His response was that if you put the waste far enough underground, like a km or more, then if an asteroid actually disturbs it, the dispersion of the waste won't actually be your biggest problem.
You could leave the world's nuclear waste and weapons on the surface and not have it be an extinction-level concern in the event of an asteroid impact.
Depleted uranium is not explosive: You will not have an explosion like a nuclear bomb.
A nuclear engineer can probably explain this better.
My understanding is that depleted uranium is a highly toxic substance, so we wouldn't want it to leech into soil / groundwater. I also doubt meltdowns are a substantial risk because the fuel pellets would have to fall / land in a way that they are concentrated together.
Funny anecdote: Two years ago I took my family on vacation, and I happened to be near a pumped hydro storage plant that was built to balance an early reactor. I couldn't find it, and when I turned around, I looked out the windshield, and across the river was a bunch of nuclear waste in dry storage: https://www.google.com/maps/place/Yankee+Atomic+Electric+Co/...
I wouldn't have even known what it was if I hadn't gone looking for it on Google Maps a few years prior.
Getting back to the point, if an asteroid hit the dry storage that I mapped above, I doubt there would be an explosion. The Deerfield river would probably become very toxic, but I wouldn't call it an extension event.
Depleted uranium is very highly toxic. "In a three-week period of conflict in Iraq during 2003, it was estimated that between 1,000 and 2,000 tonnes of depleted uranium munitions were used."
The Germans did not get a bomb because they did not concentrate their fissonable material. ( look it up )
This was similar to the confusion in Chernobyl. Engineers were saying "The reactor has exploded", while the Chief Engineers were retorting, "Tell me how it explodes. There's graphite rods and uranium rods". Tangential to the waste byproducts, absolutely.
Then, this is accurate, right?
Even a non-depleted uranium is not explosive until the pile reaches the critical mass.
In the laymen terms that reactor design would, under a very, very specific conditions, sharply increase the energy generation on the shutdown procedure (instead of decreasing), which would overheat the core, increase the fuel elements temperature and, consequently, increase the amount of steam from the water which is used as a coolant.
After the temperature and pressure rose too much the parts of the coolant and fuel systems started to break, which provided the reactor with a lot more fuel to work on, increasing the output 60 times (from 500MW to 30000MW), which insta-converted the all remaining coolant to the steam (and further raised the temperature and pressure in the reactor) and that steam pressure just ripped off the top cover of the reactor and send it flying.
There was another explosion (about two or three seconds after the first), but it's a mere consequence of the first one, happened in an already destroyed reactor and it is still not clear if that was just a plain chemical one (hydrogen) or the thermal one (ie what you would call a 'nuclear').
https://en.wikipedia.org/wiki/Chernobyl_disaster#Test_execut...
If anything, water is a neutron moderator ie it inhibits the reaction:
>> In the light-water-cooled, graphite-moderated RBMK, a reactor type originally envisioned to allow both production of weapons grade plutonium and large amounts of usable heat while using natural uranium and foregoing the use of heavy water, the light water coolant acts primarily as a neutron absorber and thus its removal in a loss-of-coolant accident or by conversion of water into steam will increase the amount of thermal neutrons available for fission
https://en.wikipedia.org/wiki/Neutron_moderator#Form_and_loc...
But in the context of the comment that sparked this,
>(A big reservoir of spent nuclear fuel consisting mainly of) depleted uranium is not explosive.
Is an accurate argument, right?
Depleted uranium is just a heavy metal, spent fuel is a heavy metal with a higher radioactivity (not suitable to handle with a bare hands) but not enough to go critical or even be close to go critical.
You can pour any amount of water on it, at best you would get a slightly radioactive water.
Just to be clear - you need something else, with a high yield, eg a chemical bomb with a high TNT equiv., to make a dirty bomb - still it wouldn't provide a self sufficient critical reaction.
Not sure I buy "Murphy predicts" but large asteroid impacts carry far more energy than any terrestrial explosion. The nuclear materials don't need to add any energy to that for the risk to be present.
Look up the PPM for uranium in ocean water, then multiply that out by how much ocean water there is. The amount of uranium naturally dissolved into the oceans is staggering. Nothing humanity can do will ever come close. Even when you throw the nasty transuranics into the consideration, it's nothing. Dump nuclear reactors straight into the oceans and stop worrying about it.
Dumping nuclear waste into the ocean will, mathematically, result in a globally acceptable concentration of nuclear waste in ocean water - but it will be locally problematic for wherever we put it, and that's assuming it stays put.
This is a question that would need to be extensively researched, before committing to something that's irreversible on a geological timescale. Handwaving arguments aren't enough, for something this important, this consequential.
(There's actually rather a lot of spent nuclear reactors littering the ocean floor, so opportunities to research this question are available right now—if any scientists could be bothered).
One of the problems associated with this option includes the difficulty of recovering the waste, if necessary, once it is emplaced deep in the ocean.
In the form of U-238? Perhaps too, but look near enrichment plants and you will see yards full of steel cylinders filled with depleted Uranium hexafluoride.
https://www.google.co.uk/maps/@53.2635448,-2.9592278,327m/da...
So the spent fuel does not contain valuable energy. Better off removing the requirement to be able to retrieve it and just get on with disposing of it.
People far overestimate the destructive power of nuclear weapons and nuclear fallout. This fear of them is of course greatly encouraged by states that hold nuclear weapons, but they're far from humanity's most destructive tendencies.