Japan's Nuclear ReactorsBorrowed from the CCO Board. Good read.
I am writing this text (Mar 12) to give you some peace of mindregarding some of the troubles in Japan, that is the safety of Japan’snuclear reactors. Up front, the situation is serious, but undercontrol. And this text is long! But you will know more about nuclearpower plants after reading it than all journalists on this planet puttogether.
There was and will *not* be any significant release of radioactivity.
By “significant” I mean a level of radiation of more than what youwould receive on – say – a long distance flight, or drinking a glass ofbeer that comes from certain areas with high levels of naturalbackground radiation.
I have been reading every news release on the incident since theearthquake. There has not been one single (!) report that was accurateand free of errors (and part of that problem is also a weakness in theJapanese crisis communication). By “not free of errors” I do not referto tendentious anti-nuclear journalism – that is quite normal thesedays. By “not free of errors” I mean blatant errors regarding physicsand natural law, as well as gross misinterpretation of facts, due to anobvious lack of fundamental and basic understanding of the way nuclearreactors are build and operated. I have read a 3 page report on CNNwhere 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 BWRfor short. Boiling Water Reactors are similar to a pressure cooker.The nuclear fuel heats water, the water boils and creates steam, thesteam then drives turbines that create the electricity, and the steamis then cooled and condensed back to water, and the water send back tobe heated by the nuclear fuel. The pressure cooker operates at about250 °C.
The nuclear fuel is uranium oxide. Uranium oxide is a ceramic with avery high melting point of about 3000 °C. The fuel is manufactured inpellets (think little cylinders the size of Lego bricks). Those piecesare then put into a long tube made of Zircaloy with a melting point of2200 °C, and sealed tight. The assembly is called a fuel rod. Thesefuel rods are then put together to form larger packages, and a numberof these packages are then put into the reactor. All these packagestogether are referred to as “the core”.
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 vessels”. That is thepressure cooker we talked about before. The pressure vessels is thesecond containment. This is one sturdy piece of a pot, designed tosafely contain the core for temperatures several hundred °C. Thatcovers the scenarios where cooling can be restored at some point.
The entire “hardware” of the nuclear reactor – the pressure vesseland all pipes, pumps, coolant (water) reserves, are then encased in thethird containment. The third containment is a hermetically (air tight)sealed, very thick bubble of the strongest steel. The thirdcontainment is designed, built and tested for one single purpose: Tocontain, indefinitely, a complete core meltdown. For that purpose, alarge and thick concrete basin is cast under the pressure vessel (thesecond containment), which is filled with graphite, all inside thethird containment. This is the so-called “core catcher”. If the coremelts and the pressure vessel bursts (and eventually melts), it willcatch the molten fuel and everything else. It is built in such a waythat 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 theweather out, but nothing in. (this is the part that was damaged in theexplosion, but more to that later).
Fundamentals of nuclear reactions
The uranium fuel generates heat by nuclear fission. Big uraniumatoms are split into smaller atoms. That generates heat plus neutrons(one of the particles that forms an atom). When the neutron hitsanother 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 wouldquickly lead to overheating and after about 45 minutes to a melting ofthe fuel rods. It is worth mentioning at this point that the nuclearfuel in a reactor can *never* cause a nuclear explosion the type of anuclear bomb. Building a nuclear bomb is actually quite difficult (askIran). In Chernobyl, the explosion was caused by excessive pressurebuildup, hydrogen explosion and rupture of all containments, propellingmolten core material into the environment (a “dirty bomb”). Why thatdid not and will not happen in Japan, further below.
In order to control the nuclear chain reaction, the reactoroperators use so-called “moderator rods”. The moderator rods absorb theneutrons and kill the chain reaction instantaneously. A nuclearreactor is built in such a way, that when operating normally, you takeout all the moderator rods. The coolant water then takes away the heat(and converts it into steam and electricity) at the same rate as thecore produces it. And you have a lot of leeway around the standardoperating point of 250°C.
The challenge is that after inserting the rods and stopping thechain reaction, the core still keeps producing heat. The uranium“stopped” the chain reaction. But a number of intermediate radioactiveelements are created by the uranium during its fission process, mostnotably Cesium and Iodine isotopes, i.e. radioactive versions of theseelements that will eventually split up into smaller atoms and not beradioactive anymore. Those elements keep decaying and producing heat.Because they are not regenerated any longer from the uranium (theuranium stopped decaying after the moderator rods were put in), theyget 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 “type” of radioactive material is the uranium in thefuel rods, plus the intermediate radioactive elements that the uraniumsplits into, also inside the fuel rod (Cesium and Iodine).
There is a second type of radioactive material created, outside thefuel rods. The big main difference up front: Those radioactivematerials have a very short half-life, that means that they decay veryfast 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-E”, they will beharmless, because they will have split up into non radioactiveelements. Those radioactive elements are N-16, the radioactive isotope(or version) of nitrogen (air). The others are noble gases such asXenon. But where do they come from? When the uranium splits, itgenerates a neutron (see above). Most of these neutrons will hit otheruranium atoms and keep the nuclear chain reaction going. But some willleave the fuel rod and hit the water molecules, or the air that is inthe 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 “type” of radiation is very important when we talk aboutthe radioactivity being released into the environment later on.
What happened at Fukushima
I will try to summarize the main facts. The earthquake that hitJapan was 7 times more powerful than the worst earthquake the nuclearpower plant was built for (the Richter scale works logarithmically; thedifference between the 8.2 that the plants were built for and the 8.9that happened is 7 times, not 0.7). So the first hooray for Japaneseengineering, everything held up.
When the earthquake hit with 8.9, the nuclear reactors all went intoautomatic shutdown. Within seconds after the earthquake started, themoderator rods had been inserted into the core and nuclear chainreaction of the uranium stopped. Now, the cooling system has to carryaway the residual heat. The residual heat load is about 3% of the heatload under normal operating conditions.
The earthquake destroyed the external power supply of the nuclearreactor. That is one of the most serious accidents for a nuclear powerplant, and accordingly, a “plant black out” receives a lot of attentionwhen designing backup systems. The power is needed to keep the coolantpumps working. Since the power plant had been shut down, it cannotproduce any electricity by itself any more.
Things were going well for an hour. One set of multiple sets ofemergency Diesel power generators kicked in and provided theelectricity that was needed. Then the Tsunami came, much bigger thanpeople had expected when building the power plant (see above, factor7). The tsunami took out all multiple sets of backup Diesel generators.
When designing a nuclear power plant, engineers follow a philosophycalled “Defense of Depth”. That means that you first build everythingto withstand the worst catastrophe you can imagine, and then design theplant in such a way that it can still handle one system failure (thatyou thought could never happen) after the other. A tsunami taking outall backup power in one swift strike is such a scenario. The last lineof defense is putting everything into the third containment (see above),that will keep everything, whatever the mess, moderator rods in ourout, core molten or not, inside the reactor.
When the diesel generators were gone, the reactor operators switchedto emergency battery power. The batteries were designed as one of thebackups to the backups, to provide power for cooling the core for 8hours. And they did.
Within the 8 hours, another power source had to be found andconnected to the power plant. The power grid was down due to theearthquake. The diesel generators were destroyed by the tsunami. Somobile diesel generators were trucked in.
This is where things started to go seriously wrong. The externalpower generators could not be connected to the power plant (the plugsdid not fit). So after the batteries ran out, the residual heat couldnot be carried away any more.
At this point the plant operators begin to follow emergencyprocedures that are in place for a “loss of cooling event”. It is againa step along the “Depth of Defense” lines. The power to the coolingsystems should never have failed completely, but it did, so they“retreat” to the next line of defense. All of this, however shocking itseems to us, is part of the day-to-day training you go through as anoperator, right through to managing a core meltdown.
It was at this stage that people started to talk about coremeltdown. Because at the end of the day, if cooling cannot be restored,the core will eventually melt (after hours or days), and the last lineof defense, the core catcher and third containment, would come intoplay.
But the goal at this stage was to manage the core while it washeating up, and ensure that the first containment (the Zircaloy tubesthat contains the nuclear fuel), as well as the second containment (ourpressure cooker) remain intact and operational for as long aspossible, to give the engineers time to fix the cooling systems.
Because cooling the core is such a big deal, the reactor has anumber of cooling systems, each in multiple versions (the reactor watercleanup system, the decay heat removal, the reactor core isolatingcooling, the standby liquid cooling system, and the emergency corecooling system). Which one failed when or did not fail is not clear atthis 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 ridof as much heat as possible, but the pressure starts building up. Thepriority now is to maintain integrity of the first containment (keeptemperature of the fuel rods below 2200°C), as well as the secondcontainment, the pressure cooker. In order to maintain integrity ofthe pressure cooker (the second containment), the pressure has to bereleased from time to time. Because the ability to do that in anemergency is so important, the reactor has 11 pressure release valves.The operators now started venting steam from time to time to controlthe pressure. The temperature at this stage was about 550°C.
This is when the reports about “radiation leakage” starting comingin. I believe I explained above why venting the steam is theoreticallythe same as releasing radiation into the environment, but why it wasand is not dangerous. The radioactive nitrogen as well as the noblegases do not pose a threat to human health.
At some stage during this venting, the explosion occurred. Theexplosion took place outside of the third containment (our “last lineof defense”), and the reactor building. Remember that the reactorbuilding has no function in keeping the radioactivity contained. It isnot entirely clear yet what has happened, but this is the likelyscenario: The operators decided to vent the steam from the pressurevessel not directly into the environment, but into the space betweenthe third containment and the reactor building (to give theradioactivity in the steam more time to subside). The problem is thatat the high temperatures that the core had reached at this stage, watermolecules can “disassociate” into oxygen and hydrogen – an explosivemixture. And it did explode, outside the third containment, damagingthe reactor building around. It was that sort of explosion, but insidethe pressure vessel (because it was badly designed and not managedproperly by the operators) that lead to the explosion of Chernobyl.This was never a risk at Fukushima. The problem of hydrogen-oxygenformation is one of the biggies when you design a power plant (if youare not Soviet, that is), so the reactor is build and operated in a wayit cannot happen inside the containment. It happened outside, whichwas not intended but a possible scenario and OK, because it did notpose a risk for the containment.
So the pressure was under control, as steam was vented. Now, if youkeep boiling your pot, the problem is that the water level will keepfalling and falling. The core is covered by several meters of water inorder to allow for some time to pass (hours, days) before it getsexposed. Once the rods start to be exposed at the top, the exposedparts will reach the critical temperature of 2200 °C after about 45minutes. This is when the first containment, the Zircaloy tube, wouldfail.
And this started to happen. The cooling could not be restored beforethere was some (very limited, but still) damage to the casing of someof the fuel. The nuclear material itself was still intact, but thesurrounding Zircaloy shell had started melting. What happened now isthat some of the byproducts of the uranium decay – radioactive Cesiumand Iodine – started to mix with the steam. The big problem, uranium,was still under control, because the uranium oxide rods were good until3000 °C. It is confirmed that a very small amount of Cesium and Iodinewas measured in the steam that was released into the atmosphere.
It seems this was the “go signal” for a major plan B. The smallamounts of Cesium that were measured told the operators that the firstcontainment on one of the rods somewhere was about to give. The Plan Ahad been to restore one of the regular cooling systems to the core. Whythat failed is unclear. One plausible explanation is that the tsunamialso took away / polluted all the clean water needed for the regularcooling systems.
The water used in the cooling system is very clean, demineralized(like distilled) water. The reason to use pure water is the abovementioned activation by the neutrons from the Uranium: Pure water doesnot get activated much, so stays practically radioactive-free. Dirt orsalt in the water will absorb the neutrons quicker, becoming moreradioactive. This has no effect whatsoever on the core – it does notcare what it is cooled by. But it makes life more difficult for theoperators and mechanics when they have to deal with activated (i.e.slightly radioactive) water.
But Plan A had failed – cooling systems down or additional cleanwater unavailable – so Plan B came into effect. This is what it lookslike happened:
In order to prevent a core meltdown, the operators started to usesea water to cool the core. I am not quite sure if they flooded ourpressure cooker with it (the second containment), or if they floodedthe third containment, immersing the pressure cooker. But that is notrelevant for us.
The point is that the nuclear fuel has now been cooled down. Becausethe chain reaction has been stopped a long time ago, there is onlyvery little residual heat being produced now. The large amount ofcooling 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 heatany more to produce any significant pressure. Also, boric acid hasbeen added to the seawater. Boric acid is “liquid control rod”.Whatever decay is still going on, the Boron will capture the neutronsand further speed up the cooling down of the core.
The plant came close to a core meltdown. Here is the worst-casescenario that was avoided: If the seawater could not have been used fortreatment, the operators would have continued to vent the water steamto avoid pressure buildup. The third containment would then have beencompletely sealed to allow the core meltdown to happen without releasingradioactive material. After the meltdown, there would have been awaiting period for the intermediate radioactive materials to decayinside the reactor, and all radioactive particles to settle on a surfaceinside the containment. The cooling system would have been restoredeventually, and the molten core cooled to a manageable temperature. Thecontainment would have been cleaned up on the inside. Then a messy jobof removing the molten core from the containment would have begun,packing the (now solid again) fuel bit by bit into transportationcontainers 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?
- 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)
- I believe the most significant problem will be a prolonged power shortage. About half of Japan’s nuclear reactors will probably have to be inspected, reducing the nation’s power generating capacity by 15%. 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. That will increase your electricity bill, as well as lead to potential power shortages during peak demand, in Japan.
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