The fear factor
As far as the diversity of fuel supply for electricity generation is concerned, nuclear power is the second most important source in the United States and it is also the single most important source in European OECD countries. Two major problem areas related to nuclear technology were identified, namely cost and public acceptance. This resulted in the rejection of conventional Light Water Reactor (LWR) technology. Nonetheless, the innovative modular High Temperature Gas-cooled Reactor (HTGR) using coated fuel particles, coupled with a closed cycle gas turbine Power Conversion Unit (PCU), was identified as a prospect to overcome these problems. Hence the Pebble Bed Modular Reactor (PBMR) project was born in 1993 with the potential to meet Eskom's requirements.
World electricity generation
Nuclear power has gained substantial acceptance worldwide and currently plays an important role in the global energy-supply mix. Despite this, arguments against nuclear power in the areas of safety and economics persist. However, as illustrated by the remarks of pre-eminent environmental leader, James Lovelock, many of these are unfortunately based on disinformation and fear, and not on sound scientific or economic arguments.
The US boasts 13 of the world's 23 top performing reactors, achieving load factors of more than 98 per cent. In 2002 five of the top 23 plants were Japanese. However, efficient operation is not restricted to the USA and Japan. Almost two-thirds of the world's reactors achieve 80 per cent while the average load factor worldwide has improved from 65 per cent in 1990 to the current value of around 90 per cent.
There is very little difference between coal and nuclear when comparing the total fuel and O&M costs, which were approximately 2.0 USą/kWh in 2001. Although it is difficult to estimate capital costs since they vary between locations and plants, OECD estimations of total costs in the US put nuclear at 3.73 USą/kWh, coal at 3.27 USą/kWh and gas at 5.87 USą/kWh.
Civil nuclear power generation has until now seen only two major accidents, namely Three Mile Island (TMI) in the USA in 1979 and Chernobyl in the Ukraine in 1986. The TMI accident was caused by a cooling malfunction, which caused a reactor core to melt, and radioactive gas was released into the atmosphere. However, the incident caused no injuries or adverse health effects. In the case of Chernobyl, the accident was much more severe. It resulted from a sequence of events that was initiated when operators conducted a series of tests after disabling many of the safety systems. The ultimate causes of the accident were a flawed reactor design, inadequately trained personnel, and a lack of a culture of safety. Thirty people were killed in the period directly after the accident and approximately ten more died later of thyroid cancer. Fortunately, the safety of all Soviet-designed reactors has improved since that time.
Despite these accidents, nuclear power is still the safest form of energy generation in terms of human fatalities. This is clearly illustrated by the results of one of the best studies on comparative safety of energy systems, the GaBE project conducted in 1998 by the Paul Scherrer Institute in Switzerland. Natural gas causes ten times more fatalities per year than nuclear, and hydro one hundred times more.
Current status of nuclear power
Of the nuclear reactors currently in operation worldwide, roughly 60 per cent are the pressurised water reactor (PWR) type, and 20 per cent are boiling water reactor (BWR) type. Both of these use enriched uranium dioxide (UO2) pellets as fuel arranged in tubular form fuel rods. Both can also be classified as light water reactors (LWR) that use ordinary water as coolant and moderator. Whereas the coolant is circulated through the reactor core to transfer heat from it, the moderator is required to slow neutrons from fission down so that they can initiate further fission reactions.
Most of these reactors are refuelled in a batch-processing mode; the reactor needs to be shut down and opened up at intervals of between one and two years to replace a quarter to one-third of the fuel rods. The PWR has a primary coolant circuit with water flowing through the reactor under high pressure. Heat is transferred to a secondary circuit via a steam generator heat exchanger where water is heated and evaporated to produce the steam that drives an ordinary steam turbine cycle.
A disadvantage of the LWR concept is that the batch-processing refuelling scheme requires some excess reactivity directly after refuelling to ensure longer continuous operation. Furthermore, the presence of water in the high-density liquid phase means that a very large volume of radioactive steam may be discharged into the atmosphere in the unlikely case of a severe incident. Besides these conventional LWR reactors, a further 15 per cent of existing reactors are either the pressurised heavy water reactor (PHWR, also known as CANDU) type or the advanced gas-cooled reactor (AGR or Magnox) type.
The PHWR employs natural UO2 fuel and therefore requires heavy water (D2O), which is a more efficient moderator. The coolant is also heavy water, and the operating principle is similar to that of the PWR, except that the pressure tube design is such that the reactor can be refuelled progressively instead of in batch-processing mode. In the AGR carbon dioxide (CO2) coolant gas is circulated through a graphite-moderated core. The heat is then transferred to a secondary cycle via a steam generator where, similar to the PWR, water is heated and evaporated and the steam drives an ordinary steam turbine cycle.
The PBMR concept
The PBMR reactor is a helium-cooled, graphite-cooled reactor (HTGR) with a diameter of approximately 0.5 mm and is contained in a coated particle made up of several different layers. The coated particles are embedded in a graphite matrix of 50 mm diameter and covered with a 5 mm thick graphite layer. This then forms the fuel sphere or so-called 'pebble'. The core is 3.7 m in diameter and 9 m in height and contains approximately 450,000 pebbles.
From the inlet manifold the coolant gas flows up the riser channels, through the horizontal inlet slots, and into the top end of the core. From there it flows down through the gaps between the pebbles during the fission process, and out through a series of channels into below the core. The core is situated inside a steel press, with circulating pebbles depleted and replaced. This of course increases availability, since the reactor need not be shut down for refuelling.
Apart from its the inherent safety properties and competitive economics, the PBMR plant has the following advantages:
This is an edited version of Pieter G Rousseau & Gideon P Greyvenstein's presentation at the Power Generation Conference
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