it to be flammable - you could use nitrogen and it’d work just fine. And, you’d only need one propellant, rather than a fuel and an oxidizer, so it vastly simplifies the design as you only need a single propellant tank.

There are some designs for nuclear rocket engines that do expel radioactive materials in their exhaust, such as the batshit crazy Nuclear Salt Water engine: it uses fissile salts dissolved in water and separated to keep them JUST below critical mass; when injected into the engine, they reach critical mass and begin a nuclear explosion, with the injection rate balanced just right to be high enough to prevent it from fizzling out, but low enough that it doesn’t destroy the entire ship. As a result, you have a continuous nuclear explosion lasting for hours propelling the ship, producing unfathomable amounts of thrust… but if you sneeze, it’ll either stop working or it’ll explode, hence why it’s still only theoretical.

However, NERVA isn’t one of these designs. Unless you end up cracking open the reactor casing and cause the nuclear fuel to spill out, the exhaust will not be radioactive. Being exposed to radiation doesn’t “infect” the exposed material and make it radioactive; this is why you can pass river water through the heat exchanger of a nuclear power plant and then channel it back into the same river, now a few degrees hotter, but not radioactive. The only time something would become radioactive is if bits of radioactive material end up inside the thing, such as pulverized dust from a fuel rod that was blown up ending up in water. So long as they don’t come into direct contact and you don’t have the chance of pieces breaking away, the coolant won’t become radioactive. Typically, as an added measure of safety, they use a heat exchanger loop where you have a water channel, a coolant channel, and a sealed reactor core: the coolant flows through its channel, through a hole in the sealed reactor, the. Exits the reactor and then flows through a sealed water channel. That way, if, say, the coolant line leaks into the water channel, you’ll have coolant in the water, but neither of the two will directly touch the reactor; or, if it leaks into the reactor, you could have contaminated coolant, but it never directly touches the water. So, the exhaust from a NERVA will never be radioactive unless the reactor has been damaged enough that the reinforced casing of the reactor has cracked open… but, if that happens, you have far bigger issues to worry about.

Plus, it’s really only meant for use in space; it’s too expensive and has too little thrust to be used in a booster stage, but once in orbit - where thrust-to-weight ratio isn’t as important - a highly-efficient engine is extremely valuable. It’s not going to be chucked away to crash into the sea during a launch.