Tag Archives: meltdown

A look at recriticality during meltdown, part 1

The issue of recriticality in the damaged reactors at Fukushima pops up every now and then (a few examples link1, link2, link3, link4). Perhaps it is worth taking a look at what recriticality means, how likely it is and what it would mean if the cores goe critical. These posts will contain some maths and give some insight into basic reactor physics. Despite what most people think it is actually quite easy as long as one can follow the solution of some simple differential equations.

We will look at two different cases, in the first case the core has melted completely and is as a molten puddle or bed of "gravel" at the bottom of the vessel. In the second case the fuel rods are still mostly geometrically intact while the control rods have melted. If I have energy I might throw in a section about criticality in spent fuel pools as well at the end. We start with the completely molten core because it is easier and highlights all the relevant physics.

What exactly is criticality?

Fission is a reaction whereby a incoming neutron hits a nucleus, the nucleus then has a certain probability (depending on the energy of the neutron, what nucleus it is etc) of splitting into two roughly equally large pieces and in the process emit 2-3 new neutrons. Those neutrons can in turn hit new nuclei that causes more fissioning and voila, we have a chain reaction. If we assume we have a system where nothing is happening and we send in a burst of neutrons, those neutrons, that we will call the first generation, will cause an initial amount of fission reactions that produce a second generation of neutrons which goes on to create a third generation etc. Criticality is simply defined as the ratio between a subsequent generation with the one preceding it, it is usually designated by the letter K. Continue reading A look at recriticality during meltdown, part 1

Examining some old Fukushima news

An old news article is circulating around that states that 68 tons of fuel has melted in reactor number 1 and that it was close to breaching the bottom of the containment. The article is several months old but for some reason I have seen it pop up again on facebook so I though it is worth examining the article briefly. In particular I want to examine this statement.

Only 37 centimeters of concrete remains between the fuel and the vessel's outermost steel wall in the most damaged area, TEPCO said.

This wording is repeatedly used by anti nuclear sources to imply that a much worse disaster was very close to happening. What the articles fail to mention however is that there is A LOT more concrete between the ground and the molten core. The reactor building itself is a very thick concrete structure. Will Davis, on his excellent blog Atomic Power Review, talked about this the first time the news about the number one vessel failure showed up last November. Some of what he wrote is worth repeating and I hope he doesn't mind me repeating it here and also posting a picture from his blog.

The NHK report indicates a melt depth of about 2.1 feet(64 cm, my note /Johan). The distance to the ground is roughly eighteen times this depth from the dry well interior floor to grade. Below is a drawing from WASH-1082 which I've marked to show the distance from the dry well floor to the grade outside, which on the particular plant shown is 39' 0"(11.8 meters, my note /Johan). I do not presently know the exact measurement at Fukushima Daiichi No. 1 but it is likely within ten percent of this measurement... meaning that in the worst case that TEPCO is describing, by its own data, the core material may have melted only about as much as 5% of the distance to the grade.

I encourage everyone to read the rest of his blog as it is by far the best information source for the Fukushima accident.


I also want to add this picture of the mark I containment that schematically shows the thick concrete even more clearly! Picture found at the blog "The capacity factor".

So we see that there is a tremendous amount of concrete below the shell of the containment structure. The hints and suggestions that the core would only have to melt another 37 centimeters for a unnamed disaster to take place is obviously false. In reality the shell of the containment is integrated into a thick concrete structure and the molten core would have to melt through several more meters, likely around 10 meters, to get out of the reactor building itself.

The cleanup of the containment is going to be a very hard and messy job, much worse than the cleanup of TMI was. But the core is still a long long way from the ground.




Day eighteen after the tsunami

Update 19:00(CET)/17:00(UTC)/02:00(JST)

New NISA and JAIF updates. NISA from 06:00 and JAIF from 20:00. Same as usual JAIF number first and NISA after within ().


Reactor 1:
Water level in the core: 1.60 (1.60) meters below the top of fuel assemblies
Flow rate of injected water: 141 liter/minute
Core pressure: 603(603) kPa
Containment pressure: 285(280) kPa
Core temperature(feedwater nozzle):323.3 Celsius
Core temperature(bottom head): 139.4 Celsius
Dose rate within containment: 36 Sv/hour

Reactor 2:
Water level in the core: 1.5 (1.5)   meters below the top of fuel assemblies
Flow rate of injected water: 117 liter/minute
Core pressure: unknown
Containment pressure: 100 (110) kPa
Core temperature(feedwater nozzle): 153.7 Celsius
Core temperature(bottom head): 77.7 Celsius
Dose rate within containment:  40.4 Sv/hour
Spent fuel pool temperature:  45 Celsius

Reactor 3:
Water level in the core: 2.3 (2.3)  meters below the top of fuel assemblies
Flow rate of injected water: 200 liters/minute
Core pressure: 135 (135)  kPa
Containment pressure: 108.5 (108.5) kPa
Core temperature(feedwater nozzle): 61.5 Celsius (obviously error)
Core temperature(bottom head): 120.9 Celsius
Dose rate within containment:  29.2 Sv/hour

Number one is quite hot, otherwise no major change. Running a bit short on time so Kyodo has a good summary of the situation and IAEA.

Update 08:30(CET)/06:30(UTC)/15:30(JST)

JAIF and NISA updates. NISA hasn't released any new information on flow rates, temperatures or containment dose rates. There is no big change in the pressures or water levels of the reactors so I will omit the normal list of parameters I write down. If NISA release temperatures I will include it.

The Asahi Shimbun reports that TEPCO admits there is possible damage to the lower parts of the reactor pressure vessels. TEPCO states that the breaches can not be very large since the pressure in pressure vessels are maintained at a higher level than in the containment structure.

There seems to be a problem again in keeping the number 1 reactor cool. The temperature of the core increased to over 300 degrees celsius again and TEPCO had to increase the flow of water into the core.

Links (English)
Reuters Japan finds plutonium at stricken plant
NEI nuclear notes We should stop running away from radiation
Union of Concerned Scientists Where did all the water in the spent fuel pools go

SvD Plutonium sprids i Fukushima
DN Plutoniumfynd vid Fukushima
Aftonbladet Plutonium i marken vid Fukushima
Expressen TEPCO chefen kan ha flytt från Japan

Röda Malmö
Röda berget
Uppkäftiga Uppsala


Day seventeen after the tsunami

Update 23:20(CET)/21:20(UTC)/06:20(JST)

NISA has not released any new update today, JAIF has released one more update since this morning. I won't paste updated tables this time, but they can be seen in the link provided. There is no major changes to any reactor. Both number 2 and number 3 reactors are now getting their freshwater pumped with temporary electric pumps that have replaced the fire pumps used up untill now. In their latest written report they say that they have found highly contaminated water in a tunnel with pipes and cables connected to the turbine hall, the activity level is similar to the water found in the turbine hall basement and they are investigating the source of the water. They continue to se high levels of, among other things, I-131 in the seawater and suspect it is due to the contamination in the tunnel.

Kyodo reports that plutonium has been found in the soil outside the reactors.  The source of the plutonium is unknown and the level is comparable to what was found in japan after the nuclear tests done by Russia and the US during the cold war. TEPCO has provided a report that gives the concentration as about 1 Bq/kg of soil of Pu-239 and less than 0.5 Bq/kg of soil of Pu-238. The levels are very low and one could practically eat tons of soil before ingesting getting a dangerous amount. There is 0.44 billionths of a gram of Pu-239 per kg of soil*, a lethal dose of plutonium-239 is half a gram. So one would have to eat more than one million metric tons of soil to get a lethal dose of the plutonium contained within the soil. That is how low the amount is (that also gives a good example how exceptionally low concentrations of radioactive materials can be detected).

NHK reports that the 3 exposed workers have been sent home and that they don't show any symptoms of radiation sickness.

It seems like the number 2 reactor is leaking quite significant amounts of radioactive water. It's not clear if it is coming through a breach in the containment or if it is through some of the pipes. If for instance the valves, closing the reactor from the pipes that are coming in through the containment, is leaking. The highly radioactive water is a big problem since it prevents access to the turbine halls and delays work to restore the reactors internal coolant pumps. They have found similar build ups of radioactive water in the number 1 and 3 reactors turbine halls, but not as strongly radioactive. Since it is a problem in all three reactors to some extent my bet would be that it is the valves that are leaking.


Update 09:30(CET)/07:30(UTC)/06:30(JST)

JAIF has released their update but not NISA. I will wait to write down the status of the reactors until the NISA update arrives and for now I just paste JAIF's tabels.

Kyodo reports that TEPCO measured the radionuclide content of the water in the turbine halls wrongly, they overestimated it by a factor of a hundred. Confirming our suspicion that they are measuring wrong. Kyodo also reports that the Nuclear Safety Comission states that there has been a partial meltdown in the number 2 reactor. I am a bit surprised by this statement because it has been quite clear since the first few days that there has been a partial meltdown.

The workers that where exposed to radiation while working in the turbine hall basement have left the hospital and are reportedly in perfect health.

According to TEPCO's latest update they have switched the cooling of the number 2 reactor from the fire pumps to temporary electric pumps.

Reuters report that the levels of radioactive material in the sea outside the plant is dropping.


Kyodo FOCUS: Little sense of crisis over tsunami in Japan's nuclear power industry
Reuters Japan raps nuclear operator over radiation mistake

DN larm om hög strålning var fel
DN samtidigt ökar oron för Fukushima
Aftonbladet strålningslarmet var ett misstag
Expressen Härdsmälta i kraftverket

Thirty years after Harrisburg, time to let go.

Thirty years ago to the day of this article, the so far worst nuclear accident in a power plant the world had ever seen took place. Unit number 2 at the Three Mile Island Nuclear Generating Station near Harrisburg, Pennsylvania, suffered a loss of coolant accident. This led to that most infamous of nuclear failure modes: a core meltdown.

But despite that "everyone knows" a meltdown supposedly is the worst that could ever happen, with millions of dead and entire states rendered uninhabitable forever and ever, the effects of the TMI-2 accident are well documented with no deaths, no injuries, no cancers. The only casualty that came from accident was said by nuclear physicist Edward Teller to be his heart attack, caused by the stress of seeing Jane Fonda using the event to unjustly trashtalk nuclear power. With this in mind, maybe it's time we had a little reality check when it comes to our nuclear fears, wouldn't you say?

Don't get us wrong, a nuclear meltdown still is no laughing matter. Having a vital energy producing unit that is supplying hundreds of thousands of citizens with electricity unexpectedly becoming permanently disabled is of course not good. But there is a huge different between "not good", and "the end of normal life as we know it".

Deriders of nuclear energy try to abuse the event by saying "They said it couldn't happen, and yet it did". This is simply not true. Noone ever said a nuclear accident cannot happen. The proof of this is in the accident itself, or rather its non-existing harmful effects. How can such a serious nuclear meltdown not harm anyone? The answer is simple: because we expected it might happen and prepared for it.

The promise that was made was not that an accident wouldn't happen, but that nuclear power would not harm anyone in the public. This promise has been kept for 55 years all throughout the world in all places except one, Chernobyl, for reasons obvious: the Soviet Union did everything wrong in ways that would have been considered appalling and shocking to the entire world, even before the accident, had we but known about them. Everywhere else, nuclear power has not harmed a single individual in the general public by cause of radioactive release. And in the thirty years that has passed since the accident, we have only become better at enforcing this promise.

It is definitely time to let go of the past and Harrisburg. The lessons have been learned. We are moving on towards creating a sustainable future for ourselves and the next generations where all forms of clean energy has their given place in the energy mix. With each coal plant we exchange for a nuclear fission reactor, we save approximately 15 000 human lives over the course of the reactor's lifetime.  Nuclear power has never been safer and cleaner that it is today. Of course we shall stop being afraid of using it, instead having a healthy amount of respect for it,  especially if the only reason we have for worrying is a thirty year old accident that didn't harm anyone.

The Forsmark incident was not Chernobyl

This is the second blog response to a blog entry made by The King of the country Lagom. The previous entry dealt with his claims that opinions are sacred and how one must not speak up against them. This entry will deal with the purely factual errors of his claims about nuclear power.

The King of Lagom claims that an incident that took place in 2006 at the Forsmark nuclear power plant could have escalated into a Chernobyl-type accident.  Well... first he says that, and then he says it could have become something entirely different. If this sounds confusing it is because the King of Lagom probably doesn't quite know what he's talking about but rather builds this statement on misconceptions about what actually happened at Chernobyl and Forsmark respectively. So let's examine the incidents and compare.

The 1986 Chernobyl accident

April 26, 1986. The night shift at reactor 4 at the V.I Lenin Nuclear power plant, 20 km north west of the town of Chernobyl, Ukraine, has been ordered to do a test. Due to operator error, they accidentally poison the RBMK-type reactor which makes it almost grind to a halt. They don't know why the reactor is giving so little power though because they were mostly coal plant workers, inexperienced with nuclear power, and oblivious to things such as nuclear poisoning. The shift boss, determined to finish the test, gives orders to proceed, telling the operators to perform actions that go against several operating rules of the reactor. This puts the reactor in an unstable state.

When the test is finished and they shut down the reactor, a fatal flaw in the reactor's control system causes the reactivity to spiral out of control, making it output between ten to onehundred times normal thermal effect. The water in the reactor flash boils and the enormous steam pressure blows the building apart. A few seconds later a chemical explosion, when water that has been split into hydrogen and oxygen burns, rocks the complex again. The reactor is on fire for ten days, resulting in a large plume of radioactive fallout.

There are several factors that allowed this accident to happen.

First it was operated by poorly educated personnel, in a political system where safety came second. In the Soviet Union, you did not rise to attractive jobs like this one by being good at your craft but by kissing up to the communist party. Also you did not stay at jobs like this by speaking up against safety issues, because such things made the party look bad. For instance this particular test was supposed to have been run years ago when the plant was commissioned. But since it failed back then, it had to be done again, this time in secret from the Soviet nuclear regulatory authorities.

This shouldn't have been a problem. But the second reason the accident could take place was the deliberate violations of the operating rules of the reactor. The test was to have taken place when the reactor was outputting at least 700 MW; they started when it was at 200 MW. They were not allowed to withdraw more than a certain number of control rods; they withdrew almost all of them. They were not allowed to increase water flow in the reactor past a certain amount when operating at low power; they did. They were not allowed to disengage the safety systems that would have shut down the reactor when they did any of the aforementioned; but they did indeed disable them.

All of this made reactor come into an unstable state that let its most critical design flaw come into play: the positive void coefficient. The void coefficient is a quality in a nuclear reactor that tells us what happens when it gets too hot. When coolant boils in a reactor that has a positive void coefficient, the nuclear reaction increases. This makes the reactor hotter, which makes more water boil. This speeds up the reaction more, making it even hotter... and so forth. And not only was the void coefficient in the RBMK-reactors of the Chernobyl plant positive, it was also dangerously high.

Finally, because the reactor had no real core vessel, nor any concrete containment, the force of the explosion wrecked the building completely. A fire started in the hundreds of tons of graphite that was in the reactor. Also the building itself that was supposed to have been made from fireproof material, was not, and the debris caught fire as well.

This is what is known as a criticality accident, when the nuclear reaction goes out of control. In this case it produced so much heat that the entire reactor blew up from all the thermal energy. This accident was not a nuclear meltdown.

The 2006 Forsmark incident

July 27, 2006. At the switch-yard for Forsmark-1, an electrical arc causes a short circuit which leads to the unit being disconnected from the power grid. This is serious as the plant relies on power to keep all pumps going.

If a nuclear reactor does not have working pumps, eventually the cooling water in the reactor will boil away. If that starts to happen you must engage the emergency core cooling, reserves of water kept for this very purpose. If this too fails and the reactor boils dry, the heat can be such that the reactor core becomes damaged, popularly called a meltdown. This can happen even when the nuclear reaction has been stopped because decay heat continues to be produced a few hours after a reactor is shut down as very short-lived nuclear waste falls apart. This is what happened at Three Mile Island in 1979.

So when a nuclear plant becomes disconnected from the power grid, the reactor is shut down and on-site diesel generators start to provide power for the pumps to deal with the decay heat, and this was what happened at Forsmark 1. However in this case, two out of the four diesel generators did not start, disabling two safety trains out off four. But the two remaining diesel generators were more than enough to drive the pumps. Hence the reactor was cooled and emergency core cooling was not necessary. The reactor shutdown proceded normally.


No, there are no similarities between these two incidents. The Chernobyl disaster was the case of a criticality accident that caused an extremely violent explosion that completely wrecked the reactor core; the building it operated in; burned for days. The Forsmark incident was the case of  slight degradation of safety features while the reactor and its cooling operated normally. The cooling system was operational the whole time; the emergency cooling did not need to be engaged; the reactor core was not damaged; the reactor tank was in no way threatened; the over one meter thick reactor contaiment remained perfectly safe. And fire? Naw... there is no graphite in Forsmark-1. Water handles that job instead.

So when the King of Lagom says that the Forsmark incident could have become another Chernobyl, he is wrong. There is no way that Forsmark-1 or any of the other Swedish nuclear reactor could undergo the process that led to the explosion in Ukraine in 1986. And this is not just because we employ people that know what they are doing; care about safety first; follow procedure; don't do things behind the back of the nuclear regulatory authorities. No, the most important reason why a Chernobyl-type criticality accident cannot happen in Sweden is the reactors themselves. Because unlike the RBMK-reactors of the Soviet Union, our boiler- and pressurized water reactors do not have a positive void coefficient. We did it the opposite way, so that when water starts boiling in the reactor, the nuclear reaction slows down because of inescapable laws of physics. It's nature's own choke collar on nuclear reactions.


The RBMK-type of reactor was employed only in the Soviet Union. The international community is working hard to get the twelve RBMK's that are still in operation closed. Even though I'm a nuclear friend I'm not an idiot, and as such I am very glad that one of the remaining RBMK's. Ignalina-2, will be shut down in 2009, meaning that Lithuania no longer operates them. Now we just need to get Russia to shut down theirs and we'll finally be rid of this blight.

When discussing nuclear safety, anyone that uses Chernobyl as an example of what could go wrong in nuclear reactors is ignoring reality. The BWR/PWR reactors of the world hold about as much in common with the RBMK-design of the Soviet Union as does slavery to common work; as does forced child soldiers to commissioned adults. There just is no comparing them as they operate differently down to subatomic level.

The Forsmark incident was not, and could not have become, another Chernobyl. This is not an opinion, it is physical reality.


ADDENDUM: As I posted a link to this entry in his blog,  and called him on his Ad hominem attacks, he first approved the entry, then he cencored it and claimed that I was violating his right to have "free opinions", i.e. he doesn't want anyone telling him he's wrong.