Nuclear salt water rocket

[kiwiak]

I was recenty reading about varius propolsion concepts.

http://en.wikipedia.org/wiki/Nuclear_salt-water_rocket

And i am wondering. How this can be controlled? After it expells fissionable materials, reaction occurs outside spacecraft. So you have no controll over strength of reaction, right? It might disperse quickly without reacting much and in another case reaction might be stronger. Isnt it kinda random?

And woulnt it require some kind of shielding from structural stress for spacecraft, like with orions pusher plate? It seems to be continuous fission explosion so it woudl be rough to spacecraft, right?

[Seret]

Well, all rocket engines are a controlled explosion, so yes they do have to be mounted to the vehicle in a strong enough manner to transmit that force. That's not a big deal though, aircraft are dragged through the air by their engines, and engine pylons don't have to be enormous either. Given that the force is (hopefully) strongly pointed in one direction you're talking about fairly linear compressive and tensile stresses, which aren't too challenging to deal with.

[kiwiak]

But what is signifcant about this drive is that most of reaction occurs outside of craft, no in some kind of combulsion chamber. Like with orion, where nukes are expelled and than detonated.

I thought that it might somehow make things different.

[TheSaint]

Contrary to popular opinion, nuclear reactions generally don't happen by accident (with the Oklo reactors being a notable exception). It all has to do with the neutrons. In order to sustain a nuclear chain reaction you need to produce a sustained population of slow (or thermal, in the technical jargon) neutrons. (Yes, that's right, slow neutrons, not fast. The faster a neutron is travelling, the less likely it will be absorbed into a U235 atom.) So the idea with the NSWR would be to store the fuel in a tank that is filled with a matrix of neutron-absorbing material, like boron-impregnated plastic. This would prevent the neutron population from increasing enough to produce a chain reaction. When the fuel is injected into the combustion chamber, the water in the fuel would act as a moderator, slowing down the neutrons produced by the fission events, allowing them to produce more fission events, and leading to a chain reaction. Once the heat from these reactions transforms the water into steam, the water will no longer be dense enough to act as a moderator and the nuclear chain reaction will stop.

All that said, I still wouldn't ride in it. Any leak of fuel would have the potential to produce an accidental reaction (essentially a criticality accident). This would produce a steam explosion near some location which stores the fuel, which would lead to more uncontrolled fuel release, which would lead to a Very Bad Day. I'll stick with good-old NTRs, thanks.

[Seret]

The problem with Nuclear Pulsed Propulsion isn't that the reaction is happening outside the vehicle, it's that you're using a very large force periodically, instead of a lower constant force. This means stresses during each very brief pulse are extremely high.

[Idobox]

But what is signifcant about this drive is that most of reaction occurs outside of craft, no in some kind of combulsion chamber.

That's not the case. The fuel would get critical only inside a reaction chamber, and most of the energy would be released there.

To put it simply, nuclear fuel is usually not very radioactive, and doesn't react much by itself. When a fissile atom receives a slow neutron, it sometimes disintegrates, releasing energy, fission products, and a few neutrons, but these released neutrons are too fast to cause other fission events.

If you put hydrogen, deuterium, carbon or a few other things in the middle, neutrons bounce off them and slow down, increasing the chances of causing more fission. It's called moderation, it obviously increase the power output, and it's very difficult to do properly because a bunch of things will absorb neutrons instead. When one spontaneous disintegration causes on average more than one other fission, you reach criticality, and you get a lot of power (nuclear bombs or power plants need that).

The goal of a properly designed nuclear reactor is to make as much stuff react while in the reactor. Typical nuclear reactors us solid fuel, which means they have to keep the concentration low (to burn slowly) and accumulate waste (which often absorbs neutrons, killing the reaction).

A salt-water reactor can use nearly pure fuel (if it can keep it from going critical when stored), can achieve much higher burn rate (less non-fissile junk, waste is ejected) and higher temperature (no need to keep the fuel and its container solid, the whole chamber can be made of graphite). Once the water is out of the chamber, concentration will fall quickly (expanding exhaust), moderation will stop (no graphite to slow the neutrons) and the waste will absorb neutrons. As a result, the reaction will grind to a halt. Of course, the exhaust will be radioactive, which means you can't use it everywhere.

[sgt_flyer]

Yup, i think you put your finger on the problem with this reactor, Thesaint

(Especially as space travels could be threatened by micrometeorite impacts - a single perforation of the fuel tank would be catastrophic indeed

[Alchemist]

As in case of combustion heat from the reaction zone heats the newly fed fuel components to the temperature they start actively react, in this system neutron flux from active zone should kickstart reaction in newly fed fuel to rates similar to nuclear explosion (that further exponentially growing chain reaction would release most of the energy in fraction of second). The good question though is piping leading into the reaction zone - you'll have to ensure the fuel won't go off before being injected into reaction zone because of all that neutron flux from there (as with any explosive monopropellant). Another question is ability to throttle (or if the continuous reaction will be stable only at some specific fuel flow rate) and the ignition procedure (that's always an issue, especially if the thing doesn't allow much throttling)

[TheSaint]

I went back and read some of the references from the Wikipedia article. The design as proposed by Zubrin actually calls for the fuel to undergo prompt criticality. For those who don't know, prompt criticality means that the neutron flux is so high that the neutrons don't need to be thermalized to sustain the reaction. The difference between regular criticality and prompt criticality is the difference between a nuclear reactor and a nuclear weapon. When you talk about prompt criticality with anyone who has worked around reactors they get that cold lump in their gut. I absolutely wouldn't ride in the damn thing.

[Alchemist]

The difference between thermal and prompt criticality is dependent on concentration of isotopes that absorb only fast neutrons. Which mostly is U-238. The thing is okay for long-term nuclear reactors (it even transforms into fissible Pu-239) but is a big problem for such fast things. So, we are not talking about going prompt critical on normal nuclear fuel, we are talking about using military grade U-235 (and/or Pu-239) - there's not much difference between these critical states in this case.

Nuclear torch is like nuclear bomb, just with continuous fuel feed not exploding all at once.