Monday, March 14, 2011

Japanese Reactors To Date

Simplifications and Oversimplifications

A friend of my daughter's wrote me an email: Can you write about those Japanese reactors in a way that ordinary people can understand? I decided to try, despite the fact that there are many excellent resources out there. I particularly recommend this post at the Energy Collective.

I am oversimplifying, and I don't have all the facts I would like to have. But waiting for all the facts would take a long time.

Starting with the Reactor

A boiling water reactor is a bunch of fuel rods (uranium oxide pellets with a zirconium coating to hold them together) and water circulating around them. The pellets get hot because a fission reaction liberates heat. The hot pellets heat the water, and the water boils and becomes steam. The steam turns a turbine and makes electric power.

Now, let's assume something goes wrong. Say, an earthquake. The sensors in the reactor feel the earthquake, and they shut down the reactor by putting control rods into it. These rods grab up neutrons and stop the fission reaction.

However, the uranium pellets are still hot, and so you have to keep water circulating to remove the heat. The circulating water is driven by pumps, the pumps are driven by electricity, but where does the electricity come from if the reactor shuts down? In general, the electricity comes from the grid when a nuclear plant stops making power.

However, in an earthquake or other emergency, the grid may well be unavailable. Therefore, nuclear plants have diesel generators, which can drive the pumps for a long time, until the grid goes back on-line, or it is safe to restart the plant again.

And now, we switch to talking about the Japanese reactors.

The Earthquake and the First Consequences

When the Japanese nuclear plants sensed the earthquake, they shut down quickly and simply, as they were designed to do. Within seconds, the diesels at all the plants started, and cooling water began to circulate. (The earthquake was about five times as strong as the design-basis earthquake, and everything worked very well. This was a triumph of engineering.) An hour went by, with the diesels working, the plants fuel rods cooling and everything fine.

Then the tsunami came. It knocked out the diesels at the three plants closest to the water, and they could not be started again. The Japanese also had back-up batteries, but they were not strong enough to keep the correct level of pumping going for very long.

Questions arise. Why couldn't they get more diesels? Why were the diesels placed where they were so vulnerable to flooding? I don't know. On the other hand, it is easy to ask questions on a nice spring day in America. It is much harder to work hard in the middle of a crisis.

The Second Consequences

Once the diesels and therefore the cooling had failed, it became very hard for the engineers to keep up with the situation. There were two problems:
  • Rising temperatures led to rising pressures of water vapor, and it is hard to pump more water IN to a situation of high pressure
  • When the zirconium fuel rods overheat, they react with water and liberate hydrogen, which is explosive.
I am not going to follow a blow by blow account of everything that happened at the plants. Basically, the engineers attempted to release pressure by releasing water vapor into the outside air. Then they added more water to make up for the water that evaporated in the bleeding-off process. There were two issues with this procedure;
  • Since the water around the fuel rods was slightly radioactive, the engineers wanted to vent as little of it as possible
  • Since the zirconium was overheating, hydrogen was being liberated and could explode.
At this point, the story gets more complicated, with hourly updates on the water levels, and hydrogen explosions that wreck parts of the reactor building, but don't interfere (as far as we can tell) with the main business of adding water and venting water vapor at the reactor. As far as I can tell, The graphic above shows the explosions taking place above the containment area.

A set of good graphics at the Capacity Factor blog show the that the explosion blew out panels, above the main containment area. The concrete area begins below the area that blew out.

Third Consequences: Borated Seawater

The next set of announcements were about adding borated seawater to the reactor vessels. Boron stops neutrons, and would probably aid the cooling of the fuel. This choice probably means that the reactor will be ruined. However, it is not a strange or unforeseen measure. Nuclear power plants have defense in depth and protocols for everything. Margaret Harding led nuclear safety assessment groups at GE for many years. I asked her about borated water addition and seawater additions. Her response:

Flooding BWR Reactor Pressure Vessel with borated water is a STANDARD part of the BWR design. We don't use it unless we have to because it does odd things in a reactor that bubbles, but it is certainly not "untested". I've explained that to several reporters.

Also, explain that some plants have seawater injection as a STANDARD emergency back-up option - it isn't used because we don't want that mess in the reactor unless we have to have it. Just like boron.

As far as I can tell, this set of techniques worked for reactors 1 and 3. They are still adding water and bleeding steam, but the temperatures are dropping within the reactor vessels and the chances of overheating are fading.

The hydrogen explosion at unit 3 was terrible, injuring 11 people. But those two reactors appear to be under control. Assuming all continues to go even moderately well, a few weeks from now the reactors will be ruined, but nothing much worse will have happened.

The situation is different at reactor 2.

Fourth Consequence: Big Problems at Unit 2

In the cases of units 1 and 3, the water flooding and bleeding have led to bringing the units close to safe shutdown. In these cases, the containment was not breached and the full rods spent most of their time covered in water. In unit 2, however, the situation is different. The fuel rods have been uncovered for long periods of time because a stuck valve defeated the feeding and bleeding operation. A hydrogen explosion may have damaged the integrity of the containment and/or the torus, which is that large donut-shaped object, filled with water, at the bottom of the drawing at the top of this blog.

Latest Developments (some updates)

There have been significant releases of radioactivity from unit 2. Bulletins about the plant are coming in practically by the minute, and some are contradictory. For example, now I have learned that a fire at unit 4 came BEFORE the explosion at unit 2. Reactor 4 had been shut down before the quake. The fire was at the fuel pool, and is apparently put out. I do not know if the radioactivity release is primarily from unit 2 or from the fire. You can watch English translations of the main Japanese news sessions at the the NHK World site. You will then know as much as I do about the latest developments.


It has been a confusing day, with fires starting and stopping. I sometimes think I should just stop writing about Japan at all for a while--it has been too hard to follow. Cheryl Rolfer has already made that decision: her post Cooling Off or Melting Down is worth a read.

To understand today's events, though, Barry Brook's summary at Brave New Climate seems the best way to update this blog post.

Wednesday Morning Photo Update

This morning, commenter Martin Langeveld sent links to some excellent pictures of the plant. Thank you Martin! I don't want the links to be buried in the comments, a section people may or may not read.

For a recent photo of the plants (you have to scroll to see them all) see this picture. For a set of pictures, go to this site.


rolf.parkerhoughton said...

Hello there, Meredith, this is Rolf writing. I have not posted anything for awhile.

Unfortunately the faultiness of this design of the boiling water reactor, such as at Fukushima and at Vernon's Vermont Yankee has been known for quite some time. Here is a quote from an article in The Telegraph, dated March 14, and written by By Nick Allen and Martin Evans.

"Prof Walt Patterson, a nuclear energy expert at
Chatham House, told Channel Four News that the
problems at the plants had been "foreseen for many
years". He said: "The design of the reactor is such
that it is inherently susceptible to the kind of
problems happening now."

Below are excerpts that laid out these concerns prior to this disaster.


"..Basic questions about the the GE
containment design remain unanswered and its
integrity in serious doubt."

"..As early as 1972, Dr. Stephen Hanuaer, an
Atomic Energy Commission safety official,
recommended that the pressure suppression system
be discontinued and any further designs not be
accepted for construction permits. Shortly thereafter,
three General Electric nuclear engineers publicly
resigned their prestigious positions citing dangerous
shortcomings in the GE design."

"..In 1986, Harold Denton, then the NRC's top safety
official, told an industry trade group that the "Mark I
containment, especially being smaller with lower
design pressure, in spite of the suppression pool, if
you look at the WASH 1400 safety study, you'll find
something like a 90% probability of that containment
failing." In order to protect the Mark I containment
from a total rupture it was determined necessary to
vent any high pressure buildup. As a result, an
industry workgroup designed and installed the "direct
torus vent system" at all Mark I reactors. Operated
from the control room, the vent is a reinforced pipe
installed in the torus and designed to release
radioactive high pressure steam generated in a
severe accident by allowing the unfiltered release
directly to the atmosphere through the 300 foot vent
stack. Reactor operators now have the option by
direct action to expose the public and the
environment to unknown amounts of harmful
radiation in order to "save containment." As a result
of GE's design deficiency, the original idea for a
passive containment system has been dangerously
compromised ..."

The above was published by The Nuclear Information and Resource Center.

Martin Langeveld said...

Agreed that the news continues to be murky and contradictory (see my comment the other day that one lesson for US plants is to get their communication plans in order).

The fire at No. 4 appears to have occurred in the spent fuel pool. This could only happen if cooling of that pool failed for some reason. See my comment yesterday about spent fuel pools on your previous post. Dealing with the spent fuel pools at No.s 1, 2 and 3 is probably going to turn out to be the biggest problem — assuming the primary containment does its job regardless of what happens in the core. So another possible lesson for US plants is that putting spent fuel so snugly next to the reactor may not be a great idea.

The explosion at No. 2 seems to be of a different nature from the hydrogen explosions at 1 and 3.

One correction to your post: the design earthquake level at the plant was 8.2. The actual quake was 8.9 or 9.0 — on the log scale, that's at least 5 times larger, not twice.

rolf.parkerhoughton said...

The situation actually is less murky in one respect.

"The crisis appeared to escalate late in the day when the operators of the facility said that one of two blasts had blown a hole in the building housing a reactor, which meant spent nuclear fuel was exposed to the atmosphere."

The cause of this breach does not appear to be the earthquake itself nor the Tsunami. Those are indirect causes. The more direct cause was the loss of cooling power combined with a fundamentally flawed reactor design.

With all due respect Meredith, the Fukushima type design is inherently flawed, and it is time to consider renaming your website, NO Vermont Yankee. I realize that is asking a lot, but really, what other calm rational reaction is there at this point than to face the failure of the design itself, and acknowledge that promoting the safety of this plant, is at this point, irresponsible.

Perhaps if we had not experienced as a state destruction caused by the Hurricane of 1938 it would still be tenable.

But it we did, and it does not tenable any longer.

Meredith Angwin said...


I have made the correction about the size of the earthquake. Thank you.

The Japanese communications on the fire have been contradictory. This is the latest I have, but it is not necessarily correct
RT @nei_media: confirmation from TEPCO that unit 4 fire was *not* in spent fuel pool but rather in corner of reactor building's 4th floor

I plan to do "lessons learned" posts only after I have "facts straight."


Thank you for your comments. You and Mr. Gundersen are on the same page, doing your best to equate a major tsunami and earthquake with a hurricane working its way inland, causing flooding and high winds. No. Flooding and tsunamis are different.

For example, with floods you get warnings.

You can read about the plant design, Gundersen's commentaries, and my commentaries in the Valley News link below. Gundersen avoids all those nasty little numbers that would get in the way of his stories of what "could" happen. As one man commented to me after the Angwin/Gundersen debate: "the guy doesn't talk like an engineer." Too true.

I am not renaming my blog. Just in case you were wondering.

Martin Langeveld said...

Best current picture of the plant take Wed. a.m.:
(shows entire complex - scroll to reactor area if necessary - no. 1 is at right)

For more photos go to Digital Imagery's main Flickr page: