Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[Angry Citizen]

Radiation is now being measured at the plant as 400-800 milliSievert, not micro!

At what distance from the plant?

 

[promecheng]

At what distance from the plant?

They said at the plant.

 

[Astronuc]

You mean to say if there are only 2 rods.. fission won't occur even without control rods? How many pieces together before they begin to fission?

Two fuel assemblies are shipped together. Each fuel assembly contains between 91 and 96 fuel rods. They will not fission outside of the reactor core. Only when they are carefully placed in a designated predetermined location, and the control rods are withdrawn, with the core under the proper conditions (including fuel surrounded by cooling water/moderator) will the fission process be allowed to commence. It is all very controlled.

 

[Astronuc]

This situation keeps getting worse. I'm watching the live press conference on NHK and they are now saying unit-4 is on fire due to a hydrogen leak from the spent fuel and unit-4 may have also suffered a hydrogen explosion inside the reactor building.

Also, they are confirming a that there is a hole in the suppression pool structure of unit-2 which is releasing radiation.

The good news is that they think that units 1&3 are effectively cool due to seawater cooling operations. Now they need to figure out how to maintain cooling.

Radiation is now being measured at the plant as 400-800 milliSievert, not micro!

Edit: They've also increased the radius from 20 km to 30km from the plant where people should evacuate, or stay indoors.

When posting activties, please cite the source and/or link.

 

[rogerl]

Two fuel assemblies are shipped together. Each fuel assembly contains between 91 and 96 fuel rods. They will not fission outside of the reactor core. Only when they are carefully placed in a designated predetermined location, and the control rods are withdrawn, with the core under the proper conditions (including fuel surrounded by cooling water/moderator) will the fission process be allowed to commence. It is all very controlled.

If that's the case. Why are nuclear reactor cores not designed such that during emergencies like occurring now.. mechanical actuators can separate the fuel rods at a distance horizontally from one another enough to stop the fission process just like during shipment?

 

[rogerl]

From NEI:

UPDATE AS OF 9:40 P.M. EDT, MONDAY, MARCH 14:
An explosion in the vicinity of the suppression pool at Fukushima Daiichi 2 just after 6:20 a.m. Japan Standard Time (5:20 p.m. EDT) may have damaged a portion of the reactor’s primary containment structure.

Pressure in the suppression pool has been reported to have decreased to ambient atmospheric pressure shortly after the blast. Plant operator Tokyo Electric Power Co. (TEPCO) has reported possible damage to the reactor’s pressure-suppression system. Radiation levels at local monitoring stations have risen but are still in flux. TEPCO has evacuated some workers from all three Fukushima reactors with the exception of approximately 50 workers involved in sea water pumping activities into the reactors as part of emergency cooling efforts.


Efforts to inject sea water into Unit 2 have been complicated by a faulty pressure relief valve. The fuel at Unit 2 has been exposed at least twice, before being re-covered with sea water.

Japan's Chief Cabinet Secretary, Yukio Edano, has said a partial defect has been found inside the containment vessel of reactor 2 at the Fukushima Daiichi nuclear power plant.

Is the primary containment vessel strong enough to avoid being ripped apart by mere steam and pressure from the water and heating fuel rods?

 

[Drakkith]

If that's the case. Why are nuclear reactor cores not designed such that during emergencies like occurring now.. mechanical actuators can separate the fuel rods at a distance horizontally from one another enough to stop the fission process just like during shipment?

I believe it is like this.

The fuel isn't what is causing the problem, it's short lived waste products from the chain reaction of the fuel that is still fissioning and producing most of the heat. All the reactors successfully shut down as soon as the earthquake was detected. Once all the waste products have been converted to other stabler elements the reactor will no longer need the active cooling. It just takes a few days for the products to run through their half lives a few times and die down.

Reference: http://en.wikipedia.org/wiki/Fukushima_I_nuclear_accidents#Earthquake_and_tsunami

 

[TCups]

This is only going to get worse, I fear. As a Radiologist, not a physicist, I am not an expert on the technology, and all of the information available is "sketchy".

That said, does this sound about right?

There are now 4 reactors and their associated spent fuel rod cooling pools at imminent risk of complete loss of coolant.

Breech of the reactor vessel and primary containment of Unit 2 seems probable.

Radiation levels at present "at the plant" are possibly 800 milliSievert (0.8 Sievert), a level would result in whole body absorbed radiation dosages sufficient to cause acute radiation sickness within hours (God help those not yet evacuated and trying to do what can be done as long as possible).

Contamination has been confirmed in US Navy personnel as far as 100 miles out to sea. The winds may be shifting to the south, toward Tokyo.

Two of the reactor cores may be "cold" or nearly cold(units 1, 3) but it is not certain that they are yet stable.

Even if the reactor cores don't melt down and remain stable, the spent fuel cooling containment within the damaged building may be an equally, if not more dangerous, problem in the coming hours and days.

 

[rogerl]

I believe it is like this.

The fuel isn't what is causing the problem, it's short lived waste products from the chain reaction of the fuel that is still fissioning and producing most of the heat. All the reactors successfully shut down as soon as the earthquake was detected. Once all the waste products have been converted to other stabler elements the reactor will no longer need the active cooling. It just takes a few days for the products to run through their half lives a few times and die down.

Reference: http://en.wikipedia.org/wiki/Fukushima_I_nuclear_accidents#Earthquake_and_tsunami

Are these short lived waste products inside the fuel rods themselves? If so, if the heat melt the fuel rods and fall down to floor, they would be separated from the control rods and the fuel rods fission again at the bottom??

 

[Astronuc]

If that's the case. Why are nuclear reactor cores not designed such that during emergencies like occurring now.. mechanical actuators can separate the fuel rods at a distance horizontally from one another enough to stop the fission process just like during shipment?

It's not that easy. The fuel/core is located in pressurized vessel. The operating pressure is 1055 - 1075 psia on the inside and 14.7 psia (1 atm) on the outside. The control rods did their job of shutdown down the fission process.

The pressure vessel containing the core is then surrounded by tons of reinforced concrete.

One does not simply pry apart fuel rods within the fuel assembly.

 

[Drakkith]

Are these short lived waste products inside the fuel rods themselves? If so, if the heat melt the fuel rods and fall down to floor, they would be separated from the control rods and the fuel rods fission again at the bottom??

I'd guess that they are inside and outside. And melting fuel is what they are trying to avoid. If the fuel melts and pools in one area, it could lead to a criticality incident. Not to mention damaged core equipment and such just from the heat.

 

[Astronuc]

Is the primary containment vessel strong enough to avoid being ripped apart by mere steam and pressure from the water and heating fuel rods?

The pressure is released through valves. The pressure inside the reactor vessel is below operating pressure, but about the pressure in containment, which is designed for a lower pressure at which the core normally operates. The containment is vented in order to maintain pressure below that which would damage containment.

The consequence of venting contaiment is the release of radioactive gases.

 

[Astronuc]

This is only going to get worse, I fear. As a Radiologist, not a physicist, I am not an expert on the technology, and all of the information available is "sketchy".

Breech of the reactor vessel and primary containment of Unit 2 seems probable.

On what is this claim based?

 

[rogerl]

I'd guess that they are inside and outside. And melting fuel is what they are trying to avoid. If the fuel melts and pools in one area, it could lead to a criticality incident. Not to mention damaged core equipment and such just from the heat.

If so, this seems to be the main danger.. that as the fuel melts, they would pool in one area and become critical again. But Astronuc said the rods have to be arrange in some symmetrical configuration to become critical.. so I wonder if the melted pooling fuel at the floor can fission again (with the control rods left above). Astronuc?

 

[Drakkith]

If so, this seems to be the main danger.. that as the fuel melts, they would pool in one area and become critical again. But Astronuc said the rods have to be arrange in some symmetrical configuration to become critical.. so I wonder if the melted pooling fuel at the floor can fission again (with the control rods left above). Astronuc?

I'm not sure actually. I'm just basing that on previous incidents I've read about where too much of a radioactive material has been accidently put in one spot and resulted in a criticality incident. If the neutrons from the fuel require a moderator to slow them down to be captured, I would think that it would be safe until you had a thick/dense enough glob of melted fuel to slow the neutrons AND capture them to chain react.

 

[TCups]

On what is this claim based?

Not intended to be so much of a claim as a question, I suppose. Sorry if my speculation is unfounded.

Wasn't there an announcement to that effect by one of the TEPCO officials. Also, the "news" such as it is that the explosion of the #2 reactor was internal - within the primary containment, not to the outside - and that the pressure levels (in the vessel?) had precipitously dropped from about 3 atm to 1 atm, this following a failure to be able to pump cooling water and perhaps as much as 2.5 hours of partial or complete exposure of the core.

I apologize again -- not my intent to make dire predictions, more to see if I had any real understanding of what the "news" is likely to mean. It keeps getting worse, it seems.

Do you get radioactive Cesium and Iodine released without melting of the core? I thought not. And if there were an internal explosion followed by a drop in pressure in the coolant and a rise in radiation levels, can that imply some other more likely scenario?

In short, that is kind of why I am here -- to find out if someone more informed than me can put it all together for me. I shall refrain from any further posts and just "listen". Thanks.

 

[MeMyself+Eye]

May I ask for a smart answer to a stupid armchair engineering question?

As hydrogen is generated from the oxidation of the Zr fuel rods, and eventually vented along with steam, to reduce reactor pressure, why isn't it flared to prevent hydrogen pressure buildup outside the reactor? Those buildings surrounding the containment seem awfully big, so it just seems to me that keeping the mean outside pressure to less than plus one or two PSI by just flaring it as it's released would prevent any kind of detonation, or large deflagration for that matter.

I'm sure there is a good reason not to do this, but I'm just a dude who grew up around the steel yards and am kind of used to the idea of flaring gasses to prevent big booms.

 

[Reno Deano]

NRC sends more BWR experts to Japan: http://www.nrc.gov/reading-rm/doc-collections/news/2011/11-048.pdf"

 

[Astronuc]

May I ask for a smart answer to a stupid armchair engineering question?

As hydrogen is generated from the oxidation of the Zr fuel rods, and eventually vented along with steam, to reduce reactor pressure, why isn't it flared to prevent hydrogen pressure buildup outside the reactor? Those buildings surrounding the containment seem awfully big, so it just seems to me that keeping the mean outside pressure to less than plus one or two PSI by just flaring it as it's released would prevent any kind of detonation, or large deflagration for that matter.

I'm sure there is a good reason not to do this, but I'm just a dude who grew up around the steel yards and am kind of used to the idea of flaring gasses to prevent big booms.

As far as I understand the current situation, the hydrogen was not supposed to get into the upper containment - but rather is was supposed to go up the stack where it could have been vented with the steam. However, I haven't been able to verify that.

I heard one comment that the duct to carry the hydrogen out safely ruptured under the higher than design pressure, so the hydrogen leaked into the upper containment.

Some plants have hydrogen recombiners which are intended to burn the hydogen in a very controlled and slow manner. I don't know about these plants.

The explosion was not intended.

 

[Reno Deano]

Normally there is only one vent path for accumalate gases off the main steam lines via an air ejector. I believe they have a major rupture in a line within the primary containment that surrounds the reactor vessel, and venting it is awkward at best to the dry well. Main isolation valves are closed so the normal vent path is isolated and they are using the safety relief valves within the primary containment surrounding the reactor vessel, and who know what path they are using to get it out of there and through the secondary containment.

 

[Astronuc]

Do you get radioactive Cesium and Iodine released without melting of the core? I thought not. And if there were an internal explosion followed by a drop in pressure in the coolant and a rise in radiation levels, can that imply some other more likely scenario?

Yes - Cs and Iodine are volatile at fuel operating temperatures, and if the ceramic fuel gets hot enough, the Cs and I can come out into the gap between pellet and cladding. If the cladding is breached - i.e., cracks - then Cs and I can be carried out into the coolant. Xe and Kr obviously come out.

In normal operation, when fuel fails - cladding is breached - the coolant can enter the fuel rod. The UO2 oxidizes which reduces the thermal conductivity, which increases temperature, which causes Xe, Kr, Cs, I and some other volatile elements to migrate out of the cermamic. Oxidation of UO₂ also increases the diffusivity of some fission products - particularly noble gases.

So once can release Cs and I without melting the fuel.

Also - I beta decays to Xe, which beta decays to Cs, so Cs, in addition to being a direct fission product, is also a daughter product of Xe decay.

 

[falcon32]

No, the radiation source is independent of geometry, and only dependent on the fission products or nature of the radionuclides decaying.

But wouldn't the neutron intensity increase by changing from spaced rods to an agglomerated molten puddle?

 

[Reno Deano]

It is sad state of affairs that the news media hypes the reactor problems when thousands are dead and missing, and 10 of thousands people homeless in Japan and suffering. If all three BWRs had a full core melt the carnage would not be a tenth as bad what is already facing those currently trying to survive.

 

[Astronuc]

If so, this seems to be the main danger.. that as the fuel melts, they would pool in one area and become critical again. But Astronuc said the rods have to be arrange in some symmetrical configuration to become critical.. so I wonder if the melted pooling fuel at the floor can fission again (with the control rods left above). Astronuc?

I doubt the fuel will melt - but it might break into pieces - which will be trapped by the channels and bottom tie plate. The control rods may still be intact.

However, even if the fuel did melt though the bottom tie plate - then the steel pad - it would fall between the control rod drive guide system and still not achieve criticality.

The term 'core melting' is used losely by some in the industry, and certainly by those outside of industry. According to my understanding of the term, core melting may not have happened - it has been very limited. If cooling is achieved, then further core melting is prevented.

However, there is still the matter of the fission products in containment and how much will be released and how.

 

[Astronuc]

But wouldn't the neutron intensity increase by changing from spaced rods to an agglomerated molten puddle?

No. And in fact, as far as I can tell - there will not be a molten puddle.

If the ceramic fuel melted, then the steel would melt sooner and it would dilute the fuel, displace the water, and there would be no moderation to make the system critical. The enrichment is too low for a fast fission system.

 

[lapsedgeo]

Thank you all for calm and informed analysis, as a member of Joe Public this has been such a helpful place to visit. Every day it seems to get a little worse and I detect a note of resignation in all your posts now. Hearing that the spent fuel pond is on fire has made my heart sink. As a geographer I want to people to realize that we are not sitting on an inanimate lump of rock but this is one heck of a reminder.

 

[rogerl]

So if what Astronuc saying is accurate. The worse is over. Even if worse case scenerio no water were added anymore and the fuel rods just dry up. They won't melt at all. And even if they melted, fission wouldn't be restarted. They would be just molten mixture of many stuff that nullify the reactions. So we expect the crisis to be over in a few days with some emission of small radiations that is not dangerous to health, the emissions lasting perhaps for a couple of days or so and then everything will be back to normal. Right guys? Many people are evacuating Tokyo now and wish Astronuc can share with them technical stuff so as to lessen the panic. Thanks to him for answering all technical questions.

 

[rogerl]

I just heard from CNN that the heat in nearby reactors could have evaporated the water that soaked the spent fuel in reactor #4 and the rods caught fire and release radioactive debris about 400 milliSiv inside the compound.

So how dangerous are spent fuel? If they are "spent", there should be few live nuclei left that can fission, isn't it. Or the decayed project is acting up and still radioactive? What elements?

Also about Reactors 1-3. Isn't it if you leave cyanide powder in the trash can with no wind in the room, it would stay there. If you instead blow it with air, it will spread. What I'm saying is that the fuel rods in reactors 1 to 3 are no longer live and just left over decay heat. So why don't they just stop putting water and releasing steam outside the plants which can spread radiation all over the country and instead just let the fuel rods rot or melt and let the mixture stay safely in the bottom of the containment shell just like cyanide power left in the bottom of the trash can and basically quarantined there? Stupid questions but this nuclear incident is unprecendented. Sorry and thanks/

 

[falcon32]

No. And in fact, as far as I can tell - there will not be a molten puddle.

If the ceramic fuel melted, then the steel would melt sooner and it would dilute the fuel, displace the water, and there would be no moderation to make the system critical. The enrichment is too low for a fast fission system.

Astronuc, many thanks for answering my questions. It does sound hopeful, in that we only have to wait for the byproducts to decay, with the main uranium fission reaction inhibited.

One more question did come to mind...suppose in the unlikely event that the core did melt -- wouldn't the mixture separate by density, with uranium on the very bottom, and everything else floating on top?

 

[promecheng]

When posting activties, please cite the source and/or link.

Sorry, I got this information from watching the live broadcast of a press conference on HNK television in Japan. They were giving English translation of what the government spokesman was saying about the situation.