Eskom takes a cool look at high temperature reactors19 October 1998
The high temperature gas cooled reactor (HTGR) has a number of attractive inherent safety features, although there are none currently in commercial operation. South African utility Eskom, which has two pressurized water reactors at Koeberg, has a major programme underway to investigate the feasibility of the HTGR as a future option for its system.
The Eskom HTGR effort is focused on a version of the technology called the pebble bed modular reactor (PBMR). This has been under active investigation since 1993 as part of Eskom's Integrated Electricity Planning process. The overall objectives of these investigations have been to establish whether such a system could form part of Eskom's expansion planning and what specific advantages it would bring over current options.
The first phase of Eskom's investigation (carried out by Eskom with IST Industrial) is now complete, and the results have been compiled in a comprehensive set of technical and costing reports.
The results so far show that the design has been established in enough detail to support safety studies, confirm operating limits and allow costs to be estimated, including production costs, O&M costs, fuel plant costs and design and development cost.
They also indicate that the technology required for the design has been sufficiently demonstrated to avoid fundamental technical risk.
The increased level of inherent safety over current nuclear plant designs is fundamental to the cost reductions achieved over other nuclear options. By demonstrating a catastrophe-free design, the requirements for both safety grade back-up systems and an off-site emergency plan are removed.
The operational requirements were established at the start of the project by the issuing of user and owner requirements documents. These objectives were developed in the light of the potential of a reactor linked to a closed cycle gas turbine system, the current cost imperatives in Eskom and the increasing need for a load following power station. This feature became a prime system driver in terms of a user requirement and led to a major effort being put into consideration of load following capabilities and maintenance requirements.
The fundamental design philosophy is aimed at achieving a system without any physical process that could cause an internally induced and/or externally induced radiation hazard outside the site boundary. This is principally achieved in the PBMR by demonstrating that the system stabilizes itself neutron physically and thermal-hydraulically by appropriate inherent feedback mechanisms. Neutron physical self-stabilization is provided by the small excess reactivity margin and by the strong negative temperature coefficient of reactivity, together with sufficient temperature margins. The thermal-hydraulic stabilization is provided by modularizing the core with a relatively low power density (less than 4.5 MW/m3), such that the integrated heat loss capability from the reactor exceeds the decay heat production of the core under all conceivable accident conditions. This means that during a design base event such as the de-pressurized loss of forced cooling, the peak temperature reached will never threaten fuel retention, which has been demonstrated to 1650 °C. The possibility of a "core melt" scenario due to a reactivity and/or heat-up event is therefore ruled out.
The use of helium as a coolant, which is both chemically and radiologically inert, combined with the high temperature integrity of the fuel and structural graphite, allows for the use of high primary coolant temperatures (900°C) that yield high thermal efficiencies. With these high temperatures, the use of a closed cycle gas turbine is justified. It results in an increased efficiency over a steam plant, thus reducing the output-specific capital cost.
The PBMR fuel is based on the proven high quality German moulded graphite sphere and TRISO coated particles. The PBMR uses fuel elements, called pebbles, in which the uranium fuel is distributed among many small fuel particles, called kernels, each coated with two high density layers of pyrocarbon and one layer of silicon carbide and which is embedded in a carbon matrix, called the fuel zone.
Probably the most important safety feature of the PBMR is that radioactive fission products produced during system operation are confined within the fuel during all operating and accident conditions in such a way that there will be no significant release of radioactivity from the fuel particles.
This safe confinement of radioactivity is assured by the design of the fuel particle coatings. The silicon carbide layer, in particular, is so dense up to temperatures of 1650°C that no radiologically significant quantities of gaseous or metallic fission products are released from the fuel elements.
The fuel pebbles have a 50 mm diameter inner fuel zone. The fuel zone is covered by a 5 mm thick fuel free graphite matrix zone, resulting in a spherical fuel pebble having a diameter of 60 mm. The fuel zone contains some 11 667 coated particles, the equivalent of 7g uranium. About 301 000 fuel pebbles and 137 000 pure graphite pebbles are required for a single core loading.
It is planned to construct the fuel manufacturing plant for the PBMR project in South Africa at the Atomic Energy Corporation within the existing BEVA facilities.
The plant typically consists of a single building approximately 50m x 26m in plan and 42m in height, with 21m below ground level. Thus the main building will be constructed with only half its height protruding above the ground surface level, depending on the specific site.
The building layout will be designed to facilitate easy access for all components and for easy handling of these components within the building. This will include maintenance laydown areas for performing maintenance tasks.
The layout also makes provision for the storage of spent fuel for the 40 years of operating life cycle of the system and after shutdown of the plant for an additional 40 years of interim spent fuel storage. This implies that no radioactive waste will be removed from the site during the lifetime of the plant.
The system will be designed to withstand specific predicted seismic conditions. The system will also be designed to withstand the direct impact of specific high speed aircraft as laid down in the stringent German aricraft crash specifications.
The process cycle used is a standard Brayton cycle with closed circuit water cooled inter-cooler and pre-cooler. The use of separate turbo-compressors and power turbine with adjustable stator blades is envisaged.
The heat generated by the fuel within the core is transferred to the helium and exits the reactor at a temperature of about 900°C and pressure of about 69 bar. The helium gas flow passes through the high and low pressure turbo units, through the power turbine which in turn drives the generator. A highly effective recuperator is used after the power turbine to recuperate the thermal energy. The lower energy helium is passed through the pre- and inter-cooler and the low and high pressure compressors. The helium is returned to the core at about 540°C and 70 bar.
The system is started up using an auxiliary compressor, nozzle and terry turbine arrangement on the low pressure and high pressure turbines. Compressed air is introduced via a nozzle and an air jet acts on terry turbine blades mounted on the axial disk. The whole turbine compressor assembly is turned in this way, which in turn causes the helium to circulate throughout the cycle.
The fuelling principle currently planned is the MEDUL (MEherfachDUrchLauf, German for re-circulation), ie on-line fuelling.
The PCU heat sink will depend on the site details, but the current design is based on cooling water inlet at 22°C and outlet at 34.6°C (this could be increased to higher temperatures for process use without dramatically impacting plant efficiency). The present options considered are sea water cooling with a closed-loop intermediate circuit and backup cooling.
Control is achieved by altering the amount of helium in the volume inside the main circuit. Adding helium increases the pressures and mass flow rate without changing the temperatures and pressure ratios. The increased pressure and subsequent increased mass flow increase the heat transfer rate, thus increasing the power. Power reduction is achieved by evacuating gas from the circuit. The power control system revolves around a series of helium storage tanks ranging from low pressure (LP) to high pressure (HP) to maintain the required pressures. Short term control is achieved by the adjustable stator blades on the turbo machinery and bypass flow.
During reactor shut down, residual heat is removed by an alternative system, namely the active and/or passive cooling system.
Energy parks consisting of a number of modules (up to 2x10) are envisaged on one site with a common control centre for every 10 modules.
Given the operating and maintenance requirements, the staffing of the stations would be comparable with a combined cycle gas fired plant.
All the estimates done on the project are based on the completion of one prototype module followed by 10 production modules, making up the first power station unit. A construction period of 24 months per module, on-site to criticality, is thought to be achievable. A key factor determining construction periods will be the availability of long lead items, in particular the reactor vessels.
The production of the first fuel load will dictate the possible starting date of the nuclear commissioning. The fuel production facility will therefore be managed as a critical facility to achieve the required system schedules.
Two identical fuel production lines are planned at this stage. The first fuel production line will be built in order to qualify the fuel manufacturing process and to deliver the first core load for the reference module. The two fuel production lines envisaged for the current project (11 modules) would be adequate, if run at 100 per cent capacity, to support the operation of up to 18 modules.
Eskom's Pebble Bed Modular Reactor programme is currently focused on the following main areas of activity:Application for nuclear licence with the South African regulator (Council for Nuclear Safety). Initiation of the environmental impact analysis process. Establishment of single programme team in Centurion (about 40 people). Finalization of basic design. Prequalification of key suppliers. Negotiations with potential joint venture partners. Tender process for detailed design of long lead items.
When combined with public and stakeholder consultations, these steps are intended to enable a decision on the potential construction of the first reactor to be taken by the end of 1999. This would include full consideration of the development, construction and operating costs, design parameters and the site location.
|Some still like it hot|
| because-of-the-high-temperature-stability-of-the-core-components-the-inert-coolant-the-power-levels-and-the-core-configuration-high-temperature-reactors-can-be-guaranteed-to-be-literally-meltdown-proof-a-characteristic-not-shared-by-any-current-reactor-type-development-of-high-temperature-reactors-goes-back-to-the-late-1950s-and-at-least-five-operated-in-the-1960s-and-1970s-including-dragon-in-the-uk-two-german-pebble-bed-plants-avr-and-thtr-300-and-peach-bottom-and-fort-st-vrain-in-the-usa-whiTablesPerformance characteristics for the first reference module, Eskom Pebble Bed Modular Reactor
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