Author Topic: NASA boosts nuclear thermal propulsion with BWXT contract  (Read 2768 times)

Offline Jim

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #20 on: 08/10/2017 05:19 PM »

IOW in principal going LEU saves a lot of money
...... a cost claim without argument.

That one is intuitively obviously and doesn't need an argument.

Offline john smith 19

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #21 on: 08/10/2017 08:35 PM »
That doesn't make any sense. Please elaborate and/or correct your comment.
There is thermal efficiency (% of heat converted to electricity) and system efficiency (Kw/Kg of mass)
Thermodynamics says the bigger the temperature difference the more efficient the heat engine. With the background temperature of the universe being around 3K that makes a temperature difference of (maybe) 2800K.

But that means a temp difference between the radiator (which in space works purely by radiation) of maybe a few Kelvin. Given that the ISS radiator is around 50W/M^2 at around 303-340K that means the hypothetical  radiator would be enormous and the system efficiency (Kw/Kg) would be very low.

For any NEP system there's a graph of radiator temp Vs overall system weight for a given output.

Improving space radiator efficiency for NEP is a really good idea (actually improving space radiator efficiency in general at various temperatures would be a good idea)

Quote from: Propylox
You're talking about HEU systems, which this is not, and contradicted your desire for efficiency by proposing a low temperature generator and associated systems.
There are actually 3 systems being talked about here. The NASA legacy HEU NTP (that's a rocket) the Russian fast NEP (electric power reactor running an ion thruster) and the NASA BWXT programme which is LEU NTP.

Since you brought the Russian system up you should keep track of what's being talked about.
NTP's have short run times. Their pressure vessel and nozzle typically needs regenerative cooling (which is what drives the turbopump). While running some of that flow can drive a generator. During shut down there would be no flow through the reactor, so how does the power get from the core to the wall?
Quote from: Propylox
Another contradiction, and a cost claim without argument.
Based on your previous posts, I'd request the old JS19 reply  ;)
There are 2 different costs here. Raw material and system costs.

KiloPower uses HEU. It is cheap because the US has a surplus of HEU. It is being tested in areas that already have high security because they deal with nuclear weapons.  It is part of the payload. It is switched off when it's fitted to the rocket.

Once you get to NTP for propulsion you have to do a lot of ground testing at NASA facilities. It's not just a % of enrichment it's a step change in the security and planning costs involved in each stage of the design and mfg process. It's like the cost differences between using Hydrogen Peroxide/Kerosene (coveralls, gloves, goggles, water shower) and  NTO//UDMH (full rubber suite with self contained air supply).
Quote from: Propylox
-- Previous quote --
Re1) Why have you proposed tungsten will be part of the core and not part of the rocket - ie; the throat and upper nozzle? Isotopic requirements on neutron absorption/reflection would be much more applicable to the rocket architecture or core's casing than the core's matrix.
While it's possible, the process is so expensive that you'd only use it if absolutely necessary. The key issue is LEU makes a design more sensitive to neutron absorption by the structure of the core. That's why you'd want to strip the more absorbent isotopes out of the core raw material.
Quote from: Propylox
Re2) Agreed, but a LEU fast reactor still doesn't produce the heat desired for NTP without reflecting (W?), or otherwise encouraging, neutrons back into the matrix to accelerate fission and temperature. This is why I asked about creating higher temps or use of tungsten around the throat - the LEU doesn't cut it otherwise. And there's still the issue of keeping propellant in contact long enough to extract temperature - conflicting with keeping it moving and building velocity. I don't see how that's solvable without a working fluid. Thoughts?
Well that's sort of the point of a design contract. To see if an NTP engine with LEU is possible.

You do realize that the "working fluid" you're talking about is the propellant in an NTP system, right?

There are multiple materials that can be used as neutron reflectors. AFAIK Isotope enriched or depleted W has never been one of them. Why bother with something when higher TRL materials already exist?

Tungsten's specific gravity is about the same as pure Uranium. The T/W of NTP systems is bad to begin with. There are easier materials to work unless you absolutely need the maximum temperature Tungsten can give you. [EDIT TBH If isotopic enrichment techniques are on the table I'd go with separating Molybdenum from Hafnium. Mo with 5%Re is quite ductile and weldable. It's thermal conductivity is good and it's about 1/2 the density of W. But W is what the previous programme focused on so that's where the knowledge base is.   :( ]

From your questions you don't know as much about this subject as you seem to think you do.  :(
« Last Edit: 08/10/2017 11:17 PM by john smith 19 »
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Offline ZachF

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You could probably make an NTR on LEU if you use heavy hydrogen (deuterium) as the remass. maybe even heavy Methane.

You'd lose a some Isp from the heavier element, but fuel would be denser.

CANDU reactors can run on natural uranium because of the use of heavy water as moderator.

Online Asteroza

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One might be able to make the argument that a NTER might fit this case better, since a NTER's turboinductor will need lots of tungsten anyways and the turboinductor overcomes some of the fuel matrix heat limitations. Though that's like arguing a LEU NTER is equivalent to a HEU NTR, which may or may not be the case, all things considered.

Online Robotbeat

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You could probably make an NTR on LEU if you use heavy hydrogen (deuterium) as the remass. maybe even heavy Methane.

You'd lose a some Isp from the heavier element, but fuel would be denser.

CANDU reactors can run on natural uranium because of the use of heavy water as moderator.
You can also make a LEU NTR using Hydrogen/protium. That's what this thread is about.
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