chamber, thus allowing the system to remain intact and safe to operate. These claims have still not been proven due to no test of such a device having ever been conducted. For example, Zubrin argues that if diluted nuclear fuel flows into the chamber at speed similar to diffusion speed of thermal neutrons, then nuclear reaction is confined in the chamber and does not damage the rest of the system (the nuclear analog of a gas burner). A possible problem in that line of thinking is that neutrons do not all diffuse at the same velocity, but have a broad distribution over several orders of magnitude. It is possible that tails of this velocity distribution would be sufficient to generate enough heat in fuel feeding system (by scattering and fission) to destroy the system. This question can be perhaps answered by detailed Monte-Carlo simulations of neutron transport. The vessel's exhaust would contain radioactive isotopes, but in space these would be rapidly dispersed after travelling only a short distance; the exhaust would also be travelling at high speed (in Zubrin's scenario, faster than Solar escape velocity, allowing it to eventually leave the Solar System). This is however of little use on the surface of a planet, where a NSWR would eject massive quantities of superheated steam, still containing fissioning nuclear salts. Terrestrial testing might be subject to reasonable objections; as one physicist wrote: "Writing the environmental impact statement for such tests [...] might present an interesting problem ...". It is also not certain that fission in a NSWR could be controlled: "Whether fast criticality can be controlled in a rocket engine remains an open question". See also Spacecraft propulsion Fission-fragment rocket Project Orion Technology readiness level References Nuclear spacecraft propulsion