Tuesday, April 5, 2011

The Nuclear Meltdown at Chernobyl

The Nuclear Meltdown at Chernobyl
How nuclear Plants Work:

·      Nuclear reactors are used to create heat by splitting atoms of elements, usually Uranium 235. This heat generated by the reactor is used to produce steam which turns a turbine that powers a generator. This generator creates electricity. Within the core of the nuclear reactor, fuel rods are inserted to control how much heat is being created within the reactor. (1) These fuel rods take away neutrons produced by the reactor to cool it down. There are also pressurized water-coolers that are used to cool down the reactor. (2) 

     Chernobyl Reactor:

       The Chernobyl reactor was a RBMK-1000 Soviet powered reactor near Pripat, Ukraine (3). This reactor is different because it was intended for plutonium and power production (2). This used a graphite moderated reactor that used water only for cooling. This reactor also used a Graphite moderator: graphite blocks that surrounded and separated the pressure tubes (the graphite slowed down the neutrons released during fission so that the fission chain reaction could be maintained). The  most dangerous part about this nuclear power plant was that it's Promt-Temperature Coefficient was positive.
                    Promt-Temperature Coefficient:
                             1. Being positive means as the reactor gets hotter, it gets more reactive and its power             increases which is very dangerous (3)
                             2. Most Plants are negative which means that the hotter the core gets, the more it wants to shut itself down (3)

      Another flaw in the Chernobyl reactor was its design of Control Rods. These control rods contained a poison (either Boron or Hafnium) that was inserted into channels between the graphite moderator and the Uranium to absorb neutrons and slow down the reaction. But, the tips of these control rods was not Boron poison, it was graphite. This means that control rods briefly increased the power of the reactor before bringing the reactor’s power, which was also dangerous. (3)
·   (7)
·     The Disaster at Chernobyl: 

     The disaster at Chernobyl occurred on April 26, 1986. On this particular day, they had scheduled a test on reactor 4 of the Chernobyl complex. They were trying to figure out how long it would take to cool the reactor if they shut it down using only the residual steam and coasting turbines for power to run the coolant pumps. (U.S. doesn’t allow these tests). In order to run the tests they had to bypass several automated safety systems (which violated protocol) which was strongly objected by the reactor operators but, an engineer was in charge. (this is one of the human mistakes, why would an engineer lead a test when it should be someone who is familiar with the reactor, like a reactor operator?)  This experiment was done several times before unsuccessfully but this time there was an 12 hour delay and they disabled safety systems. During this test the reactor was run down to a low power and held there for the experiment. A call came in ordering them to delay the experiment for several hours because of a sudden increase in power demand on the grid (nearly 12 hours). Lower power mean fewer neutrons in the core. Then the Xenon-135 started to build up in a highly accelerated rate and the power dropped further. The workers then began to withdraw the control rods in an attempt to stay at power (but the tips weren't Boron poison, they were graphite). They turned on extra cooling pumps, which cooled the core which allowed the power to drop even more (at this point they had a reactor held by low to no power by cold water and burnable poison with all of its control elements removed).  When they shut off the steam supply to the turbines, the flow of water slowed within the core increasing heat and raising reactor power because of their positive temperature coefficient. The reactor’s power increased so rapidly that someone his the SCRAM button or the emergency shutdown button. Now the graphite tips of the control rods quickly drove into the reactor, the moderator section filled the space between the fuel and further increased the reaction; this caused so much more power and heat that a feedback loop chain reaction took place. The heat and power increased so rapidly that there was a steam explosion that blew the 1,200 ton reactor head off the core and through the roof of the building.  The highly radioactive molten fuel and burning graphite was ejected into the atmosphere. (3)
     Long Lasting Health Effects: 
     The explosion triggered release of radiation into environment. There were 28 firemen and workers that died from acute radiation exposure. There was a huge increase in thyroid cancer and diagnosis of down syndrome. There were also chromosomal aberrations (chromosomes were messed up in DNA) and Neural Tube Defects (affects spinal cord, nervous system, and brain). Restriction of food even after 25 years.

     New Safety Precautions:
         Many new safety systems were created after Chernobyl in order to make sure that this never happened again. The Passive nuclear safety  system was created incase all systems fail. Pebble bed reactors were designed so that the complete loss of coolant for an indefinite period doesn’t cause a meltdown. They now have 3 sets of emergency diesel generators along with new emergency cooling systems. (Wikipedia)
·      Fukushima: 
      During the Fukushima incident the reactors themselves were taken offline, and the coolant pumps were supposed to have received  power from offsite but this option was eliminated by the earthquake itself, which apparently cut off the external power to Fukushima. The earthquake also triggered a shutdown of the reactors, removing the obvious local source of power to the pumps. At this point, the first backup system kicked in: a set of on-site generators that burn fossil fuels to keep the equipment running.Those generators lasted only a short while before the tsunami arrived and swamped them, flooding parts of the plant's electrical system in the process. Batteries are in place to allow a short-term backup for these generators; it's not clear whether these failed due to the problems with the electrical system, or were simply drained. In any case, additional generators were slow to arrive due to the widespread destruction, and didn't manage to get the pumps running again when they did.As a result, the plants have been operating without a cooling system since shortly after the earthquake. Even though the primary uranium reaction was shut down promptly, the reactor cores have continued to heat up due to secondary decay products. So far the most long-lived radioactive materials at the site appear to remain contained within the reactor buildings. Radioisotopes have and continue to escape containment, but there's no indication yet that these are anything beyond secondary decay products with short half-lives. (4)
        Since the U.S. is so far away from Japan and the reactor hasn't distributed any major signs like Chernobyl, and even though it is giving off small radioactive material with short half lives, we shouldn't be worried. 

       Footnote Citations:
       (4) http://arstechnica.com/science/news/2011/03/understanding-japans-nuclear-crisis.ars/2  
       (5) http://www.google.com/imgres?imgurl=http://www.deakwan.co.cc/myblog/wp-content/uploads/2011/03/Chernobyl.jpg&imgrefurl=http://www.deakwan.co.cc/archives/335&usg=__mdWJQyi6NIsuucWqBZHNfbBCvhs=&h=274&w=280&sz=19&hl=en&start=0&sig2=LnMn9BL7HLnZRrwjS3uXLQ&zoom=1&tbnid=mQqWhr0Py5d4FM:&tbnh=129&tbnw=130&ei=pending&prev=/images%3Fq%3DChernobyl%2Bexplosion%26um%3D1%26hl%3Den%26client%3Dsafari%26sa%3DN%26rls%3Den%26biw%3D1189%26bih%3D582%26tbm%3Disch&um=1&itbs=1&iact=hc&vpx=926&vpy=113&dur=465&hovh=219&hovw=224&tx=158&ty=156&oei=ZeWbTZWrIYS30QHWk7jjAg&page=1&ndsp=19&ved=1t:429,r:6,s:0
       (6) http://www.google.com/imgres?imgurl=http://www.thomasnet.com/articles/image/chernobyl-disaster.jpg&imgrefurl=http://www.thomasnet.com/articles/hardware/hardware-failures&usg=__19bQGAVhAWCuAtblVO1sa-hdPWs=&h=799&w=715&sz=108&hl=en&start=0&sig2=E9OMO9cilQJnNzBtDw9Lxg&zoom=1&tbnid=dpCWzi02UMicoM:&tbnh=158&tbnw=153&ei=b-ObTdXZEYa20QHy6b3jAg&prev=/images%3Fq%3Ddisaster%2Bof%2BChernobyl%26um%3D1%26hl%3Den%26client%3Dsafari%26sa%3DN%26rls%3Den%26biw%3D1189%26bih%3D582%26tbm%3Disch&um=1&itbs=1&iact=hc&vpx=469&vpy=89&dur=3286&hovh=237&hovw=212&tx=165&ty=126&oei=b-ObTdXZEYa20QHy6b3jAg&page=1&ndsp=16&ved=1t:429,r:2,s:0
       (7) http://www.google.com/imgres?imgurl=http://holbert.faculty.asu.edu/eee460/rbmk.gif&imgrefurl=http://holbert.faculty.asu.edu/eee460/eee460.html&usg=__Ean0MhZFymrbvk26THEr-ZlKwo4=&h=421&w=600&sz=47&hl=en&start=0&sig2=o_NmD27J4BJ98dZzc81sUQ&zoom=1&tbnid=vIb6RnrQ0jnvKM:&tbnh=126&tbnw=180&ei=s-KbTfWgLbC60QHxueDmAg&prev=/images%3Fq%3Ddesign%2Bof%2BChernobyl%26um%3D1%26hl%3Den%26client%3Dsafari%26sa%3DN%26rls%3Den%26biw%3D1189%26bih%3D582%26tbm%3Disch&um=1&itbs=1&iact=hc&vpx=124&vpy=95&dur=141&hovh=188&hovw=268&tx=138&ty=126&oei=s-KbTfWgLbC60QHxueDmAg&page=1&ndsp=18&ved=1t:429,r:0,s:0
      (8) http://www.google.com/imgres?imgurl=http://scienceinseconads.com/cmsFiles/pageImages/Nuclear%2520reactor.jpg&imgrefurl=http://www.scienceinseconds.com/blog/Meltdowns&h=244&w=500&sz=22&tbnid=XumcMc9M-_z3aM:&tbnh=63&tbnw=130&prev=/search%3Fq%3Dnuclear%2Breactor%26tbm%3Disch%26tbo%3Du&zoom=1&q=nuclear+reactor&usg=__6wtSqlMnZ11Qlf7Hi_BF5MArU5I=&sa=X&ei=beubTYOYGvKw0QGTnfHlAg&ved=0CEUQ9QEwBQ
     (9) http://www.google.com/imgres?imgurl=http://www.smh.com.au/ffximage/2004/08/09/nuclear_wideweb__430x376.jpg&imgrefurl=http://www.smh.com.au/articles/2004/08/09/1092022409037.html&usg=__TLB0SyBqn5E5l6mhgO5RiFzcc_0=&h=376&w=430&sz=32&hl=en&start=0&sig2=L7vsQrtwd1uJksiQ1GL2Vg&zoom=1&tbnid=G_QnNF3NjHbseM:&tbnh=135&tbnw=167&ei=u-ubTeCNK-yG0QGK3LjjAg&prev=/images%3Fq%3Djapan%2Bnuclear%2Bplant%26um%3D1%26hl%3Den%26client%3Dsafari%26rls%3Den%26biw%3D1189%26bih%3D582%26tbm%3Disch&um=1&itbs=1&iact=hc&vpx=738&vpy=90&dur=430&hovh=210&hovw=240&tx=146&ty=104&oei=u-ubTeCNK-yG0QGK3LjjAg&page=1&ndsp=17&ved=1t:429,r:3,s:0