Why is unused nuclear fuel safe, but used not?
Discussion
I am puzzled why unused nuclear fuel isn't radioactive, but used is highly radioactive. I thought it would be the other way around! i.e. the radioactivity is what generates the power til the radioactivity dies down to nothing then it is spent fuel!
Clearly I am completely wrong. In simple terms why am I wrong?
As an aside (and lifted from Quora) :
Take all the used fuel bundles that are in the core of a reactor, place them on a field.
At 600 meters (2000 ft) you will be fine, at least for a short while, because the intensity drops faster than the square of the distance.
Then you start running, as fast as you can, towards the fuel rods. You goal is to try to put your hand on the fuel.
You will fail.
Because during the last few meters, the direct radiation from the fuel will be so intense that it overloads your nervous system and you will fall down unconscious, before you reach the fuel.
And soon after, you die.
Clearly I am completely wrong. In simple terms why am I wrong?
As an aside (and lifted from Quora) :
Take all the used fuel bundles that are in the core of a reactor, place them on a field.
At 600 meters (2000 ft) you will be fine, at least for a short while, because the intensity drops faster than the square of the distance.
Then you start running, as fast as you can, towards the fuel rods. You goal is to try to put your hand on the fuel.
You will fail.
Because during the last few meters, the direct radiation from the fuel will be so intense that it overloads your nervous system and you will fall down unconscious, before you reach the fuel.
And soon after, you die.
In laymans terms,
Unused fuel needs to be "activated" to start the reaction that causes the heat that is used. It is safe to handle.
Used fuel then takes a very long time for the heat and radioactivity to die down. If you see with the naked eye it you're f
ked.
I build the stuff and put it in and out of reactors for a living.
Unused fuel needs to be "activated" to start the reaction that causes the heat that is used. It is safe to handle.
Used fuel then takes a very long time for the heat and radioactivity to die down. If you see with the naked eye it you're f
ked.I build the stuff and put it in and out of reactors for a living.
Edited by Brads67 on Saturday 25th July 22:55
eldar said:
Unused fuel is radioactive, in a predictable way. Used fuel is still radioactive, but the emissions are less predictable.
It is moderated to be sub critical outside the reactor.
Unused is not radioactive (or is very very slightly, like lots of other substances) It has an unstable nuclei that can be encouraged to split and create a "chain reaction" and give off heat. Once that has started it is then moderated inside the reactor to control said heat, and other things.It is moderated to be sub critical outside the reactor.
You start with moderately radioactive fuel. You bring enough of it together to get a self-sustaining fission chain reaction. This results in a sequence of daughter products, some of them with much shorter half lives and more intense radioactivity than the fuel. It also produces radioactive products when neutrons from the chain reaction hit things other than fuel.
This might explain it all.
https://www.osti.gov/etdeweb/servlets/purl/587853
This might explain it all.
https://www.osti.gov/etdeweb/servlets/purl/587853
OK thank you. But if used fuel is still heavily radioactive then why not keep using it til it is not radioactive? Why stop using it when it still has some radioactivity left?
Is it simply that it is well past its peak, but still sufficiently radioactive to kill life but not sufficiently radioactive to generate useful amounts of power?
Is it simply that it is well past its peak, but still sufficiently radioactive to kill life but not sufficiently radioactive to generate useful amounts of power?
Brads67 said:
In laymans terms,
Unused fuel needs to be "activated" to start the reaction that causes the heat that is used. It is safe to handle.
Used fuel then takes a very long time for the heat and radioactivity to die down. If you see with the naked eye it you're fked.
I build the stuff and put it in and out of reactors for a living.
How do you activate it? I thought highly radioactive stuff didn't need any encouraging to spit out deadly particles left right and centre, it just does it as it is unstable? Why does fuel sit there quite happily stable but when activated starts spitting out dodgy stuff?Unused fuel needs to be "activated" to start the reaction that causes the heat that is used. It is safe to handle.
Used fuel then takes a very long time for the heat and radioactivity to die down. If you see with the naked eye it you're f
ked.I build the stuff and put it in and out of reactors for a living.
Edited by Brads67 on Saturday 25th July 22:55
Apologies for such basic questions. I just thought the highly radioactive stuff at the heavy end of the periodic table didn't need any excuses to start spitting stuff out?
otolith said:
You start with moderately radioactive fuel. You bring enough of it together to get a self-sustaining fission chain reaction. This results in a sequence of daughter products, some of them with much shorter half lives and more intense radioactivity than the fuel. It also produces radioactive products when neutrons from the chain reaction hit things other than fuel.
This might explain it all.
https://www.osti.gov/etdeweb/servlets/purl/587853
OK thanks - I will read that tomorrow.This might explain it all.
https://www.osti.gov/etdeweb/servlets/purl/587853
Brads67 said:
eldar said:
Unused fuel is radioactive, in a predictable way. Used fuel is still radioactive, but the emissions are less predictable.
It is moderated to be sub critical outside the reactor.
Unused is not radioactive (or is very very slightly, like lots of other substances) It has an unstable nuclei that can be encouraged to split and create a "chain reaction" and give off heat. Once that has started it is then moderated inside the reactor to control said heat, and other things.It is moderated to be sub critical outside the reactor.
I used to be involved in fuel building / recycling. All good fun...
MikeStroud said:
How do you activate it? I thought highly radioactive stuff didn't need any encouraging to spit out deadly particles left right and centre, it just does it as it is unstable? Why does fuel sit there quite happily stable but when activated starts spitting out dodgy stuff?
Apologies for such basic questions. I just thought the highly radioactive stuff at the heavy end of the periodic table didn't need any excuses to start spitting stuff out?
Less than 1% of the uranium in the ground is 'the good stuff', so reactor fuel is enriched to get better power density (think brandy/whisky vs wine or beer). The activation is by encouraging the atoms to split by whacking them (gently Apologies for such basic questions. I just thought the highly radioactive stuff at the heavy end of the periodic table didn't need any excuses to start spitting stuff out?
) with neutrons. But when they split some of the dodgy stuff isn't good for controlling the reactor, the term is actually 'poison' as it poisons the fuel and makes life more difficult for the chaps who have chipped in with operational experience. This is one of the factors that forces refuelling. The old fuel has all sorts of rubbish in it as well as a load of unburnt good stuff, hence quite nasty.Once a reactor is going it produces plenty of neutrons to tick along, some (most? - not a reactor engineer!) need a 'starter' supply of neutrons. If things get out of hand and the neutrons have a real party: Chernobyl or similar could be an outcome - which is when the operator chaps step back as their controls aren't controlling anything any more and eventually someone like me will turn up to try to help clear things up.
[there is a little flippancy and some monstrous generalisations in the above!]
Brads67 said:
eldar said:
The risk of trying to answer the complex question in a very simplistic way
I used to be involved in fuel building / recycling. All good fun...
Indeed, it can be hard to simplify without it sounding stupid to someone with knowledge.I used to be involved in fuel building / recycling. All good fun...
NOT the other way round which would be horribly insulting
MikeStroud said:
OK thank you. But if used fuel is still heavily radioactive then why not keep using it til it is not radioactive? Why stop using it when it still has some radioactivity left?
Is it simply that it is well past its peak, but still sufficiently radioactive to kill life but not sufficiently radioactive to generate useful amounts of power?
It's not the radioactivity that generates the heat, but the uranium atom splitting. It splits into smaller atoms which are more radioactive but don't split.Is it simply that it is well past its peak, but still sufficiently radioactive to kill life but not sufficiently radioactive to generate useful amounts of power?
spikeyhead said:
It's not the radioactivity that generates the heat, but the uranium atom splitting. It splits into smaller atoms which are more radioactive but don't split.
Its the difference between fission and decay. The energy from the former is what’s being harnessed, the latter is what makes the ‘waste’ so hazardous.Brads67 said:
I've just read that back and it sounds awful. I was actually meaning me sounding stupid to someone like yourself.
NOT the other way round which would be horribly insulting
No probsNOT the other way round which would be horribly insulting
not the easiest subject to explainAnyway, the banana equivalent dose can sometimes help...
https://en.wikipedia.org/wiki/Banana_equivalent_do...
We should really start investing in Gen IV reactors - they can use "spent" fuel.
https://www.energy.gov/ne/articles/3-advanced-reac...
https://www.energy.gov/ne/articles/3-advanced-reac...
BrundanBianchi said:
spikeyhead said:
It's not the radioactivity that generates the heat, but the uranium atom splitting. It splits into smaller atoms which are more radioactive but don't split.
It's the difference between fission and decay. The energy from the former is what’s being harnessed, the latter is what makes the ‘waste’ so hazardous.Simply, natural or enriched uranium is fairly stable and gives off little radiation (Alpha from 238 wont make it out of the material so you tend to get the Thorium low energy Beta). When we build fuel, we handle the fuel elements directly (well, with gloves on and using element hoists etc, but unshielded )
When you put it in a reactor and hit it with neutrons, some of the atoms split and give off energy and also some brand new atoms.
These new atoms are frequently very unstable and spontaneously split themselves at varying times, some do it in nano seconds, some in millions of years and every delay in between.
When these new atoms split, they release more energy and Gamma ray, Beta ray/particle and Alpha particle emissions and some even give out xrays. This is what we call 'radiation' or more accurately called ionising radiation. This is massively higher in rate and energy than the radiation the natural uranium puts out and is far more dangerous (basically, we smash it with neutrons and make quite a lot of the atoms unstable).
These new atoms that split, or decay over time after the original uranium atom was split are significant in that they are the cause of what is known as decay heat and why post trip cooling is so important to a nuclear reactor. At trip, the reactor is still producing about 10% of the energy it was just before it tripped. This is not just heat within the reactor, its like having a fire that you cannot put out. If you dont remove it, the reactor will warm up and for current generation of reactors, without active cooling, the fuel WILL melt its way out of the containment.
When you put it in a reactor and hit it with neutrons, some of the atoms split and give off energy and also some brand new atoms.
These new atoms are frequently very unstable and spontaneously split themselves at varying times, some do it in nano seconds, some in millions of years and every delay in between.
When these new atoms split, they release more energy and Gamma ray, Beta ray/particle and Alpha particle emissions and some even give out xrays. This is what we call 'radiation' or more accurately called ionising radiation. This is massively higher in rate and energy than the radiation the natural uranium puts out and is far more dangerous (basically, we smash it with neutrons and make quite a lot of the atoms unstable).
These new atoms that split, or decay over time after the original uranium atom was split are significant in that they are the cause of what is known as decay heat and why post trip cooling is so important to a nuclear reactor. At trip, the reactor is still producing about 10% of the energy it was just before it tripped. This is not just heat within the reactor, its like having a fire that you cannot put out. If you dont remove it, the reactor will warm up and for current generation of reactors, without active cooling, the fuel WILL melt its way out of the containment.
Edited by Gary C on Tuesday 28th July 18:03
Edited by Gary C on Tuesday 28th July 18:04
Edited by Gary C on Tuesday 28th July 18:05
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