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In the grip of nuclear power

There's something vaguely scary about the notion of nuclear power. Images of Chernobyl, nuclear weapons and all the horror that they convey all too often spring to mind when we talk about nuclear energy.

But with an energy crisis looming in South Africa — and the possibility of blackouts becoming the norm — the government has announced an ambitious plan to build at least 24 'mini' nuclear power stations, called Pebble Bed Modular Reactors.

But do we really need them, how do they work, and should we be alarmed?

Energy crisis

In order to understand the 'why' of it all, you have to begin by examining the energy situation in South Africa. At the moment, Eskom supplies 95 percent of South Africa's energy and 60 percent of Africa's energy. At its current rate the demand for electricity is expected to exceed the supply by 2008.

South Africa is largely reliant on coal for energy, with nuclear energy contributing just six percent to the overall energy supply. However, there are a number of problems with using coal as an energy source.

* It is not sustainable and coal reserves will eventually dry up.
* It is very bad for the environment, with the carbon dioxide produced by coal contributing significantly to the greenhouse effect.
* Logistically it is impractical as the country's main coal reserves are in the north-east while the bulk of the electricity load is near the coast (Cape Town, Durban).

In an attempt to find feasible alternative energy sources (solar energy and wind energy are also being considered but are regarded as being experimental), the government has turned once more to nuclear energy.

South Africa has two nuclear reactors, which are housed at Koeberg and each produce 900 MWe. The use of nuclear energy is controversial for two reasons. The first is that it is extremely volatile and a meltdown will mean the annihilation of the surrounding population (think Chernobyl) and radioactive poisoning of the earth. The second is that the creation of nuclear energy results in radioactive waste which cannot be safely reabsorbed by the earth.

The government has been investigating the possibility of PBMRs since 1993. They have commissioned a pilot project demonstration plant which is set to be completed by 2011. If this project is a success, the first commercial PBMRs are planned for 2013.

Each PBMR will produce 165 MWe. Eventually the government plans to have 24 or more PBMRs which will produce at least 4000 MWe, amounting to a quarter of South Africa's electricity supply.

What is a PBMR?

A PBMR is a helium-cooled High Temperature Reactor. Other countries, such as Germany, Japan and China, are also developing gas-cooled HTRs, but the South African PBMR is generally regarded as the leader in this technology.

A PBMR is most simply described as a huge graphite cylinder (6.2m in diameter and 27m high), which is full of uranium enriched, graphite encased pebbles (456 000 fuel pebbles). There is a graphite column in the centre of the core and the pebbles are in the area around it.

The pebbles

Particles of enriched uranium dioxide are coated with silicon carbide and pyrolitic carbon (to make them safer) and then encased in graphite. The fuel pebbles resemble graphite tennis balls.

The coolant

Helium is used as a coolant and to drive the closed cycle gas turbine and generator. The coolant enters the top of the vessel at 500 degrees Celsius and once it has cooled the nuclear reaction, it leaves the bottom of the vessel at 900 degrees Celsius. The gas passes through a turbine which drives the electricity generator.

The process

The reactor is continuously replenished with 'fresh' pebbles as 'used' pebbles are removed from the bottom. After each pebble passes through the reactor, it is measured to ascertain how much 'fissionable material' is left. It takes a pebble about six months to travel through the reactor and each pebble contains enough 'fissionable material' to pass through the reactor six times, making the 'life' of a pebble six years.

Once the pebble is spent it will be stored in an onsite storage facility. This facility is made up of ten tanks which can each store 600 000 pebbles.
How do they compare to traditional reactors?

Most importantly, PBMRs are safer than traditional reactors. When it comes to nuclear reactors, meltdowns generally occur when there is too much heat. In a traditional reactor, there is an 'active' cooling system. If the mechanics (such as the pumps) of the system fail, then the heat in the reactor escalates causing an increase in the amount of energy released. This eventually damages the nuclear fuel and results in a radioactive explosion.

The PBMR on the other hand, has a 'passive' cooling system. The core structure is created in such a way that the heat produced by the nuclear fission is less than the heat lost through the core surface. This means that the reactor will never reach a temperature where the fuel will become damaged. Further safety measures have been put in place so that if the plant malfunctions, the reactor will stop any nuclear fission and cool itself down.

Another advantage of PBMRs is that they are more efficient than conventional reactors because the helium coolant also acts as an energy transfer medium. In other reactors the coolant gas is used to heat steam which in turn drives the turbines. Cutting out this step makes the reactor more efficient and it also means that the plant does not need to be located near large supplies of water.

The modular nature of the PBMR means that they can be set up anywhere. They are also far more efficient when it comes to space — a plant can be set up on a plot the size of a soccer field.

PBMRs are also supposed to be more environmentally friendly because the radioactive material is already encased in silicon carbide and graphite which decreases the possibility of the waste contaminating the environment.

Voices of dissent

While this picture of PBMRs seems to be rather rosy, not everyone is extolling the benefits of more nuclear power. The environmental group Earthlife objects strongly to the prospect of South Africa expanding their nuclear programme.

Firstly, they assert that the claim that PBMRs are safe is not entirely true. According to a member of the US Nuclear Regulatory Commission, the design of the PBMR is flawed because the unpredictable movements of the pebbles within the reactor lead to the possibility of core instability. Furthermore, they claim that there is no 'safe' dose of radiation and that by having nuclear plants, the public (and more specifically the workers) is being exposed to harmful radiation.

Their second qualm is that there is no way to dispose of radioactive waste — a problem which will only increase with the proliferation of nuclear energy. The Kyoto Protocol doesn't recognise nuclear energy as a clean alternative to fossil fuel, which they say suggests that claims that nuclear power is 'cleaner' than coal power are unsubstantiated.

Earthlife proposes that renewable energy sources such as wind, solar and ocean sources are feasible and cost effective alternatives to nuclear power. Although they will only be able to provide 13 percent of the electricity demand by 2020, studies have found that they should be able to supply at least 70 percent by 2050.

Allegedly, the PBMR programme has already cost R2-billion and is expected to cost another R11.3-biliion. Earthlife claims that renewable energy will be cheaper and will create 27 times as many jobs. The government however argues that because the objective is to start a 'power stations construction industry', the PBMRs hold great promise in terms of job creation.

Essentially, the whole debate comes down to a matter of expense. Do the benefits outweigh the costs? Unfortunately, in the case of nuclear power, expense is a relative term which depends very much on what you regard as valuable.

Автор: 
cooltech.iafrica.com

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