Spent nuclear fuel safety

[farmerben]

The designers of the Fukishima plant did something potentially even dumber than putting backup generators in the basement.  The spent fuel pool was located above ground on a second story.  Fortunately, they were able to keep the spent fuel covered with water.  However, if the pool had cracked it would be impossible to keep the fuel covered with water.  In that case the zirconium could catch fire and release atmospheric contamination.  

In hindsight its obvious to put the pools in the basement and the generators above ground.  What else could be done to eliminate this vulnerability?  Also does anyone know how long the spent fuel is hot enough to pose a fire risk?  

 

 

[Pixophir]

Storing radioactive waste on site at the plant is not a good idea, no matter the viewpoint. It is subject to weathering and corrosion when simply left alone, may be accessible to bad guys, needs constant  supervision, etc. How long it poses a risk depends on the isotopes and their half life.

There's really no good option what to do with waste, other from storing it in facilities, name them permanent storage, where it may rest until ... well something happens, in an ideal case enough time has passed. That's certainly not going to happen at least for the the most dangerous parts. But the people responsible have by then long gone.

Edited August 26, 2022 by Pixophir

[Nuke]

high level nuclear waste should be placed in a reactor capable of burning it. 

[RCgothic]

I have professional expertise in this area.

Fuel storage must comply with the "double contingency principle", which means that two or more unlikely independent things need to go wrong simultaneously before you end up with criticality incident.

One defence is spacing, but big pools are expensive so fuel is stored close enough together that absent any other mitigations it would go critical, so in most cases this doesn't count.

Another defense could be neutron absorbers in the racks themselves, but if there's a problem with the absorbers, that's only one contingency, so we add another - soluble boron in the pool water.

 

Soluble boron is gains its effect from its concentration. If it gets diluted, it loses its effect. What could cause it to get diluted? Flooding. What causes flooding? Big Tsunamis, for one.

 

So for me as a nuclear professional I'd say Fukushima's designers got this one right. The pools didn't get diluted by the enormous tsunami and the critically contingency was preserved.

Edited August 26, 2022 by RCgothic

[Pixophir]

Well, I am also not quite without experience.

Theoretically all nice and well. Practically, there's a human factor or simply greed and mismanagement, and like anything else, nuclear waste deposits are leaking:

https://ecology.wa.gov/Waste-Toxics/Nuclear-waste/Hanford-cleanup/Leaking-tanks

https://www.nationalgeographic.com/history/article/100708-radioactive-nuclear-waste-science-salt-mine-dump-pictures-asse-ii-germany

https://www.santafenewmexican.com/special_reports/from_lanl_to_leak/leak-confirmed-at-nuclear-waste-dump-in-carlsbad/article_85930315-701c-57ff-849d-7b42e61febfd.html

The list goes on.

 

Just like theoretically and by law, damaged valves in primary circulation systems must be exchanged, not repaired. Yet, because that stuff is extremely material intensive and expensive, they are being welded for repair. Personal observation, the plant (Grundremmingen in Germany) has been taken out of service after protests and deadly accidents.

This is mostly the human factor, but it must be taken into account to judge if a technology is actually manageable or not. Fukushima was the wrong construction for that place. Btw., it was just luck, not planned error margin, that the decay basins were unharmed by the Tsunami.

It is specifically reckless to just dump the waste into ocean now there. Of course it as all diluted won't do harm, but in principle it is a testimony of utter helplessness in the face of the uncontrollability of the technology. It'll certainly happen again.

Edited August 26, 2022 by Pixophir

[farmerben]

1 hour ago, RCgothic said:

I have professional expertise in this area.

Fuel storage must comply with the "double contingency principle", which means that two or more unlikely independent things need to go wrong simultaneously before you end up with criticality incident.

One defence is spacing, but big pools are expensive so fuel is stored close enough together that absent any other mitigations it would go critical, so in most cases this doesn't count.

Another defense could be neutron absorbers in the racks themselves, but if there's a problem with the absorbers, that's only one contingency, so we add another - soluble boron in the pool water.

 

Soluble boron is gains its effect from its concentration. If it gets diluted, it loses its effect. What could cause it to get diluted? Flooding. What causes flooding? Big Tsunamis, for one.

 

So for me as a nuclear professional I'd say Fukushima's designers got this one right. The pools didn't get diluted by the enormous tsunami and the critically contingency was preserved.

 

I'm not talking about a criticality incident  I'm talking about residual decay heat setting the zirconium on fire.  If an earthquake or an explosion prevented the pool from holding water at all, that could be a (non-critical) worst case scenario.

[Pixophir]

That specific problem can be taken care of by sorting the waste and storing it with an appropriate spacing. I mean, a barrel popping because of simple overheat from decay would be a rather dumb error.

There is though, the issue of corrosion through various physical effects, addressed in papers experiments and observed, that does cause problems that where not taken into account. This valid for waste storage as well as operation and maintenance of power plants, and currently the cause of France's problem with the majority it's nuclear power plants.

For instance:

https://www.sciencedirect.com/science/article/abs/pii/S0022311508003218

and an example:

https://www.osti.gov/biblio/21062374

 

[RCgothic]

1 hour ago, farmerben said:

 

I'm not talking about a criticality incident  I'm talking about residual decay heat setting the zirconium on fire.  If an earthquake or an explosion prevented the pool from holding water at all, that could be a (non-critical) worst case scenario.

The two choices were:

1) Protect the criticality defense against flooding.

2) Allow that in the event of a leak (and concrete structures both tend not to leak catastrophically and also have leak detectors), and also the loss of all other top-up methods such as mains water and road-tankers, a vulnerability to flooding might be useful if there is also flooding at the same time.

The first is by far the more sensible choice. We *do not* want our nuclear fuel storage facilities to be vulnerable to flooding. We *do not* design around disaster scenarios covering for other faults.

Yes, the auxiliary generators should have been higher.

[DDE]

I believe the major reason for that arrangement was to put the pool as close as possible to the reactor. Shortest path for the hot fuel.

[wumpus]

4 hours ago, Pixophir said:

Storing radioactive waste on site at the plant is not a good idea, no matter the viewpoint. It is subject to weathering and corrosion when simply left alone, may be accessible to bad guys, needs constant  supervision, etc. How long it poses a risk depends on the isotopes and their half life.

There's really no good option what to do with waste, other from storing it in facilities, name them permanent storage, where it may rest until ... well something happens, in an ideal case enough time has passed. That's certainly not going to happen at least for the the most dangerous parts. But the people responsible have by then long gone.

Leaving the rods in water makes a lot of sense for the first several years after they've been decommissioned.  It allows the worst stuff with the shortest half lives to decay, and has limited effect on the water.  I have know idea if there is ever a point where you'd want to transport the stuff *then* put it back under water.  Don't forget to have enough electricity for vitrification (see below) when you fish them out for good.  It might make sense to have two (or more) levels of storage pools.  Start them in highly secure pools that can survive earthquakes and artillery and later (once they can be left in the air with minimal issues) move them to pools closer to normal swimming pools

After that, vitrification is your best bet, presumably including (semi-accessible) underground storage.  Vitrification turns the stuff into glass (or crystal), and thus makes erosion issues *much* slower and less of an issue.  Making it less accessible to modern humans might be needed from a proliferation standpoint, but there's always the issue that you are locking up the fuel supply needed by future generations.  Sure, vitrification takes a lot of electricity, but is typically done with a handy nuclear plant nearby.

https://mo-sci.com/vitrification-nuclear-waste-management/

[farmerben]

1 hour ago, wumpus said:

Leaving the rods in water makes a lot of sense for the first several years after they've been decommissioned. 

The fuel rods must be kept submerged in cooling water for some period of time before they can go into dry storage.  I'm not sure what the period is (possibly 18 months).  The spent fuel could meltdown which might not be catastrophic or the zirconium cladding could catch fire and belch radioactive smoke into the atmosphere which is catastrophic.   This seems to be the weakest link in nuclear safety as currently designed.

[kerbiloid]

A stock of spent rods near the backup generator of a nuclear power plant at several steps far from the coast where a forty meter high tsunami had been seen.

1. What could go wrong?

2. Just a typical beginning of every second manganime about a typical Japan pupil.

3. Should seek for Godzilla. The setting is not complete.

Edited August 26, 2022 by kerbiloid

[RCgothic]

34 minutes ago, farmerben said:

The fuel rods must be kept submerged in cooling water for some period of time before they can go into dry storage.  I'm not sure what the period is (possibly 18 months).  The spent fuel could meltdown which might not be catastrophic or the zirconium cladding could catch fire and belch radioactive smoke into the atmosphere which is catastrophic.   This seems to be the weakest link in nuclear safety as currently designed.

It's 10 years in my experience.

[wumpus]

8 hours ago, RCgothic said:

It's 10 years in my experience.

But is it "exposure to air is an immediate danger" for all 10, or would it be possible to have two levels of pools.  One built to "nuke standards" (to prevent any leaks that might cause the fuel to go critical) and one for economical efforts.

[RCgothic]

Leaks actually wouldn't cause the fuel to go critical. Water is a moderator, and removing the water makes the fuel less reactive. A leak is less dangerous from a criticality perspective than dilution.

For thermal management the soonest fuel might get loaded into a dry cask is 6 years best case, but 10 years is more typical and the pools are sized appropriately for that.

Secondary pools do get considered for capacity increases sometimes, but usually they're considered undesirable. It's unnecessary handling operations, which should be minimised, and another pool facility, which has additional costs.

Pools are built to survive anticipated earthquakes, and have no possible method of siphoning out the water. No pipework is permitted below the level of the fuel for this reason. They're reinforced concrete, with a liner and leak detection channels. A catastrophic leak is extremely unlikely.

The most likely scenario for uncovering of the fuel is failure of the cooling plant and top-up supply, in which case the water simply evaporates over a period of weeks. But that's a pretty long-lead fault with plenty of opportunity for recovery.

 

In terms of placement, the pools need to be positioned so that the free surface is the same as that in the refuelling cavity above the reactor when the reactor head is unbolted for refuelling. If the pools are within reactor containment they'll just fill a canal between the two. If the pools are outside of the reactor containment then the fuel gets posted in and out through a fuel transfer port and excessive water flow through there needs to be minimised.

[Pixophir]

3 hours ago, RCgothic said:

Leaks actually wouldn't cause the fuel to go critical.

The damage is environmental, like poisoning of ground water and in extreme cases loss of land.

It is not like it is fool proof, leaks in all variants, with or without contamination, may be rare but they happen. And waste is there to remain a problem for generations to come, and it is piling up quickly. Nobody wants it on the front door.

Edited August 27, 2022 by Pixophir

[DDE]

15 hours ago, wumpus said:

After that, vitrification is your best bet, presumably including (semi-accessible) underground storage.

A more extreme proposal has been an attendant, on-site nuclear fuel reclamation and re-manufacturing facility.

Followed by off-site vitrification of any non-reclaimable waste, and centralized deep geological storage.

[wumpus]

5 hours ago, DDE said:

A more extreme proposal has been an attendant, on-site nuclear fuel reclamation and re-manufacturing facility.

Followed by off-site vitrification of any non-reclaimable waste, and centralized deep geological storage.

Extreme?  Sounds more like sane, with a slight improvement on what the French  have been doing for decades.