For a typical dispersed PWR, the primary circuit includes a reactor pressure vessel (RPV), two or more steam generators (SGs) and a pressuriser.
The RPV, comprising a main body section and removable closure head, houses the reactor core. This consists of many fuel assemblies, containing the fuel elements where heat is generated, together with control rod assemblies that can be used to adjust the power level of the reactor by capturing neutrons to regulate the rate of fission. The RPV provides a pressure boundary and a flow path into and out of the core’s fuelled region.
Penetrations in the body connect it to primary circuit pipe-work carrying primary coolant to and from the reactor core; penetrations through the closure head accommodate rod control gear that adjusts reactor power levels.
The pressuriser prevents water coolant in the primary circuit from boiling by maintaining the system well above ‘saturation pressure’. This corresponds to the prevailing primary coolant temperature at which bulk boiling occurs. Electrical heaters form a steam bubble in the pressuriser, with the steam and primary circuit water in thermodynamic equilibrium, and automatic adjustments in power input to the heaters compensate for heat losses to maintain steady pressure.
The SGs convert the core’s heat energy into steam, which powers turbines to provide electrical power. Steam then passes through a condenser and is re-circulated as feed water, through the SGs.
The reactor core, like a boiler, provides heat to the coolant. For pressurised water reactors, the coolant and moderator is light water, with heat generated within the fuel by nuclear fission (energy released by splitting large atomic nuclei into smaller ones). A heat exchanger, called a stream generator, transfers this energy to a secondary circuit, where it produces steam to drive a turbine. This in turn can generate electricity or provide propulsion – as in the case of a submarine propeller.
Reactor core designs have to balance thermal-hydraulic, reactor physics and mechanical aspects. This ensures safe and efficient operation over the lifetime of the reactor. This can be up to 60 years in a civil nuclear plant or, for naval reactors, the 30-year-plus life of the submarines they power.
Key elements of design and methods fall into three categories:
Reactor physics design considers the neutron population, fissile properties of the fuel and the amount of fissile material throughout the reactor. This ensures the safe, efficient and predictable operation of the reactor core.
The physics design seeks to produce a core with as uniform an axial and radial power distribution as possible by changing the distribution of fuel and neutron absorbers within the fuel, coolant and control rods. The distribution is obtained by performing analysis using various two and three-dimensional computer codes. This is supported and verified by results from a low-power test facility and data from existing operational plants.
The power limit for a nuclear reactor is determined by how much heat can be removed from the reactor core during safe, continuous operation. Optimisation of the reactor’s thermal hydraulic design is crucially important in establishing its overall performance.
The thermal hydraulic design is also vital for reactor safety: it must avoid both the bulk boiling of the coolant and that the core component temperature limits are not exceeded.
The optimisation and safety justification of the thermal hydraulic design of the reactor core is achieved using computer-based analytical techniques supported by extensive rig-testing.
Components within the reactor core are subject to extreme environmental conditions. Components are designed using the latest computer-based design-by-analysis techniques including finite element analysis, computational fluid dynamics and computer-aided design. This produces 3D models to aid visualisation, and support manufacturing processes. Component design is undertaken against internationally recognised codes and standards. Designs are justified to satisfy their operational requirements throughout a full service life spanning 30 years or more.
Meticulous mechanical design and the careful selection of materials guarantee structural integrity throughout the reactor’s lifetime. Component integrity is justified using finite element analysis techniques, and further verified by a range of mechanical and material rig-testing. The designs are supported by safety justifications that must comply with very stringent nuclear safety requirements set by UK safety regulators.
The harsh environment of the reactor core demands long-life materials capable of withstanding high temperatures, high pressure, corrosion and must operate effectively and reliably over long periods of time.
Many materials must meet the general criteria for high-quality engineering applications – strength, toughness, corrosion resistance and manufacturability. Others must meet the unusual demands of a nuclear reactor, the working environment of which creates issues such as irradiation damage from neutrons emitted by the core and thermal degradation.
Irradiation damage is a major issue for reactor pressure vessels (RPVs), particularly in the regions adjacent to the core, causing material hardening and reduced toughness over plant-life. An RPV is typically produced from massive forgings of a ferritic steel alloy – chosen for its strength, ductility and irradiation properties. This is joined by a welds, the number of which are limited to reduce inspection requirements. The surrounding shielding, designed to limit exposure to radiation, is made from dense materials such as steel, water and polythene with a high absorption cross-section for neutron radiation.
The steam generator shell can also be made of welded ferritic steel forgings. The generator’s heat exchange tubing and headers, containing primary coolant, are often produced from stainless steel or from nickel alloys.
Control rods can be made from materials such as SINCAD (silver, indium and cadmium), boron carbide and hafnium, whose large neutron cross-section facilitates the capture of a large proportion of neutrons from a relatively small volume.
The reactor core, though a high-integrity and radiologically hazardous engineering product, utilises a number of standard manufacturing techniques. The nuclear environment imposes constraints, that together with stringent safety requirements, demand thorough and rigorous quality systems.
Larger components, including the reactor pressure vessel, are forged in sections and machined to the required dimensions before being welded together.
With over five miles of welding present in a reactor core, it is vital to ensure the product’s structural integrity. Welds tend to be weaker than the parent components being joined, so advanced welding techniques are employed followed by secondary machining to remove the weld bead and leave a level surface. Extensive use of the latest non-destructive evaluation techniques is used to ensure product quality and integrity of the joint.
Component designs are developed using a computer-aided design system that establishes components’ geometric definitions. For manufacture, these definitions are exported directly into control software to perform automatic profile machining.
In order for nuclear regulators* to grant an operating licence for a nuclear reactor they must be satisfied that the design can both function safely during normal operation and shut down safely and controllably in an accident.
Design safety is justified using a variety of tools, and executed throughout the design process. A combination of computational tools and rig-tests verify plant safety justification, together with data from existing operational reactors.
Specialist computational tools, developed over decades, produce results that are validated by a series of physical tests of rigs varying from scale models to full-size core sections.
Like many industries, the nuclear sector is undertaking more computer-based design evaluation and validation.
A number of factors surrounding the design and material selection for reactor components come under consideration in order to minimise the plant’s environmental and disposal burden.
The volume of material is minimised by avoiding non-essential elements of the design. Care is taken in selecting materials to ensure that the level of activation – where materials become radioactive by exposure to neutrons produced in the core – is kept to a minimum. Activation of some components such as the reactor pressure vessel is, nevertheless, inevitable as they sit in high radiation zones.
Coatings, surface treatments and environmental controls are utilised to limit corrosion degradation of components during their service life This enhances the potential to reuse materials and the ability to recycle them. Potential routes for safe disposal are investigated as part of the design process, and non-hazardous materials are used in manufacture.
All nuclear plant components are subject to a rigorous verification process to demonstrate that the designs satisfy all contract requirements and meet the demands of relevant safety legislation.
Analysis and simulations show basic performance, physical testing puts a component through the extremes of its design parameters and demonstrations are undertaken to prove that components behave as intended. This thorough set of validation activities is concluded by an inspection and review to confirm all requirements have been met.