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Thread: [Fukushima, power stations, nuclear scare]

  1. #3121
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    Quote Originally Posted by Trebuchet View Post
    Just for clarity, that's a video on the Three Mile Island cleanup.
    Yes, and it shows how the same material was found after the accident, far removed from containment.
    Quote Originally Posted by geonuc View Post
    New video and preliminary analysis from TEPCO reveals an interesting substance found adhered to fuel assemblies in the Unit 3 fuel pool. It's the same stuff previously seen inside primary containment and under the reactor vessel which appears to be the result of interaction of melted fuel (called corium) and concrete. For it to be found in the fuel pool suggests there was a whole lot of corium-concrete interacting going on after the accident, further suggesting that a significant amount of fuel escaped containment after the explosion.

    Lots of photos in this link.
    The following video shows how material escaped containment, even with out a massive explosion that destroyed the building.

    Quote Originally Posted by Gigabyte View Post
    This video is educational and relevant

    https://www.youtube.com/watch?v=wY3qCKZOF30

    Of great interest is the removal of melted fuel from the cooling system and steam generation areas. This was a minor disaster compared to Fukushima, yet corium was spread all over by travelling through the pipes. There was no explosion, but melted fuel still escaped primary containment.

    There is zero doubt melted fuel escaped from primary containment at Fukushima reactor 3. And secondary containment. Since secondary containment pretty much was blown to little pieces.
    The real question is how did melted fuel get into the cooling pond? Was it through water pipes? Or did it fly through the air and land? I doubt we will know for a very long time. But the knowledge that a disaster in this type of reactor can lead to melted fuel escaping, outside containment, is valuable data to have.

    The obvious hypothesis is that the steam explosion blew melted fuel that had already escaped the reactor core (primary containment) out of the secondary containment. Which seems quite disturbing.

  2. #3122
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    Quote Originally Posted by Gigabyte View Post
    Yes, and it shows how the same material was found after the accident, far removed from containment.
    The following video shows how material escaped containment, even with out a massive explosion that destroyed the building.


    The real question is how did melted fuel get into the cooling pond? Was it through water pipes? Or did it fly through the air and land? I doubt we will know for a very long time. But the knowledge that a disaster in this type of reactor can lead to melted fuel escaping, outside containment, is valuable data to have.

    The obvious hypothesis is that the steam explosion blew melted fuel that had already escaped the reactor core (primary containment) out of the secondary containment. Which seems quite disturbing.
    I suggest as a start that we get our terms correct.

    BWRs (Fukushima) and PWRs (TMI) have different physical layouts and containment structures. The reactor core is not the 'primary containment' in either type of reactor. The reactor core is an assemblage of fuel bundles designed to allow free flow of water around the fuel rods, the opposite of containment. In both reactor types, there are three 'barriers' to prevent fission products* from escaping to the environment:

    1. the fuel cladding. This is the metal tubing that holds the fuel pellets. The cladding together with the fuel pellets make up the fuel rods. There are many of them and they are not particularly robust as their main purpose (beyond holding the fuel pellets in the proper configuration) is to transfer heat to the surrounding water. The cladding serves to prevent fission products from entering the primary coolant. Except for small instances, cladding cannot contain melting fuel.
    2. The primary system. In a PWR, this is the heavy gauge steel reactor vessel and piping system designed to transfer heat to the steam generators via pressurized water. In a BWR, it is the same vessel and piping but the water in the reactor boils, sending steam directly to the turbines. The primary system is capable of preventing fission products from escaping and dealing with limited fuel melting.
    3. Containment. In a PWR, it is the big steel and concrete building. Inside the building are all the main primary systems; outside is the environment. It is capable of preventing fission products and melted fuel from escaping to the environment in the event of a massive meltdown, such as at TMI. One of the lessons from TMI was the demonstration that the containment structure worked. In a BWR, it is a bit more complicated. There are two structures termed ‘containment’ but only one is important in this discussion. Primary containment is a robust steel and concrete structure housing the reactor vessel and primary piping and is often referred to the ‘drywell’. Attached to the drywell is the wetwell, a system designed to act as a heat sink in the event of an accident (PWRs have no analog to a BWR wetwell). The wetwell sits outside primary containment. Surrounding the primary containment and wetwell is the secondary containment. In the lower portion, this is an unreinforced concrete building. The upper portion is a building structure designed mainly to enclose the turbine and other components. It is this upper portion of secondary containment that blew apart due to hydrogen explosion (not steam explosion) at three of the units at Fukushima. In a BWR, the spent fuel pool is in secondary containment. In a PWR, the spent fuel pool is in a separate building attached to the containment building.

    * Fission products are what we want to contain - they are the highly radioactive isotopes that constitute the hazard to human life and the environment. Un-fissioned fuel (Uranium dioxide) is of no great concern.

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