Editing Nuclear thermal rocket (section)

== Risks ==
An atmospheric or orbital rocket failure could result in the dispersal of radioactive material into the environment. A collision with orbital debris, material failure due to uncontrolled fission, material imperfections or fatigue, or human design flaws could cause a containment breach of the fissile material. Such a catastrophic failure while in flight could release radioactive material over the Earth in a wide and unpredictable area. The amount of contamination would depend on the size of the nuclear thermal rocket engine, while the zone of contamination and its concentration would be dependent on prevailing weather and orbital parameters at the time of re-entry.{{citation_needed|date=June 2019}}

It is considered unlikely that a reactor's fuel elements would be spread over a wide area, as they are composed of materials such as carbon composites or carbides and are normally coated with [[zirconium hydride]].<ref>{{cite book |doi=10.1109/IECEC.1990.716860 |chapter=Safety Status of Space Radioisotope and Reactor Power Sources |title=Proceedings of the 25th Intersociety Energy Conversion Engineering Conference |date=1990 |last1=Bennett |first1=G.L. |volume=1 |pages=162–167 |isbn=0-8169-0490-1 }}</ref> Before criticality occurs, solid core NTR fuel is not particularly hazardous. Once the reactor has been started for the first time, extremely radioactive short-life fission products are produced, as well as less radioactive but extremely long-lived fission products. The amount of fission products is zero at fresh-fueled startup, and roughly proportional to (actually: limited by) the total amount of fission heat produced since fresh-fueled startup.<ref>{{cite book |doi=10.1063/1.41909 |chapter=Safety questions relevant to nuclear thermal propulsion |title=AIP Conference Proceedings |date=1992 |last1=Buden |first1=David |volume=246 |pages=648–654 |url=https://digital.library.unt.edu/ark:/67531/metadc1071526/ }}</ref><ref>{{cite journal |last1=Sforza |first1=Pasquale |title=A safety and reliability analysis for space nuclear thermal propulsion systems |journal=Acta Astronautica |date=July 1993 |volume=30 |page=68 |doi=10.1016/0094-5765(93)90101-2 |bibcode=1992wadc.iafcQX...S }}</ref> Additionally, all engine structures are exposed to direct neutron bombardment, resulting in their radioactive activation.{{citation_needed|date=June 2019}}