The Pebble Bed Modular Reactor Concept

Essay by PaperNerd ContributorHigh School, 12th grade August 2001

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The Modular Pebble Bed Reactor Concept Dr. Rudolf Schulter of Germany started the concept of the Modular Pebble Bed Nuclear Reactor (PBMR), during the late 1950's. This new nuclear power plant concept was soon underway until the Chernobyl accident, which caused a lot of anti-nuclear protest throughout Europe, which stopped the project for quite some time until more recently where the idea has once again surfaced in South Africa. The PBMR concept is one based on research and development gained in the field of high temperature reactor technology, which began in the 1980's in Germany.

The PBMR represents a new generation of advanced nuclear reactors, which sets it apart from the conventional nuclear power plants of today. Several features set this type of nuclear reactor apart from the reactors used today such as Pressurized Water Reactors (PWRs) and Boiling Water Reactors (BWRs) as examples.

The most important issues of nuclear power generation include safety and waste disposal, but before one can discuss the advantages of the PBMR in the areas of safety and waste disposal, it is important to note the inherent safety properties of the fuel used in these reactors.

The fuel used in the PBMR is a high quality graphite molded sphere, which contains coated particles. These particles consist of uranium dioxide, coated with a layer of porous carbon, which will accommodate any mechanical deformation the fuel particle may undergo during it's lifetime. The carbon layer also will accommodate any fission products released from the fuel particle. The deformation of the particle is due to a change in density, caused by the fission products. The particles are also coated with two high-density layers of pyrolytic carbon (heat treated carbon) and a layer of silicon carbide, both materials provide for an impenetrable barrier containing the fuel and radioactive products that result from the nuclear reactions. In dealing with the other safety features of the Pebble Bed Modular Reactor, the use of graphite as a structural material within the reactor implies that the moderation within the reactor along with the precise configuration of the fuel elements can not change under any circumstances. This implies that a core meltdown is not possible in this type of reactor, and can be ruled out. Helium gas is used as a coolant in the reactor, which will not change phase during any point within in the operating range of the reactor. This is another added safety feature, as helium is chemically and radiological inert, and will not react with the graphite or metallic core components used in the design. The PBMR has a low core power density, compared with other conventional nuclear reactors. This feature along with the thermal conductivity of the graphite will ensure that the fuel element temperature does not exceed 1600 degrees Celsius. It has been demonstrated that the fuel particles can operate at this high of temperature without losing its capability as an efficient barrier against the release of fission products. The peak temperature of 1600 degrees is below the temperature that may cause damage to the fuel, which is 2000 degrees. Natural convection, conduction and radiation, independent of the reactor coolant conditions achieve the removal of decay heat. This combination of the low power density of the reactor core and temperature resistance of the fuel underpins the safety characteristics of this type of nuclear reactor. The PBMR is inherently safe as result of the design, materials used, fuel, and physics involved. This means that should a worst case scenario occur, no human intervention is required in the short or medium term.

In terms of waste generation and disposal, a nuclear power plant produces less waste than a fossil fuel station generating the same amount of electricity. One kilogram of uranium has the same energy output as seventeen tons of coal. The PBMR will generate about 19 tons of spent pebbles per annum, which is less than one ton of depleted uranium. Storage of spent fuel is much easier to store than fuel rods from Pressurized Water Reactors, as the silicon carbide coating around the particles will keep the radioactive decay products isolated for one million years, This is longer than any radioactive products, which includes plutonium. Spent nuclear fuel includes simple handling, and is durable. It may be stored on site in dry storage tanks within the reactor building. All of the spent fuel that the PBMR generates will be stored on site during the plants 40 year operating period. Once the plant is shut down, the spent fuel will be stored on site for another 40 years before being sent to a final repository. Regulations for the Pebble Bed Modular Reactor are currently the same as those for the LWR, and the natural safety characteristics call for a look at a new licensing process.

The PBMR is also economically competitive, producing electricity at 3.3 cents per Kilowatt-hour as compared to 3.4 cents with natural gas. Rapid construction allows for revenues to be produced within three years, and there are low staffing (approximately 150 personnel), operating and management costs, estimated at $31.5 million per year. Assumed fuel costs include an estimated $20.00 per fuel particle and one third of the fuel is to replaced annually (360,000 fuel particles total, so 120,000 particles to be replaced per year).

* Cost estimates based on 1992 dollar