Coping with Nuclear Reactor Leakage: Lessons from Chernobyl and Fukushima

[nikkkom]

I cannot see how could those requirements met up in the air around the (old and during the process: partially dismantled) sarcophagus.

There is no need to dismantle Sarcophagus, neither partially nor fully.
The opposite: it needs to be "mantled" more. Walls reinforced with steel beams. Currently exposed debris should be covered with concrete, until a state is reached where no radioactive dust is generated. Then isolate everything against water ingress (nowadays there are dozens of various polymeric and epoxy sealers which make concrete completely water-impenetrable).

This plan has one fatal flaw: it is way too _cheap_!

 

[mfb]

Can we keep discussion on technical aspects please? This is a science forum.

 

[Bigjoemonger]

so if all can be contaminated,, why they still built a giant dome to cover it.

When the containment building exploded the melted fuel became exposed to outside air. Which is very bad. A gust of wind or water could carry it and severely contaminate a wider area. So to stop this they just dumped tons and tons of concrete on top. Calling it a tomb. But concrete directly exposed to radiation breaks down over time. Meaning one day it'd be a huge issue again. The ultimate goal is to actually clean up the corium to get rid of the problem. The process of cutting out that concrete to get to the corium will kick up a ton of radioactive dust. The dome is a seal over it so none of that radioactivity is able to further harm the surrounding area.

 

[nikkkom]

But concrete directly exposed to radiation breaks down over time.

On what timescales does concrete lose its integrity?

 

[girts]

I think that the loss of integrity depends mostly on the place where the concrete is used for example whethet it is in a structural element under stresd or whether it is simply dumped in a large pile on top of radioactive crap in ordet to contain it , anyway

Since this is a science forums I was expecting a bit of clarity in simplicity about the matters discussed.

as to the Chernobyl contamination point and contamination in general, I tend to think that the danger from the buried Chernobyl unit 4 is very low or next to nothing by now and that there is a much higher risk of contamination escaping if the surrounding forests burn in a wildfire for example.
The phrase "leaking radioactivity" is a bit foolish to my opinion simply because as we know radioactivity can't walk or talk it can only expand from a radioactive source, so when someone says that the Chernobyl reactor is leaking radiation both in air or in the ground water I want to ask by what means?

Sure after the 1986 accident it indeed emitted a lot of aerosols or in everyday language "dust" which was then the major reason for the exclusion zone contamination but after the majority of the fission reactions halted and the tangled mess cooled down it became much less dangerous in terms of contamination because hit me if I'm wrong but there isn't any noticeable amount of dust that can escape from a wet and moist pile of metal and concrete mixed with bunch of heavy metal and all that sitting at atmospheric temperatures.
Nor I think there's much that can escape into the ground either from beneath the reactor, the only thing that probably escaped was the fire and rain water mixed with some amount of radioactive particles.

I must admit I know firsthand that there is a high level of corruption in Ukraine which is usually hard to understand to most westerners, but I can't agree that the ark is simply a tax scheme, because in the long run it would be easier to dismantle the innards of the reactor using automatic cranes and a secondary containment structure to keep the dust inside because whenever you dismantle things dust appears.

I think their not repairing the turbine hall roof because there isn't any airborne pollutants inside or atleast not to any significant amount and as far as I know they are already underway in a process to dismantle the turbine and machine hall as that is the easiest part in all of the power plant to dismantle in terms of cleaning from contamination.Although ultimately in the case of Chernobyl I'm not sure whether there is any gain in dismantling or atleast trying to dismantle the reactors now because the surrounding land will still be inhabitable for the next 100 years and there isn;t anything that we can do about it realistically speaking so my opinion is we can let the reactor sit there and decrease in radioactivity over time and then in the future deal with it.
taking it away would require using up another space to store the materials and there are tons and tons of them there. plus taking them away doesn't lower the background radiation levels in the exclusion zone anyway.
but that's just my opinion.

surely the bottom line is there is only one good way of dealing with contamination - not making it in the first place, as for nuclear reactors - avoiding accidents or leaks and keeping a tight inspection schedule and operator morale

 

[Bigjoemonger]

On what timescales does concrete lose its integrity?

There's a lot of factors that play into this:
Level of exposure
Type of radiation exposure
Duration of exposure
Quality of concrete
Exposure to the elements
How the concrete is treated to protect it

How it works is when water is added to cement it reacts which causes the material to solidify in a lattice. So it's not hard because it dries out, it's hard because it's hydrated. When concrete is exposed to neutron radiation the neutrons impact with the hydrogen atoms from the water. It breaks the hydrogen bonds and knocks the hydrogen out of the lattice. This dehydrates the concrete which causes the lattice to break apart and the concrete crumbles.

A steel reinforced specially designed concrete containment building 8 feet thick is capable of withstanding the direct impact of a fully loaded passenger airliner and would probably last hundreds of years on it's own.

But a pile of cheap concrete poured directly on top of the elephants foot and exposed to the elements, would probably only last a few decades before degrading. Which is why it's important to build the dome now and keep it contained.

 

[Rive]

... When concrete is exposed to neutron radiation ...

If it is about neutron radiation, then this does not apply for the current Chernobyl.

 

[Bigjoemonger]

If it is about neutron radiation, then this does not apply for the current Chernobyl.

It was an example.

Neutrons impact protons and dislodge them from the lattice, breaking apart the water molecules.
Betas get absorbed, breaking hydrogen bonds, which breaks apart water molecules, same result.
Gammas ionized atoms ripping apart bonds, breaking apart water molecules, same result.

 

[jim hardy]

Concrete is actually a pretty decent reflector of neutrons because of its high water content. Neutrons bounce off

The concrete surrounding my reactor was in fine shape after thirty years of neutron bombardment at perhaps 10¹⁰ nv(that's neutrons / cm² / sec)
I was last down there around 2002 , the year i retired. I presume it's still doing fine for were it not surely i'd have 'heard it through the grapevine'.

@Bigjoemonger - Why do you want to appear sensational ?

When the containment building exploded the melted fuel became exposed to outside air. Which is very bad. A gust of wind or water could carry it and severely contaminate a wider area.

If you remember those days, it was only after other countries complained about fresh fission fragments floating down into their airspace that Russian officals finally admitted something was wrong.

You ought to read up before making wild assertions.
.

 

[Bigjoemonger]

Yes concrete is great at stopping neutrons. But when a neutron "bounces off" what do you think happens to the lattice of the concrete? Think it just stays put? It doesn't, it displaces and interacts with other atoms. One neutron can have an impact on thousands of atoms. Most of it resettles back in it's place but some don't and cause defects. It's ability to reorganize is dependent on the quality of the material. Like I said before, a nice well built reactor vessel or building or what have you could last a really long time before seeing any degradation. Which is why all these plants are getting relicensed way past their original operating timeline because they're still in pretty good shape. But hastily made concrete dumped directly on top of melted nuclear fuel would not last as long.

As for the rest. I don't know what wild assertions you're referring to. And its hard for me to remember those days considering i wasn't born yet when chernobyl happened.

 

[mfb]

Thermal neutrons don't have the energy to induce chemical reactions, for example.

 

[Rive]

And its hard for me to remember those days considering i wasn't born yet when chernobyl happened.

Well. To start with, neutron radiation is significant only in/around working cores.

 

[nikkkom]

Yes concrete is great at stopping neutrons. But when a neutron "bounces off" what do you think happens to the lattice of the concrete? Think it just stays put? It doesn't, it displaces and interacts with other atoms. One neutron can have an impact on thousands of atoms. Most of it resettles back in it's place but some don't and cause defects. It's ability to reorganize is dependent on the quality of the material. Like I said before, a nice well built reactor vessel or building or what have you could last a really long time before seeing any degradation. Which is why all these plants are getting relicensed way past their original operating timeline because they're still in pretty good shape. But hastily made concrete dumped directly on top of melted nuclear fuel would not last as long.

Again, melted nuclear fuel does not emit significant amounts of neutrons.
Betas affect only a few first millimeters of concrete.
Gammas can not dislodge atoms, they only knock out electrons. Depending on material's chemical properties, this may be ineffective at destroying its structure: for example, metal objects will not be damaged by it at all. I don't know about concrete.

I am not convinced that Chernobyl's concrete is seriously affected by radiation.

 

[Rive]

I am not convinced that Chernobyl's concrete is seriously affected by radiation.

It is not.

It is affected by the fact that it was a more heroic than thorough build due the radiation (way back), but that cannot be measured by any dosimeter.

 

[jim hardy]

I am not convinced that Chernobyl's concrete is seriously affected by radiation.

I agree.

Actually concrete is affected more quickly by high temperature than by radiation. That's how you make cement, cook limestone to dehydrate it.

For that reason our reactor shield had coils embedded for cooling water .

What radiation damage there is drops off quickly with distance because of self shielding.