RACA Journal June 2016 - Page 57

Getting Technical Continued from page 53 nuclear plants under construction. Nuclear generated power now provides more than 10% of the world`s electricity as reliable base-load power with virtually negligible carbon dioxide emissions. General safety has steadily improved worldwide to levels comparable or better than many long established industrial and manufacturing processes. After the successful establishment of Koeberg, the next major nuclear power project adopted by South Africa was the Pebble Bed Modular Reactor (PMBR) which began during the 1990s. Technically, the project was based on over 20 years of German development and application of pebble bed reactors into their national power grid. The PBMR design was, and still is, inherently very safe in that cooling system failure alone cannot result in reactor meltdown due to the composition of the fuel pebbles and the annular configuration of the reactor. Figure 1 illustrates how small enriched uranium particles are coated with protective layers of carbon and silicon carbide and then assembled into either pebble or prismatic form. Figure 1. 0.5mm diameter uranium particles coated and processed into pebbles or prismatic elements. In pebble configuration, each 60mm diameter pebble is a sphere of graphite containing about 15 000 evenly distributed tiny fuel particles of low level enrichment uranium oxide. This even distribution of fuel particles embedded in graphite neutron moderator was tested for cooling failure in a German installation by deliberately shutting down the circulation of the cooling system. The reactor temperature rose from normal operating at around 1 000°C to a maximum of 1 400°C, which is well below temperatures in excess of 2 600° C which would begin to affect the graphite pebble spheres. Thereafter, the reactor was switched off and allowed to cool down to normal shut-down status. Figure 2 is a record of the reactor temperatures for 120 hours after cooling had been deliberately stopped. www.hvacronline.co.za Figure 2. Temperature profiles during deliberate cooling shutdown. From its establishment in 1999, Pebble Bed Modular Reactor (Pty) Ltd grew into a sizeable nuclear power reactor development and management team comprising some 900 people at the PBMR head-office in Centurion near Pretoria. In addition, more than a thousand people at universities, private companies and research institutes were involved with the project. In 2010 the PBMR project ended when the South Africa government announced it had stopped funding the development of the pebble bed modular reactor. The reasons given were mainly large unforeseen escalations in both development and projected operating costs. Also, difficulties involving technical systems were mentioned but not detailed. The total cost of the PBMR episode has been estimated conservatively as being in excess of R10-billion and since it was abandoned before any modules began to be construct ed the project has been widely regarded as an expensive failure. However, for more than a decade, the PBMR project trained a broad spectrum of South African people in nuclear power technology, many of whom will now be available for the new power stations currently being finalised. In this context, opinions on the PBMR episode will probably become less critical, particularly in the light of growing awareness of how complex and costly it has turned out to be in putting nuclear energy to practical use within acceptable safety standards. In fact, since 2010 when the South African PMBR project came to a full stop, costs of new greenfields base load nuclear power stations from site preparations including environmental impact investigations and clearances, inclusion of mandatory additional surrounding perimeter areas in safety provisions and through Continued on page 57 RACA Journal I June 2016 55