A gaseous radioactive isotope is used to fill a chamber at a density at which the reactor is non-critical. Then hydrogen is pumped in through the porous walls of the chamber creating a swirling vortex, radioactive gas in the center, hydrogen on the outside. The increased density of the gases causes the reactor to go critical and heat up to ~25,000 deg.C roughly. The hydrogen gas being pumped in through the chamber walls prevents the plasma from contacting the walls, and melting down, since no known material could withstand such heat. Heat is transferred from the plasma to the hydrogen which escapes out the nozzle.

There is no way to prevent some leakage of radioactive isotope. A gas-core nuclear thermal rocket (controlled nuclear meltdown and gas explosion) would probably cause more environmental damage during a single launch than Chernobyl, which was also an explosion.