Questions on the Fukushima Reactors

Tales from the Nuclear Age

Copyright © 2011 by Charles Glassmire

 

Mar. 22, 2011

Questions on the Fukushima Reactors

 

We will continue the saga of Soviet Submarine K-19 soon, but the current situation at Fukushima does need some discussion. Not all is being told there and there are some factors which everyone needs be aware of.

 (A Disclaimer: I am not party to any information beyond what I am hearing from the (sometimes hysterical) media, and they in turn, seem to be getting a limited story from the utility and the government there, while trying to do a difficult job. However, there are pieces of the story which have not been explained by the media, and need to be told – Auth.)

 

Some History:

Starting in the early 1950’s, the Boiling Water Reactor (BWR) was designed by the General Electric Corporation in league with the U.S. Government laboratory at the Idaho National Laboratory (INL). It was then in competition with the Pressurized Water Reactor (PWR) already successfully marketed by Westinghouse.

 

Safe Design?

From the beginning the GE design had unusual features and was questioned by some knowledgeable in the field. This design uses boiling water and a mix of steam (bubbles) and hot water to remove the fission heat. This creates a complex environment for nuclear fluid flow calculations. The presence of a mix of bubble voids could cause unstable core criticality. Additionally fluid circulation within the core relied critically upon fluid pumps to push the coolant upwards through the core fuel rods, working against natural gravity flow. If the coolant pumps should breakdown, a dangerous reactor LOCA (Loss of Coolant Accident) could occur. Most reactors rely upon gravity to enhance flow downwards in event of an emergency.

 

“Unacceptable Safety Risk?”:

In 1972 the Atomic Energy Commission (predecessor to the NRC) reviewed the BWR design and safety official Stephen Hanauer recommended that the BWR Mark 1 design be discontinued because it presented unacceptable safety risks. This objection was overruled by consensus. In the mid-1980’s, an NRC official named Harold Denton said that Mark 1 reactors had a 90 percent probability of bursting should the fuel overheat and melt down. Multiple backup systems three deep were engineered into the reactor design to maintain safe operation during emergency shutdown. Ultimately these and other problems caused the U.S. Navy to choose the Westinghouse design for the nuclear submarine fleet.

 

Successful Marketing:

The BWR design was tested thoroughly and finally has sold successfully around the globe. (Sweden uses almost entirely BWR reactor generation – the Germans have invested heavily in BWR). In the early years of sales, General Electric’s Nuclear Energy Division merged with Hitachi Corporation to form GE Hitachi, which markets the BWR system. The early units were known as “Mark 1” designs. Since then the design has evolved through several major modifications (such as ABWR – Advanced Boiling Water Reactor), and newer designs were submitted for approval to the NRC for use in the U.S. The latest (ESBWR) are still under review. Japan has invested heavily in the BWR and the PWR, recently having some 37 BWR’s operating.

 

What’s at Fukushima?

There are six Boiling Water Reactors located at the Fukushima site, near the city of Okuma on Japan’s northeast coastline. Four of these are Mark 1 type designs. Reactor number 1 first generated commercial power in March of 1971 at a peak load of 4,700 megawatts. It was scheduled for permanent end-of-life shut down early in 2011. Over the ensuing years, numbers 2 thru 6 were constructed, (numbers 3 and 5 supplied by Toshiba) and were currently operating. In February of this year, Japanese authorities extended number 1’s license for an additional ten years. Number 3 uses an unusual mixture of Uranium and Plutonium fuel, known as MOX (mixed oxide). Two additional, numbers, 7 and 8, are in planning stages to begin construction in 2012. The site is protected by seawalls, and is operated by Tokyo Electric Power Company (TEPCO).

 

An Early Warning:

Before construction began, the Japanese were warned that the Japanese islands were located on one of the most geologically active sites on the face of the earth, and construction of nuclear reactors there was not recommended. Nevertheless the construction went ahead. Unit 1 was designed for an earthquake based on the 1952 Kern County earthquake, with a peak ground acceleration of 0.18 g. They survived nicely in a 1978 quake of 0.125 g, which lasted for 30 seconds showing no damage to critical components. The recent 2011 Sendai earthquake recorded an acceleration of 0.35g, almost double the design limit.

 

So What Happened?

        Most of us are familiar with the bare facts reported by the media. A magnitude 9 earthquake struck followed almost immediately by a raging tsunami impacting the north eastern coast. Thousands of lives were taken, mostly by the tsunami, and tens of thousands are still listed as missing. Zero lives were lost at the Fukushima reactor site. Reactors 5 and 6 were shutdown safely. But what happened at numbers 1 thru 4? Hydrogen explosions? Fuel meltdown? Reactors “out of control? ” The media has certainly focused on this problem despite all the tsunami deaths, and continually demonstrate their lack of knowledge increased by the lack of (and sometimes contradictory) information from the utility and the government.

          There are two rather separate problem areas in this event not much discussed by the media. One involves the design of the fuel element, and a second but related problem lies in the spent fuel storage area.

 

Tsunami and electric power:

          The reactor containment vessels seem to have tolerated the earthquake with no catastrophic ruptures. As of today (March 20) there is a confused announcement that number 3 containment “may have” been breached slightly. If such is the case radioactive fission products would escape to the environment. If high level radiation has been leaking, it is puzzling that such has not been detected in the ensuing ten days. It is also suggested that one spent fuel storage container has a “tear” in the side and is leaking water. Again, despite pouring in thousands of gallons, no hugh volumes of water have been detected running/leaking from the reactor grounds.

The real difficulty arose from the tsunami. It destroyed the electric tower connection to the power grid, cutting off all electric power to the reactor site. No problem. There are more levels of backups engineered in for just such an event. So activate the backup diesel generators. These then generate electricity on site and then can allow the pumps to continue to pump coolant water over the fuel rods to keep them cool in the reactor core. But the tsunami had put the diesel generators out of commission.

No problem. Activate the backup emergency system of storage batteries to keep the pumps working. This battery backup system did indeed function for several days keeping the pumps working. But eventually the batteries ran out of juice, as all batteries will. The assumption was that more batteries could be trucked in, but the roads were devastated by the flood. The pumps stopped, the cooling water no longer removed heat from the fuel rods and they began to increase temperature.

 

Why still heating after shutdown?

We may logically ask why, if the reactors are shut down, does the fuel continue to generate heat and, actually increase in temperature to several thousand degrees F with no coolant water flow? When the reactor shuts down, doesn’t all fission in the core cease – thus no more heat is generated? The answer is no.

The effect is known as decay heat. Throughout the reactor fuel cycle lifetime, as the Uranium atoms break apart, they give off lots of usable energy. After fission, the broken pieces of the U atom are now smaller, and have become other elements such as Iodine and Strontium. These are called fission products,  and some of these smaller atoms are radioactive themselves. Some of them emit neutrons along with other radiations. These emitted neutrons can still hit other Uraniums and cause the U atoms to fission and release heat, even though the reactor is turned off. So heat continues to build up in the core even after the reactor is shut down. (Decay heating.)  If the reactor has operated for a long time, lots of fission products have built up in the fuel rods, causing much decay heat generation. The fuel then stays hot unless cooled by circulating water.

 

But Where Does the Hydrogen Explosion come from?

          There is no Hydrogen in the core, so how does it explode? Where does it come from? In the emergency with no cooling TEPCO had to control the temperature in the reactor core by some means. The pumps were dead. The answer was to pour in sea water. This is a last ditch solution. The salt in sea water will corrode the reactor core and destroy its capability for use later, ruining the company’s investment. When this step is taken the reactor becomes effectively useless. And still hot.

           But something else happens as a result. When your pour water onto red hot over-heated fuel rods there is so much energy present that it actually breaks apart the water molecules (H₂O) into the component elements of Hydrogen and Oxygen. This in turn is a VERY explosive mixture of gases, and all in the presence of the red-hot fuel rods! But there is still debate that the reactor containment vessel should be strong enough to contain such explosions inside the steel bottle. (To date the only containment breach is suggested at reactor 3, the MOX Plutonium design.)

          But you then get another problem. Gas pressure builds up inside the sealed containment vessel. So it is necessary to vent some of this pressure into the air to prevent the containment vessel from bursting due to pressure. This in turn might release some fission products into the air. Depending on the internal piping, it may be possible to vent this gas through carbon filters and remove most of the radioactivity before the gas is released into the air. We have little information from officials on this venting beyond that it is occurring.

 

The Second Problem – What is “Fuel Cladding”?

        When the Uranium fuel rods are manufactured, its necessary to encase each rod in a metal sheath called “cladding”. This metal sheath around the Uranium rods keeps fission products from escaping from the fuel rod into the coolant water. Coolant flows around the fuel, and moves the generated heat out of the core to be turned into electricity. One doesn’t want radioactivity in the coolant water.

          So what metal do we choose for cladding? This metal needs be strong, but transparent to neutrons so core reactivity won’t be hurt. It has to be easily machinable and able to conduct heat out of the fuel rod easily. Very few metals pass this test. A metal called Zirconium is chosen. More accurately, an alloy of Zirconium known as “Zircaloy” is used to clad the fuel.

 

Where do the Fires come from?

But along with its good characteristics, Zircaloy brings along with it one problem. During a reactor accident, if the fuel is allowed to get very hot, when heated to thousands of degrees Zircaloy spontaneously catches fire in the presence of air or water. It turns into a Halloween sparkler – burning hot and bright. So you can’t fight a Zirc fire with water. It makes things worse. This property only becomes a problem in an accident; this is the source of the fires you have been hearing about in the used fuel storage rooms in the reactors. During the burning we get more Hydrogen generated and that’s how the roofs were blown off in the spent fuel storage rooms near the top of the reactor …

 

(to be continued …)