Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[falcon32]

Layman's question...doesn't the rate of radiation greatly increase when a core melts into a blob at the bottom of the reactor because of the inverse square law? So wouldn't a lone melted rod give off much less radiation than a bunch of them?

And at what time is the maximum amount of radiation released? Pre-core meltdown when all coolant is gone? Post?

Thanks for the help!

 

[Astronuc]

Layman's question...doesn't the rate of radiation greatly increase when a core melts into a blob at the bottom of the reactor because of the inverse square law? So wouldn't a lone melted rod give off much less radiation than a bunch of them?

And at what time is the maximum amount of radiation released? Pre-core meltdown when all coolant is gone? Post?

Thanks for the help!

No, the radiation source is independent of geometry, and only dependent on the fission products or nature of the radionuclides decaying. If one looks at the decay heat curve as a function of time, one sees that it is decreasing, and the radionuclides decay to long-lived radionuclides, or inert (non-radioactive) nuclides.

The maximum release would be dependent on a number of variables including the fission product inventory (source term) and the rate at which fuel rods are breached.

 

[Astronuc]

In a nuclear reactor, there are as many as a thousand fuel rods. But there only appears to be a dozen or so control rods which are put amongst the thousands of fuel rods. How does this stop nuclear reaction within each rod? Or in a nuclear reactor. Do neutrons from different fuel rods hit other fuel rods at a distance and the control boron rods are supposed to block or absorb them? But how can the few control rods absorb the neutrons from thousands of fuel rods?

In a BWR, there is one control rod for four fuel assemblies. Most modern BWRs use a 10x10 array of fuel rods, but some fuel rods are part-length rather than full-length (core height), and there are 'water rods' or 'water channels' within the assembly in order to introduce water for moderation in the interior rods of the assembly. So while a 10x10 fuel assembly has 100 lattice positions, some designs have 96 rods, some 91 rods, and other 92 rods, and some of those fuel rods might be 2/3 of the full-length or core height.

The are local differences in neutron flux and power generation, which is more or less proportional to local neutron flux (we call this power peaking, and we can describe local power in terms of an average power and local peaking factor). The fuel rods in the four assemblies are most affected by the control blades, while those on the opposite side of the fuel assembly are less affected - but that is only important when the reactor is critical, and then only when the reactor is at power.

We know the control rods were inserted, which means the fission reaction shutdown, i.e. the fission reaction or power geneation went to essentially zero. However as mentioned elsewhere, there is decay heat from the beta and gamma decay of fission products, which decay well after the fission reactions stop.

Control rods contain a neutron absorber B-10 and/or Hf, which is very efficient at absorbing neutrons. The B-10 is a much better absorber of neutrons than U-235 or Pu-239, so the cores are always designed such that when the control rods are inserted, minus the strongest control rod, the core goes subcritical. Core and fuel design is a very mature technology.

 

[Reno Deano]

Core covered with seawater, and still a continuing melt and release of vapors? Sounds like something is missing, such as a control rod or 2, or the core was never covered. BTW, salt water is a great neutron moderator. Remember BWR control rods need to be held "up" inplace by an intricate hydraulic system.

BWR system description: http://www.nrc.gov/reading-rm/basic-ref/teachers/03.pdf"

 

[ferrelhadley]

Dumb question time (but its the right forum for it I guess)
Where have I gone wrong with this.
If the fuel rods in the core are about 1300K then pluggin into stefan boltman I get
At 1300K I a get c 1.62*10^5 J/s per square meter assuming emissivity of 1.
j = (1300K)^4* 1 * (5.67*10^-8)

So why has there not been a reasonable amount of cooling due to radiative rather than conductive heat transfer? Have I got the calculations wrong?

 

[promecheng]

Just read that there was a third explosion at unit-2. The article mentioned that the explosion created a "defect" in the torus pressure suppression pool. I'm trying to confirm this from a credible source. Isn't the suppression pool considered part of primary containment? I really hope they are mistaken.

http://www.ft.com/cms/s/0/cdc71436-4e59-11e0-98eb-00144feab49a.html#axzz1Gci1ultJ

 

[Astronuc]

I have been looking for info on the risks of a dry spent fuel pool. Unfortunately, there is little middle-of-the road info out there. Lots of the worst-case scenarios, though many of them have the fingerprints of Fairewinds Associates on them. Phrases like "Chernobyl on steroids" etc. The NRC materials I found were comforting about the level of safety required, but very light on the risks.

This software simulator site quiz says that the severity of an SFP failure would be on a par with a "worst case power accident".
http://www.microsimtech.com/sfpquiz/default.htm

Can you help clarify, Astro?

As gmax137 indicated, they plant personnel will make every effort to ensure that the spent fuel pools are filled and cooled. We have no info on that.

The consequence of an SFP fire could be comparable - give or take - to a core accident. But it's not so simple, and no one has done an actual test, although we know something about the physics behind Zr combustion. That's actually something we'll be looking at in more detail.

I have seen Zr alloys inductively heated to temperatures above 1000C, and the protective oxide prevents oxidation. A mass of Zr alloy, like tubing or sheet, is not the same as fine wire in a flash bulb. The oxide won't necessarily 'burn' - uncontrolled or rapid combustion.

I've done an demonstration in high school with magnesium ribbon. We put a flame/torch to an oxidized ribbon - and it just got hot. But once we scratched the oxide, we could burn the metal. In a spent fuel pool, the cladding already has a protective oxide, and it would have to get very hot before the oxide would break and rapid oxidation would occur. How hot the fuel would get depends on the burnup and how long it has been sitting in the pool (i.e., how much decay heat has dropped off).

 

[Astronuc]

Dumb question time (but its the right forum for it I guess)
Where have I gone wrong with this.
If the fuel rods in the core are about 1300K then pluggin into stefan boltman I get
At 1300K I a get c 1.62*10^5 J/s per square meter assuming emissivity of 1.
j = (1300K)^4* 1 * (5.67*10^-8)

So why has there not been a reasonable amount of cooling due to radiative rather than conductive heat transfer? Have I got the calculations wrong?

1300 K for what? Also, if two surfaces are at the same or similar temperature, there is not differential to drive radiative cooling - the two surfaces radiate to each other.

 

[CAC1001]

Quick question, but when they say a nuclear plant is for example "Designed to withstand up to a 7.0 earthquake," do they mean the plant could go into meltdown beyond that level of earthquake, or just that the plant is designed to keep operating up to that level of earthquake, but might have to be shutdown if the earthquake is greater?

Because everyone seems to assume it means beyond that level of quake, the plant could go into meltdown, but for all I know that could just mean the plant is designed to keep operating to that level.

 

[ferrelhadley]

1300 K for what? Also, if two surfaces are at the same or similar temperature, there is not differential to drive radiative cooling - the two surfaces radiate to each other.

I was working on open literature that stated an uncovered fuel rod would reach a temperature of 1100C, but thinking on the issue I've spotted a couple of flaws including that as you say most of the fuel rods will be radiating at other fuel rods so no net loss and as the rods are not really uncovered for all that long they are likely to be significantly cooler plus off course the containment vessel will heat up and radiate back at the fuel rods so this is not really an effective mechanism for losing heat.

I was just wondering why the rods had not lost significant amounts of heat over 3 days and still posed a melting risk unless the moderators had not been fully inserted.

Thanks anyway.

 

[Ferrus]

Can someone answer a basic chemistry question here: what is the redox potentials of the zirconium-water reaction? Is it thermodynamically infeasible under standard conditions, or does the oxide coating merely form a high activation energy barrier ala TST?

 

[edgepflow]

1300 K for what? Also, if two surfaces are at the same or similar temperature, there is not differential to drive radiative cooling - the two surfaces radiate to each other.

For this reason, the only place in the reactor core I could see radiation heat transfer being significant would be the fuel rods on the perimeter. These could radiate to the lower temperature of the vessel. The fraction of the total surface area of the rods is small however, and convenction to the fluid from interior fuel would probably still dominate. A view factor would need to be applied and you could assume the vessel is a blackbody. It would be interesting to calculate. Will try to find some time to do this. I will bet it will be a very small fraction of the decay heat.

 

[edgepflow]

I was just wondering why the rods had not lost significant amounts of heat over 3 days and still posed a melting risk unless the moderators had not been fully inserted.

Thanks anyway.

They have lost significant amounts of heat over 3 days, but the power output is still large. You can back-of-the-envelope estimate the decay heat power with infinite fuel exposure from (Ref. "Nuclear Heat Transport" El-Wakil):

P(t) = 0.095 Po ts ^ -0.26

Po = power before shutdown
ts = shutdown time is seconds

And you can integrate this to get the total energy released.

For ts = 2 days, and Po = 2000 MWt (not sure if this is the actual power before shutdown), there is still 8.3 MW of thermal power. And integrating to 2 days gives 22.3 MW-day of released energy.

 

[NeoDevin]

Why couldn't they have used the remaining heat/steam to run the generator to produce enough electricity to power the cooling system, once the diesel generators had failed?

 

[turbo]

Why couldn't they have used the remaining heat/steam to run the generator to produce enough electricity to power the cooling system, once the diesel generators had failed?

T-G sets are designed to operate in pretty narrow parameters regarding feed pressure, temperature, superheat, etc, and they have intermediate stages and controls. Also, the pressure drop across the turbines has to be enhanced by cooling/vacuum in final stages to make sure that the turbine actually operates at all. You can't pump wet steam into a turbine with insufficient stage-to-stage steam control and expect it to work without tearing itself apart. Please bear in mind that I am used to studying and documenting much smaller (often 30-60 Mw) turbines in single T-G sets, but I don't believe that the laws of physics can be violated when you scale up to larger turbines.

Edit: there are probably emergency pumps that can be driven by robust turbines with with less restrictive feed-quality requirements, like the line-shaft turbine driving my old paper machine, but you have to have electrical power to control those systems, too. If your battery backup fails and you have no access to the AC grid, good luck controlling those. I should mention that such really primitive turbines might be regulated by mechanical governors, but I don't know enough about nukes to know if that kind of low-tech was implemented 40 years ago.

 

[rogerl]

I'm wondering. Right after uranium fuel rods are manufactured in the factor. Do they start to fission and produce nuclear reactions? Then how do the manufacturers store the fuel rods before sending them out? By putting them in live reactor with boron control rods and water
to cool them? For example, when they are shipped out at sea. Do they have to be put in a reactor like configuration inside the ship with running coolant water and stuff just like in normal reactor or do they put them in wooden crate and send them out?? How then do the nuclear plant enable them or turn them on to begin the nuclear reactions?

 

[promecheng]

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.

 

[Astronuc]

I'm wondering. Right after uranium fuel rods are manufactured in the factor. Do they start to fission and produce nuclear reactions? Then how do the manufacturers store the fuel rods before sending them out? By putting them in live reactor with boron control rods and water
to cool them? For example, when they are shipped out at sea. Do they have to be put in a reactor like configuration inside the ship with running coolant water and stuff just like in normal reactor or do they put them in wooden crate and send them out?? How then do the nuclear plant enable them or turn them on to begin the nuclear reactions?

BWR assemblies are shipped in pairs. They are in a dry sealed metal inner container in a outer container. They are subcritical.

Fission does not start occurring until the fuel is in the core, and the control rods are withdrawn to preset levels.

Shipments of fuel go by truck usually, sometimes by ship, or by cargo aircraft. Normally within a country, they go by truck.

 

[promecheng]

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.

 

[rogerl]

BWR assemblies are shipped in pairs. They are in a dry sealed metal inner container in a outer container. They are subcritical.

Fission does not start occurring until the fuel is in the core, and the control rods are withdrawn to preset levels.

Shipments of fuel go by truck usually, sometimes by ship, or by cargo aircraft. Normally within a country, they go by truck.

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?

 

[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.