In the year 2069AD, a spaceship is transporting hundreds of people in cryogenic sleep to a distant star. The spaceship engine is a nuclear fission reactor and its thruster is based on an ion drive for acceleration.

The problem is, why not dump the nuclear waste into the vast emptiness instead of threatening the crews with radiation danger? There is no legal responsibility and not even a cultural obligation to properly dispose of nuclear waste so why do people continue this perilous practice regardless of duty and border?

  • $\begingroup$ You're asking this question like there's a known practice of interstellar ships retaining their nuclear waste. Perhaps you might want to reword this so we have a better idea what you're getting at here? $\endgroup$
    – Cyn
    Commented May 4, 2019 at 4:25
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    $\begingroup$ I've changed your title to reflect the specific question more closely. The previous title was, in my opinion, misleading, as spaceships indeed do eject their spent fuel - it's how spaceship propulsion works in the current day. $\endgroup$
    – dot_Sp0T
    Commented May 4, 2019 at 5:21
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    $\begingroup$ Downvoted due to insufficient research. With a properly-designed system, there IS no radiation danger. The running reactor produces more "radiation" (which is not a single, simplistic thing) than the spent fuel, and there is a non-negligible amount of it in space, as "cosmic rays". $\endgroup$
    – jamesqf
    Commented May 4, 2019 at 16:48

6 Answers 6


Ejecting spent nuclear fuel is not as simply as dragging things to the airlock and giving them a push. You need to be able to enter the reactor, extract the spent fuel, replace it with fresh fuel, carry it to the exterior of the ship, and give it enough of a push that it won't damage the ship. You need to either make provisions for protecting the crew member(s) carrying out the operation, or for an automated system that can survive the hazardous environment within the reactor. You need provisions to ensure that the fuel is not dangerous to the crew, passengers, and other ship systems while moving it.

In contrast, if you keep the spent fuel contained in the reactor until arriving at your destination, you don't need to make the reactor accessible. You don't need to have equipment and procedures for moving the spent fuel around an active starship. These savings in complex machinery translate directly into savings in weight throughout the whole trip, which could easily outstrip the weight savings from ejecting fuel, which would only be over some of the trip.


Because recycling?

Spent nuclear fuel rods can be reprocessed to get nuclear fuel. This is fairly efficient AFAIK but in the real world comes with the issue that some of the material generated could also be used for nuclear weapons. The efficiency apparently depends on the reactor type, so that has to be part of the design. This might make sense for a long range spaceship as it might need good fuel economy.

The reactor itself could be built with "additives". Many elements increase in value due to nuclear reactions that happen as a side effect inside the reactor. Basically the atom absorbs a neutron, becomes radioactive and decays via positron, beta or alpha decay to another element that has higher value.

This is of marginal value to us but colony ship reactors might be designed to allow production of rare elements that a starting colony might not otherwise have access to. Or the additives might be materials that neutron activate to usable nuclear fuel.

Additionally such materials would protect the actual structural materials from radiation which would make the reactor last longer in use. Seeing as being stuck on a spaceship with broken nuclear reactor would be "annoying" such extra protection would have more value than on Earth. And the produced "waste" would be too valuable to throw away.

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    $\begingroup$ Exactly. The rods are no good powering the ship but could power a thorium reactor once you get to the destination. $\endgroup$
    – Thorne
    Commented May 4, 2019 at 9:27

Why you would dump it ...

It's radioactive, it's dangerous to ship systems and could lead materials and equipment to fail. It's also got mass, and if you eject mass it can give you extra momentum (a boost) and reduce the ship mass further to make acceleration (and the eventually required deceleration) more efficient.

However you can store the waste a long way from the main body of the ship, so it's not really dangerous to keep it.

Why you would not dump it ...

It's going to decay into useful materials that will be useful to get your colony started. You don't want to expend your early colonization effort gathering difficult to gather (and possibly very rare) materials, so (carefully chosen) the "waste" of your fission process could, after hundreds or more likely thousands of years of travel, decay to a very valuable collection of materials for a starter colony.

And the Winner is ... eject to decelerate a little (?).

The payload-fuel balance is a very complex one and it would require very, very detailed calculations knowing the precise performance of the propulsion system to work out which mix is optimal in any specific case. My gut feeling would be that the deceleration phase (to slow down on the long approach to the destination and not whizz by it !) means that loosing the mass of spent fission fuel is likely better. Ejecting the fuel can even be done in a way to decelerate the ship a little, making it even more advantageous to eject the spent fuel.

At the end of the day space is vast and it really make no difference at all to the universe if you dump nuclear waste in it. However the efficiency of the mission is absolutely critical and anything that makes it more efficient is going to be done.

Only the most critical need (can't think of one) or the most strict of religious or ethical desires would over-rule the cold, hard mathematics of the engineering of such a project. Perhaps not even such concerns would be enough.

So it's dump the waste (to accelerate and decelerate), IMO.

  • $\begingroup$ The problem with this is, in order to decelerate, the fuel needs to be ejected in the direction the ship is travelling in. What this then means is you have debris floating right in front of where you are trying to go, preventing you from accelerating in the future without manoeuvring around or ramming into the nuclear debris you left there earlier. $\endgroup$ Commented May 4, 2019 at 10:47
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    $\begingroup$ @LiamMorris Remember space is BIG. You don't have to direct it directly in front of you - a little off to one side is fine. Over the distances involved even a minute difference will mean you couldn't find it again if you tried. But even if you fire it straight ahead when decelerating, it makes no sense to accelerate again anyway - that's simply not the way you need to operate on this kind of trip. $\endgroup$ Commented May 4, 2019 at 11:30
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    $\begingroup$ Thats true, to be honest i did consider that, however i felt the need to point it out as it is still a problem that you may need to over come. My solution would have been to fire half of the payload out on each side of the ship because remember, every action has an equal and opposite reaction. As much as you have pushed that payload away, it has pushed your ship away. Whilst it may have only adjusted your course by a fraction of a degree, at high speeds and long distances that fraction could turn into several degrees off course. -continued- $\endgroup$ Commented May 4, 2019 at 11:38
  • $\begingroup$ What was a small fraction of a degree difference could mean your trajectory is now thousands of miles away from where you wanted to be. However, by firing an equal amount out of each side of the ship (Mirroring the angle and force on the other side of the ship) the forces have cancelled each other out and allowed you to stay on course (or at least more than you would have been, requiring less course correction and thusly saving fuel). $\endgroup$ Commented May 4, 2019 at 11:42

The Debris Will Follow You

Whilst i posted the answer about ethics first, this came to me as an after thought. Jettisoning something in space is not the same as on Earth. In space, we must remember Newton’s first law “an object in motion stays in motion unless it is acted upon” greatly applies here. In space, there is nothing to act upon the object in motion, the jettisoned nuclear waste. Whilst you may have jettisoned it and saw it drift away in the distance, that doesn't mean it has lost all momentum, it is still travelling at the same speed it was (minus the speed of your arms pushing it away).

If you were travelling at 10,000 mph and jettisoned the waste, that waste is also going to travel at 10,000 mph (minus the speed you took away by jettisoning it which would be negligible anyway). If you then accelerated to 11,000 mph, that waste is still travelling towards you at 10,000 mph.

When you eventually reach your destination and set up your colony, that waste is still following you and will one day catch up. All of a sudden your ancestors are now having to deal with the nuclear waste you left behind many years ago. Whilst the material may have decayed to the point of being non-hazardous by the time it reaches your planet, you will still have potentially hundreds of heat-resistant containers travelling towards your colony at, in this example, 10,000 mph.

Whilst this could be avoided by changing course after jettisoning or slowing down, you will have to accelerate again and use up a huge amount of fuel if you stop and start every time you jettison waste (leading to more stops and starts, causing more fuel to be used, requiring more jettisons, leading to more stops and starts etc.) Changing course would require you to at least slow down and use up some amount of fuel.

Even if it doesnt hit your planet of colonists though, by jettisoning the debris you have set a minimum speed you need to travel at. If you go any slower, the debris will catch up and hit the back of your ship. However, if you overcome all of issues these through course corrections, who’s to say the debris won’t crash into another planet like some sort of accidental orbital strike?


Although they have no legal obligation to not dump the nuclear waste, to leave highly radioactive material unchecked in the middle of nowhere is ethically wrong. It is one of the reasons why the Waste Isolation Pilot Plant has created a system to inform future generations about radioactive waste.

Explained in this answer about how to leave a message for humans 50,000 years in the future the WIPP said this:

Each component of the marking system should be made of material(s) with little intrinsic value. The destructive (or recycling) nature of people will pose a serious threat to the marking system. We decided against simple "Keep Out" messages with scary faces. Museums and private collections abound with such guardian figures removed from burial sites. These earlier warning messages did not work because the intruder knew that the burial goods were valuable. We did decide to include faces portraying horror and sickness (see Sections 3.3 and 4.5.1). Such faces would relate to the potential intruder wishing to protect himself or herself, rather than to protect a valued resource from thievery.

Here is the message that they proposed:

This place is a message… and part of a system of messages… pay attention to it!

Sending this message was important to us. We considered ourselves to be a powerful culture.

This place is not a place of honor… no highly esteemed deed is commemorated here… nothing valued is here.

What is here is dangerous and repulsive to us. This message is a warning about danger.

The danger is in a particular location… it increases toward a center… the center of danger is here… of a particular size and shape, and below us.

The danger is still present, in your time, as it was in ours.

The danger is to the body, and it can kill.

The form of the danger is an emanation of energy.

The danger is unleashed only if you substantially disturb this place physically.

This place is best shunned and left uninhabited.

Whilst we have no legal responsibility for humans 10,000 years in the future, the fact that we have developed a way to warn them about the dangers of this nuclear waste we have left behind shows we have an ethical responsibility. It would be morally wrong to leave the nuclear waste unattended and unchecked, even if it is in space.

Risk of The Unknown

Additionally, for all we know, by dumping that waste there we may damage the ecosystems of other planets, potentially ones with life on them. Would you want to be responsible for the accidental genocide (xenocide?) of alien life forms? I would hazard a guess at “probably not”.

Or there may be an intergalactic Green Peace which we’ve now angered by leaving that nuclear waste there. It would not be a good idea to start a war with an alien civilisation over littering of all things.


If the nuclear waste is a "radiation danger", then it is generating heat. This heat can be put to good uses, such as keeping the ship from completely freezing, or powering the ship's computer.

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    $\begingroup$ It may seem hard to believe, but getting rid of heat in space is just as hard a problem (arguably harder) as staying warm. $\endgroup$ Commented May 4, 2019 at 7:04
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    $\begingroup$ Adding to what @StephenG said, ships may need external radiators to prevent the crew inside from overheating. Space is a vacuum meaning heat can not travel via conduction or convection so instead it must travel via radiation (though not to be confused with the radiators i mentioned earlier). On the contrary, the inside of the ship has air, allowing for convection to occur. Without external radiators, the inside of the vessel may become an oven as heat is trapped inside with nowhere else to go. $\endgroup$ Commented May 4, 2019 at 7:21
  • $\begingroup$ @StephenG -- I am familiar with the problem. Heat pipes and radiator fins are attached to lots of parts of satellites. $\endgroup$
    – Jasper
    Commented May 4, 2019 at 7:48

Radiation is useful

You can use it to sterilise things. For example, if there is a mutiny, you can lock the opposition leaders inside the reactor and they will be pretty much sterilised.

You can booby trap sensitive parts of the ship with very high level of radiation. Aliens boarding your ship uninvited will no longer be a problem.

You can keep whatever it is that you are smuggling in the reactor. Cops never check the nuclear reactor.

And if push comes to shove, a kamikaze radioactive ship is more of a menace than a non-radioactive one, which is good for diplomacy. Pax armada and stuff.

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    $\begingroup$ Im not sure i like your definition of “sterilisation”, i’d hate to be a scientist working in one of your “sterile” labs :P $\endgroup$ Commented May 4, 2019 at 14:20

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