Taylor Wilson and NERVA; check out his TED talk.

[Dispatcher]

Here's a 12 minute video:

While Taylor's concept allows for dumping the critical elements of a fission core into a neutron absorbing pit in the event of "out of tolerance" performance (yes, its a molten salt and gas design; rejoice thorium fans), he proposes (but does not detail) a NERVA version.

For propulsion, its primary use is understandably for transfer stages (and possibly for use at destination for power generation).

For a NERVA in nil-G conditions, its not clear how the dump function might work. Perhaps utilizing neutron moderating rods instead?

[Wahgineer]

He does realize that the guys who built the NERVA new what they were doing, right? The reason the NERVA wouldn't need this system is because it used a solid, power plant style core, which only needs to be replaced every couple of years.

[Accelerando]

Whoa. He could do modeling work on the side.

Moderating rods, aye. Maybe moderating rods built into a form like nhnifong's baffles and vanes for attracting propellant engine-ward in microgravity.

[Idobox]

Dumping fuel in your nuclear rocket is not really useful. If your engine malfunctions so bad you need to dump fuel, the whole mission is screwed anyway.

If you are in an ascent phase, you can use gravity + acceleration. If you are in orbit or on an intercept trajectory, you might as well keep the fuel in rather than disperse it.

He does realize that the guys who built the NERVA new what they were doing, right? The reason the NERVA wouldn't need this system is because it used a solid, power plant style core, which only needs to be replaced every couple of years.

I think he uses the Nerva picture because it's the only nuclear rocket to have actually been tested. A liquid core rocket would have a much higher ISP than NERVA, and some designs could also have a higher power output.

[Dispatcher]

I'm not suggesting dumping the radioactive fuel "overboard", rather wondering about how to moderate the activity down quickly in an emergency. I agree that the NERVA representation should be taken as a generalization for fission propulsion; in fact, the acronym NERVA could also be (mis)understood to be so by the layperson. Clearly he is proposing a molten approach, probably using thorium as radioisotope.

[Ambulatory Cortex]

I'm not suggesting dumping the radioactive fuel "overboard", rather wondering about how to moderate the activity down quickly in an emergency.

You could perhaps use a solid neutron poison that melts at a dangerous temperature and mixes with the fuel as a failsafe. Alternately, have more than one engine, and eject the malfunctioning one.

[Idobox]

I'm not suggesting dumping the radioactive fuel "overboard", rather wondering about how to moderate the activity down quickly in an emergency. I agree that the NERVA representation should be taken as a generalization for fission propulsion; in fact, the acronym NERVA could also be (mis)understood to be so by the layperson. Clearly he is proposing a molten approach, probably using thorium as radioisotope.

Thorium cycle, while great for power generation, is terrible for rockets. He also compares the activity of plutonium RTGs to the uranium fuel used in the proposed engine, so I guess he envisions uranium fuel.

In my way, the easiest way to safely dump the fuel would be to have some solid that turns into a lot of gas when heated to unsafe temperature, as well as a heat plug that would melt (possibly made of the same stuff to ensure the same critical temperature). That way, when you overheat, your reaction chamber suddenly gets a hole on one end and high pressure on the other end, forcing the fuel and coolant out. You just need to store said fluid in long tubes to cool it while keeping it subcritical.

[Dispatcher]

Thorium cycle, while great for power generation, is terrible for rockets. He also compares the activity of plutonium RTGs to the uranium fuel used in the proposed engine, so I guess he envisions uranium fuel.

That sounds better. I think the greatest hurdles to attaining practical use of such propulsion technologies are due to public/ governmental fears rather than inherent production limitations.

[Streetwind]

Guys, pay attention to what the young man is saying He has not developed a new nuclear reaction, or even the concept of molten salt reactors (those were already being tested in live reactors in the 70's).

What he was presenting there is a feat of engineering, not one of nuclear physics. He has taken the existing parts (the molten salt reactor, the brayton cycle etc. etc.) and put them together to form a nuclear power plant that significantly improves over currently running nuclear power plants in several fields - notably security and thermodynamic efficiency.

In the part about the NERVA engine, he says that he is interested in doing the same for nuclear engines - that is, he wants to engineer something out of existing parts that works better than the old NERVA, but he has not yet done so. I'm fairly confident that he (or anyone else attempting this) has a pretty good shot at it, simply because no nuclear thermal rocket has been built since the cancellation of the NERVA program, and we've had a lot of technological advances since then. It's a given that there will be several low-hanging fruit to pick that instantly provide a sizable boost over NERVA. There just needs to be someone committed to building it. And as we all know, that's facing the very same problem that's kept the development of these engines down all this time: political and social backlash against nuclear technology.

[Rakaydos]

His new design uses a gravity drain of low pressure molten fuel into a neutron absorbing tank in the event of a failure.

Would the space equilvilant be a neutron absorbing torus around the reactor, with a couple sepratrons to spin the reactor to drain into it?

[einsteiner]

Just to be clear, it isn't Taylor Wilson's design and it isn't new. Anybody interested in this should look up FLiBe energy and Kirk Sorensen.

[Dispatcher]

Correct. None of his ideas is new. However, he's a visible proponent who seeks the funding to make it happen.

[Duxwing]

Just to be clear, it isn't Taylor Wilson's design and it isn't new. Anybody interested in this should look up FLiBe energy and Kirk Sorensen.

I was wondering about whether he had invented this design. Thanks!

-Duxwing

[Nuke]

His new design uses a gravity drain of low pressure molten fuel into a neutron absorbing tank in the event of a failure.

Would the space equilvilant be a neutron absorbing torus around the reactor, with a couple sepratrons to spin the reactor to drain into it?

i think i would use vacuum actuated pistons in the sub critical tanks. their natural tendency would be to evacuate the core (there would be passive displacement devices to take up the space of the extracted fuel in the core or other parts of the reactor). they would be blocked by freeze plugs and so would remain in a primed state. if the plugs melt, then they are allowed to draw off the fuel. resetting would require pressurizing the vacuum side of the pistons to force them back into their primed state, and to re-inject the fuel into the reactor (and this would just take a couple of valves and some pressurant). the freeze plugs would then be allowed to reform and the pressure evacuated from the pistons. it at least seems simpler than a centrifuge.

but thats only a minor problem. the big problem is that the molten salt tends to be rather corrosive. so designing materials that are both light enough for use in space and also can resist corrosion from the salt would be a huge challenge. i dont even think we have the materials for a terrestrial msr. we have run experimental msrs before, and they worked great, but i dont think we have ever ran them long enough to fail due to corrosion. its certainly a problem that needs to be resolved before we can design one to run for a few decades.

Edited March 20, 2014 by Nuke

[Idobox]

but thats only a minor problem. the big problem is that the molten salt tends to be rather corrosive. so designing materials that are both light enough for use in space and also can resist corrosion from the salt would be a huge challenge. i dont even think we have the materials for a terrestrial msr. we have run experimental msrs before, and they worked great, but i dont think we have ever ran them long enough to fail due to corrosion. its certainly a problem that needs to be resolved before we can design one to run for a few decades.

For a spacecraft, lead might be a better option that molten salt. It has an even higher boiling point, is non corrosive and an excellent gamma ray shield. The main problem, its high melting point, wouldn't be an issue in this case. Of course, the mass is an issue.

Anyway, corrosion of the evacuation tubes is not a big problem. The fluid will quickly solidify, and won't have time to corrode anything much. Also, you don't really care about contaminating the fluid in this type of situation.

[Nuke]

im not sure if you can use lead in the fuel loop, you kinda want reactivity there, and you need something in which the fuel can dissolve (idk if lead meets any of these requirements). but liquid metal would be good in the coolant loop. NaK alloy would be a better coolant, its liquid at room temperature and has been space proven thanks to TOPAZ. its very reactive to water and air, so use in terrestrial reactors is limited, but that is not an issue for a space reactor.

Edited March 21, 2014 by Nuke