Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[SteveElbows]

I will look into that. (The TEPCo documents I just transcribed do not list the temperatures, hopefully some other document does.)

As part of the same release of data some weeks back, there was temperature data in another file, covering a similar time period. This is the file for reactor 3:

http://www.tepco.co.jp/nu/fukushima-np/f1/images/syusei_temp_data_3u.pdf

Unfortunately temperature data of this variety only became available on the 19th, so we don't get a great look at the temperature trends long before the 21st.

The picture is rather messy, but a few of the temps hit new highs around the 01:25 time on the 21st.

 

[SteveElbows]

I know this is speculation, but I can't help myself. Those readings... are they continuous? If not, it may be that any neutron spikes could have been "lost" just like the pressure/temp readings.

Well the data has long been available, I only looked at it again recently because TEPCO published some corrections & additional data. I don't think the corrections/additions make any difference to the neutron figures but I could be wrong, its been a while since I looked at the original data.

http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110528e14.pdf

Neutron readings during this period were mostly taken near the main gate every 10 minutes.

 

[GJBRKS]

That is 2x10⁹ Bq/Litre and safe limit is 60 or 90 Bq/L for Cs-134 and 137 respectively.

Why are Tepco reporting sea water contamination in Bq/L and ground water, basement water etc as Bq/cm³?

with a mean gamma ray energy of 662keV per decay ,

this would amount to 18.33 J/day per liter ,

or (230mSv / day) / ingested liter ( for a man of 80 Kg)

or (9.6 Sv / hour ) / ingested cubic meter ( for a man of 80 Kg)

http://www.wolframalpha.com/input/?i=2.0e09+bq+*+662+kev&a=*C.bq-_*Unit-&a=ListOrTimes_Times

(This is only the cesium component , there would be other additional sources , and not counting any betaradiation that would add to the Sv)

 

[jim hardy]

Jorge wrote:
"""However, on the morning of March 20th the containment vessel pressure started decreasing and it was announced that the plan to vent had been set on hold. There was a sustained decrease in containment vessel pressure throughout March 20th and 21st, ultimately leading to its being at atmospheric pressure, so there is little reason to think that the plan to vent was invigorated and enacted during this period.""



That is a very confusing time.
I was and remain puzzled by that day.
Your graph shows i believe a LOT of water injection. Nootice the pressures track injection flow...


""""I still cannot fgure out where the RPV pressure gauges A and B are located exactly. Is it possible that the space inside the RPV got partitioned by the meltdown into two fairly airtight spaces, with one gauge in each? "

What i believe is they used local pressure gages on pipes that go into the RPV from whatever temporary pumps they had jury-rigged. One of those pipes i believe had a valve that was closed or nearly so, so the pump pressure was high and it took them a while to figure that out. when they opened it the situation "stabilized", so valve they opened was afill valve not a vent valve. That's my best guess at a hypothesis that fits observations.

amen to your torus sensor being under water.
What i did not realize until recently is how high above ground the reactor vessel is. My PWR was at ground level. I am wondering if the ~1.5 atm pressures in drywell could be from their flooding it , and the pressure indication due to pressure drop in the fill lines and elevation. Both pressures tracked injection flow for a while.

about a month ago i posted some thoughts about unit 3 and your charts here,
http://tickerforum.org/akcs-www?singlepost=2541679

at time i thought the rpv and containment were both open at top. Less convinced of that now.

 

[NUCENG]

Jorge wrote:
"""However, on the morning of March 20th the containment vessel pressure started decreasing and it was announced that the plan to vent had been set on hold. There was a sustained decrease in containment vessel pressure throughout March 20th and 21st, ultimately leading to its being at atmospheric pressure, so there is little reason to think that the plan to vent was invigorated and enacted during this period.""



That is a very confusing time.
I was and remain puzzled by that day.
Your graph shows i believe a LOT of water injection. Nootice the pressures track injection flow...


""""I still cannot fgure out where the RPV pressure gauges A and B are located exactly. Is it possible that the space inside the RPV got partitioned by the meltdown into two fairly airtight spaces, with one gauge in each? "

What i believe is they used local pressure gages on pipes that go into the RPV from whatever temporary pumps they had jury-rigged. One of those pipes i believe had a valve that was closed or nearly so, so the pump pressure was high and it took them a while to figure that out. when they opened it the situation "stabilized", so valve they opened was afill valve not a vent valve. That's my best guess at a hypothesis that fits observations.

amen to your torus sensor being under water.
What i did not realize until recently is how high above ground the reactor vessel is. My PWR was at ground level. I am wondering if the ~1.5 atm pressures in drywell could be from their flooding it , and the pressure indication due to pressure drop in the fill lines and elevation. Both pressures tracked injection flow for a while.

about a month ago i posted some thoughts about unit 3 and your charts here,
http://tickerforum.org/akcs-www?singlepost=2541679

at time i thought the rpv and containment were both open at top. Less convinced of that now.

For information, RPV Pressure instruments sense steam dome pressure on the reactor water level condensing chamber instrument tap outside of the shroud and dryer skirt at about the elevation of the tops of the steam separators. The instruments themselves are in the reactor building outside of containment'

 

[Jorge Stolfi]

Jorge on the anomalous pressure readings around 21 March, from my recollections at the time (i noticed it too): I think they were inferring pressure from a gage on a pipe through which they were pumping water. Possibly a gage on a fire truck pump.

There's one more failure mode, wet insulation.
Being dissimilar metals [...] they'll also make a battery if placed in an electrolyte. That electrochemical effect makes tens of millivolts and will cause substantial error if the insulation gets compromised and water gets to the conductors. The chemical millivolts overwhelm and bury the temperature microvolts. [...] Given all the steam in there it could be a wet terminal block in a flooded junction box. Might dry out and start working on its own.

Good explanation, thanks!

One thing I don't understand about theormocouples is why the bimetal wires are usually extended all the way to the voltmeter. Why couldn't they be extended only to some cooler place nearby (such as just outside the concrete enclosure), and then have the signal be carried by copper wires to the meter? That would result in lower resistance for the signal and reduced risk of electrochemical effects along the way.

Or is that in fact how it is done?

About a month ago i posted some thoughts about unit 3 and your charts here,
http://tickerforum.org/akcs-www?singlepost=2541679
at time i thought the rpv and containment were both open at top. Less convinced of that now.

Thanks for the compliments and for the thorough analysis. As for them being "open", there is a continuum between having a small leak and being wide open, so it may not be a simple yes/no question. Also, for a small leak, the degree of opening may be sensitive to pressure, temperature, flooding, clogging, etc., and so may vary erratically with time.

As part of the same release of data some weeks back, there was temperature data in another file, covering a similar time period. This is the file for reactor 3:
http://www.tepco.co.jp/nu/fukushima-np/f1/images/syusei_temp_data_3u.pdf

Thanks! I think I saw mention of it in this forum, but hadn't the time to check it out then.

[Gamma and neutron] data has long been available [...]
http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110528e14.pdf

Thanks again. I am tempted to include those readings in my plots too, but first one question: do they reflect the conditions inside the reactor, or only of the external contamination? In other words, are those gammas and neutrons mostly created by fission and decay inside the reactor's concrete enclosure? If so, does the spent fuel in the SFP contribute to those readings?

For information, RPV Pressure instruments sense steam dome pressure on the reactor water level condensing chamber instrument tap outside of the shroud and dryer skirt at about the elevation of the tops of the steam separators. The instruments themselves are in the reactor building outside of containment'

Thanks, that is important information.

As for the temperature sensors, I have seen several diagrams showing their approximate location on the RPV, drywell and torus; but I still miss the key details. Namely, where precisely are the RPV temperature measured: on the outside surface of the RPV, or embedded into its wall? If the former, woud the reading be affected by the drywell atmosphere or by water leaks above the sensor? How far is the "water nozzle" temperature sensor from the nozzles and their feedpipes? And so on...

These details are important, for example, to analyze the pressure x temperature plots. The red boiling curve in those plots is relevant only if the temperature and pressure are measured at the same spot in the fluid. Barring gauge malfunctions, the pressure must be indeed that of the fluid at the gauge's intake point, which should be valid for the bulk space inside (except for the hydrostatic pressure gradient in the liquid-filled part). On the other hand, if the temperature is measured on the outside of a 15 cm thick wall, or even embedded into it, it will be some value intermediate between the temperatures of the two fluids in immediate contact with the wall. Thus, one can easily have superheated steam inside the RPV with a temperature reading well below the boiling curve, or (less likely) liquid water inside with a temperature reading well above the boiling curve.

 

[SteveElbows]

On the other hand, if the temperature is measured on the outside of a 15 cm thick wall, or even embedded into it, it will be some value intermediate between the temperatures of the two fluids in immediate contact with the wall. Thus, one can easily have superheated steam inside the RPV with a temperature reading well below the boiling curve, or (less likely) liquid water inside with a temperature reading well above the boiling curve.

The early NRC assessment that was leaked said 'Vessel temperature readings are likely metal temperature which lags actual conditions.'

http://cryptome.org/0003/daiichi-assess.pdf page 2 reactor 1 assessment first mentions this, I believe the same sentence is also used for reactors 2 & 3 later on in the document.

 

[Quim]

Here is a paper which discusses "aquifer recharge" which has been used to combat salt water intrusion into the underground aquifer in Japan. The Japanese have been fighting saltwater intrusion into their aquifer by injecting fresh water into wells and by creating freshwater ponds (basins) over porous stratum. This paper discusses the basin method of aquifer recharge.
http://www.igme.es/internet/Boletin/2009/120_2_2009/311-320.pdf

Current circumstance in Fukushima suggests that the corium have now reached (become at one with) the groundwater. This is a first ever, in this respect the accident in Fukushima has surpassed events in Chernobyle.

Possibly, it is time for those managing the accident to get ahead of events and do whatever is necessary to prevent the spread of contamination to the deeper stratum of groundwater in Honshu.

This forum is sorely lacking the expertise of a geologist as shown in the previous mudstone/bedrock discussion.

Could a poster from Japan find a knowledgeable contributer to join the discussion?

The "battle of Fukushima" would seem to now be an underground battle.

 

[biffvernon]

This forum is sorely lacking the expertise of a geologist as shown in the previous mudstone/bedrock discussion.

It's not so much the lack of a geologist as a lack of any detailed geological information about the site.

 

[jlduh]

Here is a paper which discusses "aquifer recharge" which has been used to combat salt water intrusion into the underground aquifer in Japan. The Japanese have been fighting saltwater intrusion into their aquifer by injecting fresh water into wells and by creating freshwater ponds (basins) over porous stratum. This paper discusses the basin method of aquifer recharge.
http://www.igme.es/internet/Boletin/2009/120_2_2009/311-320.pdf

Current circumstance in Fukushima suggests that the corium have now reached (become at one with) the groundwater. This is a first ever, in this respect the accident in Fukushima has surpassed events in Chernobyle.

Possibly, it is time for those managing the accident to get ahead of events and do whatever is necessary to prevent the spread of contamination to the deeper stratum of groundwater in Honshu.

This forum is sorely lacking the expertise of a geologist as shown in the previous mudstone/bedrock discussion.

Could a poster from Japan can find a knowledgeable contributer to join the discussion?

The "battle of Fukushima" would seem to now be an underground battle.

Welcome to the forum first!

I agree with your view that we are lacking specific knowledge about the geology and watertable in this area, I already mentionned it and personally, i haven't been able, despite some research on the net, to find precise information on this matter (in this specific area i precise). I just found that Tepco has conducted in 2008 and 2009 many tests with soil drilling and test explosions and shocks to better assess the seismic properties of the area regarding to resistance to earthquake of the Daichi plant, but I have not been able to find the reports and data of this studies.

I think that nothing says at this time that the corium/debris entered the water table and escaped from the containment, we just don't know where are relocated the cores that have melted, based on Tepco explanations. This is a serious problem not to know what's happening inside...

The only very factual element that let us think that maybe "some corium went through something" has been the black smoke from reactor 3, around March 21/23. I don't think we can say more than this.

But whatever the situation of the corium is, it is very important to have a better knowledge of the underground area around the reactors, also to better assess how the basements can leak and contaminate the surroundings or even the sea.

So if somebody knows someone in a University in Japan with some knowledge of the geology and or the aquifer there, just try to convince him or her to come on this forum!

 

[elektrownik]

If core would hit groundwater we would see KABOOM, but anyway it is hard to say where cores are, sensor data are different for A and B, and also there is not much sensors at all in drywell. If someone would trust current data then it could say that unit 1 core is in drywell, unit 2 part of core in torus, and unit 3 is unknown, temperatures are increasing there but radiation isnt, so we can assume that it is cooling problem. Also parts of cores can be in concrede under drywell or around torus in the basement...

 

[robinson]

I've read research that says the amount of steel and concrete mixed in with a melted core makes it improbable it could melt through the concrete floor. The more concrete and steel mixed in with the fuel decreases the temperature, so it slows the whole mess down.

 

[Quim]

If core would hit groundwater we would see KABOOM

The KABOOM theory seems to have been dis-proven along with the China Syndrome.
When corium meets water, the Leidenfrost effect comes into play.
We are learning so much from Fukushima!

 

[maddog1964]

Here is a paper which discusses "aquifer recharge" which has been used to combat salt water intrusion into the underground aquifer in Japan. The Japanese have been fighting saltwater intrusion into their aquifer by injecting fresh water into wells and by creating freshwater ponds (basins) over porous stratum. This paper discusses the basin method of aquifer recharge.
http://www.igme.es/internet/Boletin/2009/120_2_2009/311-320.pdf

Current circumstance in Fukushima suggests that the corium have now reached (become at one with) the groundwater. This is a first ever, in this respect the accident in Fukushima has surpassed events in Chernobyle.

Possibly, it is time for those managing the accident to get ahead of events and do whatever is necessary to prevent the spread of contamination to the deeper stratum of groundwater in Honshu.

This forum is sorely lacking the expertise of a geologist as shown in the previous mudstone/bedrock discussion.

Could a poster from Japan find a knowledgeable contributer to join the discussion?

The "battle of Fukushima" would seem to now be an underground battle.

Could you please show the information, link or theory that suggest the corium has now reached the groundwater. I would need to understand your basis before coming to the same conclusion.

 

[Quim]

concrete and steel mixed in with the fuel decreases the temperature, so it slows the whole mess down.

In post #7728 Jorge Stolfi offered this version of events:

"If you dilute very hot molten metal with cooler molten stuff, such as concrete, it will immediatly cool down and remain cool. If you confine a ton of liquid metal in a closed container, it will stay there and slowly cool down. If you cool the surface of a lump of lava, it will form a solid, relatively cool crust and then slowly cool down throughout.

None of these "common sense facts" seem to apply to corium, because its radioactive contents will continue to generate heat from "nowhere" at the same total rate, no matter how much it is diluted or how it is confined. (Mixing with boron can prevent it becoming critical but has absolutely no effect on the decay heat generation.) If that heat has nowhere else to go, the corium will keep getting hotter and hotter until it boild away. (And even then the vaporized material will continue generating heat at the same rate.) If you dlute the corium 100 fold with molten concrete, and then keep that mass isolated, the rate at which its temperature increases with time will be reduced a 100 fold perhaps, but it will remain positive.

So the entire mass --- original corium plus mixed concrete --- will continue to get hotter and hotter without limit; it will only take 3 months to reach the boiling point, instead of a day."



Also, in this case, the groundwater has apparently risen up to make contact with the corium instead of the more expected version of the process.

 

[etudiant]

I've read research that says the amount of steel and concrete mixed in with a melted core makes it improbable it could melt through the concrete floor. The more concrete and steel mixed in with the fuel decreases the temperature, so it slows the whole mess down.

That sounds plausible for this situation as well.
The slightly larger reactor at TMI did not melt through the reactor pressure vessel, despite the meltdown from a mistaken cooling cutoff. The Fukushima reactors were battered by the earthquake beforehand, so they may be more leaky than the TMI reactor was, but the only precedent we have suggests the bulk of their cores are still collected at the bottom of their pressure vessels.

 

[Jorge Stolfi]

If core would hit groundwater we would see KABOOM

Would we?

The groundtable is not liquid water, just an area of "very wet" dirt with a rather fuzzy top boundary. So, if the molten corium managed to bore into the ground and get down tho the water table at all, I would expect a gradually increasing release of steam, as the dirt is first heated then melted into slag (somewhere between 1000C to 1500C, I guess). That steam would create bubbles in the molten slag and will presumably contribute to insulate the corium from the dirt below it, slowing its descent. The steam pressure will rise until it can push the frothy slag back up through the hole created by the corium, creating a miniature volcano on the drywell floor.

On the other hand, the increased insulation provided by the frothy slag will cause the corium to get hotter. Presumably things will reach an equilibrium where the lava's temperature and pressure are just enough to keep the channel open, and the rate of conversion (dirt+water) ==> (slag+steam) is just enough to carry away the heat produced by the corium.

However the corium may also mix gradually with the slag and thus be carried up with it, until the massa that remains in the hole is too small to melt the dirt around it. Uranium oxide has a much higher density than molten dirt, and its melting point is much higher; so it may be slow to dissolve, as a drop of honey in a glass of water. If the fuel mass is mostly metallic, it may not mix with the slag at all.

 

[maddog1964]

In post #7728 Jorge Stolfi offered this version of events:

"If you dilute very hot molten metal with cooler molten stuff, such as concrete, it will immediatly cool down and remain cool. If you confine a ton of liquid metal in a closed container, it will stay there and slowly cool down. If you cool the surface of a lump of lava, it will form a solid, relatively cool crust and then slowly cool down throughout.

None of these "common sense facts" seem to apply to corium, because its radioactive contents will continue to generate heat from "nowhere" at the same total rate, no matter how much it is diluted or how it is confined. (Mixing with boron can prevent it becoming critical but has absolutely no effect on the decay heat generation.) If that heat has nowhere else to go, the corium will keep getting hotter and hotter until it boild away. (And even then the vaporized material will continue generating heat at the same rate.) If you dlute the corium 100 fold with molten concrete, and then keep that mass isolated, the rate at which its temperature increases with time will be reduced a 100 fold perhaps, but it will remain positive.

So the entire mass --- original corium plus mixed concrete --- will continue to get hotter and hotter without limit; it will only take 3 months to reach the boiling point, instead of a day."



Also, in this case, the groundwater has apparently risen up to make contact with the corium instead of the more expected version of the process.

not sure that i would be total agreement that the above quote is what is going on, and there are three reactors... each in different stages and locations, IMOP that is... but would you please clairify what your definition of groundwater is? There seems to be different definitions used by dif. sources.

 

[Quim]

Could you please show the information, link or theory that suggest the corium has now reached the groundwater. I would need to understand your basis before coming to the same conclusion.

There has been a recent (last three days) discussion of a large amount of water in the basement of the #1 building. This water appears to be groundwater seeping in through an earthquake damaged building foundation.

(posts 8559. 8601, #8607 etc)

 

[etudiant]

In post #7728 Jorge Stolfi offered this version of events:

"If you dilute very hot molten metal with cooler molten stuff, such as concrete, it will immediatly cool down and remain cool. If you confine a ton of liquid metal in a closed container, it will stay there and slowly cool down. If you cool the surface of a lump of lava, it will form a solid, relatively cool crust and then slowly cool down throughout.

None of these "common sense facts" seem to apply to corium, because its radioactive contents will continue to generate heat from "nowhere" at the same total rate, no matter how much it is diluted or how it is confined. (Mixing with boron can prevent it becoming critical but has absolutely no effect on the decay heat generation.) If that heat has nowhere else to go, the corium will keep getting hotter and hotter until it boild away. (And even then the vaporized material will continue generating heat at the same rate.) If you dlute the corium 100 fold with molten concrete, and then keep that mass isolated, the rate at which its temperature increases with time will be reduced a 100 fold perhaps, but it will remain positive.

So the entire mass --- original corium plus mixed concrete --- will continue to get hotter and hotter without limit; it will only take 3 months to reach the boiling point, instead of a day."



Also, in this case, the groundwater has apparently risen up to make contact with the corium instead of the more expected version of the process.

Absent any outside cooling, this would surely be true.
However, TEPCO has been jumping through hoops for the past 12 weeks to ensure that outside cooling was always present. That suggests a truce of sorts is currently in effect, with the corium getting enough cooling the keep it in place.
Of course, TEPCO is now struggling to find places to put the massively contaminated water produced by this cooling. If the ground water rises up as suggested, it might have the unwanted effect of reducing the corium cooling, as there would no longer be a flow to carry the hot water away. Not sure how that plays out, sort of a steam volcano, but only powered by 4-6 megawatts of decay heat.

 

[elektrownik]

We can't say what is under reactors now, science 3 months they don't pump off ground water, if we add that water level increased after earthquake, plus water which is leaking from reactors, plus reactors location not far from sea... There could be a lot of water...

 

[maddog1964]

There has been a recent (last three days) discussion of a large amount of water in the basement of the #1 building. This water appears to be groundwater seeping in through an earthquake damaged building foundation.

(posts 8559. 8601, #8607 etc)

There has been water in the basement for as long as I can remember. There has been way to many reports as to the cause of the water to list. I would agree that some maybe going to be groundwater, not sure if we are using the same defintion as to what is groundwater.
But one would also have to figure that the water going into the reactors for cooling is going to collect in the buildings, along with several other things that could be possible but I don't have time to post the proper links, so I'll leave them out

 

[Jorge Stolfi]

The slightly larger reactor at TMI did not melt through the reactor pressure vessel, despite the meltdown from a mistaken cooling cutoff.

However the TMI core was only partially, uncovered through the whole incident, so only a cubic meter or so of the fuel actually melted down. At Fukushima it seems that most of the core melted, at least in one reactor. (Isn't this TEPCo's own assessment now?)

Although the molten fuel at TMI did not breach the RPV, it did melt its way through internal shrouds and baffles on its way to the bottom head. So a breach of the RPV at Fukushima does not seem so unlikely.

 

[elektrownik]

Also in TMI if I understand correct only upper part of core melted and corium doesn't hit RPV

 

[maddog1964]

However the TMI core was only partially, uncovered through the whole incident, so only a cubic meter or so of the fuel actually melted down. At Fukushima it seems that most of the core melted, at least in one reactor. (Isn't this TEPCo's own assessment now?)

Although the molten fuel at TMI did not breach the RPV, it did melt its way through internal shrouds and baffles on its way to the bottom head. So a breach of the RPV at Fukushima does not seem so unlikely.

just a question of curiosity, do you know "abouts" not exact the thickness of the bottom head? the rod intrusion ports seem quite congested and robust.. also when the statement is made that the ...fuel is melting through... are we talking "glob" .. "small particles" ... or "fluid such as water would be" what size would be pertent to flow around/through the rod ports? thanks

 

[~kujala~]

There has been a recent (last three days) discussion of a large amount of water in the basement of the #1 building. This water appears to be groundwater seeping in through an earthquake damaged building foundation.

(posts 8559. 8601, #8607 etc)

We were discussing about the unit #6. You must be careful not to make conclusions about the unit #1 based on that conversation

In the unit #6 it really makes sense that some groundwater might be seeping in although other explanations also exist. But in the unit #6 there is no corium.

In the unit #1 all water in the basement of the reactor building could have come from the cooling water they have injected into the reactor.

On the other hand groundwater has been medium-level contaminated below #1 - #4. But we don't know the source. It might be that only one of them is leaking and contamination has spread all over (best case) or all of them are leaking (worst case).

Nevertheless, the main thing is to keep eye on the inland deep well which so far has not been contaminated suggesting that groundwater is moving towards sea. The last page here:
http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110602e15.pdf

 

[elektrownik]

Maybe I am stupid but I can't understand how unit 5 have 300m3 from ground water and unit 6 13500m3 from ground water, they are so close and 45 times difference...

 

[~kujala~]

Maybe I am stupid but I can't understand how unit 5 have 300m3 from ground water and unit 6 13500m3 from ground water, they are so close and 45 times difference...

That's a good question.
I guess the level of groundwater doesn't have to be the same although they are near each other.
Also the size of leak (through damaged waterproof systems or through concrete only) can differ - it can be "small", "big" or something in between.
Also the tsunami might have left more water in one place than other. Remember that two dead guys were found in the unit #4 turbine building and they were probably killed by tsunami waters. So the tsunami was probably able to hit directly at least turbine buildings.
Tsujitsu proposed this direct hit might also have happened in the reactor buildings. About that I don't know.

 

[Jorge Stolfi]

TEPCO has been jumping through hoops for the past 12 weeks to ensure that outside cooling was always present. That suggests a truce of sorts is currently in effect, with the corium getting enough cooling the keep it in place.

True, but a compact molten mass is much harder to cool down than an intact core. Even if completely immersed in water, it may still remain molten and able to flow; it all depends on its size and heat generation. A solid crust will form, but it may be too thin to hold the mass in place. (Volcanic lava will flow underwater for a while, even at the bottom of the ocean.)

In the best scenario, the fuel did not manage to breach the RPV (yet), and the drywell is flooded until the RPV bottom is underwater. The molten core is sitting at the bottom of the RPV, covered by water. Water pumped into the RPV mixes with the water inside, and the steam plus any excess water exits through some leak/pipe/breach on the side wall. Most of the heat produced by the corium will go into boiling the water inside the RPV, but some will be conducted through the RPV wall and heat/boil the surroudning water.

A previous post analyzed this situation and (IIRC) concluded that heat conduction through the RPV wall woud be so low that the water next to its outside surface may not even get to boil. However, if the corium is molten, the steel on the inside surface of the RPV will be at the same temperature as the corium. If that is over 1500 C, then the steel will melt locally. Depending on the corium's density, this layer of molten steel may float out of the way, exposing the steel underneath to the corium.

Since the corium is producing heat at a nearly costant rate, its temperature will be nearly constant too; so this process could go on indefinitely, until the wall is breached.. If this is happening at all, the rate of progress may be very slow, and it may perhaps take several months for the RPV to be breached.

Thus, IMHO, the current situation of apparent stability --- with low pressures and temperatures near 100 C --- does not guarantee that things are under control.

 

[NUCENG]

Good explanation, thanks!

One thing I don't understand about theormocouples is why the bimetal wires are usually extended all the way to the voltmeter. Why couldn't they be extended only to some cooler place nearby (such as just outside the concrete enclosure), and then have the signal be carried by copper wires to the meter? That would result in lower resistance for the signal and reduced risk of electrochemical effects along the way.

Or is that in fact how it is done?


Thanks for the compliments and for the thorough analysis. As for them being "open", there is a continuum between having a small leak and being wide open, so it may not be a simple yes/no question. Also, for a small leak, the degree of opening may be sensitive to pressure, temperature, flooding, clogging, etc., and so may vary erratically with time.


Thanks! I think I saw mention of it in this forum, but hadn't the time to check it out then.

Thanks again. I am tempted to include those readings in my plots too, but first one question: do they reflect the conditions inside the reactor, or only of the external contamination? In other words, are those gammas and neutrons mostly created by fission and decay inside the reactor's concrete enclosure? If so, does the spent fuel in the SFP contribute to those readings?

Thanks, that is important information.

As for the temperature sensors, I have seen several diagrams showing their approximate location on the RPV, drywell and torus; but I still miss the key details. Namely, where precisely are the RPV temperature measured: on the outside surface of the RPV, or embedded into its wall? If the former, woud the reading be affected by the drywell atmosphere or by water leaks above the sensor? How far is the "water nozzle" temperature sensor from the nozzles and their feedpipes? And so on...

These details are important, for example, to analyze the pressure x temperature plots. The red boiling curve in those plots is relevant only if the temperature and pressure are measured at the same spot in the fluid. Barring gauge malfunctions, the pressure must be indeed that of the fluid at the gauge's intake point, which should be valid for the bulk space inside (except for the hydrostatic pressure gradient in the liquid-filled part). On the other hand, if the temperature is measured on the outside of a 15 cm thick wall, or even embedded into it, it will be some value intermediate between the temperatures of the two fluids in immediate contact with the wall. Thus, one can easily have superheated steam inside the RPV with a temperature reading well below the boiling curve, or (less likely) liquid water inside with a temperature reading well above the boiling curve.

Couldn't find exact locations of individual thermocouples, but extracted the attachment from an old training reference. It shows that RPV thermocouples are surface mounted.