The Magnox reactor type was the first gas-cooled reactor to be produced in any quantity. However, almost all designs used on individual sites were unique. This was in part no doubt due to the rapidly developing technology of nuclear power and also the fact that there were a number of different consortia involved in the construction. This approach resulted in a lack of economy of scale such as that seen in the building of series of PWR and BWR plants in France and the USA.

11.3.1 Main plant features

The majority of Magnox reactors constructed were of the steel pressure vessel (SPV) type. The exceptions are Oldbury and Wylfa (UK) and Chinon A3, St Laurent des Eaux and Bugey 1 (France), which had concrete pressure vessels.

Steel pressure vessel reactors

The general layout of most SPV reactors was of a spherical vessel of diameter typically ~20 m containing the reactor core, with typically six hot and six cold ducts leading to external vertical heat exchangers. However, the early reactors (Chapelcross, Calder Hall and Berkeley) had cylindrical pressure vessels. In those early designs each of the heat exchangers was in a separate building, fed with external, exposed ductwork. Later designs saw the heat exchangers housed in the same building as the reactor and bioshield, giving notionally better protection from external impact events.

The material used was essentially a mild steel, of thickness in the range 50-100 mm. The construction was by welding plates in situ, with the reactor pressure vessel (RPV) being supported either on a cylindrical skirt or on a system of rollers to allow for expansion movement.

The cylindrical graphite core, typically of around 15 m diameter, was mounted on support plates, in turn sitting on the diagrid. The latter was a massive steel support structure, the name apocryphally coming from an abbreviated note indicating ‘diameter of grid’ written on an engineering drawing. The diagrid and support structure had many penetrations to allow coolant flow to the fuel channel in the graphite stack.

The graphite core could be as massive as 4000 Te and the fuel weight totalled between 113 Te and 350 Te (uranium only). Earlier designs of reactor used a ball­bearing interface between the graphite and support structure to accommodate differential thermal expansion, whereas later designs used spigots to locate the graphite. The latter was enabled by development of a core restraint very similar to that used in the later AGR design. Both designs used quite complex linkages to tie the graphite to a steel structure, which expanded at different rates. The core restraint of the earlier reactors used a temperature compensated steel circumferential

hoop, designed to expand at the same rate as the graphite (using similar techniques to those deployed centuries earlier in pendulum clocks). Later designs used a steel restraint structure in which the graphite expands radially at the same rate as the steel. This was accomplished by rigid tie rods set between the core restraint and the outer bricks, with differential vertical expansion being accommodated by pivoting ‘Warwick’ links.

Access for fuelling and servicing of control rods for example, is via a series of standpipes at the top or bottom of the vessel. Magnox reactors have a large number of fuel channels, varying in UK designs from 1700 for early reactors to 6150 at Wylfa. In order to reduce the number of penetrations, each standpipe served a number of fuel channels (varying from 16 to as many as 60). Most reactors were fuelled from above the reactor, but Hunterston A was unique in the UK in being fuelled from beneath; the reactor pressure vessel and bioshield were suspended above a large refuelling hall.

The integrity of the pressure vessel was a key part of the reactor safety case, resulting in complex rules being introduced to keep the vessel operating in a region where brittle fracture was not possible — i. e. in a region where, in extreme conditions, leak before break would occur. The pressure vessel temperature and pressure operating envelopes were revised over the lifetimes of the stations as understanding of radiation embrittlement developed. The pressure vessels are exposed to neutron irradiation, causing displacement of atoms in the steel grains and building up defects. Above a certain temperature the defects are annealed out of the steel and the fabric is maintained in a ductile region. Extensive programmes of monitoring and research were undertaken to understand the phenomena.

Corrosion of reactor steels was also an issue, with early limits being placed on some reactors due to failure to control the steel grade of bolts and washers adequately. Fatigue failure was also an issue, but limited in likelihood by keeping the number of major changes in temperature to a minimum (i. e. keeping the number of start-up/shut-down cycles to a minimum, and not load-following to any extent).

Expansive forces on the ductwork leading to the boilers were either taken up by designing the possibility of movement into the structure, using a system of ductwork hangers, or by use of bellows joints with rigidly fixed ductwork.

The biological shield (bioshield) comprised a separate, thick concrete cylinder surrounding the SPV. In order to control the temperature of the concrete, a shield cooling air flow was maintained by passing air between the SPV and the bioshield. This air was exposed to neutron irradiation, resulting in discharges from the cooling air stack of short-lived N-16 (7 s half-life), O-19 (26 s half-life) and Ar-41 (1.8 h half-life).

Concrete pressure vessel reactors

In the UK, only four Magnox reactors were constructed using pre-stressed concrete pressure vessels (PCPVs): two each at Oldbury and Wylfa (Fig. 12.1).

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CAD REF. WYA/A2Z/031

72.7 A diverse range of fuel elements was produced for the Magnox reactors in the UK.

These represented the latest designs of the Magnox series, and had many features in common with the slightly later AGR reactors. In France, Chinon A3, St Laurent des Eaux and Bugey 1 were also of a concrete pressure vessel design.

The advantages of the PCPV over SPVs were many, including:

• lower cost

• greater strength (so higher coolant pressures were possible)

• combining the bioshield and pressure vessel into one structure

• avoiding working at the limits of weld technology

• flexibility of shape

• enabling much larger volumes to be incorporated, so the boilers could be built into the pressure vessel avoiding the need for external ductwork

Pre-stressed concrete pressure vessels (PCPVs) are constructed of concrete, many metres thick (5 m being a typical dimension). Each vessel contains a large number (thousands) of steel tendons in a helical formation from top to bottom threaded through mild steel tubes, which are embedded in the concrete. Each tendon consists of a number of strands of wire and each strand is of seven-core construction. Stressing galleries above and below the top and base caps of the vessel permit tensioning of the steel cables used to maintain the concrete in compression.

The construction comprises a cylindrical steel liner with lugs to locate it once the concrete is poured, with cooling pipework and penetrations (in particular for water and steam, gas circulator penetrations and refuelling standpipes) built into it.

The liner is internally insulated with foil mesh to keep hot gas away from the surface, and cooling water flowing through pipes in the concrete keep the temperature to typically ~40 °C. As repair of these pipes is to all intents impossible once the concrete has been poured, a degree of redundancy is incorporated to enable plugging of defective tubes, and strict chemical control is applied to prevent corrosion. Radiolytically generated oxygen is inhibited or removed by application of an overpressure of hydrogen or continuous degassing of pressure vessel cooling water.

As mentioned above, the boilers of Magnox reactors with a PCPV are contained within the PCPV cavity. The reactor core of these reactors was surrounded by a boiler shield wall, keeping neutron activation of the boilers (and the pressure vessel) to a minimum and permitting manned access to the boilers for inspection and repair. The refuelling arrangements and fuel channels were generally similar to the older SPV stations (bearing in mind that no two were identical).