Sunday, March 13, 2011

A Little More Information About Boiling Water Reactors

First off, there are two distinct sites that are owned by Tokyo Electric Power Company TEPCO in the Fukushima Prefecture. Fukushima Daiichi (literally One) and Daiini (Two):

Daiichi is the home to six separate reactor cores while Daiini has four. Currently, the reactors in question are units 1 and 3 of Daiichi. Unit 1 is a General Electric BWR/3 design while Unit 3 is a BWR/4 design. The major difference is an increased power density of 264 MWe (Mega Watts electric).

Japanese power plants take advantage of using the ocean as their natural heat sink. All power plants require a large supply of water to disperse unusable heat. Typically either the ocean, a lake, river or a picturesque cooling tower is used for this purpose.

At a glance, a nuclear power plant is similar in many ways to its fossil fuel cousins. A heat source, whether it be the burning of coal, oil, natural gas or the fissions of uranium from a nuclear plant, provide energy to increase the temperature of water.

The energy density though of uranium is orders of magnitude higher than any fossil fuels. This means that a few tons of fuel material (uranium dioxide and zirconium cladding) can supply the same amount of energy as massive heaps of coal.

The concept is not unlike that of a pressure cooker in yours grandmother's kitchen. We can use the pressure differential of the built up steam to turn the blades of a turbine. This turbine is connected by a huge shaft to a generator; basically a massive rotating magnet. Using good ole' Faraday's Law we can induce a movement of electrons which is currently powering that glowing box you are staring into.

When the first earthquake occurred off the coast, all nuclear generation facilities 'tripped' automatically, meaning that seismic sensors located on each of the unit's site felt the earthquake and told the reactor to initiate an immediate shutdown sequence. Keep in mind these are big machines. The main jet pumps that drive the coolant (pure deionized water) need massive amounts of electricity themselves to stay running.

With a shutdown sequence initiated, control blades, which look like long three dimensional plus signs, are inserted from the bottom up into the reactor core between the fuel. These blades are filled with boron, basically the same stuff your mom uses to get out those tough stains in the laundry. Boron has a very high affinity for absorbing neutrons which in turn robs the uranium atoms. Since the uranium atoms absorb and produce more neutrons the chain reaction is broken the with boron.

Keep in mind that fissioning is not the only reaction occurring inside the reactor core. There are other events including mainly alpha, gamma and beta decays. Think of it this way: uranium is a BIG atom and as such has a lot of protons in its nucleus. Protons, having a positive electrical charge, do not like each other and need someone in the middle to keep them stabilized (enter mom into a quarreling sibling's bedroom). Mom, in this case, happens to be the neutrons. The neutrons, have no electrical charge and act as a mitigator allowing a heavy atom's nucleus to stick together. As this uranium is fissioned, the atom is split into usually two distinct pieces. These pieces obviously are smaller and have roughly half the number of protons of the original uranium atom. It actually takes less neutrons to keep these protons together. As a result, radioactive decay in the form of alphas occurs. This decay produces a helium atom, the same stuff that goes in your kid's balloon. Another reaction, gamma, produces photons to get rid of excess energy. This is light, we just can't see it because of the wavelength. Beta decays are the ejection of electrons usually when a neutron has an identity crisis and turns into a proton.

The three of these decay mechanisms continue to occur after the control rods are inserted into the reactor core. As such, all of these particles are moving around, banging into other atoms tend to slow down. This process of slowing down imparts decay energy and the energy is heat. This process slows exponentially over time. My friend Alan is going to talk more on this in an upcoming post.

Typically, when the reactor is tripped this sends a signal to extremely big diesel engines to start up. There are redundant engines in case one doesn't start or one is down for maintenance. In the case of the the Daiichi site there are multiple diesel engines which can service the multiple reactors. At the beginning of the earthquake the engines were able to start and provide electricity to the the pumps that circulate water in the reactor cores.

It wasn't until the tsunami came a little less than an hour later that the diesels were forced to shut down because of a massive 30 foot wall of water. I don't have specifics on the complications and I am not qualified to speculate, so I won't.

Now, even though the diesels are down, there are banks of batteries which will provide power for around 8 hours to critical systems. Because of the design of a BWR, as long as it has enough water, it's happy. The thing can sit there and boil all day long just fine (unit 1, in fact had been doing this for over 40 years).

So, the name of the game is get water into the reactor pressure vessel where the core sits. If water is not added, just like a pot on the stove, eventually it will all boil away. Now, just like that pot, it takes a while for this to happen. Most older systems are designed such that operators have around eight hours to figure out the best course action to take. Newer, generation III+ systems are passive, meaning the operators could no nothing and the laws of gravity would keep the reactor doing what its supposed to do via natural circulation.

In the event of the reactor core being uncovered and the fuel exposed to just steam and air the amount of heat removal is lessened. Water is much better at removing heat than air, just think about when you jump into the ocean in January; you would only have a few minutes in the water as opposed to a little longer just exposed in air.

The major thing that can quickly speed up when the fuel is uncovered is a chemical reaction called hydriding. The uranium fuel is sheathed in a metal zirconium sleeve that lines up all the uranium fuel pellets and serves as a first layer of protection. This cladding gets hot enough that it will actually reduce a water molecule to its components hydrogen and oxygen. The oxygen is really corrosive and will oxidize with the zirconium created the equivalent of zirconium rust. The hydrogen is released into the pressure vessel and eventually finds its way to the top of the dry well (a containment structure).

There is speculation that the first few feet of the reactor core in unit 1 was uncovered for a short time. IF that is the case, then one could further speculate this is where some of the hydrogen came from in the explosion.

Keep in mind, even if the fuel is uncovered there is the buzz phrase "Defense in Depth". If the operators cannot keep water covering the core there are still many physical barriers that must fail before anything would pose a radiological issue to anyone.

One, a great many of the ~12,000 or so fuel rods would have to be pretty fully uncovered for a decent amount of time before getting up to the 5189 degrees Fahrenheit needed to begin to melt. Even then, it would need to become a molten ball of what is called 'corium' that then slumps to the bottom of the pressure vessel. This massive structure would then dissipate a tremendous amount of heat due to conduction with the molten fuel. A typical pressure vessel is around eight inches thick made of steel with an outer liner on top of that. Just think about that. That is a lot of steel to go through. Even if it gets that far, it then gets to dissipate its heat through the pedestal and eventually the concrete base-mat that's around 20 feet thick (to stabilize against earthquakes mind you).

On top of that, there are multiple systems within containment that are designed to keep the gaseous fission products in. There are series of HEPA and charcoal filters that catch quite a bit.

On top of that, there are perimeters established at nuclear facilities to keep the public at a safe distance in case of a release. On top of that, the Japanese government has take the precaution of moving everyone a LONG ways away.

The bottom line:
Is this a significant occurrence: yes; one I will certainly always remember in my career.
Is there going to be a catastrophic event where the deaths of many people are involved with the nuclear facilities: there is a finite probability that tends to nil.
Should the news agencies be focusing on the reactors instead of other events: no, let the engineers do their job and we will let the journalists do theirs.
Should the engineers keep everyone informed: yes, and they are but with the internet media these days its a little hard to find Waldo (TEPCO, NEI, IAEA) in this sea of ramblings and misinformation.

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