What is the Pebble Bed Modular Reactor?
In a nutshell, the Pebble Bed Modular Reactor (PBMR) is a small, clean, cost effective and inherently safe nuclear power plant. It uses coated uranium particles encased in graphite to form a fuel sphere (60 mm in diameter). The PBMR design makes use of helium as the coolant and energy transfer medium to a closed cycle gas turbine and generator.

The PBMR represents the new generation of advanced nuclear reactors characterized by their inherent safety properties. This feature, which renders the need for safety grade backup systems and off-site emergency plans obsolete, is fundamental to the cost reduction achieved over other nuclear reactor designs.

Based on the belief that new generation nuclear reactors should be small, the PBMR is being designed in a modular form. This design not only allows the erection of small power plants to serve local needs, but also makes provision for expansion as demand grows. Dry cooling, although more expensive, is an option to provide even more freedom of location.

Which features ensure the safety of the PBMR?
The inherent safety of the PBMR is a result of the design, materials and fuel used, and the natural physics involved. It does not rely on engineered active safety systems like those used in a Pressure Water Reactor (PWR).

Coatings surrounding the uranium fuel particles within the pebble form a miniature pressure vessel, preventing the release of any of the fission products generated during operation. Graphite used as moderator and structural material remains stable up to temperatures of 2800 °C. This is significantly higher than the normal operating temperature, as well as the maximum temperature of 1600 °C during the event of accidental depressurized loss of forced coolant, which prevents a core meltdown.

Helium, used to transfer heat from the core to the power-generating gas turbine, is chemically and radiologically inert. It cannot combine with other chemicals, is non-combustible and cannot become radioactive when passed through the core.

If a fault occurs during reactor operation, the system, at worst, will come to a standstill and merely dissipate heat on a decreasing curve without any core failure or release of radioactivity to the environment.

What is the project's potential?
Following widespread international interest in the PBMR project, the PBMR company was formed between Eskom, South Africa's Industrial Development Corporation, British Nuclear Fuel and the United States utility Exelon, to build and market PBMR-based power plants. The intention being to build and operate a single module to serve as a demonstration plant and launch platform for local and international sales. The successful completion of the demonstration phase will be followed by commercialization, with Eskom likely to be the first customer, with an order of ten production modules.

The world market for new power stations is about R700 billion per year. Independent assessments show that the PBMR should be capable of capturing a promising foothold in this market - at least 20 modules per year could be exported once the technology has fully proved its worth.

Into the future
With PBMR shareholder approval - after evaluating the Detailed Feasibility Study, the target for commencement of the detail design of the PBMR demonstration module is well on track for early 2002.

More Information
For more information on the project, technical overviews and the functionality of the PBMR, visit the PBMR web-site: http://www.pbmr.com

PBMR Sub-systems Designed and Developed by IST

Fuel Handling and Storage System (FHSS)
The FHSS is the largest PBMR support system, responsible for:

• Supplying the reactor with fresh fuel
• Removing spent fuel from the reactor
• Storing spent fuel for the complete plant operating life and until plant decommissioning
• Circulating the fuel spheres through the reactor for multi-pass burnup
• Loading fuel and graphite spheres in such a manner that the two-zone core is maintained
• Defueling and refueling the reactor core during special maintenance work

IST successfully completed the FHSS basic design project and began with detailed design aspects in preparation for the demonstration module detail design, which will commence early in 2002.

PBMR Sub-systems Designed and Developed by IST

Reactivity Control and Shutdown System (RCSS)
The RCSS forms an integral part of the PBMR control and comprises of the reactivity control sub system and reserve shutdown subscription.

The reactivity control sub-system is made up of control rods and a control rod drive system employed to control the reactivity of the reactor. A total of eighteen sets of control rods are used to control the reactivity by virtue of the depth they are driven into the control rod sleeves.

The reserve shutdown sub-system is an additional safety and control system that releases boron rich pebbles into special sleeves on the side of the reactor core. The boron serves as a neutron absorber and diminishes the reactivity of the core until reactor shutdown is achieved.

IST successfully completed the RCSS basic design phase, and as the RCSS forms an integral part of the reactor core internals, started with the development of both the reactivity control and reserve shutdown subsystems.

PBMR Sub-systems Designed and Developed by IST

Helium Inventory Control System (HICS)
One of the main features of the PBMR is its rapid load following characteristics. Load following is made possible by the inventory control system, which injects into and extracts helium from the PBMR's main power system. The inventory control system has a booster function to cater for rapid step load changes.

Additional functions of the HICS through its inventory control, helium purification and helium make-up sub-systems, are:

• Storage of helium during Main Power System maintenance
• Control of chemical impurities in the helium
• Removal of radioactive species from the helium
• Supply of the initial helium fill
• Supply of fresh helium to balance any helium leakage during operation

The scope of this project fitted well with IST's experience in the industrial arena, contributing to the successful completion of the HICS basic design project.

Conditioning gas systems
Resulting from the integrated design requirements of the conditioning gas systems, IST worked closely with several of the other suppliers of PBMR sub-systems. IST successfully completed the basic design on the following systems:

Reactor Pressure Vessel Conditioning System (RPVCS)

The RPVCS ensures a homogenous temperature distribution within the reactor pressure vessel during normal operation.

Core conditioning system (CCS)

The CCS is used to remove heat from the reactor core when the power generation cycle is inactive. The CCS is thus used during PBMR maintenance with the further ability to cool the core, in the unlikely event of loss of forced cooling.

Start-up Blower System (SBS)

The SBS is a pair of blowers used for conditioning the main power system before start-up as well as to start the main power system by initiating the transfer of thermal energy from the core to the power generating components. The cycle quickly becomes self-sustaining and the SBS can then be stopped.