Summary of nuclear accident -1/2

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Dear Shoji,

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The US-Japan discussion forum to which I subscribe has, of course, hosted a lot of discussion about the Fukushima incident.  One of the contributors forwarded the following story written by the dean of an American college of engineering. It is in English and it is quite long, but it gives a good technical summary for non-engineers:

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This is an E-mail from the Dean of the University of Washington College of Engineering　to the students - March 17

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*What happened at Fukushima*

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I will try to summarize the main facts. The earthquake that hit Japan was 5times more powerful than the worst earthquake the nuclear power plant wasbuilt for (the Richter scale works logarithmically; the difference betweenthe 8.2 that the plants were built for and the 8.9 that happened is 5 times,not 0.7). So the first hooray for Japanese engineering, everything held up.

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When the earthquake hit with 8.9, the nuclear reactors all went intoautomatic shutdown. Within seconds after the earthquake started, the controlrods had been inserted into the core and nuclear chain reaction of theuranium stopped. Now, the cooling system has to carry away the residualheat. The residual heat load is about 3% of the heat load under normaloperating conditions.

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The earthquake destroyed the external power supply of the nuclear reactor.That is one of the most serious accidents for a nuclear power plant, andaccordingly, a “plant black out” receives a lot of attention when designingbackup systems. The power is needed to keep the coolant pumps working. Sincethe power plant had been shut down, it cannot produce any electricity byitself any more.

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Things were going well for an hour. One set of multiple sets of emergencyDiesel power generators kicked in and provided the electricity that wasneeded. Then the Tsunami came, much bigger than people had expected whenbuilding the power plant. The tsunami took out all multiple sets of backupDiesel generators.

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When designing a nuclear power plant, engineers follow a philosophy called“Defense of Depth”. That means that you first build everything to withstandthe worst catastrophe you can imagine, and then design the plant in such away that it can still handle one system failure (that you thought couldnever happen) after the other. A tsunami taking out all backup power in oneswift strike is such a scenario. The last line of defense is puttingeverything into the third containment, that will keep everything, whateverthe mess, control rods in our out, core molten or not, inside the reactor.

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When the diesel generators were gone, the reactor operators switched toemergency battery power. The batteries were designed as one of the backupsto the backups, to provide power for cooling the core for 8 hours. And theydid.

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Within the 8 hours, another power source had to be found and connected tothe power plant. The power grid was down due to the earthquake. The dieselgenerators were destroyed by the tsunami. So mobile diesel generators weretrucked in.

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This is where things started to go seriously wrong. The external powergenerators could not be connected to the power plant (the plugs did notfit). So after the batteries ran out, the residual heat could not be carriedaway any more.

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At this point the plant operators begin to follow emergency procedures thatare in place for a “loss of cooling event”. It is again a step along the“Depth of Defense” lines. The power to the cooling systems should never havefailed completely, but it did, so they “retreat” to the next line ofdefense. All of this, however shocking it seems to us, is part of theday-to-day training you go through as an operator, right through to managinga core meltdown.

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It was at this stage that people started to talk about core meltdown.Because at the end of the day, if cooling cannot be restored, the core willeventually melt (after hours or days), and the last line of defense, thecore catcher and third containment, would come into play.

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But the goal at this stage was to manage the core while it was heating up,and ensure that the first containment (the Zircaloy tubes that contains thenuclear fuel), as well as the second containment remain intact andoperational for as long as possible, to give the engineers time to fix thecooling systems.

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Because cooling the core is such a big deal, the reactor has a number ofcooling systems, each in multiple versions (the reactor water cleanupsystem, the decay heat removal, the reactor core isolating cooling, thestandby liquid cooling system, and the emergency core cooling system). Whichone failed when or did not fail is not clear at this point in time.

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So imagine a pressure cooker on the stove, heat on low, but on. Theoperators use whatever cooling system capacity they have to get rid of asmuch heat as possible, but the pressure starts building up. The priority nowis to maintain integrity of the first containment (keep temperature of thefuel rods below 2200°C), as well as the second containment, the pressurecooker. In order to maintain integrity of the pressure cooker (the secondcontainment), the pressure has to be released from time to time. Because theability to do that in an emergency is so important, the reactor has 11pressure release valves. The operators now started venting steam from timeto time to control the pressure. The temperature at this stage was about550°C.

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This is when the reports about “radiation leakage” starting coming in. Ibelieve I explained above why venting the steam is theoretically the same asreleasing radiation into the environment, but why it was and is notdangerous. The radioactive nitrogen as well as the noble gases do not pose athreat to human health.

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At some stage during this venting, the explosion occurred.. The explosiontook place outside of the third containment (our “last line of defense”),and the reactor building. Remember that the reactor building has no functionin keeping the radioactivity contained. It is not entirely clear yet whathas happened, but this is the likely scenario: The operators decided to ventthe steam from the pressure vessel not directly into the environment, butinto the space between the third containment and the reactor building (togive the radioactivity in the steam more time to subside). The problem isthat at 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, damaging thereactor building around. It was that sort of explosion, but inside thepressure vessel (because it was badly designed and not managed properly bythe operators) that lead to the explosion of Chernobyl. This was never arisk at Fukushima. The problem of hydrogen-oxygen formation is one of thebiggies when you design a power plant (if you are not Soviet, that is), sothe reactor is built and operated in a way it cannot happen inside thecontainment. It happened outside, which was not intended but a possiblescenario and OK, because it did not pose a risk for the containment.

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So the pressure was under control, as steam was vented. Now, if you keepboiling your pot, the problem is that the water level will keep falling andfalling. The core is covered by several meters of water in order to allowfor some time to pass (hours, days) before it gets exposed. Once the rodsstart to be exposed at the top, the exposed parts will reach the criticaltemperature of 2200 °C after about 45 minutes. This is when the firstcontainment, the Zircaloy tube, would fail.

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And this started to happen. The cooling could not be restored before therewas some (very limited, but still) damage to the casing of some of the fuel.The nuclear material itself was still intact, but the surrounding Zircaloyshell had started melting. What happened now is that some of the byproductsof the uranium decay – radioactive Cesium and Iodine – started to mix withthe steam. The big problem, uranium, was still under control, because theuranium oxide rods were good until 3000 °C. It is confirmed that a verysmall amount of Cesium and Iodine was measured in the steam that wasreleased into the atmosphere.

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It seems this was the “go signal” for a major plan B. The small amounts ofCesium that were measured told the operators that the first containment onone of the rods somewhere was about to give. The Plan A had been to restoreone of the regular cooling systems to the core. Why that failed is unclear.One plausible explanation is that the tsunami also took away / polluted allthe clean water needed for the regular cooling systems.The water used in the cooling system is very clean, demineralized (likedistilled) water. The reason to use pure water is the above mentionedactivation by the neutrons from the Uranium: Pure water does not getactivated much, so stays practically radioactive-free. Dirt or salt in thewater will absorb the neutrons quicker, becoming more radioactive. This hasno effect whatsoever on the core – it does not care what it is cooled by.

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Michael Molenda.

コメント 2

私もこのforumのsuscriberですが、寄稿を読むだけです。
別の寄稿者が、この解説の筆者はMITの人だといっている寄稿を見ました。

by i watanabe (2011-04-06 07:50) 

I watanabe さん
貴兄は特別の関心をもってTEPCOの原発事故をみておられると
推察しておりました。これを読むと、TEPCOやメーカの技術者たちが
極限状態の中で、その時その時の危機脱出の為の操作をしている
時に、菅首相および首脳が政治主導の名のもとに、障害になって
いるか判ります。

by ぼくあずさ (2011-04-06 08:15)