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Summary of what really happened in Japan Nuclear Plant


Don Paul
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Subject: Fukashima Summary

A few days old but pretty good!

This guy seems to have pieced together the probable accident scenario at

Fukashima. Probably better than anything that the media has put out. It's

a good read.

Below is a summary [for the general public] on the Fukushima situation

prepared by Dr Josef Oehmen, a research scientist at MIT, in Boston.

He is a PhD Scientist, whose father has also extensive experience in

Germanys nuclear industry.

~~~~~~~~~~~~

What happened at Fukushima

I will try to summarize the main facts. The earthquake that hit Japan was

7 times more powerful than the worst earthquake the nuclear power plant

was built for (the Richter scale works logarithmically; the difference

between the 8.2 that the plants were built for and the 8.9 that happened

is 7 times, not 0.7). So the first hooray for Japanese engineering,

everything held up.

When the earthquake hit with 8.9, the nuclear reactors all went into

automatic shutdown. Within seconds after the earthquake started, the

control rods had been inserted into the core and nuclear chain reaction of

the uranium stopped. Now, the cooling system has to carry away the

residual heat. The residual heat load is about 3% of the heat load under

normal operating conditions.

The earthquake destroyed the external power supply of the nuclear reactor.

That is one of the most serious accidents for a nuclear power plant, and

accordingly, a plant black out receives a lot of attention when

designing backup systems. The power is needed to keep the coolant pumps

working. Since the power plant had been shut down, it cannot produce any

electricity by itself any more.

Things were going well for an hour. One set of multiple sets of emergency

Diesel power generators kicked in and provided the electricity that was

needed. Then the Tsunami came, much bigger than people had expected when

building the power plant (see above, factor 7). The tsunami took out all

multiple sets of backup Diesel generators.

When designing a nuclear power plant, engineers follow a philosophy called

Defense of Depth. That means that you first build everything to

withstand the worst catastrophe you can imagine, and then design the plant

in such a way that it can still handle one system failure (that you

thought could never happen) after the other. A tsunami taking out all

backup power in one swift strike is such a scenario. The last line of

defense is putting everything into the third containment (see above), that

will keep everything, whatever the mess, control rods in our out, core

molten or not, inside the reactor.

When the diesel generators were gone, the reactor operators switched to

emergency battery power. The batteries were designed as one of the backups

to the backups, to provide power for cooling the core for 8 hours. And

they did.

Within the 8 hours, another power source had to be found and connected to

the power plant. The power grid was down due to the earthquake. The diesel

generators were destroyed by the tsunami. So mobile diesel generators were

trucked in.

This is where things started to go seriously wrong. The external power

generators could not be connected to the power plant (the plugs did not

fit). So after the batteries ran out, the residual heat could not be

carried away any more.

At this point the plant operators begin to follow emergency procedures

that are in place for a loss of cooling event. It is again a step along

the Depth of Defense lines. The power to the cooling systems should

never have failed completely, but it did, so they retreat to the next

line of defense. All of this, however shocking it seems to us, is part of

the day-to-day training you go through as an operator, right through to

managing a core meltdown.

It was at this stage that people started to talk about core meltdown.

Because at the end of the day, if cooling cannot be restored, the core

will eventually melt (after hours or days), and the last line of defense,

the core catcher and third containment, would come into play.

But the goal at this stage was to manage the core while it was heating up,

and ensure that the first containment (the Zircaloy tubes that contains

the nuclear fuel), as well as the second containment (our pressure cooker)

remain intact and operational for as long as possible, to give the

engineers time to fix the cooling systems.

Because cooling the core is such a big deal, the reactor has a number of

cooling systems, each in multiple versions (the reactor water cleanup

system, the decay heat removal, the reactor core isolating cooling, the

standby liquid cooling system, and the emergency core cooling system).

Which one failed when or did not fail is not clear at this point in time.

So imagine our pressure cooker on the stove, heat on low, but on. The

operators use whatever cooling system capacity they have to get rid of as

much heat as possible, but the pressure starts building up. The priority

now is to maintain integrity of the first containment (keep temperature of

the fuel rods below 2200°C), as well as the second containment, the

pressure cooker. In order to maintain integrity of the pressure cooker

(the second containment), the pressure has to be released from time to

time. Because the ability to do that in an emergency is so important, the

reactor has 11 pressure release valves. The operators now started venting

steam from time to time to control the pressure. The temperature at this

stage was about 550°C

.

This is when the reports about radiation leakage starting coming in. I

believe I explained above why venting the steam is theoretically the same

as releasing radiation into the environment, but why it was and is not

dangerous. The radioactive nitrogen as well as the noble gases do not pose

a threat to human health.

At some stage during this venting, the explosion occurred. The explosion

took place outside of the third containment (our last line of defense),

and the reactor building. Remember that the reactor building has no

function in keeping the radioactivity contained. It is not entirely clear

yet what has happened, but this is the likely scenario: The operators

decided to vent the steam from the pressure vessel not directly into the

environment, but into the space between the third containment and the

reactor building (to give the radioactivity in the steam more time to

subside). The problem is that at the high temperatures that the core had

reached at this stage, water molecules can disassociate into oxygen and

hydrogen an explosive mixture. And it did explode, outside the third

containment, damaging the reactor building around. It was that sort of

explosion, but inside the pressure vessel (because it was badly designed

and not managed properly by the operators) that lead to the explosion of

Chernobyl. This was never a risk at Fukushima. The problem of

hydrogen-oxygen formation is one of the biggies when you design a power

plant (if you are not Soviet, that is), so the reactor is build and

operated in a way it cannot happen inside the containment. It happened

outside, which was not intended but a possible scenario and OK, because it

did not pose a risk for the containment.

So the pressure was under control, as steam was vented. Now, if you keep

boiling your pot, the problem is that the water level will keep falling

and falling. The core is covered by several meters of water in order to

allow for some time to pass (hours, days) before it gets exposed. Once the

rods start to be exposed at the top, the exposed parts will reach the

critical temperature of 2200 °C after about 45 minutes. This is when the

first containment, the Zircaloy tube, would fail.

And this started to happen. The cooling could not be restored before there

was some (very limited, but still) damage to the casing of some of the

fuel. The nuclear material itself was still intact, but the surrounding

Zircaloy shell had started melting. What happened now is that some of the

byproducts of the uranium decay radioactive Cesium and Iodine started

to mix with the steam. The big problem, uranium, was still under control,

because the uranium oxide rods were good until 3000 °C. It is confirmed

that a very small amount of Cesium and Iodine was measured in the steam

that was released into the atmosphere.

It seems this was the go signal for a major plan B. The small amounts of

Cesium that were measured told the operators that the first containment on

one of the rods somewhere was about to give. The Plan A had been to

restore one of the regular cooling systems to the core. Why that failed is

unclear. One plausible explanation is that the tsunami also took away /

polluted all the clean water needed for the regular cooling systems.

The water used in the cooling system is very clean, demineralized (like

distilled) water. The reason to use pure water is the above mentioned

activation by the neutrons from the Uranium: Pure water does not get

activated much, so stays practically radioactive-free. Dirt or salt in the

water will absorb the neutrons quicker, becoming more radioactive. This

has no effect whatsoever on the core it does not care what it is cooled

by. But it makes life more difficult for the operators and mechanics when

they have to deal with activated (i.e. slightly radioactive) water.

But Plan A had failed cooling systems down or additional clean water

unavailable so Plan B came into effect. This is what it looks like

happened:

In order to prevent a core meltdown, the operators started to use sea

water to cool the core. I am not quite sure if they flooded our pressure

cooker with it (the second containment), or if they flooded the third

containment, immersing the pressure cooker. But that is not relevant for

us.

The point is that the nuclear fuel has now been cooled down. Because the

chain reaction has been stopped a long time ago, there is only very little

residual heat being produced now. The large amount of cooling water that

has been used is sufficient to take up that heat. Because it is a lot of

water, the core does not produce sufficient heat any more to produce any

significant pressure. Also, boric acid has been added to the seawater.

Boric acid is liquid control rod. Whatever decay is still going on, the

Boron will capture the neutrons and further speed up the cooling down of

the core.

The plant came close to a core meltdown. Here is the worst-case scenario

that was avoided: If the seawater could not have been used for treatment,

the operators would have continued to vent the water steam to avoid

pressure buildup. The third containment would then have been completely

sealed to allow the core meltdown to happen without releasing radioactive

material. After the meltdown, there would have been a waiting period for

the intermediate radioactive materials to decay inside the reactor, and

all radioactive particles to settle on a surface inside the containment.

The cooling system would have been restored eventually, and the molten

core cooled to a manageable temperature. The containment would have been

cleaned up on the inside. Then a messy job of removing the molten core

from the containment would have begun, packing the (now solid again) fuel

bit by bit into transportation containers to be shipped to processing

plants. Depending on the damage, the block of the plant would then either

be repaired or dismantled.

Now, where does that leave us?

The plant is safe now and will stay safe.

Japan is looking at an INES Level 4 Accident: Nuclear accident with local

consequences. That is bad for the company that owns the plant, but not for

anyone else.

Some radiation was released when the pressure vessel was vented. All

radioactive isotopes from the activated steam have gone (decayed). A very

small amount of Cesium was released, as well as Iodine. If you were

sitting on top of the plants chimney when they were venting, you should

probably give up smoking to return to your former life expectancy. The

Cesium and Iodine isotopes were carried out to the sea and will never be

seen again.

There was some limited damage to the first containment. That means that

some amounts of radioactive Cesium and Iodine will also be released into

the cooling water, but no Uranium or other nasty stuff (the Uranium oxide

does not dissolve in the water). There are facilities for treating the

cooling water inside the third containment. The radioactive Cesium and

Iodine will be removed there and eventually stored as radioactive waste in

terminal storage.

The seawater used as cooling water will be activated to some degree.

Because the control rods are fully inserted, the Uranium chain reaction is

not happening. That means the main nuclear reaction is not happening,

thus not contributing to the activation. The intermediate radioactive

materials (Cesium and Iodine) are also almost gone at this stage, because

the Uranium decay was stopped a long time ago. This further reduces the

activation. The bottom line is that there will be some low level of

activation of the seawater, which will also be removed by the treatment

facilities.

The seawater will then be replaced over time with the normal cooling water

The reactor core will then be dismantled and transported to a processing

facility, just like during a regular fuel change.

Fuel rods and the entire plant will be checked for potential damage. This

will take about 4-5 years.

The safety systems on all Japanese plants will be upgraded to withstand a

9.0 earthquake and tsunami (or worse)

I believe the most significant problem will be a prolonged power shortage.

About half of Japans nuclear reactors will probably have to be inspected,

reducing the nations power generating capacity by 15%. This will probably

be covered by running gas power plants that are usually only used for peak

loads to cover some of the base load as well. That will increase your

electricity bill, as well as lead to potential power shortages during peak

demand, in Japan.

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