Editing Nuclear salt-water rocket (section)

==Limitations==
The propellant used in the initial design would contain a rather large amount of the relatively expensive isotope [[uranium-235|{{sup|235}}U]], which would not be very cost effective. However, if the use of NSWR began to rise, it would be possible to replace this with the cheaper isotopes [[uranium-233|{{sup|233}}U]] or [[plutonium-239|{{sup|239}}Pu]] in either fission breeder reactors or (much better) [[nuclear fusion–fission hybrid]] reactors. These other [[fissile material|fissiles]] would have the right characteristics to serve nearly as well, at a relatively low cost.<ref name="zubrin91"/><ref>{{cite journal |last1=Kang |first1=Jungmin |last2=von&nbsp;Hippel |first2=Frank N. |year=2001 |title=U-232 and the proliferation-resistance of U-233 in spent fuel |journal=Science and Global Security |volume=9 |issue=1 |pages=1–32 |doi=10.1080/08929880108426485 |bibcode=2001S&GS....9....1K |s2cid=8033110}}</ref>

Another major limitation of the nuclear salt-water rocket design by [[Robert Zubrin]] included the lack of a material to be used in the reaction chamber that could actually sustain such a reaction within a spacecraft. Zubrin claimed in his design that the apparatus was created so that the liquid flow rate or velocity was what mattered most in the process, not the material. Therefore, he argued that if the proper velocity was chosen for the liquid traveling through the reaction chamber, the site of maximum fission release could then be located at the end of the chamber, thus allowing the system to remain intact and safe to operate. These claims have still{{As of?|date=June 2024}} not been proven due to no test of such a device having ever been conducted.<ref name=cramer92/>

For example, Zubrin argues that if diluted nuclear fuel flows into the chamber at speed similar to diffusion speed of [[thermal neutron]]s, then the 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 [[Maxwell–Boltzmann distribution|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.{{citation needed|date=July 2018}} This question can be perhaps answered by detailed [[Monte Carlo method in statistical physics|Monte Carlo simulations]] of neutron transport.

The vessel's exhaust would contain [[radioactive isotope]]s, 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). However, this is 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 ...".<ref name=cramer92>{{cite magazine |author=Cramer, J.G. |author-link=John G. Cramer |title=Nuke your way to the stars |date=December 1992 |department=Alternate View Column |id=AV-56 |magazine=[[Analog Science Fiction and Fact]] |url=http://www.npl.washington.edu/AV/altvw56.html |access-date=2012-03-07}}<br/>&nbsp; &nbsp; ''text also available at''<br/>{{cite web |title=Alternate View Column AV-56 |website=npl.washington.edu |url=https://www.npl.washington.edu/av/altvw56.html |access-date=2017-04-18}}</ref>
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".<ref>{{cite report |author=McNutt, Ralph L. Jr. |title=A Realistic Interstellar Explorer |date=31 May 1999 |series=Phase I Final Report |publisher=NASA Institute for Advanced Concepts |url=http://www.niac.usra.edu/files/studies/final_report/76McNutt.pdf |access-date=14 November 2012}}</ref>