Future

Pebble Bed Reactor Technology

PBMR Development

The Pebble Bed Modular Reactor (PBMR) is a new type of high temperature helium gas-cooled nuclear reactor, which builds and advances on world-wide nuclear operators' experience of older reactor designs. The most remarkable feature of these reactors is that they use attributes inherent in and natural to the processes of nuclear energy generation to enhance safety features.

It is also a practical and cost-effective solution to most of the logistics of generating electricity, with particular reference to South Africa today.

PBMR's are designed to produce 110MW each which means that 30 000 average homes could be sustained by one such reactor. More than one PBMR can be located in a facility thus creating energy parks. It is possible for a PBMR energy park to be made up of a maximum of 10 modules which share a common control centre. This system allows sequential construction of modules to match users' growth requirements; as the area grows, so more modules can be added to meet the industrial and domestic needs for electricity in an area

A single PBMR reactor would consist typically of a single main building, covering an area of 1 300 square metres (50 x 26 m). This area is far less than the area covered by a rugby field or even a soccer field. The height of the building would be 42m, some of it below ground level, depending on the bed rock formations as the building would sit on bed-rock. The part of the building that would be visible above ground is equivalent to a six storey building. There would be a unit control room, a high voltage switch yard, and a cooling tower for inland facilities and a sea pump-house for coastal facilities.

Ten PBMR reactors produce 1 100 MW would occupy an area of no more than three football fields.

These relatively small power stations would be versatile and flexible. They could be erected anywhere there is a steady and ready supply of water. They could be used as base-load stations or load-following stations, and could be adjusted to the size required by the communities they serve.

By building a PBMR energy park near a base-load centre, such as a town or an energy intense manufacturing area such as the Hillside Aluminium Smelter in Kwa-Zulu Natal, Eskom would be able to improve the reliability of the electricity supply to locations that are currently remote from existing power stations. A PBMR close to such an energy-thirsty base load centre would also limit the need for transmitting power over long distances and obviate the need for contructing and strengthening pylons and high voltage cable systems.

In particular the PBMR's would provide the country with a competitive option for coastal generation.

Eskom has been investigating the PBMR option since 1993. At this time Eskom began its Integrated Electricity Plan (IEP) which examines on an on-going basis the supply of electricity on one hand and the expected demand for electricity on the other.

By 1993 it had become clear that building a new traditional Pressurised Water Reactor (PWR) such as Koeberg would be prohibitively expensive. Eskom therefore began to consider the PBMR option - along with other forms of conventional and alternative electricity generation - as part of the IEP..

In 1995 Eskom initiated a concept design and costing exercise with local and overseas contractors. The results of the exercise over the next two years confirmed the basic validity of the PBMR parameters. The South African government was kept up to date on all Eskom's findings.

During 1997 and 1998 Eskom undertook extensive reviews (internal and external) of the project and began discussions with potential local and overseas partners.

It was found that the PBMR would be a cost effective option. Consequently, in 1998, the Eskom Council formally launched the PBMR as a priority project. The National Nuclear Regulator was approached in 1998 to review the project and issue a statement on the licensability of the PBMR.

Operation

(Need diagram to explain the flow system. I will bring a CD Rom disk with me that has a diagram.)

Helium gas is passed into the reactor (1) and flows over the fuel pebbles in which a chain reaction is taking place. The helium is heated to a temperature of 900 degrees and pressure increases to 69 bars inside the reactor. The heated helium gas flows through to the turbine (2) which in turn drives a generator (3). The helium gas then goes through to a very effective recuperator (4) which gives up much of its heat to the helium which is just about to re-enter the reactor - pre or re-heating the helium. The lower-energy helium gas is then passed through the pre-intercooler (5) and inter-cooler (6) and low pressure compressor (7) and high pressure compressor (8) before returning to the reactor core at 540 degrees. Water is used only on the cooling systems.

The reactor is a helium-cooled, graphite-moderated unit of 100MWe

The turbo generator weighs approximately 28t and rotates at 3000rpm. It is mounted vertically and is suspended by two radial magnetic bearings and an axial magnetic bearing which will have to support the load of 28t.

The recuperator is the piece of equipment which gives up much of its heat to the helium which is just about to enter the reactor.

Two turbo compressors pump helium around the PBMR circuit. Each turbo-compressor sits on its own shaft.

Fuel Cycle

The nature of the chain reaction that takes place in the PBMR is exactly the same as the one that takes place at Koeberg. (Refer to Koeberg experience - Fuel )

The fuel used in a PBMR consists of "spheres" which are designed in such a way that they contain their radioactivity. The PBMR fuel is based on proven high quality fuel used in Germany.

Each sphere is about the size of a tennis ball and consists of an outer graphite matrix (covering) and an inner fuel zone The fuel zone of a single sphere can contain up to 15 000 "particles". Each particle is coated with a special barrier coating, which ensures that radioactivity is kept locked inside the particle. One of the barriers,the silicon carbide barrier, is so dense that no gaseous or metallic radioactive products can escape. (it retains its density up to temperatures of over 1 700 degrees Celsius). The reactor is loaded with over 440 000 spheres - three quarters of which are fuel spheres and one quarter graphite spheres - at any one time. Fuel spheres are continually being added to the core from the top and removed from the bottom. The removed spheres are measured to see if all the uranium has been used. If it has, the sphere is sent to the spent fuel storage system, and if not, it is reloaded in the core. An average fuel sphere will pass through the core about 10 times before being discharged. the graphite spheres are always re-used. The graphite spheres are used as a moderator. They absorb and reduce the energy of the neutrons so that these can reach the right energy level needed to sustain the chain reaction.

Diagram of Pebble Bed Reactor fuel

Waste

The design of the of PBMR fuel makes it easy to store the spent fuel, because the silicon carbide coating on the fuel spheres will keep the radioactive decay particles isolated for approximately a million years, which is longer that the activity even of plutonium.

Because the PBMR fuel can be stored on site for at least 80 years, special casks for transporting the spent fuel and storing it at a remote location such as the nuclear waste disposal site at Vaalputs, 100km South-East of Springbok in the Northern Cape, will not have to be bought from overseas or manufactured locally. There is no intention to reprocess the spent fuel as this is more difficult than with Koeberg-type fuel. The PBMR fuel also has a greater "burn-up" than Koeberg-type fuel, which makes it less valuable to recycle. More of the useful uranium present in the fuel is used while in the reactor

The spent coated particle fuel can be disposed of in a deep under-ground repository. (Coated particle fuel will maintain its integrity for up to ~ 1 million years in a repository, ensuring that spent fuel radionuclides are contained for extremely long periods of time. The plutonium will have decayed away completely in 250 000 years)

Safety

Any PBMR station built in South Africa will adhere to the stringent local and international safety standards that are laid down for nuclear stations in South Africa and throughout the world.

The PBMR is walk-away safe. Its safety is a result of the design, the materials used and the physics processes rather than engineered safety systems as in a Koeberg type reactor.

The peak temperature that can be reached in the reactor core (1 6000 degrees Celsius under the most severe conditions) is far below any sustained temperature (2 000 degrees Celsius) that will damage the fuel. The reason for this is that the ceramic materials in the fuel such as graphite and silicone carbide - are tougher than diamonds.

Even if a reaction in the core cannot be stopped by small absorbent graphite spheres (that perform the same function as the control rods at Koeberg) or cooled by the helium, the reactor will cool down naturally on its own in a very short time. This is because the increase in temperature makes the chain reaction less efficient and it therefore ceases to generate power. The size of the core is such that it has a high surface area to volume ratio. This means that the heat it loses through its surface (via the same process that allows a standing cup of tea to cool down) is more than the heat generated by the decay fission products in the core. Hence the reactor can never (due to its thermal inertia) reach the temperature at which a meltdown would occur. The plant can never be hot enough for long enough to cause damage to the fuel.

Radiation Leakage

The helium itself, which is used to cool the reaction, is chemically and radiologically inert: it cannot combine with other chemicals, it is non-combustible, and non-radioactive.

Because oxygen cannot penetrate the helium, oxygen in the air cannot get into the high temperature core to corrode the graphite used in the reaction or to start a fire. If, through some accident, the helium gas duct (inlet and outlet lines) is ruptured, it would take some nine hours for natural air to circulate through the core. Even if this could happen, it would only lead to less than 10-6 (one millionth) of the radioactivity in the core being released per day. That means that the amount of activity released in 24 hours under this very severe (and recoverable) situation would be some 10 000 times less than that requiring any off-site emergency actions. To avoid such a total failure of the main gas ducting it is designed to leak before it breaks, so that the depressurisation will be gradual and cannot lead to such a rupture.

The helium pressure inside the closed cycle gas turbine is higher than the air pressure outside it, so nothing can get inside the nuclear circuit to contaminate it.

Other Technological Innovations

Besides the safety improvements of the PBMR design there has been a major effort to improve efficiency and to remove systems whose complexity could lead to operational mistakes (human error or machine failure). This has led to a higher power output with the same amount of nuclear fuel being used, and reduced the maintenance and operating requirements.

The use of a continuous fuelling regime has removed the need to shut the reactor down every 12 - 18 months to change the fuel, as is the case at Koeberg.

The use of a closed cycle gas turbine with helium and magnetic bearings has meant tha the thermal efficiency is higher (~45 per cent c.f. to 33% for Koeberg) - because gas turbines are more efficient than steam and also with magnetic bearings there is less friction - this has reduced the major maintenance requirements to once every six years.

All components are built as replaceable modules, so they are changed for spares which are then refurbished for the next machine. This shortens the maintenance periods significantly.

All these improvements enable the PBMR to be operated with little human intervention. The staff are there to monitor and supervise the plant rather than to operate it.

All this means that PBMR's need fewer safety and fall-back systems, without compromising either worker or public safety. This also reduces the cost of building PBMR's and the time needed to plan, build and commission them.