TEPCO Rose

[PamW]

40. Actually...

RobertEarl,

The MOX is a ceramic. When ever I explain this, I think of a couple of blue glass tumblers that my brother and I got when we were children. One of the soda fountains we used to go to had an offer in which you got to keep the glass the float was served in. My parents still have those blue glass tumblers.

What makes the glass blue is a blue pigment. Suppose you had some blue glasses like my parents have. Are you concerned that the blue pigment is going to come off on your hand when you handle the glasses; even when wet? Are you afraid that the blue pigment is going to come off when you wash the glasses?

The blue pigment is trapped in the glass.

The same is true for the uranium and plutonium in the ceramic fuel; they are trapped in the ceramic.
As long as you don't melt the ceramic; the heavy metals are trapped.

However, the ceramic fuel pellets are encased in zirconium tubes. In order to load the tubes with the fuel pellets, the pellets just slide into the tubes. That's because the outer diameter of the pellets is just a shade smaller than the inner diameter of the zirconium tube. So there's a little gap between the pellets and the tubes. When uranium and plutonium fission, the fission products, which are the remnants of the split nucleus repel each other after the fission. Those remnants get a bunch of kinetic energy in the process, and when they slow down, that energy becomes heat, and that's the heat that we ultimately turn into electric energy. These fission products usually come to rest somewhere in the ceramic fuel pellet, and are trapped. However, sometimes they come to rest in that little gap between the pellet and the tube. Some of the fission products are Iodine-131, Strontium-90, and Cesium-137.

So in that little gap between the pellet and the tube, you have radioactive "fission gas". When the zirconium overheats and oxides, and the tube falls apart, the fission gas that used to be trapped by the tube is no longer trapped, and gets into the coolant water. In a BWR, like Fukushima, the coolant water for the reactor is the same water that gets turned into steam for the turbine. So there is a path for these released radioactive fission gases to contaminate the turbine steam piping. However, there is not normally a path from that loop to the environment.

However, when Fukushima lost its electric power, it lost the capability to use coolant pumps to keep the reactor cool. So the operators vented that contaminated coolant loop to the atmosphere as a way of cooling the reactor. That is why Fukushima released Iodine-131, Strontium-90, and Cesium-137.

However, the Plutonium is trapped in those fuel pellets. There can be a small amount of Plutonium that escapes from the pellet surface. That's why the measurements show that the amount of Plutonium that was released to the environment due to the 4 reactors and the fuel pools was about 2 grams.

Very little of the Plutonium will escape the pellets. That's why nobody should be surprised that the Fukushima accident only released a couple grams of Plutonium.

As far as Iodine, Strontium, and Cesium; which were in those gaps; the calculations / measurement of those releases are as detailed in this post from the University of California - Berkeley:

http://www.nuc.berkeley.edu/node/5774#comment-21635

PamW