Energised for the future with BBR

 

Since the epic failure, in 1979, at the nuclear plant on Three Mile Island (TMI), Pennsylvania – and indeed the Chernobyl disaster in 1986 – many lessons have been learned. One of these is about the importance of strong nuclear containment vessels, as the strength of the structure at TMI potentially prevented a greater disaster. With increasing pressures to slow climate change, governments around the globe are now showing interest in commissioning new nuclear plants. We examine the shape of the current market, contemporary nuclear vessel design and practice – as well as the BBR Network’s range of nuclear products and capabilities.

 

Nuclear technology was first developed in the 1940s and uses the energy released by splitting the atoms of certain elements. Research initially focused, during the Second World War, on producing bombs by splitting either uranium or plutonium atoms. It was only in the 1950s that attention turned to the peaceful purposes of nuclear fission, notably for power generation.Today, the world produces as much electricity from nuclear energy as it did from all sources combined in 1960. Civil nuclear power now supplies 16% of global needs, in 30 countries – which together have some 439 commercial nuclear power reactors. Over 30 further power reactors are under construction, equivalent to 7.5% of existing capacity, while over 80 are firmly planned, equivalent to 24% of present capacity. Sixteen countries depend on nuclear power for at least a quarter of their electricity. France and Lithuania obtain around three quarters of their power from nuclear energy, while Belgium, Bulgaria, Hungary, Slovakia, South Korea, Sweden, Switzerland, Slovenia and Ukraine get one third or more. Japan, Germany and Finland get more than a quarter of their power from nuclear energy, while the USA gets almost one fifth.

From 1980 to 2004, total world primary energy demand grew by 54% – and to 2030, it is projected to grow at much the same rate. Electricity growth is even stronger, and is projected to almost double from 2004 to 2030. Increased demand is most dramatic in developing countries – and that is projected to increase. Currently some two billion people have no access to electricity and remedying this lack is a high priority. With the United Nations predicting world population growth from 6.4 billion in 2004 to 8.1 billion by 2030, demand for energy must increase substantially over that period. Both population growth and increasing standards of living for many people in developing countries will cause strong growth in energy demand.

Nuclear power generation is an established part of the world's electricity mix and is especially suitable for large-scale, base-load electricity demand. The World Energy Outlook 2006 from the OECD's International Energy Agency (IEA) highlights the increasing importance of nuclear power in meeting energy needs while achieving security of supply and minimising carbon dioxide emissions.The report demonstrates that nuclear power could make a major contribution to reducing dependence on imported gas and curbing CO2 emissions in a cost-effective way, since uranium fuel is abundant.

On a global scale, nuclear power currently reduces carbon dioxide emissions by some 2.5 billion tonnes per year. Carbon dioxide accounts for half of the human-contributed portion of the global warming effect of the atmosphere – over one-third of human-induced greenhouse gases come from the burning of fossil fuel to generate electricity. Nuclear power plants do not emit these gases. In 2007, the UN Intergovernment Panel on Climate Change (IPPC) produced a report on mitigation of climate change which says that the most cost-effective option for restricting the temperature rise to under 3°C will require an increase in non-carbon electricity generation from 34% (nuclear plus hydro) now to 48 – 53% by 2030, along with other measures.With a doubling of overall electricity demand by then, and a carbon emission cost of US$ 50 per tonne of CO2, nuclear's share of electricity generation is projected by IPCC to grow from 16% now to 18% of the increased demand, representing more than a doubling of the current nuclear output by 2030. These projected figures are estimates, and it is evident that if the renewable energy sector – such as solar, wind and water power – fails to grow as much as hoped, it means that other noncarbon sources will need to play a larger role. Thus nuclear power's contribution could triple or perhaps quadruple to more than 30% of the global generation mix in 2030.

Worldwide Nuclear Reactors.

In nearly all projects, the basic structural design is a cylindrical vessel with flat end slabs or a convex dome. Practically all recent nuclear structures consist typically of two shells, the inner and the outer containment. Today, the post-tensioned nuclear structures have reached an outside diameter of 50m and a height of 70m of the inner shell with a capacity of 1600MW.

Three types of BBR post-tensioning tendons are typically used the inner containment, as follows:

• Vertical tendons for wall post-tensioning
  The lower anchorage is situated in the roof of the tendon gallery and the higher anchorage is placed on the ring in the base level of the dome.
• Horizontally looped tendons (hoop tendons)
  These can accommodate a varying number of vertical buttresses (ribs) – for example, four buttresses with 180° tendon hoops or three buttresses with 240° tendon hoops.The tendons are anchored alternately in one of the vertical ribs and both anchorages are situated on opposite sides of the same rib.
• Cane shaped tendons
  These are placed in two right angled planes in the dome, which have a varying length.The lower anchorage is situated in the roof
  of the PT gallery and the upper anchorage is placed under the
  retaining ring of the dome.

During the entire lifetime of the pressure vessels, the greatest attention must be paid to the protection of the prestressing steel from corrosion. The BBR Network uses three different types of corrosion protection for nuclear tendons:

• Cement grouted tendons
  The cementitious grout surrounds the prestressing steel in an alkaline environment that inhibits corrosion. If there is a local fracture of a tendon, part of the prestressing force remains transmitted to the concrete due to the bond with the grout.
• Grease or wax grouted tendons
  For these unbonded tendons, the prestressing force is transmitted to the concrete primarily at the location of the anchorages. Corrosion is prevented by grease, corrosioninhibiting compounds or wax. The tendons can be mechanically tested in-situ and the actual prestressing force can be monitored by lift-off tests or through permanently installed equipment.They can be retensioned and they can also be removed for visual inspection and eventually replaced.
• Tendons protected with circulating dry air
  For these unbonded tendons, the same properties and advantages apply as for the grease grouted tendons.

For the continuous safety of service of large tendons, accurate measurement of the stressing force and regular controls are of great importance. For many years now, BBR Network members have been carrying out lift-off tests and the experience of Norwegian BBR Network member, Spenntekknik, is typical. Our periodical surveillance procedure is as follows:

• Lift-off of the anchor head with the BBR SA 500 automatic stressing device to determine the actual prestressing force – this is digitally recorded by an x-y-writer.
• The tendon is subsequently released and a single tensile element is extracted for corrosion examination and further testing in the technical laboratory.
• A new single tensile element is installed and the tendon is stressed again to its original prestressing force.

Utilising the latest technology for monitoring prestressing force, all types of BBR VT CONA CMI anchorages can be equipped with the BBR WIGAring ELASTO force measuring system. It allows for longterm tensile force measurements with digital control, memory, WINDOWS compatible data files and printing facilities. In addition, Spenntekknik has been assisting with the replacement of steam generators at one of the plants. This work requires a large hole to be cut in the one metre thick secondary containment wall. The BBR team removes both horizontal and vertical tendons before the cutting process – and destresses the wall on the opposite side to avoid unexpected forces developing in the wall – keeping it in balance – during the replacement work. As well as preparing Risk Assessment and Health & Safety statements for each job, our staff have to undergo a three day training course which covers behaviour inside the special facility. They are carefully screened and personal radiation logs are maintained and provided by the owner.

BBR engineers were pioneers in the truest sense of the word when it came to developing high capacity tendons for nuclear applications. As early as the 1960s, they tested the first generation of large tendons for nuclear power plants – BBR wire tendons with button heads and BBR strand tendons with wedges. This early BBR testing of tendons involved the construction of a special facility at the tendon assembly plant in Switzerland, in order that large tendons could be tested in conditions which simulated the environment of a reactor vessel. Although in the sixties, at the beginning of this new application, some pressure vessels had been realised with tendons which are today considered quite small, the use of large capacity tendons soon became common practice. Since 1965, prestressing by post–tensioning has become a clearly established technique for pressure confinement in the nuclear power industry. Today, the recognised adaptability and reliability of BBR Prestressing Systems for the post-tensioning of nuclear power plant pressure and containment vessels has made them some of the best known internationally. The BBR team has now completed close to 100 nuclear energy projects in 15 countries.

 

The BBRVT CONA CMI system has passed all prescribed European Organisation forTechnical Approvals (EOTA) tests with flying colours – and has even withstood more stringent voluntary testing commissioned by the dedicated BBR engineers. From the very beginning, ever since the foundation of the BBR partnership in 1944, many hundreds of tests have been executed in different technical laboratories for the approval of various anchorage types in many countries. However, since 2002, BBR VT CONA CMI bonded post-tensioning anchorage types have been developed and tested extensively in accordance with standards set by EOTA – and have secured European Technical Approval. Static load, resistance to fatigue and load transfer tests were completed successfully according to ETAG guidelines. Although well in excess of the ETAG requirements, the team decided to go above eight million load cycles – already over four times more than required – in one additional voluntary test run. It was amazing that, even under these extremely hard conditions, the tendon endured this gruelling fatigue testing without any strands breaking. It is worthy of note that these tests were carried out as part of the certification of the largest post-tensioning tendon ever to be tested to these guidelines – anywhere in theWorld. In addition, a full scale installation, replacement and grouting test was undertaken for the 6106, similar to the trials carried out on smaller tendons by BBR forefathers.