Good News On Radiation from Japan, from a research scientist at MIT

[Guest guest]

You can read the beginning and the

list of points at the end to get the idea.

Theta

This post is by Dr f Oehmen, a

research scientist at MIT, in Boston.

He is a PhD Scientist, whose father

has extensive experience in Germany’s nuclear industry. I asked him to write

this information to my family in Australia, who were being made sick with worry

by the media reports coming from Japan. I am republishing it with his

permission.

It is a few hours old, so if any

information is out of date, blame me for the delay in getting it

published.

This is his text in full and

unedited. It is very long, so get comfy.

I am writing this

text (Mar 12) to give you some peace of mind regarding some of the troubles in

Japan, that is the safety of Japan’s nuclear reactors. Up front, the situation

is serious, but under control. And this text is long! But you will know more

about nuclear power plants after reading it than all journalists on this planet

put together.

There was and will

*not* be any significant release of radioactivity.

By “significant†I

mean a level of radiation of more than what you would receive on – say – a

long

distance flight, or drinking a glass of beer that comes from certain areas with

high levels of natural background radiation.

I have been reading

every news release on the incident since the earthquake. There has not been one

single (!) report that was accurate and free of errors (and part of that problem

is also a weakness in the Japanese crisis communication). By “not free of

errors†I do not refer to tendentious anti-nuclear journalism – that is

quite

normal these days. By “not free of errors†I mean blatant errors regarding

physics and natural law, as well as gross misinterpretation of facts, due to an

obvious lack of fundamental and basic understanding of the way nuclear reactors

are build and operated. I have read a 3 page report on CNN where every single

paragraph contained an error.

We will have to

cover some fundamentals, before we get into what is going

on.

Construction of the Fukushima nuclear

power plants

The plants at

Fukushima are so called Boiling Water Reactors, or BWR for short. Boiling Water

Reactors are similar to a pressure cooker. The nuclear fuel heats water, the

water boils and creates steam, the steam then drives turbines that create the

electricity, and the steam is then cooled and condensed back to water, and the

water send back to be heated by the nuclear fuel. The pressure cooker operates

at about 250 °C.

The nuclear fuel is

uranium oxide. Uranium oxide is a ceramic with a very high melting point of

about 3000 °C. The fuel is manufactured in pellets (think little cylinders the

size of Lego bricks). Those pieces are then put into a long tube made of

Zircaloy with a melting point of 2200 °C, and sealed tight. The assembly is

called a fuel rod. These fuel rods are then put together to form larger

packages, and a number of these packages are then put into the reactor. All

these packages together are referred to as “the coreâ€.

The Zircaloy casing

is the first containment. It separates the radioactive fuel from the rest of the

world.

The core is then

placed in the “pressure vesselsâ€. That is the pressure cooker we talked

about

before. The pressure vessels is the second containment. This is one sturdy piece

of a pot, designed to safely contain the core for temperatures several hundred

°C. That covers the scenarios where cooling can be restored at some

point.

The entire

“hardware†of the nuclear reactor – the pressure vessel and all pipes,

pumps,

coolant (water) reserves, are then encased in the third containment. The third

containment is a hermetically (air tight) sealed, very thick bubble of the

strongest steel and concrete. The third containment is designed, built and

tested for one single purpose: To contain, indefinitely, a complete core

meltdown. For that purpose, a large and thick concrete basin is cast under the

pressure vessel (the second containment), all inside the third containment. This

is the so-called “core catcherâ€. If the core melts and the pressure vessel

bursts (and eventually melts), it will catch the molten fuel and everything

else. It is typically built in such a way that the nuclear fuel will be spread

out, so it can cool down.

This third

containment is then surrounded by the reactor building. The reactor building is

an outer shell that is supposed to keep the weather out, but nothing in. (this

is the part that was damaged in the explosion, but more to that

later).

Fundamentals of nuclear

reactions

The uranium fuel

generates heat by nuclear fission. Big uranium atoms are split into smaller

atoms. That generates heat plus neutrons (one of the particles that forms an

atom). When the neutron hits another uranium atom, that splits, generating more

neutrons and so on. That is called the nuclear chain

reaction.

Now, just packing a

lot of fuel rods next to each other would quickly lead to overheating and after

about 45 minutes to a melting of the fuel rods. It is worth mentioning at this

point that the nuclear fuel in a reactor can *never* cause a nuclear explosion

the type of a nuclear bomb. Building a nuclear bomb is actually quite difficult

(ask Iran). In Chernobyl, the explosion was caused by excessive pressure

buildup, hydrogen explosion and rupture of all containments, propelling molten

core material into the environment (a “dirty bombâ€). Why that did not and

will

not happen in Japan, further below.

In order to control

the nuclear chain reaction, the reactor operators use so-called “control

rodsâ€.

The control rods absorb the neutrons and kill the chain reaction

instantaneously. A nuclear reactor is built in such a way, that when operating

normally, you take out all the control rods. The coolant water then takes away

the heat (and converts it into steam and electricity) at the same rate as the

core produces it. And you have a lot of leeway around the standard operating

point of 250°C.

The challenge is

that after inserting the rods and stopping the chain reaction, the core still

keeps producing heat. The uranium “stopped†the chain reaction. But a number

of

intermediate radioactive elements are created by the uranium during its fission

process, most notably Cesium and Iodine isotopes, i.e. radioactive versions of

these elements that will eventually split up into smaller atoms and not be

radioactive anymore. Those elements keep decaying and producing heat. Because

they are not regenerated any longer from the uranium (the uranium stopped

decaying after the control rods were put in), they get less and less, and so the

core cools down over a matter of days, until those intermediate radioactive

elements are used up.

This residual heat

is causing the headaches right now.

So the first “typeâ€

of radioactive material is the uranium in the fuel rods, plus the intermediate

radioactive elements that the uranium splits into, also inside the fuel rod

(Cesium and Iodine).

There is a second

type of radioactive material created, outside the fuel rods. The big main

difference up front: Those radioactive materials have a very short half-life,

that means that they decay very fast and split into non-radioactive materials.

By fast I mean seconds. So if these radioactive materials are released into the

environment, yes, radioactivity was released, but no, it is not dangerous, at

all. Why? By the time you spelled “R-A-D-I-O-N-U-C-L-I-D-Eâ€, they will be

harmless, because they will have split up into non radioactive elements. Those

radioactive elements are N-16, the radioactive isotope (or version) of nitrogen

(air). The others are noble gases such as Argon. But where do they come from?

When the uranium splits, it generates a neutron (see above). Most of these

neutrons will hit other uranium atoms and keep the nuclear chain reaction going.

But some will leave the fuel rod and hit the water molecules, or the air that is

in the water. Then, a non-radioactive element can “capture†the neutron. It

becomes radioactive. As described above, it will quickly (seconds) get rid again

of the neutron to return to its former beautiful self.

This second “typeâ€

of radiation is very important when we talk about the radioactivity being

released into the environment later on.

What happened at

Fukushima

I will try to

summarize the main facts. The earthquake that hit Japan was 5 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 5 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

Depthâ€.

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 eventâ€. It is again a step along the “Depth of Defenseâ€

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 defenseâ€), 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 rodâ€. 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? My assessment:

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)

(Updated) I believe

the most significant problem will be a prolonged power shortage. 11 of Japan’s

55 nuclear reactors in different plants were shut down and will have to be

inspected, directly reducing the nation’s nuclear power generating capacity by

20%, with nuclear power accounting for about 30% of the national total power

generation capacity. I have not looked into possible consequences for other

nuclear plants not directly affected. 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.  I am not familiar with Japan’s energy supply chain for

oil,

gas and coal, and what damage the harbors, refinery, storage and transportation

networks have suffered, as well as damage to the national distribution grid. All

of that will increase your electricity bill, as well as lead to power shortages

during peak demand and reconstruction efforts, in Japan.

This all is only

part of a much bigger picture. Emergency response has to deal with shelter,

drinking water, food and medical care, transportation and communication

infrastructure, as well as electricity supply. In a world of lean supply chains,

we are looking at some major challenges in all of these areas.

If you want to stay

informed, please forget the usual media outlets and consult the following

websites:

http://www.world-nuclear-news.org/RS_Battle_to_stabilise_earthquake_reactors_120\

3111.html

http://www.world-nuclear-news.org/RS_Venting_at_Fukushima_Daiichi_3_1303111.html

http://bravenewclimate.com/2011/03/12/japan-nuclear-earthquake/

http://ansnuclearcafe.org/2011/03/11/media-updates-on-nuclear-power-stations-in-\

japan/