Sunday, May 15, 2011

Fukushima Reactor Meltdown- More Thoughts

Japan Probe, an interesting website I found during the earthquake/tsunami/nuclear disaster, reported recently that the Fukushima event was a confirmed meltdown. That is not particularly surprising, Chernobyl and Three Mile Island (TMI) were also meltdowns. The Fukushima situation ranks in the middle of Chernobyl and TMI as far as meltdowns go.

In the Japan Probe post, I saw my first artist's depiction of the core for reactor 1 at Fukushima Daiichi. Wikipedia has a similar depiction for TMI and a real picture of Chernobyl.

Chernobyl, most people will tell you, was an extremely poor design. There was not much thought put into containment.

As you can see from the Wikipedia picture above, the design lived up to expectations and did a poor job of containment. Unfortunately, for the Russians, the Chernobyl accident improved the World's understanding of the risks of nuclear power. It did too good of a job at that really. Only people that can compare the design realities get the real picture.

One thing that Chernobyl should have taught people is that the China Syndrome movie was just that, a movie meant to be exciting and suspenseful. The China Syndrome is a ludicrous description of more than a worse case. A nuclear meltdown will not burn its way to China. It can cause a very nasty situation.

TMI was a frightening situation with little nasty radiation fallout. The totally different design of the TMI reactor shows that responsible design makes a huge difference in end results.

The artist depiction above illustrates that a fifty percent meltdown was contained in the pressure vessel as it was designed to do.

The photo of the artists depiction of reactor 1 at Fukushima is not as detailed as the TMI one, it will take some time to determine more exactly what happened, but it is the best we have for now. The report Japan probe links to indicates that the plant operators think the melted fuel may have burn a or several small holes in the bottom of the reactor pressure vessel which allowed radioactive water to leak into the reactor dry well, then the containment building and then underground to the turbine building. This seems a little unlikely to me, but possible I guess.

My theory was that attempts to cool the reactor with limited water caused leakage at the piping connections to the reactor near the penetration of the high pressure steam pipe penetration of the containment building. That would give the radioactive water an easier path to the turbine building.

During the attempt to cool the reactor, the operators pumped huge amounts of water which could over flow the dry well, which could fill the containment building, but that does not appear to be the case. How the water leaked out of the dry well is a mystery to me, but the operators are there and I am not. I still have my doubts, but that does not mean anything.

This drawing of the Fukushima reactor containment design looks to me like leakage from the containment structures to the turbine room would be a little difficult. Leaking into the dry well though would not be. The same thermal expansion and contraction probable with the sporadic and inadequate water available could somewhat easily cause leakage at the control rod penetration of the reactor pressure vessel. The two penetrations that the control rods move through complicate the leakage path, but it is possible.

The interesting part of the design to me is that even with the lower control rod mounting, it would be extremely difficult for molten fuel to burn through the pressure vessel containment. The main pressure vessel is made of layers of steel which greatly reduces the chance of thermal stress fracturing and burn through. I am not sure, but the second penetration is probably layered steel as well.

In any case, the amount of water they were pumping in had to go somewhere. Whether I am right or not does not matter, what does matter is the that the old design basically did what it was designed to do, contain the fuel.

The situation with the spent fuel pools, especially at reactor 4, interests me. All of the pools should have been safe for at least 6 days without cooling or make-up water. SFP 4 had the highest decay heat load, so it is logical that it would have become a problem first. The SFP 4 situation may have exposed a design or operational flaw. Generally, make-up water flow, nearly twice the evaporation rate, would be available. The volume and depth of the pool should allow more than enough time to re-establish make-up water flow. This may have been over looked as a priority, operationally or due to the overall situation, which exposes a design issue. In turn, that may have exposed a near empty design flaw.

The pools are designed for safety when full without cooling and when empty. When full, make-up water is all that is required and there is nearly a week to find it. With the pool completely empty, the storage racks have enough ventilation for passive cooling. The problem is that when nearly empty, the ventilation is blocked by water at the bottom of the racks, which can cause over heating of the rod assemblies. This issue is sure to be investigated as a priority concern.

No comments: