The first commercially operational nuclear reactor in the world was the Calder Hall station, which became part of the Sellafield site. Calder Hall operated between 1956 and 2003. Its gas-cooled Magnox design was used for 22 reactors at 10 other sites in the UK. It also formed the basis for the second-generation advanced gas-cooled reactor (AGR) that has become the standard for the UK’s nuclear power since the 1980’s.
The AGR improved on the Magnox by increasing the gas temperature to 648°C for more efficient steam turbine operation. In fact a standard turbo-generator plant from a coal-fired power station is used. Both designs use a graphite core as the neutron moderator, slowing down the neutrons so they are more able to collide with atomic nucleus, and CO2 gas as the coolant. Magnox reactors used use natural (unenriched) uranium as the fuel, while the newer AGRs require enriched uranium.
Created during the cold war, Britain’s Magnox reactors were designed to have a dual purpose, producing electrical power while also enriching uranium to create weapons grade plutonium. They were so effective that the UK ended up with a large surplus of bomb-grade material. The design, which the UK released into the public domain, is still used by North Korea for this reason.
Modern nuclear reactors typically use normal water as a neutron moderator (they are called PWRs, pressurised water reactors). However, water absorbs some of the neutrons. Graphite more efficiently conserves neutrons, allowing reactors to produce weapons-grade plutonium from natural uranium. Because graphite easily oxidizes, carbon dioxide is used as the coolant, transferring heat from the reactor core to a heat exchanger, where water is superheated and used to drive a turbine.
The six remaining operational AGR power stations are all approaching the end of their lives, with closures scheduled between 2022 and 2030 (see table). Free from the need to produce weapons-grade material, new power stations currently under construction use the EPR design. This is a pressurised water-cooled design focused on safety and economical operation. Other designs are also being considered, such as the Chinese Hualong planned for Bradwell.
GRAPHITE ISSUES
In an AGR, the uranium fuel rods are surrounded by a graphite core, which acts as the neutron moderator. The core is constructed as a loose stack of thousands of graphite bricks arranged in a cylinder approximately 12m in diameter and 10m high. The graphite bricks are bound by a steel restraint and contained within a three metre-thick concrete pressure vessel. Control rods and the gas coolant also pass through vertical channels in the core. It is vital that the bricks remain aligned so that the channels remain clear for the movement of control rods and gases.
The configuration of fuel rods within the core is designed to achieve a stable critical reaction. If the reactor was operating at a perfectly steady state, each fission event would trigger exactly one other fission event. However, natural variations mean that the reaction might fizzle out (go subcritical) or become dangerous (go supercritical). Additional neutron absorbing control rods provide a way of controlling the speed of the reaction in real-time. The control rods in an AGR use boron to absorb the neutrons. These are moved up and down through channels in the graphite core to provide this control.
Over time, two changes have been seen in the graphite core – cracking and aging. The big problem here is that cracks in the core may impede the movement of control rods. If the control and shutdown rods get stuck, it becomes more difficult to control and shut down the reactor. The secondary safety system is to inject nitrogen into the CO2 coolant gas, and the tertiary shutdown involves injecting boron beads. All of these systems require access to the core through the vertical channels. Maintaining channel alignment in the core is safety critical, and the core must be able to sustain this even in the event of an earthquake. In a worst-case scenario, it would not be possible to shut down a reactor as it goes supercritical, which could have disastrous consequences.
Although some loss of strength in the reactor core was expected during the life of the AGR reactors, during operation there has been increasing uncertainty over the structural integrity and residual strength of the cores. Initial planned periodic shut-downs of reactors allowed for limited in-core inspections in a sample of channels. These found extensive cracking which could not be fully explained, and it was not possible to extrapolate how many of the uninspected bricks may also have cracked. It was also not possible to determine a safe limit for cracking.
The Nuclear Installations Inspectorate therefore imposed more frequent inspections and longer shutdowns. However, even with these measures the Nuclear Safety Directorate noted, “an increased likelihood of increased risk should we agree to continued operations.” These issues and dangers were initially kept out of the public domain by the operators. They were first revealed by a Freedom of Information request relating to Hinkley Point nuclear power station, documented in a 2006 report by Large and Associates. The creation of the government’s Office for Nuclear Regulation (ONR) in 2011 has improved transparency in the inspection processes. (Electrical utility EDF, the operator of all of these reactors, declined to comment for this article).
Work over the last two decades has improved fundamental understanding of cracking in the graphite core. Aging mechanisms include radiolytic oxidation, the effects of neutron dose, irradiation creep, onset of cracking and cracking rate, crack progression and geometrical shape changes of core components. During operation, the graphite can be oxidised due to the extreme conditions inside the core and so undergo weight loss. Differential shrinkage caused by neutron interaction throughout the brick can also cause radial cracking to occur. Articulated control rods have also been installed in some reactors to give improved tolerance to movement of the core. Improved inspection methods, such as eddy current devices, allow sub-surface crack detection. However, even in its latest report, the ONR acknowledges that: “Given the large numbers of channels and bricks present, it may not be reasonably practicable to inspect statistically significant sample sizes for an entire reactor core.”
INSPECTIONS
The ONR lists the changes in graphite cores that may affect safety and therefore must be considered:
- The size, shape and position of graphite components
- Properties - including stored (Wigner) energy
- Developing internal stresses
- The initiation and growth of cracks
- Developing forces and moments between components
- The formation of potentially mobile debris
- Additional aspects of structural integrity such as oxidation.
Because the channels in the reactor core are arranged vertically, all inspection instruments can be remotely inserted into the core using lifting cables. It is understood that direct in-service inspection may not be able to determine all of these parameters and other means may therefore be employed to give confidence. These include mechanistic understanding of material behaviour, information on how the cores were constructed and predictive modelling.
The main inspection method for cracks is to insert an eddy current probe into the vertical channels of the graphite core. A simple eddy current probe consists of a conducting coil to which an alternating current is applied, inducing eddy currents in the material being tested. The conductivity and permeability of the material cause a change in the eddy currents. This can be measured as a change in the phase and amplitude of the coil impedance. This technique is commonly used to detect cracks and defects in conducting materials. This is a non-contact technique.
Where samples must be removed for further analysis, a graphite trepanning tool (GTT) is used. This is a steel cylinder with two sets of three arms which deploy radially to clamp the tool in position. A cylindrical hollow steel tube cutter can then be deployed to remove a small core of graphite material. An ‘air knife’ is used to contain graphite dust during the cutting operation.