If you are not familiar with nuclear technology in power generation, I would encourage you to read my writeup in Nuclear Power before proceeding, it will help this to all make sense. Most of the information in this writeup was compiled from an article in the September 2001 issue of Nuclear News as well as various websites.

Background

The Pebble Bed Modular Reactor (PBMR) is a high pressure reactor currently being proposed for use in South Africa. The reactor is being designed by Eskom, South Africa's largest energy producer, with the cooperation of the South African Industrial Development Corporation, British Nuclear Fuels, and the Exelon Corporation. The design utilizes a series of 120 MWe reactor "modules" composed of fuel pebbles with helium used to transfer heat directly to the power generation system.

Currently, South Africa has only 1 nuclear power plant which produces around 7% of the nations electricity. The remainder of the power is produced by coal power plants located far away from the coast. This presents a bit of a problem as most of the future growth of South Africa is expected to happen near the coast. Currently, Cape Town requires the majority of it's power to be transmitted from power plants 1400 km away which requires expensive and complicated transmission systems. In addition to this, South Africa is expected to exceed it's current electrical generation capacity around 2008 and the majority of the nations coal power plants are scheduled to be shut down around 2025. The country needs a way to generate power for the future, away from the coal deposits, and that is how the development of the PBMR came about. Construction is planned to begin in 2002 provided the government investigations find that it is safe and cost effective.

The Fuel

As the name suggests, the PBMR uses fuel pebbles rather than traditional pellets housed in fuel rods such as a light water reactor would use. The pellets are about the size of a tennis ball (60 mm in diameter) and are composed of many coated fuel "kernels". The core of the kernel is composed of uranium dioxide with an 8% U-235 enrichment. It is formed into a .5 mm diameter sphere and baked before the coating process is begun using chemical vapor deposition (CVP).

The first coating to the kernel is a layer of porous carbon. This layer is used primarily to collect fission products from the fuel and to accommodate any deformation of the kernel during the fuel life. After this, a layer of pyrolytic carbon (a dense heat treated form of carbon is added, followed by a layer of silicon carbide, and finally another layer of pyrolytic carbon. These layers function as an insulator to contain any radioactive decay particles from the fuel core.

After these coatings, the kernel is approximately 1 mm in diameter. About 15,000 of these are then mixed with a graphite powder and formed into spheres 50 mm in diameter. A layer of pure carbon is added as a buffer, and the entire pebble is then heat treated and hardened. The final product is then machined to exactly 60 mm in diameter. The mass of a pebble is about 210 grams, with 9 g of uranium fuel per pebble.

How the reactor works

Now we know what the fuel is composed of, you are probably wondering how is it used and what makes this different from a light water reactor. Well, hopefully since you have read this far you are at least a little curious...

The reactor core "module" is a steel pressure vessel about 6 m in diameter and 20 m in height. The walls are lined with graphite bricks which act as a passive heat transfer system. Holes are drilled through the bricks to hold standard control rods. The core holds about 440,000 pebbles, of which about 330,000 are fuel pebbles with the remainder being graphite. The graphite spheres are located in the center of the core as well as around the outside and act as a moderator (to slow down nuetrons to a lower energy level where they can cause a U-235 fission) as well as heat dissipators. They are arranged in a geometry that limits the maximum temperature the reactor core can reach should the coolant system fail.

Helium is used as a coolant and heat transfer mechanism, and it pumped into the core at about 500 °C and a pressure of 7 MPa. It travels from the top down through the fuel pebbles and exits the bottom of the reactor at about 900 °C. The helium then travels through three turbines in series. The first two are used to drive compressors which compress and cool helium before it is pumped back into the core. The third is used to drive an electrical generator for power production. The helium exits the last turbine at about 530 °C and a pressure of 2.6 MPa. It is then cooled, compressed, and heated before being pumped back into the core.

Benefits of the PBMR

Efficiency

Turbines operate more efficiently with higher pressure fluids. The upshot of this is that a typical PWR reactor is about 33% efficient (meaning that around 33% of the energy generated is converted to electrical energy, the rest is lost as heat). The PBMR is expected to be approximately 40% efficient, with 50% expected to be possible with continual improvements to the design.

Fuel Refilling

A conventional light water reactor must be shut down to refuel every few years. Given that the intended purpose of a power plant is to make money by selling energy, shutting down a plant means the loss of several million dollars. In addition, it's a lengthy process because the reaction slowly slows down and you have to wait for decay heat from fission decay products to stop. The PBMR is designed to allow online refueling by using a fuel cycling system. This is accomplished by cycling pebbles through the core. As pebbles are cycled out, they are measured to determine the fissionable material left. If they can still be used, they are cycled back into the core. If not, they are removed and fresh fuel is added. Thus, the reactor never needs to be shut down for refueling, only for periodic scheduled maintenance.

Fuel Depletion

Because of the fuel pebble design, the fuel is much more depleted after it is removed from the reactor (meaning there is less fissionable material in the fuel than in a light water reactor). In addition to less wasted fuel, this also means that it would be unlikely that the fuel could be stolen for it's fissionable material, especially since the layered pebbles would require significant technology to remove the fuel from the pebble.

Adjustable Power Load

Because the design of the power station is modular, you have a lot more capability to adjust the power output. Individual modules can change the amount of helium in the power generation loop as well as change the rate fuel is added and remove to control power output. Of course, you can also just bring a portion of the modules online to meet power demands, and more modules can always be added later.

Safety

Of course, the biggest benefit of the PBMR is the safety aspects of the design. The danger in nuclear power generation is the possibility of a meltdown. This occurs when the cooling system fails and the fuel begins to heat up to a point where it breaks down. This reduces the capacity of the fuel to hold on to fission products, causing a great deal of radioactive fission products to be released. Light water reactors use redundant cooling systems to compensate for this, which of course does not eliminate the possibility that the redundant systems will fail.

In a PBMR, the modules are designed to that they can be cooled primarily by heat radiation, convection and conduction. This means that even if the cooling helium is disrupted, the reactor can still cool through other means (primarily convection in the event of a helium failure). In addition, since the individual fuel pellets are hardened and coated with carbon, they are very heat resistant. Combined with the graphite pebbles in the core designed to absorb and disperse heat, it is nearly impossible for the fuel to heat up to a point where it would degrade and release radiation. Thus, a meltdown is supposedly impossible in a PBMR which has been demonstrated in several small test cases.

Furthermore, the design causes the core to naturally shut down on its own should a failure in the control mechanisms occur. The cooling helium is also inert and cannot carry radiation outside of the core. If there should be a rupture in the helium lines, it would take about 9 hours for air to circulate through the core, and the radiation released would be about 10,000 times less than the amount which would require an offsite emergency to be declared. It has been calculated that during a worst case accident at the plant, the radiation dosage to a person standing at the fence of the plant would be about the same amount as you naturally receive in background radiation every day.

Criticisms of the PBMR

Of course, there are people who don't agree with all these safety aspects. I am not a nuclear expert, so I really can't comment too much on how valid these points are, but I will list some of the arguments I have found against the PBMR.

No containment vessel

To date, there have been two widely known nuclear accidents, Three Mile Island and Chernobyl. If you look at these two incidents, the Three Mile Island meltdown had almost no environmental impact and no loss of life. The Chernobyl meltdown had a devastating impact on the surrounding people and environment. Why is this? Because Three Mile Island had a containment vessel and Chernobyl did not. A containment vessel is basically a large reinforced concrete structure which is designed to contain any radiation in the event of a failure. All US power plants have containment vessels. Russian plants, however, only have confinement vessels which will not hold radiation in the event of a failure.

Because the PBMR is a modular design, it is impossible to have a containment vessel since it would prohibit you from being able to add more modules. In addition, because the modules need to be cooled by convection, you can't really put a containment vessel around it. Some critics feel that the design is not as inherently safe as claimed, and that without a containment vessel it would still be possible for a serious environmental impact in the event of an accident.

Graphite is flammable

The fuel spheres all contain graphite, which is a flammable material. If there were defects present in the fuel pebble that would allow heat to transmit more readily to the graphite, the fuel could ignite. If this should happen, it could potentially cause a meltdown. In addition, burning graphite would spread radiation to a significant area surrounding the plant. Thus, perfectly manufactured fuel pellets are essential to the safety of the reactor, and we all know there are almost always defects in any manufacturing process.

Greater amount of nuclear waste

The pebbles are less radioactive than conventional fuel rods, however there is a far greater volume of them. This means there will need to be more space to store the fuel as well as a more transportation of spent fuel to storage facilities.

There has already been an accident with a similar design

A High Pressure Research Reactor in Germany used fuel pebbles and had been running for quite some time. In 1986, a pebble became lodged in the feeder tube to the core. In trying to free the pebble, it was damaged releasing radiation to the environment up to 2 km away. The plant tried to cover up this accident because the design was also supposedly accident proof, however word got out and the reactor was shut down.

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