Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[Jorge Stolfi]

Why not let [corium] reach its own equilibrium?
Hot sand, gravel, concrete or dirt is no threat to anything or anyone.
But there is no need to "move" any heat. The heat is fine where it is.
Once the water is removed from the nuclide piles, heat becomes an academic subject.

The problem is that you don't have a fixed amount of heat (as in ordinary molten iron), but rather megawatts of heat being produced continuously. If you enclose wht remains of the fuel (in whatever form or mixture) within a poorly conducting material, the mass will keep getting hotter until the uranium oxide itself boils.

 

[biffvernon]

Indeed. It's a question of whether the heat conduction through the sand or whatever is fast enough to stop the corium boiling. If yes, that's fine. If no, then that very much not fine at all.

 

[hbjon]

Er...I don't think heat can be contained. You might slow down it's transfer for a while but that just stores up a bigger problem for later.

The quantity of fuel is an important factor to consider. Jim aint a whistling dixie when he says there is a lot of potential energy at this site. I remember reading about the Mass Defect and the conversion of matter to energy in high school. If you bury everything in sand, I believe the fuel will realize it's full potential in time. I side with Jim on this. But I am just one of the backseat drivers around here.

 

[Astronuc]

'SOLUTION?' "There is an appropriate engineering SOLUTION?"

With all due respect, Astronuc, you need to use a different word.
There may be a best 'response.' There may be wiser 'options.'

But a 'SOLUTION?'

Is there a 'solution' that'll magically erase the millions of terabecquerels already released?
That'll decontaminate every fish, and every piece of seaweed in 10,000 square miles of ocean?
That'll somehow neatly suck up the layer of Cs-137 over hundreds of square km of countryside, that's already heavier than that of the Exclusion Zone in Chernobyl?
That'll perfectly clean up the contaminated spinach and tea 200 miles from the site?

I'm sorry, but talk of a 'solution' to Fukushima is the kind of Orwellian language that makhes reasonable people around the world so profoundly distrust the nuclear industry.

I was referring only to the current state of the damaged cores and SFPs at Fukushima units 1-4. I do not include the contamination due to the release of fission products so far; this is an entirely different problem, although one rooted in the same precursor.

I appreciate the mistrust/distrust of the nuclear industry. The event at Fukushima has betrayed whatever trust had been established.

My immediate concern is the situation at hand, and the minimization of further contamination - aside from the technical considerations.

The imperative is to cool the cores in order to reduce/mitigate further release of fission products. Then, to the extent possible, a closed system for cooling and prodessing of the radwaste must be established. To the extent possible, a containment system must be established to prevent further releases into to the atmosphere. These are the technical considerations.

 

[Quim]

The problem is that you don't have a fixed amount of heat (as in ordinary molten iron), but rather megawatts of heat being produced continuously. If you enclose wht remains of the fuel (in whatever form or mixture) within a poorly conducting material, the mass will keep getting hotter until the uranium oxide itself boils.

You raise a valid concern.
Yet, these "puddles" are already three months old and they will decline in heat generation from here on out (sans criticalities.)

But you are absolutely correct that somebody needs to attempt the actual math behind the premise I am proposing.

A problem could be expected to develop if the corium is still contained in the RPVs and all the water goes away - the corium would then only have air as a heat sink. That wouldn't be acceptable for the reason you raise.

But I bet there are ways of dealing with that circumstance cheaply and without endangering anyone's health. Be it lead or sand or something else, the RPVs will need something in them other than corium and air.

I believe that can be done.

 

[Quim]

Indeed. It's a question of whether the heat conduction through the sand or whatever is fast enough to stop the corium boiling. If yes, that's fine. If no, then that very much not fine at all.

Agreed!

 

[SteveElbows]

Well as expected we now learn how the humidity has changed in reactor 2 since the successful pool cooling.

Not terribly surprisingly the news is bad, it has not changed. It was probably always wishful thinking that the pool was responsible for most of the humidity inside reactor 2 building, but they had to rule it out.

One alternative plan of action they are talking about seems to involve opening the door!

http://www3.nhk.or.jp/daily/english/06_02.html

 

[jim hardy]

""But I am just one of the backseat drivers around here.""

thanks hbjon i too am out of my league here. i do remember my high school physics though.

So did Frank Sinatra - to paraphrase:

"When an unstoppable force like (decay heat) meets an immovable object like (perfectly insulating mountain of sand), something's got to give".

not trying to start a war of words here, just we got to keep our thinkin' straight.

i appreciate when people point out my slips.

And i am not talking down to anyone. i try always to put things in simple language because it keeps my thinking straight.
Isaac Asimov was the master at explaining things, and i always try to imitate the 'one step at a time' thinking i learned from his writings. One comes to appreciate just how imprecise and inconsistent English language can be at painting accurate pictures in our mind.

old jim

 

[Borek]

But there is no need to "move" any heat. The heat is fine where it is.
Once the water is removed from the nuclide piles, heat becomes an academic subject. Containment is achieved.

Nope. There is simply too much heat produced (and will be produced). What is there is still able to get through many meters of whatever you want to put under. Note, that the fuel has very high density, so whenever it will melt substances surrounding it it will sink deeper, replacing them. Contrary to what you think it will be not contained, sooner or later it will end in the groundwater.

 

[Bioengineer01]

There is the saying: never say never. Certainly this recent tragedy is once again a testament to this philosophy and a stark reminder to all engineers and scientists.

Given that disclaimer, these civilian nuclear reactors are designed with contingency in place to ensure they (for the most parts) do not go critical when they aggregate into a lumped mass corium at the bottom of a reactor containment and beyond. This is because firstly the nuclear fuel is of low purity in terms of fissile elements. Secondly, neutrons generated directly from fission of say uranium have too high energy - this results in a low probability of causing other fission events of other uranium atoms thereby eventually freezing out a chain reaction.

These reactors are purposely designed to have neutron moderators placed between fuel rods to slow down and reduce the energy of these neutrons from fission, so that they can cause fission and sustain a chain reaction. In other words, when fuel rods are melted down into a corium, there is presumably no more neutron moderators between the fuel rods and fuel elements (that is, the water), thereby from that point on you are primarily concerned with high energy neutrons generated from fission which have low probability of sustaining a chain reaction while thermal neutrons (lower energy) that can sustain a chain reaction are reduced in a hypothetical corium configuration.

This is the best case scenario of course. If it so happens that the civil structure of the plant itself unknowingly had neutron reflecting material nearby, it may be able to sustain a reaction to a degree. Theres also the possibility that the corium achieves localized critical mass configurations within the corium that can sustain a reaction (though not likely as the fuel is purposly designed to have low purity and is assumed to be homogenous in composition). All of this is purely hypothetical of course as are any other plausible scenarios people can draw up.

That is also not the end of the story of course, as life is never that perfect: it is not just 100% fuel configuration with control rods in place or 100% corium lump mass. If it is partially melted with a corium and some spent fuel assembies, presumably with neutron moderating water inbetween, this can again lead to criticality, especially since control rods that would have otherwise prevented criticality may be ineffective to block neutrons between the corium and the fuel assemblies though they will be far away in distance and the reaction rate should not be very high.

Arm chair, I understand what you say and I don't argue with it, but what happens with Unit 3 Corium? That has MOX in it, do we know what percentage of MOX they used? And what type of Plutonium they used? 90% weapons grade? if so, what was the composition of the MOX. I am extremely concerned about the high experimental nature of the MOX as a fuel, due to the higher cross sectional area of Pu-239 for neutron absorption, and the zero data that we have on accidents of this nature involving MOX... Also, the short lived use of the reactor since it was restarted in Setember 2010 up to March's accident means that most Pu was still there...
I'd love to hear other experts comments on this topic...

 

[Quim]

Nope. There is simply too much heat produced (and will be produced). What is there is still able to get through many meters of whatever you want to put under. Note, that the fuel has very high density, so whenever it will melt substances surrounding it it will sink deeper, replacing them. Contrary to what you think it will be not contained, sooner or later it will end in the groundwater.

That's not what happened in the only other comparable circumstance.
Why would it happen here?

These core loads have had three months to bleed off nuclides.
At that other place the dried out material started out in life as a core developing full power.


Let's see some math, or are the formulas yet to be developed? (as a result of the Fukushima incident)

 

[Astronuc]

Arm chair, I understand what you say and I don't argue with it, but what happens with Unit 3 Corium? That has MOX in it, do we know what percentage of MOX they used? And what type of Plutonium they used? 90% weapons grade? if so, what was the composition of the MOX. I am extremely concerned about the high experimental nature of the MOX as a fuel, due to the higher cross sectional area of Pu-239 for neutron absorption, and the zero data that we have on accidents of this nature involving MOX... Also, the short lived use of the reactor since it was restarted in Setember 2010 up to March's accident means that most Pu was still there...
I'd love to hear other experts comments on this topic...

There has been considerable use of MOX fuel in light water reactors (LWRs).

It was reported that there were 32 MOX fuel assemblies in Unit 3, or about 6% of the core of 548 assemblies. They were operating in their first cycle, so they didn't have much operation/exposure. Ostensibly, the MOX fuel was derived from spent fuel from the Fukushima reactors, thus it was reactor grade, and the Pu isotopics would have reflected that legacy. The MOX fuel would be designed to match the enrichment of the U-235 assemblies, which is about 4% U-235. So likely the MOX would be about 6% Pu, with a mix of Pu239, 240, 241 and 242.

Spent fuel contains Pu isotopes. The use of MOX is rather insubstantial to the event and the current state of Unit 3.

 

[tsutsuji]

A new theory for unit 1 explosion :

TEPCO officials believe hydrogen gas that should have been released from the vent pipe flowed back into the reactor building through the open SGTS valve after reaching the point where the two exhaust systems converge.
http://www.asahi.com/english/TKY201106040165.html

 

[etudiant]

You raise a valid concern.
Yet, these "puddles" are already three months old and they will decline in heat generation from here on out (sans criticalities.)

But you are absolutely correct that somebody needs to attempt the actual math behind the premise I am proposing.

A problem could be expected to develop if the corium is still contained in the RPVs and all the water goes away - the corium would then only have air as a heat sink. That wouldn't be acceptable for the reason you raise.

But I bet there are ways of dealing with that circumstance cheaply and without endangering anyone's health. Be it lead or sand or something else, the RPVs will need something in them other than corium and air.

I believe that can be done.

My $0.02 is that someone did do the math and concluded that your belief is mistaken.
We are looking at 4 to 6 megawatts per core of residual heat per core, more than enough to boil that core if the heat is not removed continuously. Adding lead is useless, it may be a great shield but it melts at low temperature and emits toxic fumes. Ditto sand, no toxic fumes but no cooling either. Because it is decay heat, no outside material such as boron will help. We are lucky that water is cheap, non toxic and has a high heat capacity.. It is cooling this mess reasonably well and our only problem is that we are running out of places to store the used water. That is a high class problem relative to the question of how do you control boiling radio nucleotide vapors.

 

[rmattila]

In an BWR, I believe, the feedwater line also enters above the top of the fuel.

Above the top of the core, but outside the shroud (=in the downcomer region) where the cooler feedwater mixes with the recirculation flow to provide some suction head for the recirculation pumps. At least this is the case with reactors with internal recirculation pumps, which I am familiar with.

The core spray system, which I believe the fire extinguisher system at Fukushima is lined up to, ends up inside the shroud and actually sprays the water on top of the fuel, not into the downcomer (from where it may flow out of the reactor through leaking main circulation pump seals and never reach the actual core region).

 

[Quim]

My $0.02 is that someone did do the math and concluded that your belief is mistaken.

"Your assumption is that someone did the math....."I assume otherwise.Or to take the advice of Bill Gates: "assume nothing"**This is a assembly directive in the MS assembly language which exists in the form of "assume nothing."

To make a long story short, it accomplishes nothing, it would have only been included as an assembly directive because Mr. Bill was trying to tell us something.

 

[Luca Bevil]

Quick question:

Fission yield from U 235 are quite similar for Cs 134 (6,78%) and CS 137 (6,08%) according to wikipedia.

Given the shorter half life of CS134, though, activity of CS134 is much higher: 47.864 Tbq/g (CS 134) vs 3.214 Tbq/g (Cs 137).

Even after taking into account the faster decay, during the average time likely spent since fission, I would have expected CS134 contribution to water radioactivity being quite higher than CS137 contribution.

However data TEPCO released a few days ago about basement 1 contamination had estimates for CS137 bq/cc and CS 134 Bq/cc very close to each other.

How would that be explained ?

 

[rmattila]

Quick question:

Fission yield from U 235 are quite similar for Cs 134 (6,78%) and CS 137 (6,08%) according to wikipedia.

Very little Cs-134 is produced directly in fission, and most of it is produced by neutron capture of the more common (stable) fission product Cs-133. The value 6,78 % is the combined yield of Cs-133 and Cs-134. How much of Cs-133 is converted to Cs-134 depends on the burnup of the fuel.

 

[Astronuc]

Quick question:

Fission yield from U 235 are quite similar for Cs 134 (6,78%) and CS 137 (6,08%) according to wikipedia.

Those should be the cumulative yield based on the decay chains (and transmutations) leading to the respective isotopes.

Given the shorter half life of CS134, though, activity of CS134 is much higher: 47.864 Tbq/g (CS 134) vs 3.214 Tbq/g (Cs 137).

Even after taking into account the faster decay, during the average time likely spent since fission, I would have expected CS134 contribution to water radioactivity being quite higher than CS137 contribution.

However data TEPCO released a few days ago about basement 1 contamination had estimates for CS137 bq/cc and CS 134 Bq/cc very close to each other.

How would that be explained ?

The Cs-134/Cs-137 ratio depends upon burnup. In the Fukushima cores, there are perhaps four batches of fuel representing four populations of burnup, and each will have a different accumulation of the Cs-134 and Cs-137. The actual ratio of Cs in the coolant will be a weighted sum of the source terms of those assemblies that failed.

 

[etudiant]

"Your assumption is that someone did the math....."


I assume otherwise.


Or to take the advice of Bill Gates: "assume nothing"*


*This is a assembly directive in the MS assembly language which exists in the form of "assume nothing."

To make a long story short, it accomplishes nothing, it would have only been included as an assembly directive because Mr. Bill was trying to tell us something.

Excellent approach. Mr Gates is a good guide.
So we have 3 reactor cores, each dissipating about 6 megawatts of residual decay heat (4 in the case of unit 1, which is much smaller). That will continue for the next few years largely unchanged, albeit with a modest downward trend. Assume the cores are hot, but not yet melted, at around 1000 degrees Kelvin.
A reactor core is about 100 tons of uranium oxide, plus additives. Uranium oxide has a specific heat of 240 joules/kg per degree K. So we have 100,000 kg of fuel producing 6 megawatts of heat, or 6 million joules/second. Heating the fuel 1 degree will take 240 * 100,000 joules, or 24 million joules, about 4 seconds worth of heat output. So to heat the fuel to the boiling point, somewhere around 4000 degrees Kelvin, will take 3000 times that long, about 12,000 seconds, less than 4 hours.
I'd recommend sticking with the water option.

 

[MiceAndMen]

The spent fuel must be removed from the SFPs of Units 1-4 in and placed in casks. That can only happen after the debris is removed, and most likely will have to be done remotely, and possibly robotically.

That is going to be a challenging task, to say the least. At some point the bundles are going to have to be lifted out of the SFP water completely and moved through the open air, at least for a short period of time. Assuming they are not dropped while in transit, what are the consequences of a spent fuel bundle being transported through the air for a short time without any water shielding it?

 

[jim hardy]

""I'd recommend sticking with the water option.""

Thank you Etudiant. That is the kind of step-by-step thinking and explanation that made Isaac Asimov so popular. Nice post.

old jim

 

[Astronuc]

That is going to be a challenging task, to say the least. At some point the bundles are going to have to be lifted out of the SFP water completely and moved through the open air, at least for a short period of time. Assuming they are not dropped while in transit, what are the consequences of a spent fuel bundle being transported through the air for a short time without any water shielding it?

Not necessarily. If there is water in the SFP and the cask pit, the assemblies can be transferred underwater. However, if the SFP water is contaminated, i.e., if it has dissolved fission products and some fuel in it, then decontamination of the case becomes an issue. However, there is no fuel handling machine in place with which to do this work, so it would have to be done with some kind of crane based on the grounds outside of the reactor building, unless TEPCO somehow removes the debris from the reactor service floor and basically builds a new transfer system.

We still don't know the state of the fuel in the SFPs of units 1, 2 and 3. The images of SFP #3 show a lot of debris in the pool. I have not seen images from SFP of Units 1 or 2.

 

[Atomfritz]

Why?
Why not let it reach its own equilibrium?
Hot sand, gravel, concrete or dirt is no threat to anything or anyone.

This also has been done in Chernobyl.

As the molten mass gets in touch with other stuff, it appears very probable that it sucks up impurities, reducing heat density until the point where it gets so cold that no more melting does happen.

Everybody knows the infamous "Elephant Foot".
Two lesser known images of molten mass from the (in)famous Ukrainian accident:

Isn't it amazing that the tubes where this hot (2300 C) mass flowed out didn't break?
( Pictures taken from page 28 of this http://tec-sim.de/images/stories/severe-accident-phenomenology.pdf" )


I start asking myself how much a part of the short-lived fission products already has been washed out from the core, and now flooding the basement etc.
Maybe the "corium" possibly already is way less "hot" than "freshly molten core"?

If, say, 5 MW of total 6 MW of decay heat has been dispersed in 10,000 cubic meters of water this would equal 500W per cubic meter. Probably not a big problem, probably the heat is easily dissipated by big surface, convection and (slow) vaporization, and so practically gets unnoticed.

So, if the residual heat of a hypothetical complete core is about 6MW now (taking into consideration the fission product decay heat) then the actual heat being developed where the (remaining) core is, could be substantially less, maybe 1 MW?

If the molten mass gets sufficiently dispersed with other stuff it will eventually get below the melting point.
Even if the mass tends to keep a inner liquid hot core, it eventually will lose mass due to parts it loses on its way. "China Syndrome" is just panic-mongering imo.

They are just caught in the problem that they do not know in what shape the core remains are.
It just depends on the geometry.
If there is a very big very flat splash of metal on the floor, then the situation is way different if it's concentrated in a near-round drop-like form.
Probably very good also could be if the floor is covered with lots of small blobs with large crusty surface, dispersed regularly on the floor, as was observed when TMI cleanup people finally got to the RPV floor.

I guess they just cannot stop watering the cores until they can be sure that there is no longer a risk of more melting stuff dropping into water, causing a steam explosion?

Too bad there are no optical means of remote-inspecting the containment without opening/entering it.

Maybe the Tepco strategy is
-to just wash out the cores from the short-lived fission products to...
-then to be able to let the core remains dry out soon
-finally to leave them alone, to be entombed asap?

 

[Bioengineer01]

There has been considerable use of MOX fuel in light water reactors (LWRs).

It was reported that there were 32 MOX fuel assemblies in Unit 3, or about 6% of the core of 548 assemblies. They were operating in their first cycle, so they didn't have much operation/exposure. Ostensibly, the MOX fuel was derived from spent fuel from the Fukushima reactors, thus it was reactor grade, and the Pu isotopics would have reflected that legacy. The MOX fuel would be designed to match the enrichment of the U-235 assemblies, which is about 4% U-235. So likely the MOX would be about 6% Pu, with a mix of Pu239, 240, 241 and 242.

Spent fuel contains Pu isotopes. The use of MOX is rather insubstantial to the event and the current state of Unit 3.

Do you have any links to this data? Early on in the accident I saw some export permits issued to AREVA from the USA to export MOX to Fukushima Daiichi, Japan that don't fit into your description, but eventhough, the timing was correct, I have no way of knowing whether they ended up in Unit 3 or not. That was NOT fuel derived from spent fuel...

 

[Quim]

This also has been done in Chernobyl......

I guess they just cannot stop watering the cores until they can be sure that there is no longer a risk of more melting stuff dropping into water, causing a steam explosion?

Too bad there are no optical means of remote-inspecting the containment without opening/entering it.

Maybe the Tepco strategy is
-to just wash out the cores from the short-lived fission products to...
-then to be able to let the core remains dry out soon
-finally to leave them alone, to be entombed asap?

Thank you for putting that in writing.
I am in agreement with your assessment.
I bet everyone else agrees too.

I suspect that the "plant guys" (there and here) are somewhat stuck in a vision of units 1, 2 and 3 which no longer exists. The "plant guys" want to put three nuclear reactors into cold shutdown.

But there are no longer any reactors in buildings 1,2 and 3 and there never will be any reactors in those buildings ever again. The paradigm has shifted, and to some extent we are all on equal footing here, plant guys and non-plant guys.

This is a corium isolation problem, not a NPP problem anymore.
Plant guys are very procedure oriented, they have to be in order to be plant guys.
But this situation has no procedures to follow.
It requires a different kind of analysis.
IMO.

 

[Astronuc]

Do you have any links to this data? Early on in the accident I saw some export permits issued to AREVA from the USA to export MOX to Fukushima Daiichi, Japan that don't fit into your description, but eventhough, the timing was correct, I have no way of knowing whether they ended up in Unit 3 or not. That was NOT fuel derived from spent fuel...

AREVA does not fabricate MOX fuel in the US.

Please post the export permits that one 'saw'.

TEPCO has reprocessing contracts with AREVA. There is strict control of spent fuel and MOX fuel. TEPCO indicates that MOX is derived from spent fuel.
http://www.tepco.co.jp/en/challenge/csr/nuclear/cycle-e.html

The status of MOX fuel is posted earlier in this thread.

 

[SteveElbows]

I suspect that the "plant guys" (there and here) are somewhat stuck in a vision of units 1, 2 and 3 which no longer exists. The "plant guys" want to put three nuclear reactors into cold shutdown.

I for one am neither a 'plant guy' nor am I stuck with some false and outdated vision of the reactors in my mind.

I believe the cores require cooling. You have not demonstrated otherwise, and attempts to dismiss this issue by using some dismissive 'plant guys' label do not help your case any.

Only as the story unfolds will we truly be able to judge whether a terrible mistake has been made with balancing the issues of cooling and containment. Given that it does not appear that large quantities of radioactive horror are escaping the plant daily by air, how the ever-growing quantities of water are managed will likely be a key factor in determining whether the approach taken has been a mistake or not.

 

[jim hardy]

it's a problem of moving 6 megawatts of heat while minimizing water.

to that end letting it make steam is best solution.

A kilogram of 100C steam carries away 2676 Kilojoules, but the same weight of 100C water only 419. Less if you started with already warm water.

So by steaming they reduce the amount of water to be handled probably tenfold.
and they get benefit of distillation, to help contain the contamination.

give them some credit for knowing what they are doing. they weren't born yesterday.

i learn a lot by watching intelligent people work and figuring out why they do what they do..
Mother Nature is a tough schoolmistress - she makes one work for her lessons.

 

[Atomfritz]

I suspect that the "plant guys" (there and here) are somewhat stuck in a vision of units 1, 2 and 3 which no longer exists. The "plant guys" want to put three nuclear reactors into cold shutdown.

But there are no longer any reactors in buildings 1,2 and 3 and there never will be any reactors in those buildings ever again.

Really good observation.
You remind me what happened in russia then...
Mr. Dyatlov, manager of the blown-up block in Chernobyl insisted that the reactor was still there, intact, being cooled.
He was unable to be convinced that the reactor was damaged until helicopters filmed the situation from above next noon.
This delayed many emergency measures.

The paradigm has shifted, and to some extent we are all on equal footing here, plant guys and non-plant guys.
...
But this situation has no procedures to follow.
It requires a different kind of analysis.

In Russia military quickly took over to remedy the situation.
In Fukushima also constantly new procedures are being developed.

First step should always be a thorough assessment of the situation.
Then the necessary measures must be developed or improvised.
Layman input at least is not restricted by procedure thinking obsoleted by new situations.

So to get back to the reactor problem:
There are questions not asked before even in this thread.

What fraction of the short-lived fission products can be expected to already have left the reactors and taken away by the cooling water?

If, for example, as some sources say, almost all iodide and cesium has been dissolved into the tens of thousands cubic meters of water, then a big part of the residual heat could now have left the reactor remains.

So my second and third question:
Does this bleed-out of FPs reduce residual heat substantially?
If so, what magnitude could be the probable reduction of core remains' residual heat?

Why I ask this:
There will eventually be a point of equilibrium when the heat can sufficiently passively dissipate through floors, walls etc slowly without melting anything more.
From this point on, the way most economical solution would be entombing.I'd be happy if some nuclear professional could comment on how much of the FP inventory is still in the reactors.
Thank you!