PBMR: the future is now20 August 2001
The detailed feasibility report on South Africa’s proposed Pebble Bed Modular Reactor (PBMR) is nearing completion and is due to be considered by the project’s stakeholders over the next few months. Tom Ferreira, PBMR (Pty) Ltd, Centurion, Pretoria, South Africa
Although the South African PBMR is not the only high-temperature, gas-cooled nuclear reactor currently being developed in the world (China, for instance, started up a 10 MWt research reactor in December 2000), the South African project is regarded as the leader in the field.
Under development by the South African utility Eskom, the South African Industrial Development Corporation (IDC), British Nuclear Fuels (BNFL) and the US utility Exelon Corporation, the intention is to build a 120 MWe demonstration module at Koeberg, near Cape Town, where Africa’s only nuclear power station is situated. Eskom is the dominant voice with a 30 per cent share. The IDC has 25 per cent, BNFL 22.5 per cent and Exelon 12.5 per cent. The remaining 10 per cent is reserved for black empowerment investment.
The design target construction cost for a commercial-scale PBMR plant, consisting of 10 modules with a total output of about 1 200 MWe, is about US $1000 per kWe installed, compared with US $900 per kWe for a new coal-fired power station in South Africa. If this target is achieved, the PBMR’s output cost should be well below the world average cost of U.S. 3.4 cents/kWh.
The first phase of the project, which was given the go-ahead by the South African government in April 2000, involves undertaking a detailed feasibility study, an environmental impact assessment (EIA) and a public participation process.
Once the shareholders have considered the detailed feasibility report (DFR) and given their stamp of approval, the DFR will be submitted to the South African government for review by an independent team of experts.
The environmental impact assessment (EIA), including public participation, started in June 2000. The current target date for a record of decision from the South African government on the EIA is February 2002. Environmental challenges centre largely on the disposal of spent fuel, which, in South Africa, could be disposed of at Vaalputs, where there are appropriate underground granite formations. The PBMR fuel spheres are already an ideal container.
Assuming shareholder approval, a favourable outcome of the environmental impact assessment, the issuing of a construction licence by the South African national nuclear regulator and government consent, it is envisaged that construction work could begin by the second half of 2002. If so, the first PBMR would be due for completion by the beginning of 2005 and operational by 2006.
Building on experience
Although the precise arrangement being used by the PBMR is new and will require further development (such as the use of low-maintenance magnetic bearings), much of the technology and many components are identical to what has been developed for ship propulsion systems in recent years.
While the PBMR draws heavily on German experience, notably with the AVR research reactor and the THTR power reactor, and a major technology transfer licence was negotiated in 1999 with the developers of the original design, ABB and Siemens, the PBMR project seeks to avoid the principal operating problems that the Germans encountered by strictly limiting unit size and simplifying other systems.
The PBMR consists of a vertical steel pressure vessel, 6m (19.7 ft) in diameter and about 20m (65 ft) high. It is lined with a 100 cm (39 inch) thick layer of graphite bricks, which serves as a reflector and a passive heat transfer medium. The graphite brick lining is drilled with vertical holes to house the control rods.
The PBMR uses silicon carbide and pyrolitic carbon-coated particles of enriched uranium oxide encased in graphite to form a fuel sphere, or pebble, about the size of a tennis ball. Helium is used as the coolant and energy transfer medium to a closed cycle gas turbine and generator system.
During normal operation, the PBMR core contains a load of 440 000 spheres, 330 000 of which are fuel spheres. The balance are solid nuclear grade graphite spheres, which serve the function of an additional nuclear moderator.
The graphite spheres are located in the centre of the core and the fuel spheres in the annulus around it. This geometry limits the peak temperature in the fuel following a loss of cooling.
To remove the heat generated by the nuclear reaction, helium coolant enters the reactor vessel at a temperature of about 500°C and a pressure of 70 bar (7 MPa). It then moves down between the hot fuel spheres, after which it leaves the bottom of the vessel having been heated to a temperature of about 900°C.
The hot gas then enters the first of three gas turbines in series, the first two of which drive compressors and the third of which drives the electrical generator. The coolant leaves the last turbine at about 530°C and 26 bar (2.6 Mpa), after which it is cooled, recompressed, reheated and returned to the reactor vessel.
The process cycle used is a standard Brayton cycle with a closed circuit water-cooled intercooler and precooler. A high efficiency recuperator is used after the power turbine generator to recuperate the thermal energy. Lower energy helium is passed through the precooler and intercooler and the low- and high-pressure compressors before it is returned through the recuperator to the reactor core.
The significance of the high pressure and high temperature of the helium coolant lies in its superior thermal efficiency. By comparison, the steam turbines for light water reactors (LWRs) operate at such low temperatures and pressures that they are more costly to build and less productive than the turbines for a fossil-fired plant, where temperatures and pressures may be several times higher.
While a typical LWR has a thermal efficiency of 33 per cent, a heat efficiency of over 40 per cent is anticipated in the basic PBMR design. Increases in fuel performance leading to higher operating temperatures, offer the prospect of up to 50 per cent efficiency.
On-line refuelling is another key feature of the PBMR. The reactor will be continuously replenished with fresh or reusable fuel from the top, while used fuel is removed from the bottom. The fuel pebbles are measured to determine the amount of fissionable material left. If the pebble still contains a usable amount of the fissile material, it is returned to the reactor at the top for a further cycle. Each cycle is about three months.
The PBMR fuel is based on the proven high-quality German moulded graphite sphere and triple-coated particles (Triso). Essentially, the fuel elements are multilayer spheres consisting of enriched uranium and various forms of carbon.
For the PBMR fuel, the first layer deposited on the kernels is porous carbon, which allows fission products to collect without overpressurising the coated fuel particle. This is followed by a thin coating of pyrolitic carbon (a very dense form of heat-treated carbon), followed by a layer of silicon carbide (a strong refractory material), followed by another layer of pyrolitic carbon.
The porous carbon accommodates any mechanical deformation that the uranium oxide particle may undergo during the lifetime of the fuel. The pyrolytic carbon and silicon carbide layers provide an impenetrable barrier designed to contain the fuel and the radioactive decay products resulting from the nuclear reactions.
Some 15 000 of these coated fuel particles, about a millimeter in diameter, are mixed with a graphite phenol powder and pressed into the shape of 50-mm diameter balls. A 5 mm thick layer of high-purity carbon is then added to form a “non-fuel” zone, and the resulting spheres are then pressed, sintered and annealed to make them hard and durable.
Finally, the fuel elements are machined to a uniform thickness of 60 mm, about the size of a tennis ball. Each fuel sphere contains 9 g of uranium, which means that the total uranium in one fuel load is 2.79 tons. The total mass of a fuel sphere is 210 g. The U-235 in the pebbles is enriched, on average to 8 per cent.
The spent fuel storage consists of 10 tanks, each able to store 500 000 spheres. Storage is dry at all times, with decay heat carried away by natural convection.
The South African Nuclear Energy Corporation (NECSA), where fuel rods for the Koeberg nuclear reactor were manufactured in the past, is under contract from the PBMR project team to develop the fuel manufacturing capability using the technology established in Germany.
The PBMR is inherently safe as a result of the design, the materials used, the fuel and the physics involved. This means that, should a worst-case scenario occur, no human intervention is required in the short or medium term.
In the PBMR, the removal of decay heat is achieved by radiation, conduction and convection. The combination of the very low power density of the core (1/30th of the power density of a PWR), and the temperature resistance of the fuel (in billions of independent particles) to high temperature, underpins the superior safety of this type of reactor.
The peak temperature that can be reached in the reactor core (1600°C under the most severe conditions) is below the temperature that may cause damage to the fuel. The silicon carbide layer is extremely good at withstanding high temperatures.
Even if there is a failure of the active systems designed to shut down the nuclear reaction and remove core decay heat, the reactor will stop any nuclear fission and cool down naturally because of a strong negative temperature coefficient of reactivity and the inherent heat-removal mechanisms of convection and conduction.
This inherent safety renders the need for safety grade backup systems and most aspects of the offsite emergency plans that are required for conventional nuclear reactors obsolete and is fundamental to the cost reduction achieved over other nuclear designs.
To demonstrate inherent safety the German AVR underwent a public and filmed plant safety test, when all cooling to the reactor core was stopped and the control rods were left withdrawn. The reactor shut itself down within a few minutes and there was no deterioration of the nuclear fuel.
Similar to an aircraft manufacturer introducing a new type of plane, a project of this magnitude requires a “launch customer”. Therefore, Eskom has conditionally agreed to buy ten PBMR modules if the demonstration plant meets financial expectations.
The involvement of a major utility such as Exelon could accelerate the marketing process. Exelon has already indicated that it may want to apply for a licence to construct PBMRs in the USA as early as 2004.