Thoughts on Fukushima-Daiichi
In the mid-afternoon on Friday, March 11, 2011, the seismic sensors at the Fukushima-Daiichi nuclear power plant in the Fukushima Prefecture of Japan registered the earliest indications of the largest earthquake in modern Japanese history. They executed a preprogrammed response and began to drive all of the long control rods into the three reactors that were currently operating at the site. The control rods caused each generation of fission to produce fewer neutrons and fewer fission reactions. In three minutes the reactors were making 10% of their rated power from fission; in six minutes they were making 1%, and within ten minutes nuclear fission as a source of heat had ended in the first three units at Fukushima Daiichi. It would never begin again.
Each fission reaction splits the nucleus of an atom of uranium-235 or plutonium-239 into two smaller atoms and releases a great deal of energy. The energy release from nuclear fission is roughly a million times greater per unit weight than fossil fuels, which is why nuclear fission is such a compelling long term energy source. The two “fission products” that result are highly radioactive but decay towards stability very quickly. There are about 80 different sequences of decay that fission products can follow, and roughly a quarter reach a completely non-radioactive state within a day. Within a month, about three-quarters are stable, and within a year, about 80%. But in the first few hours after a nuclear reactor shuts down these fission products are producing significant amounts of heat and, unlike fission, this heat generation can’t be turned off. It has to run its course to completion. Therefore, managing what is called “decay heat” is one of the most important aspects of operating a nuclear reactor safely. To remove the heat, today’s reactors have an abundance of safety systems, all of which have the same mission—keep removing decay heat from the nuclear fuel. As the reactors at Fukushima-Daiichi cooled down, the tsunami hit.
The tsunami destroyed the diesel generators that provide power to drive the pumps that circulate the water coolant through the reactor that removes decay heat. Without an active removal of decay heat, the reactor was adding heat to the water faster than it was taking it out, and the temperature was rising. Because this was a reactor that operated on water that was already at its boiling point, this also meant that the pressure inside the reactor was rising as well.
The reactors at Fukushima-Daiichi are called boiling-water reactors (BWRs) and were manufactured by General Electric. They have a primary and a secondary containment structure, both made from thick reinforced concrete, to protect against the release of radioactive materials. Inside the primary containment are two vessels called a “drywell” and a “wetwell.” The drywell is a large steel pressure vessel that looks like a giant upside-down pear and holds the reactor and primary pumps, and the wetwell is a large toroidal vessel that looks like a donut. The wetwell is connected to the drywell by a number of wide pipes. Both the drywell and the wetwell are surrounded by a secondary containment vessel (or shield building) also built from reinforced concrete about a meter thick. This rectangular secondary containment building is the structure that most people have seen in pictures of the reactor. At the top of the secondary containment building is a steel frame structure with “blowout” panels that holds the crane used to remove solid nuclear fuel during fueling and refueling.
The designers of the reactors at Fukushima-Daiichi had anticipated situations where pressure was rising in the core. So long as power was available, pumps would circulate hot fluid from the reactor to the wetwell where it would be condensed. Heat removal could continue indefinitely in this way. But it all relied on a power source, and power had been lost due to the tsunami’s destruction of the diesel generators.
The water in the reactor is susceptible to damage from radiation, causing it to split into its components, hydrogen and oxygen. Normally, circulation would channel the hydrogen and oxygen to a recombiner where they would be restored back to water, but in the hours after the reactors were shut down, hydrogen was accumulating and separating in the wetwell and reached a point where it was vented into the sparse steel-frame structure at the top of the reactor building. It was only a matter of time before the hydrogen reached a level where it would detonate, and one after another, the first unit, then the third unit, and finally the second unit, suffered hydrogen explosions that blew off the steel panels and left the top of the reactor building exposed. The reactor vessels remained intact as did the reinforced concrete containment buildings, but each reactor building lost its hat due to the hydrogen explosions.
Initially there was hope of saving the reactors to generate power again after the crisis had passed. But as that hope faded and the need to remove the steadily-decreasing decay heat remained, operators at Fukushima-Daiichi took measures that would cool the reactors but would ruin them for future operation, such as the decision to try to cool the reactors with seawater. It will be necessary for some time to actively cool the reactors while the decay heat continues to decrease, but within a few months it will be possible to depressurize the reactors and assess their internal states. There may have been some melting and damage to the fuel—it is not known at this time.
What is known is that this is a situation very different than Chernobyl or Three Mile Island. There was no operator error involved at Fukushima-Daiichi, and each reactor was successfully shut down within moments of detecting the quake. The situation has evolved slowly but in a manner that was not anticipated by designers who had not assumed that electrical power to run emergency pumps would be unavailable for days after the shutdown. They built an impressive array of redundant pumps and power generating equipment to preclude against this problem. Unfortunately, the tsunami destroyed these systems.
There are some characteristics of a nuclear fission reactor that will be common to every nuclear fission reactor. They will always have to contend with decay heat. They will always have to produce heat at high temperatures to generate electricity. But reactors do not have to use coolant fluids like water that must operate at high pressures in order to achieve high temperatures. Other coolant fluids like fluoride salts can operate at high temperatures yet at the same low ambient pressures as the outside. Liquid fluoride salts are impervious to radiation damage, unlike water, and don’t evolve hydrogen gas which can lead to an explosion. Conventional solid nuclear fuel like that used at Fukushima-Daiichi can melt and release radioactive materials if not cooled consistently during shutdown and for a cool-down period thereafter. Liquid fluoride salts can carry fuel in a chemically-stable form that can be passively cooled without the need for pumps driven by emergency power generation. There are safe nuclear solutions even in extreme situations like those encountered at Fukushima-Daiichi, and it may be in our best interest to pursue them.
Good post.
I hope the event doesn't scare people off nuclear for good.
Thank you for this measured summary of current events. I am hopeful that this event will "scare" people away from existing designs and will cause them to consider the advantages of the LFTR design in a more serious context.
I hope so too, Phred.
Excellent article.
You should seriously consider adding social media sharing buttons (Facebook, Twitter, etc.) so people can more easily spread the word.
I am in favor of nuclear power… if all government subsidies for nuclear power are eliminated, and the liability cap is lifted. Let's see how much actually nuclear costs when you take away the welfare.
I might also be in favor of nuclear power if all government loan guarantees and subsidies for nuclear power were matched dollar-for-dollar with subsidies towards non-nuclear renewable power. It absolutely in our best interests to pursue new solutions. So let's see which ones are cost effective when they're all on a level field.
I am really saddened by the damage the media has done.
Have been in correspondence with my many Japanese friends explaining to them what is currently happening to calm their fears.
Thank you for your informative posts and fantastic site as always.
Tom – the plant having difficulties in Japan at the moment may be an "existing design" but it is certainly not a "current design". As such it should not be used as a basis for fearing "current designs". Having said that I agree that the LFTR is a promising design.
jb, the subsidies for "renewable" (unreliable, intermittent, diffuse) energy far outstrip nuclear on a electricity-generated basis. You're already getting what you want and far more.
"on a electricity-generated basis."
Yeah, no.
Subsidize the research, financing, construction, insurance and maintenance of renewable energy at exactly the same level as all aspectes of nuclear power. Let's do that for 5-10 years. Then let's compare.
If that's simply not feasible, then we should cut all the subsidies for nuclear. If it really is such a safe, cheap, viable energy option, it should do fine.
Good post, finally real graphic along with facts.
Fox News had one anchor call the containment building the reactor vessel.
A CNN anchor standing in front of graphics said
'you can burn coal or uranium to make steam' for the
turbines to run the generators.
More 'expert talking heads' on FOX and CNN who have no
idea what they are talking about. CNN had one of the top engineers who helped design the plant. He spoke well and knew what he was talking about.
Keep the Japan people in your thoughts.
We've been subsidizing wind and solar at ruinous levels since the 1970s and they've still never accounted for a pittance of global energy production. There's a good reason for that that will never change. They're diffuse and unreliable, which means they take too much capital to harness and require energy storage or simply can't be counted on. They destabilize electrical grids upon which their unreliable energy is forced by government dictate. And the windy/sunnie crowd demands endless subsidies forever to force consumers to buy their lousy product.
I spent years at NASA working with solar panels. I'm not unfamiliar with the technology. We used them in space because there was no other choice.
Excellent post. The hysteria that the media has injected into all of this is totally irresponsible and maddening. More broadly, the whole thing speaks to a sad lack of quality in education systems.
Well done and keep the objective sense coming.
Well written article, nice and appropriate plug for LFTR.
I agree with Dave – “You should seriously consider adding social media sharing buttons (Facebook, Twitter, etc.) so people can more easily spread the word.”
I know you have a demanding day job, but in the light of this incident, a more detailed article on what would have happened to a LFTR plant under these circumstances would be invaluable.
From what I understand, LFTR fluid expands as it is heated, automatically slowing the reaction. Does this mean LFTR reactions do not need power to run/cool and would be impervious to this sort of accident?
I agree with atomicrabbit on both of his exhortations. Winning this battle for the truth is a matter of life and death for millions of people as well as sustaining the freedom and prosperity of our country. Putting an end to oil imports and showing the rest of the world how to do the same, ends, the jihad waged against us, without firing a shot, as the funding for that effort is from petrodollars. ww2 ended by the splitting of the atom, and the current war, as well, would end by the splitting of the atom. this time, though, the atom split will light homes and fuel vehicles and power industy, all over the globe, for rich and poor.
Good to see an informative discussion of alternative reactor designs and technologies.
But disagree on "this is a situation very different than Chernobyl or Three Mile Island. There was no operator error involved at Fukushima-Daiichi". Whether it is operator error or engineering error is irrelevant at the end of the day. Putting a nuclear power station behind a tiny breakwall on a tsunami-prove shore is dumb. Putting the diesels cheek by jowl with the reactors whose cooling they are supposed to support is dumb, especially when they can be taken down en masse.
I am eagerly waiting for more details to come out about what happened during the first hours. Supposedly there were batteries, providing enough power to the pumps for at least 4-6 hours. That should have been enough time to get new generators flown in by helicopter. Couldn't they have called the military "get some generators in here, now !" At that point, the reactors still could have been saved.
Ron Jenkins, the fact that it wasn't operator error meant that the decay heating rate had moved much farther down the curve when disaster struck. The heat release rate within the first few hours after shutdown is much higher than any time later. That probably made the most difference in terms of whether the fuel would sustain damage or not.
Jordan, a LFTR has a totally passive approach to decay heat removal that involves using a freeze plug and a drain tank. The mechanism you mentioned concerns how the LFTR controls itself during operation. Fukushima-Daiichi was successfully shut down. The problem came about later, when power was lost to pump the water that would remove decay heat.
If anyone has a tip for how to add those "social media" buttons on a WordPress blog, I would appreciate it and try to implement it.
There were several human errors that cost Japan the nuclear power plants and have done major damage to nuclear power for the rest of us.
So far as I understand the batteries are sufficient to operate the valves and safety equipment but not the pumps. I presume the water onsite is sufficient to coast for many hours without cooling.
1) They did bring in replacement generators (by truck I believe) but there was a problem with incompatible plugs! This delay exposed the fuel.
Why they weren't flown in and why the plugs weren't checked before hand I do not know.
2) Ran out of fuel. 2.5 days after the quake they ran out of fuel to run the pumps and again the fuel was exposed.
3) They did not remove the steel panels on the top of the building to avoid the hydrogen explosions – even after the first two. The third explosion took out 4 of 5 fire trucks pumping water and again the fuel was exposed.
While not operator errors they are human errors.
I wonder if you might help me with one element of understanding the sequence of events that unfolded at Fukushima-Daiichi Unit-1. It is my understanding that one of the cooling backup systems included in this reactor is a battery system capable of powering the coolant pumps for eight hours. It is my understanding that the battery functioned after the earthquake and tsunami and drove the pumps for the specified interval which covered the 8 hour period when the decay heat generation was most intense. I understand that during the time the battery drove the coolant pumps additional backup diesel generators were trucked in to relieve the battery and from that point supply the power needed to keep cooling water flowing through the core.
Why was it not possible during the 8 hour interval when the battery functioned to startup and connect the additional diesel generators that had been trucked in before the 8 hour battery was exhausted and thereby keep the coolant pumps continuously working while keeping cooling water flowing past the core to save the reactor?
If the additional trucked in generators were not functional, why were not additional batteries flown in to provide extended operation of the coolant pumps from batteries until some functioning diesel generators could be brought in?
What key element in saving the reactor(s) did I miss?
Kirk,
In a LFTR, when the fuel goes into the drain tank, what happens to the decay heat there? You would have to cool it, or it could also reach boiling point (much higher than water I understand). Does a LFTR have as large an inventory of decaying products as a uranium fuelled reactor?
Also, lets remember the prime cause of the accident – a magnitude 9 earthquake. A LFTR reactor would fare no better in this circumstance. I don't even know if you can design anything to withstand that and the tsunami. You can only be a bit more clever about where you site your plant.
Finally, lets hear it for the real heros here – the operators of the plant who are risking their lives to bring this situation under control.
Chris, yes, the boiling temperature of the salts is about 1600 C versus 100 C for water. Since the effectiveness of heat transfer is proportional to the temperature difference, the salt will never even get close to this temperature.
Kirk
Heat transfer to what? Is the heat to be rejected the same as for a solid fuel reactor? I don't think it is a problem because Oak Ridge used to do this regularly to shut down the reactor. I guess you can pre-charge a lot of solid salt into the drain tank to absorb the heat by melting until you can get some cooling on, and you can add additional salt into the tank to a limit.