Restoration at nuke plant disrupted, radiation fears spread to Tokyo

Kyodo News Japan ^ | 24 March 2011

[Natural Born 54]

You’re welcome. Here’s something else useful:

http://www.radiationnetwork.com/

(better than nothing which is what the government’s network is - given that 8 monitoring stations are “nonfunctional” at this time)

[buzzer]

“Pilgrim is graphite moderated?”
It doesn’t need graphite moderation to cause a fuel burn.
The zircalloy casing of our nuclear fuel is also able to burn, and it burns even more hot then carbon.
So: Yes it could happen. Is is likely to happen ? No. But if it happens then help us god.

[buzzer]

“Radiation burns?”
Ever heard about thermal radiation ?
Gamma ray is nothing but thermal radiation at a very short wave length.

[Errant]

You're right

Now that's scary! :)

I've been doing a little research on zirconium. My thought, that ziconium (a tranistion metal), released from melting ZrO, would "mix" with the uranium/plutonium and slow reaction.

Zr 101:

"Nuclear applications Consuming about 1% of the Zr supply, zirconium is used for cladding nuclear reactor fuels.[15] For this purpose, it is mainly used in the form of zircaloys. The benefits of Zr alloys is their low neutron-capture cross-section and good resistance to corrosion.[5][6] The development of efficient methods for the separation of zirconium from hafnium was required for this applications.

One disadvantage of zirconium alloys is their reactivity toward water at high temperatures leading to the formation of hydrogen gas and to the accelerated degradation of the fuel rod cladding:

Zr + 2 H2O → ZrO2 + 2 H2 This exothermic reaction is very slow below 100 °C but rapid at higher temperatures. Most metals undergo similar reactions. The redox reaction is relevant to the instability of fuel assemblies at high temperatures,[27] This reaction was responsible for a small hydrogen explosion first observed inside the reactor building of Three Mile Island accidented nuclear power plant in 1979, but then, the containment building was not damaged. The same reaction occurred in the reactors 1, 2 and 3 of the Fukushima I Nuclear Power Plant (Japan) and in the spent fuel pool of reactor 4 after the reactors cooling was interrupted by the earthquake and tsunami disaster of March 11, 2011 leading to the Fukushima I nuclear accidents. After venting of hydrogen in the maintenance hall of these three reactors, the explosive mixture of hydrogen with air oxygen detonated, severely damaging the installations and at least one of the containment buildings. To avoid explosion, the direct venting of hydrogen to the open atmosphere would have been a preferred design option. Now, to prevent the risk of explosion in many pressurized water reactor (PWR) containment buildings, a catalyst-based recombinator is installed to rapidly convert hydrogen and oxygen into water at room temperature before explosivity limit is reached."

Nuclear fuel cycle

Fuel cladding interactions:

The study of the nuclear fuel cycle includes the study of the behaviour of nuclear materials both under normal conditions and under accident conditions. For example, there has been much work on how uranium dioxide based fuel interacts with the zirconium alloy tubing used to cover it. During use, the fuel swells due to thermal expansion and then starts to react with the surface of the zirconium alloy, forming a new layer which contains both fuel and zirconium (from the cladding). Then, on the fuel side of this mixed layer, there is a layer of fuel which has a higher caesium to uranium ratio than most of the fuel. This is because xenon isotopes are formed as fission products that diffuse out of the lattice of the fuel into voids such as the narrow gap between the fuel and the cladding. After diffusing into these voids, it decays to caesium isotopes. Because of the thermal gradient which exists in the fuel during use, the volatile fission products tend to be driven from the centre of the pellet to the rim area.[16] Below is a graph of the temperature of uranium metal, uranium nitride and uranium dioxide as a function of distance from the centre of a 20 mm diameter pellet with a rim temperature of 200 oC. The uranium dioxide (because of its poor thermal conductivity) will overheat at the centre of the pellet, while the other more thermally conductive forms of uranium remain below their melting points.

Release of radioactivity from fuel during normal use and accidents:

The IAEA assume that under normal operation the coolant of a water cooled reactor will contain some radioactivity[18] but during a reactor accident the coolant radioactivity level may rise. The IAEA state that under a series of different conditions different amounts of the core inventory can be released from the fuel, the four conditions the IAEA consider are normal operation, a spike in coolant activity due to a sudden shutdown/loss of pressure (core remains covered with water), a cladding failure resulting in the release of the activity in the fuel/cladding gap (this could be due to the fuel being uncovered by the loss of water for 15–30 minutes where the cladding reached a temperature of 650-1250 oC) or a melting of the core (the fuel will have to be uncovered for at least 30 minutes, and the cladding would reach a temperature in excess of 1650 oC).[19]

Based upon the assumption that a PWR contains 300 tons of water, and that the activity of the fuel of a 1 GWe reactor is as the IAEA predict,[20] then the coolant activity after an accident such as the three mile island accident (where a core is uncovered and then recovered with water) can be predicted (I don't think this would hold true for Fukushima).

[Errant]

Interesting read (Click on view document):

BWR REACTOR VESSEL BOTTOM HEAD FAILURE MODES

[Errant]

SUMMARY

This paper has briefly described the approach taken by the BWRSAT Program
at Oak Ridge National Laboratory towards understanding the probable
sequence of events for an unmitigated BWR severe accident. There are
many associated uncertainties, and experimental verification of the
approach is certainly desirable.

For an unmitigated BWR severe accident involving the progressive relocation
of material from the core region into the lower plenum of the reactor
vessel, the control rod guide tube structure and the large amount of
water in the lower plenum would be expected to provide for distribution
and quenching of the relocating debris. Since the earliest relocation of
materials from the core region would consist of metals from the control
blades, channel boxes, and cladding, the lower portion of the bottom head
debris bed should be metals-rich. The subsequent collapse of fuel pellet
stacks into the lower plenum would provide an underwater decay heat
source and provide for continuous boiloff of the surrounding water.
After bottom head dryout, the debris bed temperature would begin to
increase.

The cluster of control rod guide tubes in the lower plenum would be
heated by the surrounding debris bed and would be weakened at high temperatures
to the point of failure. Loss of control rod guide tube
strength would cause collapse of the remaining standing outer regions of
Che core that: are supported by the guide tubes. This coLlapse would form
the upper portion of the bottom head debris bed while the stainless steel
mass of the control rod guide tubes would be subsumed into the surrounding
debris bed as they melt. Thus, there is expected to be a large
amount of stainless steel included in BWR bottom head debris.

As the bottom head debris reaches high temperature, failure of the bottom
head pressure boundary would occur at some point. Penetration failures
can occur by weakening of the stub tube welds supporting the control rod
drive mechanism assemblies or by failure of the instrument tube welds at
the reactor vessel wall. However, failure of a stub tube weld would only
cause a small downward motion of the associated control rod drive mechanism
assembly, and therefore, although gas blowdown would be initiated by
such a failure, gross release of debris from the vessel would not.

For the instrument tube, although there is nothing to prevent its complete
detachment from the vessel given weld failure at the vessel wall,
it seems probable that an earlier failure would be by opening of the tube
in the middle (hottest) point of the bottom head debris bed with subsequent
spillover of molten material into the tube with passage through the
vessel wall, causing heatup and creep-rupture of the tube just outside
the wall. Instrument tube failures in this manner would provide pathways
for release of molten debris from the vessel.

The individual components of the debris bed would be expected to leave
the vessel in the order in which they reach their melting points and
transform to the liquid state. Solid metallic material surrounding the
lower portion of the original instrument guide tube locations would be
ablated into the molten material flowing from the reactor vessel via
these pathways.

Gross failure of the portion of the reactor vessel bottom head underneath
the vessel support skirt would be expected to occur long after the penetration
failures discussed above. The reactor vessel bottom head wall is
thick, and there is relatively little wall stress after the vessel is
depressurized. BWR severe accident sequence calculations with the BWRSAR
code predict failure of the bottom head wall only after the majority of
the metallic debris has left the vessel.

[chimera]

So the instrument tube penetration failure may be a possible pathway for release, according to this analysis. From what I know of BWR instrument tubes, those do not have a very large diameter. So we may be talking about several small holes rather than bulk failure of the pressure vessel. That could very well be the case. The point of those early vapor releases was to keep internal pressures done owing to lack of residual heat removal and subsequent coolant vaporization. We know that was the case, so they were following the guidelines in the BWR Owner’s Group specifications. Until they can get some up-close inspection of the lower vessel structure, it is somewhat of a guess.

[Errant]

Yes, but possibly lots of small holes. Reading additional reports located at the same site, I'm coming to the conclusion that salt water may not have been the best coolant to use (of course it's the only thing they had). The sodiumchoride (lower boiling point of 1413 °C) left behind after dry out, may act as a much more effective vehicle for radioactive particle transport than steam.

Too bad the fuel storage pools aren't better contained. This could be a problem if the radiation levels increase and the entire site has to be abandoned.

[chimera]

They are going to have to do something to flush that salt out, no question. Hopefully sooner than later. I read a report where there were efforts to get either pure water or borated water (weak boric acid solution) going into those pressure vessels, or storage pools (maybe both?).

Maybe it was the best trade-off at the time. They had to do something to reduce that heat load or risk challenging the pressure/temperature limits of the vessel and/or containment. If the normal supply of emergency coolant was unavailable, they had to go with something, and that meant sea water. I am curious to see if there wasn't damage to the neutron-absorbing baffles of the storage pools. If there was that could account for some of the neutron flux readings (not accidental criticality so much as photoneutron production).

[Errant]

I think I read that the US is sending a barge with over 1 M gallons of fresh water. Time is not on our side. If/when the site is abandoned due to higher radiation levels, it's sayonara to a large part of Japan.

I am curious to see if there wasn't damage to the neutron-absorbing baffles of the storage pools.

Saw some recent video of the damage to the roof area (forget which reactor) this morning. Doesn't look good for the storage pool.

[chimera]

Well, I guess its problematic at this point. I'll just say I'm not as pessimistic. It don't think this will reach INES Level 7 (like Chornobil) even in the worst case. They've still got containment (unlike Chornobil) and I don't think we'll see catastrophic failure of that.

That's not to say it won't be a long road to recovery. Remember to keep things in perspective. I just saw on Drudge that they have 27,000 reported dead or missing in the larger disaster. That is the real tragedy. The reactors have caused no fatalities and a handful of injuries. That said, they'll have to keep working as hard as they can on the Fukushima situation, but on balance they have a lot of other things on their plate as well.

[Errant]

Certainly the death and destruction from the tsunami is the larger disaster for now and I suppose it makes many uncomfortable to think things could get worse (especially those in that field). Don't buy trouble, so to speak ...

I think it's just the nature of some of us to try to think several steps ahead and evaluate possible outcomes. It probably has more to do with the type of careers we've had than some personal trait. Depending upon your own situation, that can be either a good or bad thing.

[chimera]

Since we were talking earlier about the salt vs. freshwater, this is what I have on the latest news feed:

UPDATE AS OF 1:30 P.M., MARCH 25:

Workers have switched from sea water to fresh water to cool reactor 1 and were expected to make the same change for reactors 2 and 3 at the Fukushima Daiichi nuclear power plant by Saturday. Pressure and temperature inside reactor 1 were declining on Friday.

Lighting has been restored in the control rooms of reactors 1 through 4 at the plant, which lost electric power after the March 11 earthquake and tsunami, the Japan Atomic Industrial Forum said. With offsite electric service connected to all the units, workers are attempting to connect plant safety equipment. Some pumps and other equipment that were damaged in the earthquake and tsunami must be repaired or replaced.

Water spraying to maintain cooling of used uranium fuel rods in the reactor 3 used fuel storage pool was suspended because of high radiation levels near that building, but spraying into the reactor 1 and 4 storage pools continued.

Reactors 5 and 6 are safely shutdown and are being cooled with pumps using offsite electricity.

Radiation dose rates at the Fukushima Daiichi site boundary continue to range from 1 millirem to 3 millirem per hour.

[Errant]

Radiation dose rates at the Fukushima Daiichi site boundary continue to range from 1 millirem to 3 millirem per hour.

Now that is some really good news.

[chimera]

Here’s hoping for the best. They are really doing yeoman’s work over there. Heroic is more accurate. I’m outta here for the weekend...

[Munz]

again many thanks
I need to get educated on this stuff. I really am a neophyte on it all.

[Natural Born 54]

Fortunately, us neophytes have the internet.

[Errant]

Agree

[chimera]

Here is a more recent newsfeed:

UPDATE AS OF 5 P.M. EDT, FRIDAY, MARCH 25:

Fresh water is being injected into the reactor pressure vessel at reactor 3 at Fukushima Daiichi nuclear power plant, Japan’s Nuclear and Industrial Safety Agency said.

TEPCO said that radioactive materials discovered at the reactor 3 turbine building possibly came from water from the reactor system, not the spent fuel pool. TEPCO made that statement after collecting samples of contaminated water in the reactor 3 turbine building and conducting a gamma-emitting nuclide analysis of the sample. The reactor pressure and drywell pressure at reactor 3 remained stable on Friday, leading TEPCO to believe that “the reactor pressure vessel is not seriously damaged.”

Cooling efforts at Reactor 1 already had switched back to fresh water cooling. Reactor 2 is still being injected with seawater, but is expected to switch to fresh water soon.

Tokyo Electric Power Co. said that crews continued spraying water into the used fuel storage pools at reactors 3 and 4 on Friday to keep the used uranium fuel rods safe. Also on Friday, the heat removal system at reactor 6 was switched to a permanent power supply, NISA added.

TEPCO said it was assessing the radiation dose to two workers who were contaminated while laying cable in the turbine building of reactor 3. TEPCO said it had instructed its employees and contract workers to pay attention to their personal radiation dosimeter alarms and evacuate when necessary.

On-site radiation monitoring at the Fukushima Daiichi nuclear power plant indicates that radiation dose rates continue to decrease, the International Atomic Energy Agency said.

Radiation Monitoring Update

Air and seawater sampling continues by the Japanese government. Measurements in the ocean were taken 30 kilometers off-shore and 330 meters from the discharge points on March 23 and March 24. Results indicate concentrations of iodine-131 at 2,162 picocuries per liter and cesium-137 at approximately 703 picocuries per liter. Adult consumption of 1,000 picocuries (1 picocurie is one-trillionth of a curie) per liter concentration for 30 days will result in 24 millirem of radiation dose. For comparison, a typical dose from a chest x-ray is 10 millirem.

The concentrations found in the seawater samples are most likely “due to atmospheric fallout rather than just ocean currents,” IAEA said. Dilution is expected to rapidly decrease this surface contamination, IAEA added.

Iodine-131 was detected in drinking water in 13 prefectures and cesium-137 was detected in drinking water in six prefectures. All results remained below the limits set by the Japanese government, IAEA said. Iodine-131 levels in drinking water in Tokyo are now below limits for consumption by infants set by the Japanese authorities and restrictions have been lifted.

On March 25, the IAEA radiation monitoring team made additional measurements at distances from 34 to 62 kilometers from the Fukushima Daiichi nuclear power plant. At these locations, the radiation dose rate was at extraordinarily low levels, ranging from 0.073 millirem per hour to 0.88 millirem per hour.

[Errant]

Thanks!

Friday, leading TEPCO to believe that “the reactor pressure vessel is not seriously damaged.”

Your previous post that indicated radiation levels were holding, lead me to believe that any breach must not be as serious as first reported.

I'm really confused by the reports of contaminated drinking water though. This has to be some kind of chronic contamination. Even if there was some kind of subsurface release of high pressure contaminates, it doesn't seem possible to affect water aquifers miles away in so small amount of time.

Certainly it's possible if the drinking water comes from lakes, streams or is treated in open storage systems.

All in all, very encouraging news.