What the F... is Happening at Fukushima?

As a retired physicist (long time ago), but with a PhD in Computational Atomic Physics, and a daughter living in Sapporo (400kms north of the Fukushima Nuclear Power plant), I run this blog to help explain a few things....

BBC News this morning:

The crisis at the Fukushima plant - which contains six nuclear reactors - has mounted since Friday's 9.0-magnitude earthquake knocked out the cooling systems.

Explosions rocked the buildings housing reactors one and three on Saturday and Monday.

On Tuesday morning a third blast hit reactor two's building.

A fire also broke out at a spent fuel storage pond at the power plant's reactor four. Reactor four had been shut down before the quake for maintenance, but its spent nuclear fuel rods were still stored on the site. Officials said the explosions were caused by a buildup of hydrogen.

Chief Cabinet Secretary Yukio Edano said they were closely watching the remaining two reactors at the plant, five and six, as they had begun overheating slightly.

He said cooling seawater was being pumped into reactors one and three - which were returning to normal - and into reactor two, which remained nstable.

So what's a containment vessel anyway? Well, I explored Patents on line (All the inventions of mankind).
There is a patent there 3,258,403.
The brief for the patent goes like this:

A nuclear containment vessel houses an inner reactor housing structure whose outer wall is closely spaced from the inner wall of the containment vessel..... The inner reactor housing structure is divided by an intermediate floor providing an upper chamber for housing the reactor and associated steam generators and a lower chamber directly therebeneath containing a pressure suppression pool.... The inner reactor housing structure is preferably constructed of reinforced concrete and the intermediate floor and the outer wall thereof surrounding the lower chamber are preferably faced with a continuous steel lining so as to be substantially gas-tight....

The associated six page document then goes on to describe the design, function and structure of this invention. I have extracted helpful bits here...

These text extracts and the diagram below are from the reactor container vessel patent documents. This explanation gives a sense of why the invention was needed. But it also conveys the matter-of-fact approach of the US nuclear reactor designer. It also introduces the economics of design and location. It doesn't speak of the usefulness of co-locating six of these in one place, but you can imagine the economies of scale and savings....

Here we are introduced to the patent number and to the inventor, part of Westinghouse Electric. In fact the reactors for Units 1, 2, and 6 at Fukushima were supplied by General Electric, those for Units 3 and 5 by Toshiba, and Unit 4 by Hitachi. All six reactors were designed by General Electric. Containment vessels are not all the same - but they deal with the same challenges. Westinghouse and General Electric have competed for years to supply nuclear reactors around the world. Now for a diagram:

Before we explain the diagram, note that one of these containment structures is inside each of the large square box like buildings you have seen on TV at the Fukushima facility. This one is designed to be 170 feet high, with an inside diameter of 150 feet. For you metric types the containment structure is more than 50 metres high. The inner container (10 in the diagram) is made from 3/8 inch stainless steel - ie steel plate 1 cm thick.

The actual reactor vessel - where the nuclear reaction takes place and where the heat is generated is in that bottle like structure (20) in the diagram. Quite small compared with the containment structure. Which itself is inside a building. The reactor vessel is where the radioactive rods are placed, along with control rods which moderate the speed of the nuclear reaction. The vessel is very hot and heats - in this case - water to steam, which is piped outside the containment vessel and is used to turn the turbines to generate electricity.

Continuing with this particular invention, we have seen that the inner container (a cylinder about 50 metres high and 40 metres diameter) is made of 1 cm thick stainless steel. Then outside this are several other layers. If you look back at the diagram, you will see that the outer layer (36) is reinforced concrete, 6 feet thick (2 metres thick). My recollection of Chernobyl is that there was no outer concrete container.

You can see in this invention claim, that essentially what this inventor's container was intended to achieve is reduced pressure in the space between steel container 10, and the next container (which I think is 22). And the whole operation critically depends upon pumps and compressors of enormous power, and which require vast amounts of electricity. Just think what was happening in the control room at Fukushima. You could not make this up. Magnitude 8.9 earthquake. There may have been automatic shut down plans for all 6 reactors in this event. Which may have got started. Perhaps some control rods were being withdrawn. But there may not have been any time. A few minutes later a 10 metre Tsunami hits the facility broadside on. All 6 reactors are located together right on the coast, close to sea water for coolant. Everything outside the reactor buildings is washed away. It appears that backup power supplies were damaged. Employees inside have no idea what has happened to their families. Their job now is to manage an event which only time will tell was foreseen and planned for. And just in case you were wondering - I am not a supporter of nuclear power. When I went to university I thought it was fantastic. The mass of a sausage to take the Queen Mary round the world. Fantastic. It wasn't until I spent time in London that I took off those rose-tinted spectacles. But that's for another blog.