fredag 29. april 2016

10 differences of the RBMK and a PWR

Since it was 30 years since the Chernobyl accident on Tuesday, I was thinking it would be a good idea with 10 facts related to that as a little "comeback" of Friday Facts (so sorry that I don't manage to make these facts every week, it's just that lately I've either been travelling, or really busy with my PhD, which I sort of have to prioritize sometimes ;) ). Or, not just ten facts, but ten differences between the Chernobyl type RBMK reactor ("reaktor bolshoy moshchnosty kanalny", meaning high-power channel reactor), and the standard pressurized water reactor (PWR). 

Ready? 

Let's go!
  1. PWR is the most common type of reactor in the world operated in countries like USA,  Belgium, Brazil, China, Finland, France, Germany, India, Japan (the Fuksuhima reactor was not a PWR, though), Russia, Spain, and Sweden, and several more. The RBMK was a Soviet develloped design - only built in the former Soviet Union.
  2. the PWR uses water as both moderator (for slowing down all the neutrons from really high energies, to really low energy - which is what we want <3 ) and as cooling medium, but the RBMK uses graphite as moderator, and water as cooling medium. Normally we say that the PWR is light water (light water is what we normally just call "water", instead of heavy water) moderated and cooled, and the RBMK is graphite moderated and light water cooled.
  3. the RBMK was designed with a positive void coefficient; I'll don't go in detail on that now (if you want me to, I can make a separate blogpost about what this means), but in short it is the reason why the RBMK is unstable under certain conditions
  4. the tip of the control rods of the RBMK actually didn't control the reactor/absorb the neutrons -it was made out of graphite that speeds up the fission process, instead of a material that actually shuts it down
  5. the control rods of the RBMK could be withdrawn completely from the reactor - even if it wasn't allowed (no one should EVER be able to overrun safety systems, like it was done the night of the accident)
  6. it took almost half a minute to insert the control rods into the RBMK reactor; on a PWR it takes around a second or so
  7. a PWR needs fuel which is enriched to 5% uranium-235, but the RBMK only needed 2% - so it was economical with the fuel
  8. the RBMK could have its fuel changed while it was running. This, together with the low enrichment (no 7) made it ideal as a producer of weapons plutonium 
  9. a PWR is passively safe, but the RBMK definitely wasn't
  10. the Chernobyl reactor didn't have any outer barrier; meaning the reactor was placed more or less in a warehouse rather than a full containment building. Therefore, when the reactor actually exploded, the radioactive inside of it could get out, and fresh air (oxygen...!) could get in, making a strong fire that lasted for days


These are just the first ten big differences I could think of, but there are even more. 
When I, or other nuclear scientists, say that Chernobyl could never happen in a modern, Western reactor, it's not because we just don't want to see reality or something silly like that, but it's because of these facts listet above - which makes that accident physically impossible in, for example a PWR...!

testing of reactor grade concrete - the concrete stays intact, as the plane is just disintegrated (plane vs concrete: plane 0, concrete 1)


PS: There are still some RBMKs operating in the world today, but major modifications have been made to these reactors.


7 kommentarer:

  1. Very interesting post, Sunniva! I was curious about the animation with the plane hitting the concrete: Is this used to secure the reactor from terrorist attacks? Are there more ways that you know of to make the reactors safe from terrorist attacks?

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    Svar
    1. Hei Maria :)
      Ja, den gifen viser hva som skjer når et fly treffes betonginneslutningen rundt reaktoren - den er med andre ord laget for å tåle ganske mye...;) Den ytre barrierer/betonginneslutningen er laget så den både skal beskytte reaktoren mot terrorisme utenfra, og fra evt stor skade innenfra. Hadde Tsjernobyl-kraftverket hatt en slik betonginneslutning så ville eksplosjonen kunne ha skjedd, men den hadde da vært contained, og man ville ikke ha fått nevneverdige utslipp til omverdenen.

      Hvis du ikke har lest denne saken, så står det litt om hvordan (amerikanske) kjernekraftverk beskyttes: http://www.cracked.com/personal-experiences-1848-i-work-in-nuclear-power-plant-5-insane-realities.html

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  2. Jaja, nå viser det seg vel at ingen av disse designene er særlig sikkre,
    containment eller ikke containment. Alt tyder på at "contaimentene" på Fukushima ikke holdt tett. (noen eksempler på dette)

    http://www.theecologist.org/News/news_analysis/2300846/the_us_navy_knew_fukushimas_hard_rain_on_uss_ronald_reagan.html

    http://america.aljazeera.com/watch/shows/the-stream/the-stream-officialblog/2013/12/16/uss-reagan-sailorsreportcancersafterfukushimarescuemission.html

    Det er herved besvist at atomkraft ikke er sikkert. Sånn er det bare.

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    Svar
    1. Det er absolutt ingenting som er 100% sikkert ;)
      Hvis vi skal snakke om det som skjedde i Japan kan vi like gjerne si at det på ingen måte er trygt å bo i Japan, siden noe mellom 15000 og 20000 mennesker mistet livet pga naturkatastrofen (til sammenlikningt er det svært lite sannsynlig at man kommer til å få ekstra dødsfall pga stråling fra Fukushima).

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    2. Det er absolutt ingenting som er 100% sikkert ;)
      Hvis vi skal snakke om det som skjedde i Japan kan vi like gjerne si at det på ingen måte er trygt å bo i Japan, siden noe mellom 15000 og 20000 mennesker mistet livet pga naturkatastrofen (til sammenlikningt er det svært lite sannsynlig at man kommer til å få ekstra dødsfall pga stråling fra Fukushima).

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    3. Det var en drøy påstand å si at: Siden det uansett er så utrygt i Japan, kan man neglisjere den økte risikoen med å ha kjærnekraft. Utlippene fra Fukushima har kjenner ingen grenser, og du vet at det er registrert nedfall fra Fukushima rund om i Europa. Utslippene fra Fukushima kommer til å fortsette i lang tid ennå, spesielt til havet. Hvor mange som kommer til å få plager og eventuellt dør av strålingen kommer vi sikkert aldrig til å få vite. Både Tsjernobyl og Fukushima er dysset ned....... store områder kommer til å ligge brakk i lang lang tid.

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    4. Hmm merkelig resonement. Hvis contaiment vesselen tåler en flykrassj, da er reaktoren sikker. Det kaller jeg bull.... Hva med spendt fuel pool, kjøleannlegg, muligheten for å kunne kontrollere reaktoren etter ett fly treff.. SVAR: TOTAL meltdown+++ desverre..... og bullshitt resonement!

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