Short Lifetime For Nuclear Thermal Rockets In Space?

[Spacescifi]

 

I wonder how long a NTR engine relying on ISRU propellant in the solar system can last before the reactor gives out?

Obviously a nonmanned vessel can afford to be more picky, but a manned group of astronauts rely so heavily on speed that they may not always have the time to afford being picky about ISRU. Add to that the high thrust requirements of manned flight and I think a reactor may not even last four years of use.

Not to say we could not build one that could, but I reckon being picky about propellant and thrust ould play a big part in whether or not the reactor survives for longer or shorter periods.

 

Am I wrong?

This is not for scifi, this is consideration for real manned nuclear flight.

 

What do you know about reactor expiration dates in space using them with high thrust and whatever ISRU water you can grab?

Edited May 20, 2021 by Spacescifi

[cubinator]

Quick google says fuel rods last about six years in a nuclear reactor*. Not sure if they would last longer in a NTR because they're not being 'used' most of the time, but I'm no expert on the physics involved.

*Followed by a series of headlines about how scary and dangerous the waste is. 

Edited May 20, 2021 by cubinator

[Spacescifi]

46 minutes ago, cubinator said:

Quick google says fuel rods last about six years in a nuclear reactor*. Not sure if they would last longer in a NTR because they're not being 'used' most of the time, but I'm no expert on the physics involved.

*Followed by a series of headlines about how scary and dangerous the waste is. 

 

I see, so spaceships need a reactor replaced in six years.

Unless you have spare fuel rods I guess.

Hmmm...what do you even do with the waste?

Do we start polluting space with spent uranium fuel rods LOL?

One more bit of space debris to watch out for, only radioactive too!

 

I am really begining to think that perhaps slow space travel as reasonably possible is the safest for humans.....if you have a kilometer rotating tether for 1g spin and sustainable replenishable food source.

The requirements for not needing MAX speed travel time are 1g gravity via rotation and your own garden aboard the vessel to grow fruits and veggies.

 

So any manned vessel meant to endure as a slower 'boat' must be massive to have all the stuff we need to live as comfortably as possible.

Get there ASAP kinds of manned craft can afford to have just stored food and basic life support, but if something goes wrong with the engine they won't be able to survive indefinitely like the 'slow boats' can.

 

Those are the two types of manned vessels I foresee coming.

Edited May 20, 2021 by Spacescifi

[SpaceFace545]

2 hours ago, Spacescifi said:

Hmmm...what do you even do with the waste?

The spent reactors or fuel rods would either be sent down to earth for recycling but more realistically they would just be placed in a special graveyard orbit.

[Brotoro]

I doubt they'd bring used fuel rods down to Earth. Too much radioactive decay products in them that you wouldn't want to get loose in a reentry accident. Better to keep them in space until you get reprocessing facilities in orbit or on the Moon.

[SOXBLOX]

Check here?

[magnemoe]

5 hours ago, cubinator said:

Quick google says fuel rods last about six years in a nuclear reactor*. Not sure if they would last longer in a NTR because they're not being 'used' most of the time, but I'm no expert on the physics involved.

*Followed by a series of headlines about how scary and dangerous the waste is. 

However this is with constant use.  An nerva style engine will burn for less than an hour, then shut down, then burn again at destination. 
Even if you do an LEO to moon orbit route who run all the time your reactor will not use up their rods in an lifetime. 

[cubinator]

3 minutes ago, magnemoe said:

However this is with constant use.  An nerva style engine will burn for less than an hour, then shut down, then burn again at destination. 
Even if you do an LEO to moon orbit route who run all the time your reactor will not use up their rods in an lifetime. 

That's what I guessed - that the fuel rods in a NTR would last considerably longer, lasting plenty of time for your trip to Pluto.

[Spica]

A not completely unreasonable guess would be to assume a burnup threshold beyond which the reactor can not be started, due to a buildup of fission products.

I really have no idea what that usable fraction of fuel energy is, and it will surely depend on the reactor design. Let's use 5% as a value for now.

1 kg of fissile material contains around 80 TJ of nuclear energy within, and with a 5% usable burnup that gives us 4 TJ/kg.
Hydrogen takes around 44 MJ/kg to be heated to 3,000K, a warm but not unreasonable NTR core temperature.
This means that 1 kg of fissile material provides the energy to expel 90,000 kg of hydrogen propellant, that's a lot.

This leads me to think that the limit is not one of time, but one of "propellant expended", and it will be fairly long.
But beware the safety hazard that large amounts of long lived fission products create, I wouldn't want to be caught near one unshielded, and the shielding will be thick and heavy (and usually just a puck above the engine, since only the ship that carries it needs to be shielded).

 

Edited May 21, 2021 by Spica
additions

[DDE]

1 hour ago, magnemoe said:

However this is with constant use.  An nerva style engine will burn for less than an hour, then shut down, then burn again at destination. 
Even if you do an LEO to moon orbit route who run all the time your reactor will not use up their rods in an lifetime. 

Not entirely. A true NERVA would have a staging event and jettison the Earth departure engine(s).

Edited May 21, 2021 by DDE

[magnemoe]

2 minutes ago, DDE said:

Not entirely. A true NERVA would have a staging event and jettison the Earth departure engine(s).

Yes most use cases for them was single use who solve most of the radiation issues an reusable nuclear engine has. 

[SOXBLOX]

Summarizing the AR link I gave above, as fuel rods are used in chain reactions, they fill with "nuclear poisons" (so called because they poison the reactions), which hinder further reactions. A Dr. John Schilling says maybe a full day of full power operation would create enough poisons to require reprocessing (i.e. melting down the rods and removing the poisons). Some other guy says you could get perhaps 20k hours from a reactor using HEU and special techniques. Higher enrichment means longer runtimes. Usually only 15% of the available fuel is actually consumed; the rest is just trapped in poison.

[kerbiloid]

They should build a D+D fusion reactor and keep killing all radiowastes with neutrons  until they get turned into stable lightweight elements.

[monophonic]

 

11 hours ago, Spacescifi said:

I wonder how long a NTR engine relying on ISRU propellant in the solar system can last before the reactor gives out?

Newer US submarines have their reactor cores built to last for the entire lifetime of the submarine. That will be 30 to 50 years at current estimates with just the original fuel load. Unless the NTR is the only power source on your spaceship it can be run for a considerably lesser fraction of the ship's operational life than a submarine's, which (abnormal situations aside) is only shut down while pierside and on shore power. Therefore I would expect that other systems on the space ship would start to expire long before the reactor core dies.

Of course such a long life reactor cores require fuel refined to a very high purity. While I have no knowledge I suspect it could be considered "bomb-grade" even. This constrasts with the French who are determined to use the same civilian grade fuel their nuclear power plants burn. Thus they have to refuel their submarines (and the only non-US nuclear powered aircraft carrier in the world) every ten years or less. So if politics restrict the purity of fuel used, refueling the NTR may become an issue to be solved.

Edited May 21, 2021 by monophonic
Stray half sentence

[Starhelperdude]

10 hours ago, cubinator said:

Quick google says fuel rods last about six years in a nuclear reactor*. Not sure if they would last longer in a NTR because they're not being 'used' most of the time, but I'm no expert on the physics involved.

*Followed by a series of headlines about how scary and dangerous the waste is. 

it depends on what reactor type and what fuel and what NTR

[kerbiloid]

https://translate.google.com/translate?hl=&sl=ru&tl=en&u=https://ru.wikipedia.org/wiki/Кампания_ядерного_реактора

(Can't find an English equivalent)

[wumpus]

10 hours ago, SOXBLOX said:

Summarizing the AR link I gave above, as fuel rods are used in chain reactions, they fill with "nuclear poisons" (so called because they poison the reactions), which hinder further reactions. A Dr. John Schilling says maybe a full day of full power operation would create enough poisons to require reprocessing (i.e. melting down the rods and removing the poisons). Some other guy says you could get perhaps 20k hours from a reactor using HEU and special techniques. Higher enrichment means longer runtimes. Usually only 15% of the available fuel is actually consumed; the rest is just trapped in poison.

So I guess it comes down to how much delta-v is in a "full day" of thrust, and how often you have to change the rods.  Also the design of your reactor changes just how quickly you want to change those rods as the big boys used for power generation last much longer (one day sounds short for even plutonium production). I still think that the "single use set of rods" simplifies the design enough for serious consideration.

[SOXBLOX]

One day sounds short to me, too, but that's what the page said. IDK.

[Spacescifi]

5 hours ago, wumpus said:

So I guess it comes down to how much delta-v is in a "full day" of thrust, and how often you have to change the rods.  Also the design of your reactor changes just how quickly you want to change those rods as the big boys used for power generation last much longer (one day sounds short for even plutonium production). I still think that the "single use set of rods" simplifies the design enough for serious consideration.

 

1 hour ago, SOXBLOX said:

One day sounds short to me, too, but that's what the page said. IDK.

 

Future manned spaceflight efforts I think will have something in common with our history of technological development.

We started off walking, but that took too long so we tamed horses to ride. We also made the wheel to make the wagon so the horse can pull it instead of us, because the horse is better at it.

We also made paved roads, I reckon likely after the horse and wagon, as having both available allows the process of making roads to be a lot more efficient than it would be otherwise.

Nuclear manned spaceflight will require a lot of space infrastructure in place and awaiting the manned vessels to assist them.

I think nuclear manned spaceflight will need or could at least use.... and this is by no means an exhaustive list:

 

1. Nuclear powered robot spaceships without crew.  This will be the backbone of ANY nuclear space fleet. The robot fleet can go places and do stuff a manned one would not. They will also be key to starting the process of setting up bases offworld on  dead worlds like the moon, mars, etc.

2. Nuclear fuel recycling bases: For purifying 'poisoned'  fuel rods so they can be used again. It may be a royal pain, but the good news is that we can use boh nuclear and solar power to power the base, even using the virtually unlimited reaction mass from the local dead world to help supply station needs as well.

3. Magnetic railways or launch tubes. For launching mass into orbit of low gravity bodies. The moon would be perfecr for that. Nuclear could power this as well.

Unlike scifi, an awesome engine is not all you need for manned fast space travel, as power sources wear out fast. So until we get a new kind more potent than nuclear then a targeted infrastructure to support the nuclear reliance is needed to make the most of nuclear spaceflight without wasting recyclable fuel rods.

 

Edited May 21, 2021 by Spacescifi

[mrfox]

On 5/20/2021 at 10:26 PM, DDE said:

Not entirely. A true NERVA would have a staging event and jettison the Earth departure engine(s).

 

On 5/20/2021 at 10:32 PM, magnemoe said:

Yes most use cases for them was single use who solve most of the radiation issues an reusable nuclear engine has. 

Could you elaborate on this? Is this a scenario where there is an established infrastructure of nuclear space tugs? or is the mass trade off for shielding so great that it is  better to have a (baseball analogy) pitcher/catcher setup on either end even for a single use mission?

[DDE]

27 minutes ago, mrfox said:

Could you elaborate on this? Is this a scenario where there is an established infrastructure of nuclear space tugs? or is the mass trade off for shielding so great that it is  better to have a (baseball analogy) pitcher/catcher setup on either end even for a single use mission?

Most NERVA scenarios are classic, Apollo-style, non-reusable trailblaser missions assembled from parts launched using, again, non-reusable Saturn V derivatives.

[Starman4308]

On 5/21/2021 at 1:22 AM, SOXBLOX said:

Summarizing the AR link I gave above, as fuel rods are used in chain reactions, they fill with "nuclear poisons" (so called because they poison the reactions), which hinder further reactions. A Dr. John Schilling says maybe a full day of full power operation would create enough poisons to require reprocessing (i.e. melting down the rods and removing the poisons). Some other guy says you could get perhaps 20k hours from a reactor using HEU and special techniques. Higher enrichment means longer runtimes. Usually only 15% of the available fuel is actually consumed; the rest is just trapped in poison.

I suspect that "full day of full power operation" might be "if it's run for a full day, the fuel rods are no longer safe to handle without full-on nuclear-waste precautions".

Either way, I'd be surprised if an NTR had to be refueled at all unless it was also being used for electrical power during downtime, as it's trivial to run a nuclear reactor for far longer than an NTR will be used as a rocket engine.

[SOXBLOX]

Dr. What's-his-face may have meant very low enriched uranium. I could believe almost natural-grade material filling with poisons after a day, and using LEU was discussed on that page...

But one day for a Virginia-class's reactor? Ehm, no. I agree.

[Starhelperdude]

35 minutes ago, SOXBLOX said:

Dr. What's-his-face may have meant very low enriched uranium. I could believe almost natural-grade material filling with poisons after a day, and using LEU was discussed on that page...

But one day for a Virginia-class's reactor? Ehm, no. I agree.

yeah, and we are only talking about uranium-using NTRs, and prob. only about Solid-Core NTRs too

[Nuke]

On 5/20/2021 at 5:45 PM, SpaceFace545 said:

The spent reactors or fuel rods would either be sent down to earth for recycling but more realistically they would just be placed in a special graveyard orbit.

really depends on if the ship is a one off or if there is a fleet of them and how many space ports there are in the solar system with a nuclear reprocessor.  even a spent fuel rod is valuable, especially in space. its something you can reprocess and are therefore a commodity you can trade for things like remass or air or whatever currency you may be using.