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What is happening at Japan’s Fukushima Daiichi power plant? March 12, 2011

Posted by apetrov in Near Physics, Physics, Science, Uncategorized.

This is a good question to ask — especially amid speculations about “possible Chernobyl-like nuclear meltdown” and pictures of explosions at the plant.  Knowing a little bit of physics (and reading press-releases from TEPCo — Tokyo Electric Power Company), one can make some initial analysis. Clearly, a complete picture will follow in the near future.

So, first of all, what is the problem? To understand this, let me note that nuclear reactors at the Fukushima Daiichi plant are of the Boiling Water Reactor (BWR) type — quite different in design from Three Mile Island’s PWR-type reactor and  Chernobyl’s RBMK reactor. In order to see the logic of what Japanese engineers are doing, it is useful to see how the BWR reactor works.

Here is the scheme of BWR-type reactor, taken from the Wikipedia page on BWR. The physics here is very simple. Fission reaction in the uranium fuel assemblies (2) heat water (blue stuff, 7), which turns into steam (red stuff, 6) in the reactor vessel (1). The steam exits the vessel and spins the turbine (8 and 9) that generates electricity. That steam is cooled down and returned into the reactor vessel (as water) and the process begins again.

Simply speaking, fission reaction happens when a slow (thermal) neutron is absorbed by a uranium (U-235) nucleus, which then splits into several (two) lighter daughter nuclei, and several fast neutrons (about 3), releasing energy that is converted into heat. In order to have sustained nuclear reaction one needs to slow down those produced neutrons so that they could be absorbed by other U235 nuclei to initiate chain fission reaction. Different reactor designs use different moderators to do that: water (BWR, PWR), graphite (RBMK), etc.

This simple excursion into nuclear physics tells us that the rate of power generation can regulated by controlling the flux of thermal neutrons. This is indeed what is done by the control rods (3) that are usually made of a material (boron) that absorbs neutrons.

What happens in case of an earthquake? Well, the automatic control systems first and foremost would kill the sustained fission reaction that is going in the fuel elements. This was done at the Fukushima plant immediately by inserting the control rods (notice that the control rods are inserted from below). So, what’s the problem then? Why is the water vapor’s pressure rising?

The problem is that during the fission reaction one also produces a lot of short-lived nuclear isotopes. Normally, if you would like to shut down a reactor (say, to refuel), you need to allow for some time (several days) for those isotopes to decay. During that time, water is still being circulated through the reactor core in order to take away the heat produced in the decays of those short-lived isotopes. This is done via pumps that are operated via electricity from (a) power grid or (b) diesel generators or (c) batteries. After the earthquake, the grid was  knocked out and the diesel generators got damaged. The pumps are now running on the batteries and the water vapor pressure inside the reactor vessel is rising — by the way, the normal operating pressure there is about 75 atmospheres!!! TEPCo reports that the pressure there rose twice that, so the plant operators decided to release steam from the vessel.  Now, to cool down the reactor (until those short-lived isotopes decay) they decided to flood the containment vessel with sea water.

So, as you see, the Chernobyl-type of explosion is highly unlikely at the Fukushima plant. I think the reactor will cool down in a couple of days. BTW, it appears that the reported explosion happened at the pumping system

The only troubling news is that the monitoring stations appear to detect small amounts of iodine and cesium isotopes (to quote TEPCo’s press release “The value of radioactive material (iodine, etc) is increasing according to the monitoring car at the site (outside of the site). One of the monitoring posts is also indicating higher than normal level.”). Those isotopes are normally produced in the nuclear fuel rods. This might indicate that one or more rods are damaged.

Update (3/14/2011): It appears that water circulation systems in reactors 1 and 3 of the power station failed on 3/12-13. The reactor containment is now cooled by sea water (with added boric acid to further capture neutrons). Sea water is not an ideal coolant — purified fresh water is — sea water contains salt and other things that can become radioactive in the core of a reactor. Thus, the spent water will most likely be transferred to the spent fuel pools (place on the power station’s campus where spent fuel rods spend some time before being transferred to the permanent storage facility). It also appears that there were two hydrogen explosions in Units 1 and, recently, 3. Where did the hydrogen come from? It most likely came from the chemical reaction on the zircalloy’s casings of the fuel assemblies. Zircalloy, an alloy of zirconium, tin and sometimes other things, contains zirconium. That zirconium  reacts with oxygen in water and releases hydrogen. It is, however, believed that in both cases the containment vessels held up. Those containment vessels did not exist at the Chernobyl’s power station.

Update (3/15/2011): I decided to put updates in the separate post.

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1. Martin Lueker - March 12, 2011

Thank you so much for making this technical summary! It is so much better to know what is really going on, than to get media reports of “the engineers are really worried”.

2. Rick - March 12, 2011

Does anyone in the world considered the cummulative effect of Sun + Moon in the Nuclear
Risk Evaluation ,at any Nuclear Power Plant ?

3. apetrov - March 12, 2011


Sun and/or Moon do not affect nuclear reactors. You see, the only way Moon affects anything on Earth is via gravitational interactions, whose effect is minuscule at the nuclear length scales (abot 10^(-15) meters). The Sun’s story is pretty much the same. While our Sun also expels neutrinos and charged particles their effect is also negligible.

4. EH Simmons - March 12, 2011

Thanks very much, for this great post about the reactors!

5. Karsten Evans - March 12, 2011

In the UK there are plans to build a type 3 which uses caustic soda as the cooling agent..

What scenerio would happen if the same failures happened in a plant not near the coast that uses caustic soda as the cooling agent?

In the UK we had Windscale where a firechief risked his life to insert hoses to blow out fule rods using hoses and start cooling the stack..

with caustic soda involved those sort of workarounds would be never be possible.

I bet , in Japan using sea water is not an autorised procedure in the reactor’s safety manual.

Chernobyl and 3 mile island had similar workarounds.
Will the new types of Reactors really be safe in unforeseen situations like earthquakes and tsjunamis?

6. apetrov - March 12, 2011


Ok, there are several issues here. First of all, as far as I know, the proposed new UK nuclear reactors are of the PWR-type, i.e. with pressurized water as coolant (EPR design from Areva or AP1000 design from Westinghouse). They are third-generation reactors, which is probably what you meant by “type 3.” The water pressure in those reactors (as well as in other currently-operating PWR reactors) is more than hundred times higher than that of the atmospheric pressure to keep the circulating water liquid.

Second, as far as I know, there are no reactor designs that use caustic soda as a coolant. There are no advantages in doing that. There is a type of reactor called “molten salt reactor” that uses molten salt (FLiBe compound) as a coolant. That design is believed to be safer than the PWR one, since molten salt circulates under pressure close to atmospheric, so no problems with pressure build-up (as what is happening now in Japan) should happen.

Finally, normally, you would not want to build a reactor in a seismically-unstable zone. Japan is an exception, as they don’t have seismically-stable territory plus no large fossil fuel repositories. So nuclear power plants are the only way for Japan to have energy independence.

P.S. Japan’s reactors are built on the seashore to have water available to cool down reactor’s cores in case of emergency.

7. Satyajit - March 13, 2011

This post was really good. Thank you.

8. Mark - March 13, 2011

Great post… I work at a BWR in the US. So hard to get accurate information from our media on the true state of the plants there. Flooding the primary containment is very extreme measure…

9. jessica - March 13, 2011

hello im a 17 year old and im alittle scared cause of what happened in japan people why will u have nuclear bombs knowing that something will happen or go wrong please stop with the bombs because your just going to hurt ur self and other innocent people that dont deserve to die

10. jessica - March 13, 2011

people in Japan im srry for wats going on over there just try hard to keep moving on dont look back just keep look towards the positive side :)

11. AsmedOfNeoAmericans - March 13, 2011

Hey Jessica, you’re pretty stupid for a 17 year old. My 10 year old would know better than you, and I will make sure she is educated enough to not end up like the clueless broad you are. First of all, WHO SAID ANYTHING ABOUT BOMBS? Have you ever heard of nuclear power plants? It’s quite possible that the electricity in your own house comes from a nuclear power plant. Not everything nuclear is bad.
Nobody is doing anything with nuclear bombs in Japan ok? Japan doesn’t have any other efficient means of creating electricity for themselves, except nuclear plants. What happened is that the earthquake knocked out the electricity that powered the pumps that cool the nuclear reactor, and the tsunami damaged the generator the emergency generators that would be used if the power went out. They ran on batteries for a while but those don’t last long. I’m pretty confident things will be ok. Don’t worry, nobody is dropping nuclear bombs on poor japanese people. Go throw away your cellphone, stop watching Princess Bride and get an education. There is still time if you’re 17… at that age you should know the difference between a bomb and a power plant.

12. Heather Logan - March 13, 2011

Hi Alexey,

Thanks for the technical details, this is much more informative than the newspapers (to say the least). Your post got circulated around the physics department at Carleton U.

13. jessica - March 13, 2011

blah blah blah

14. AsmedOfNeoAmericans - March 13, 2011

>blah blah blah
let me guess. You must be one of the “Paris Hilton” wannabes. Or maybe a cross between Paris and a pothead hippy. (appologies to hippies. Some of them are actually quite smart). You should start a protest against those horrible nuclear bombs in Japan.
If I was 17 and someone told me what told you, I would be thanking them.

15. AsmedOfNeoAmericans - March 14, 2011

and apetrov thank you for the explanation. It answered my question about why inserting the control rods isn’t enough to stop the generation of heat immediately.

16. andycanuck - March 14, 2011

Not another Chernobyl from the WSJ.


None of this amounts to “another Chernobyl.” The Chernobyl reactor had two crucial design flaws. First, it used graphite (carbon) instead of water to “moderate” the neutrons, which makes possible the nuclear reaction. The graphite caught fire in April 1986 and burned for four days. Water does not catch fire.

Second, Chernobyl had no containment structure. When the graphite caught fire, it spouted a plume of radioactive smoke that spread across the globe. A containment structure would have both smothered the fire and contained the radioactivity.

17. Ivor Hugh Jarse - March 14, 2011

With all the worlds nuclear spin doctors writing this disaster down to nothing much at all! will we all be clambering for a power station in our own back garden?…the thing with these sites is that the spin doctors also operate on line too..the truth is out there, but not much of it…time always tell us who the liars are so in 5 years time the deaths in Japan by radiation will be classed as natural causes and the deformed babie will be classed as average for the state….20 years and the truth will then start to come out

18. Rick - March 14, 2011

Dear Professor,thank you again for yours updates
in this issue, and we all are thinking to the
great sacriffice and excellency of Fukushima Control
Room Operators ! Again,the people are saved by the
anonims !

19. Mark - March 14, 2011


Also the reactor design at Chernobyl had a positive moderator coefficient which caused power to increase on the loss of coolant (a very bad design feature!). LWR reactors in the US have a negative moderator (and void) coefficient which causes power to drop on a loss of coolant.

20. Mark - March 14, 2011


So strange, the likely death toll from the quake/tsunami is already in the thousands and the long term effects of the toxic mess left in its wake we will never know, but at the exposure levels being currently reported from the plants (highest I have heard anyway was 100 mr/hr) will know result in zero deaths or birth defects.

21. Rick - March 14, 2011

Most dangerous is Unit 2 :Core cooling system 2 (RCIC/MUWC)-Not functional(at 20.30 local),Core cooling system 1 (ECCS/RHR)-Not functional,but also the fuel is a mixture of Uranium+Plutonium.
At Unit 1 and 3 there is only Uranium.

22. Taymere - March 14, 2011

Thanks Apretov.

Why aren’t thorium reactors used? Who is working on developing that commecially?

23. Julia - March 14, 2011

Thank you for the post! please keep updating it as new info comes in!!!

24. apetrov - March 14, 2011

Thanks for your comments!

@Taymere: as far as I know, there were/are some experimental reactors that used thorium. Thorium itself is not a fissile material, but is what is called “fertile” material, i.e. it can be used to obtain nuclear fuel right inside of the reactor. Those reactors are called breeders — they were used extensively in the early years of nuclear programs to produce, for instance, plutonium. I think they are not so popular now because of some nuclear proliferation issues. Thorium could be used in, for example, in the molten salt reactor that I mentioned in the comments above. I don’t know if anyone is currently working on making a real commercial thorium reactor…

@andycanuck: graphite did catch on fire at Chernobyl, but it was not the reason for an explosion there, but rather an aftermath. Most common explanation is a hydrogen explosion, which threw the reactor core out. Since Chernobyl did not have a containment shell, the core was dispersed around Pripyat’. This was a result of a bad design (e.g. design with positive void coefficient — a quantity related increase of neutron flux (reactivity) if coolant is lost) and bad decisions.

Taymere - March 15, 2011

Nuclear power is a national security nightmare. Rouge states need not have their own nuclear weapons they can simply make disasters happen at our nuclear power plants. “Special” teams regularly test the security at nuke plants and regularly easily overcome it. While, as you say, the core is very physically strong the spent fuel pools are right out in the open. All that a terrorist requires to create a meltdown are home made thermite bombs tossed in the spent fuel pool to boil the water out of the pool, cut through the rod casings and start a fire and nuclear meltdown all at once. Or a terrorist could use a pump to spew radioactive water from the pool all over the place then watch the rods fuse and melt down. Right now the water has boiled off and the rods are fused and melting down in the spent fuel pools in Japan http://english.kyodonews.jp/news/2011/03/78267.htmlboiling

We try so hard to keep Iran from having a nuclear bomb, what we don’t realize is that by using domestic nuclear power we provide terrorist and rogue states with free easily accessible nuclear dirty bombs.

I’m going to buy a bunch of iodine tabs today because I live on the West coast and I am worried that what’s going on in Japan will reach me here.

Mark - March 15, 2011

Taymere – buy the KI if you think you must… but please don’t take it unless directed to do so by the state health officials…

Taymere - March 16, 2011

Thanks Mark, I won’t start taking them until needed. The fuel rods in #4 pool are fresh, not spent, the fresh rods had been removed from the core for maintenance when the Tsunami hit. They are steaming off radioactive water and other waste right now, so several feet must be exposed. Furthermore the cladding is making H2 which has been causing explosions. The police have brought in a riot control water cannon truck, but the pool is on an upper floor so I don’t know how successful a water cannon will be. Perhaps if they can cut a hole in the side of the building where the pool is and the water cannon can reach it will help. However can the water cannon pump handle boric acid? Very dicey. They may need to cut off the roof and drop boric acid into the pool from an aircraft in combination with the water cannon. Tokyo’s future is dicey, I bought SPY puts this morning and will buy KI tabs today.

25. Taymere - March 14, 2011

Oh I see, thanks. Plutonium is a nightmare, such toxicity that a dirty bomb is a severe threat. You’d have to built the reactor deep underground for national security reasons. It’s much simpler to dispose of DU, we sadly spray it all over the middle east through A-10 cannons. Put the flouride in the water and the DU in the kiddies overseas, there’s really nothing clean about nuke power. Give me natural gas over nuke any day.

Mark - March 14, 2011

On September 9, 2010, a natural gas pipeline in San Bruno California exploded, killing 8 people, injuring dozens, and destroying over 38 homes.

Hmmmm, in 2010 natural gas 8 – nuclear 0

Give me a nuke any day.

Taymere - March 14, 2011

Hi Mark, I was more referring to power generation with dedicated pipelines rather than the municipal octopus of pipelines for heating and cooking. But let’s ask insurers about the relative risks of nuclear vs natural gas. Would any insurer insure nukes without government backstopping? Would anyone build a nuke without government backstopping? Much like ethanol in gasoline nobody would choose it without government subsidies and intervention. The costs and liabilities just don’t pencil out for private sector money. Without the keen skills of congress allocating capital, nuclear power, like so many other boondoggles would mercifully fade away.

26. Scott - March 14, 2011

I have a few questions about flooding the primary containment of a BWR.

At TMI there were some poor decisions made about how much coolant they should add to the reactor. In a PWR like TMI there was a risk of going “solid” where the primary coolant system including the pressurizer would be completly full of liquid. You need to have some steam in the system to act as a cushion. If there is no steam in the system that could lead to loss of primary containment if there was a pressure surge because piping could break.

Is there a similar risk in a BWR?

Also a well designed PWR can maintain cooling by natural convective circulation so some level of cooling can occure without power. Does a BWR have any similar mechinism for passive cooling?

Also does Japan have measures to track and control the amount of hydrogen generating metals (zinc, aluminum ect..)? Did the salt in the sea water contribute to accelerating the rate of hydrogen generation?

What were the explosions? did they have a loss of secondary containment?

Was there a way they could have had a more controled release of pressure?

Do we know the status of the fuel, it apears that some fission products have been detected in the atmosphere but does this indicate a signifigant fuel failure?

Sorry for the long post but I always have questions.

27. James Parsons - March 14, 2011

Apetrov, Thanks for your insights. It appears that the secondary containment concrete structure(s) did explode (see dust, debris and compression wave visible in videos). Was this due to the primary containment relief/control valves discharging H2 to the secondary containment? Also why would he H2 be allowed to mix with 02 in the secondary containment or elsewhere and be exposed to an ignition source? It appears that this is very poor engineering design as the H2 factor is well known and should have been antcipipated and designed around. [I am not referring to the cubicle flat sided facade, tertiary napkin structure surrounding the inner two structures]

I have read that ~3% of fission products are I-131. Given we have 4 reactors farting their gasses to the wind what exposure to I-131 with half life of 8d can cause Thyroid cancer? Assume 3 scenarios, 100% worldwide dilution (unrealistic fantasy), 10:1 and 100:1 worldwide concentration based on meteorlogical channeling to other populated areas. ie Japan to West Coast US.

As a final comment it now appears that locating nuclear power plants in low lying coastal areas due to the abundance of cheap cooling water is a bad idea.

apetrov - March 15, 2011

> Was this due to the primary containment relief/control valves discharging H2 to the secondary containment? Also why would he H2 be allowed to mix with 02 in the secondary containment or elsewhere and be exposed to an ignition source?

A: Yes and no. H2 comes from the core venting. It is believed that it found its way into the reactor building (secondary containment), which is held at “negative pressure” — i.e. the pressure is lower than the atmospheric. This is done to minimize fire hazard (it is commonly done for the stairways of high-rise buildings to ensure evacuation route). The building contains air, which obviously has oxygen, so it is hard to not to allow them to mix…. then any spark would ignite the fire… There were reports that they tried to prevent H2 buildup at the Unit 2 (after seeing what happened to Units 1 and 3), but obviously they were not successful…

apetrov - March 15, 2011

> I have read that ~3% of fission products are I-131. Given we have 4 reactors farting their gasses to the wind what exposure to I-131 with half life of 8d can cause Thyroid cancer? Assume 3 scenarios, 100% worldwide dilution (unrealistic fantasy), 10:1 and 100:1 worldwide concentration based on meteorlogical channeling to other populated areas. ie Japan to West Coast US.

A: This is not quite correct. Radioactive Iodine is produced in the fuel casings when reactor is in operation. It is never meant to leave the fuel rods. When the chain reaction is shut down, we are talking about small quantities that are already present in the rods. Finally, fuel casings need to be broken and exposed to atmosphere. Now, that might have happened, as they detected some Iodine outside. But the (so far) released quantities of Iodine are quite small. I think the danger to the US West Coast is currently minuscule, as Iodine will dilute to natural levels and eventually decay away.

28. strlinmoss - March 15, 2011

I just read in the New York Times that the workers were distracted and didn’t notice that the water in the spent pool was low……….causing the fire in #4

29. James Parsons - March 15, 2011

Makes sense, the fuel storage pool outside of the reactor looses its water moderator and now we have a second reactor outside of the main reactor and containment vessel. This is totally forseeable and the lack of foresight in this design is murderous.

30. James Parsons - March 15, 2011

Apetrov, Please give us more details on the reaction inertia that continues and generates thermal energy after the control rods are put back in. For example based on the thermal rate of the reactor at full output how much thermal energy must be dissipated after the reactor is “shutdown” by the control rods? It appears that dealing with this energy is paramount in reactor design and should be passive in nature since any and all man made active backup cooling systems will always fail reliably at some time. Next should reactor fuel storage containment pools on existing reactors have backup cooling sources as well? Pretty obvious I’d say. I guess the bean counters have been doing our engineeering for quite a few decades.

apetrov - March 15, 2011

There is a very well written description of the cooling systems available for the BWR-type of reactors which can be found here:


As you will find there, the cooling system design has several back-ups.

31. Rick - March 15, 2011

I have a question : What could happened if the level of “water” is too low in a fuel storage facillity ?

Li - March 15, 2011

I am an engineer designing PWR. However the fundamentals of PWR and BWR are of the same. In FUKUSHIMA disaster, we need some coolant (sea water) to take away the residual heat of the core (fuel rods). The fuel will be exposed in the air when the water level is too low. As we know, the air take away heat slower than water. Therefore the temperature of the core will rise and melt the nuke fuel cladding, which is the first defense. Then the radioactive isotope will be released in the containment.

apetrov - March 15, 2011

Rick was actually asking about the water level in the fuel storage pool. I made a comment about that in the update http://apetrov.wordpress.com/2011/03/15/update-on-the-situation-at-japans-fukushima-nuclear-plant/

In two words, the spent fuel rods contain radioactive material (reaction products) which decays, producing heat. That’s why they are put in the pool, where convection cools them down. If you remove water (as what happened in Unit 4), they start heating up and might catch on fire… which is what happened. Now, there is no containment for the spent fuel pool, so everything that burns goes straight into the atmosphere. This is bad — but I believe that they now have control of the situation.

32. Rick - March 15, 2011

How is MOX fuel behavior in tis case ?

apetrov - March 15, 2011

MOX fuel (mixed oxide — uranium plus plutonium) behaves pretty much the same way in the reactor as the pure uranium fuel. The bad thing about MOX is that plutonium is toxic, so you don’t want it in the atmosphere in case of a release…

33. Rick - March 15, 2011

Unit 1,2 3 was in service at the time of disaster,this I understand.
But now,Unit 4,5,6(who were shutted down !) have cooling problems ???
Only explication is that “something”(i don t know what)produced very large oscillations of reactivity with heat generation.
Is possible that Unit 1,2,3 have direct impact on Unit 4,5,6 ?

apetrov - March 15, 2011

Units 4-6 have no cooling problems, they are not operational at the moment. There was a reported problem with the spent fuel pond on Unit 4 (will see if it’ll hold up), which is not related to reactor operations.

34. Update on the situation at Japan’s Fukushima nuclear plant « Symmetry factor - March 15, 2011

[...] Update on the situation at Japan’s Fukushima nuclear plant March 15, 2011 Posted by apetrov in Near Physics, Physics, Science, Uncategorized. trackback The situation at Japan’s Fukushima nuclear plant remains fluid, but it makes sense to do an update. It turns out that the situation is more challenging then I originally thought. To recreate what is happening (based mainly on TEPCo’s press releases and Japan’s Nuclear and Industrial Safety Agency (NISA) press releases), let us take a look at the Mark 1 BWR reactor (for a short description of physics of the nuclear power generation and schematics, please see my earlier post): [...]

35. Shane - March 15, 2011

I have at least 3 questions: 1) How much seawater is being used to cool the containment vessel? 2) Does the seawater become contaminated? 3) What happens to the spent seawater? Is it pumped back into the ocean? What happens when the spent fuel ponds fill? Thanks in advance for any help.

apetrov - March 15, 2011

1) I don’t know how much seawater they’ll use.
2) Yes. This is the reason why they use purified water in the reactor. Of course, if the fuel casings are gone (bad scenario) you might get pieces of fuel in the water.
3) I don’t think they’d dump radioactive water into the ocean.

36. Chris - March 15, 2011

Given the violence of the earthquake shaking described by workers who were on-site at that plant when it struck, maybe we should assume that the plumbing inside the reactor vessels was severely damaged from the beginning. How would this affect the prognosis?

apetrov - March 15, 2011

TEPCo claims in their press releases that there is no coolant leakage in any of the reactors. But the water level is certainly too low — they believe that the fuel rods are exposed. So it is not clear if the water level is low because of the steam releases or there is something else…

37. James Parsons - March 15, 2011

Apetrov, Please address question in 27, 29, 30

apetrov - March 15, 2011

Please see above… and a new post.



38. Mark - March 15, 2011

FYI.. Tokyo Electric Power Co. said that an oil leak in a cooling water pump was the cause of a fire that burned for approximately 140 minutes. The fire was not in the spent fuel pool, as reported by several media outlets.

39. James Parsons - March 15, 2011

Apetrov, Thanks for clarifying the H2 release from the primary to the secondary containment, presumably via a relief or control valve. It is inexcusable that any Oxygen is in the secondary containment when innerting with N2, Ar etc. is a trivial task.
That said isn’t it obvious that the explosions, especially that of #3 is clearly the secondary concrete containment vessel being blasted apart from the H2 explosion inside? Why is this being avoided or lied about in the media?

apetrov - March 16, 2011


Secondary containment is the reactor’s outer building. I don’t think it is practical to keep it filled with N2 or argon, as people sometimes go inside the building. But, in principle, it can be implemented as an emergency measure, I guess…

The explosions on Units 1 and 3 most probably happened inside the secondary containment because only the roofs are blown off.


Mark - March 17, 2011

In the US the Mark 1 primary containments are purged with N2 (drywell and torus)it is not practical to inert the secondary containment… also a hardened vent was added (in the late 80s) from the primary containment so that venting from the primary into secondary containment is not necessary.

40. ace - March 16, 2011

its good step to humanity i.e u rgivingus a good and rightknowleedge

41. Vish - March 19, 2011

Its are very useful and detailed description. Thanks. Allow me one question: Several sources report, that a nuclear power plant cannot turn explosive (like a nuclear) bomb by design. But in Japan, the several tons of nuclear fuel, including plutonium, could melt down into one large mass. Could that lead to a critical mass similar to a bomb? could it explode in a matter of time?

Mark - March 20, 2011

No! I takes highly enriched u235 (or Pu239) to cause an explosive chain reaction (like 98% U235).. a comerical BWR is only approx 3% enriched.

42. Vish - March 20, 2011

Thanks Mark.
What could be the worst of all szenarios in Fukushima, in case everything melts down? Could it mean that japan abandons a large part of its land around Fukuschima “for ever” or something even worse?

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