Fuel design

A diverse range of fuel elements was produced for the Magnox reactors in the UK alone (Fig. 12.2). Magnox reactor fuel channels contained typically eight elements,

although up to 13 were used per channel in Berkeley. Magnox fuel was of natural enrichment, other than in two of the Chapelcross reactors used for tritium production and in Oldbury towards the end of its life due to graphite corrosion having reduced the amount of moderation available.

The cladding alloy of Magnox fuel was principally magnesium with small levels of aluminium (AL80 alloy) or zirconium (ZR55 alloy) in the UK and magnesium/zirconium in French reactors. The Italian and Japanese reactors (Latina and Tokai Mura respectively) used UK-manufactured fuel whereas Vandellos 1 used French fuel. Other fuel element components used a number of different composition alloys. The various Magnox alloys all have low neutron absorption cross sections.

Most Magnox fuel produced in the UK fell into two categories, known as ‘herringbone’ and ‘polyzonal’ (also known as helical).

Polyzonal fuel cladding was ribbed in a helical pattern around the fuel element to increase heat transfer between the uranium and the coolant. The helical pattern resulted in a rotational force being applied to the fuel elements, and so in many designs a spring-loaded arm was added to the top fitting to hold the element stationary in the channel (using friction). The spring was made of a Nimonic alloy containing high levels of stable cobalt: as a result, the Nimonic springs became highly activated with Co-60 in the neutron flux.

Herringbone fuel had ribs running diagonally, with each quadrant of the fuel having ribs running in opposite directions. Thus, there was no net rotational force and no requirement to prevent rotation.

Rib height varied between 7.6 mm and 11.7 mm.

Fuel was located centrally within the channel in a variety of ways:

• Berkeley fuel had bridge pieces located at top and bottom holding a pair of graphite struts running the length of the fuel element.

• Hunterston, Tokai Mura and some Chapelcross fuel was loaded inside a graphite sleeve.

• All fuel was equipped with either ‘splitters’ (four longitudinal Magnox alloy strips held in place by a small number of circumferential braces) or, in the case of herringbone fuel, ‘lugs’ (five or seven longitudinal raised sections on each quadrant of the fuel element, integral with the rest of the cladding). These served to keep the fuel centred in the channel or in the graphite sleeve, as well as providing a means of breaking the flow and increasing heat transfer.

French Magnox fuel was principally of herringbone design.

A variety of end fittings were used to enable fuel elements to be grabbed for refuelling purposes. Fuel elements were individually handled in each channel, i. e. they did not latch onto each other. All fuel elements were fitted with internal ceramic insulating discs at the top and bottom of the uranium rod to protect the cladding from the hot inner regions of the fuel.

The bottom of each fuel channel was fitted with a gag and fuel support unit, incorporating a shock-absorber to protect dropped fuel. The gag was pre-set to restrict flow in each channel, and was not capable of subsequent adjustment. Hunterston A had a separate cast-iron fuel element support member at the bottom of each channel, which was replaced during each refuelling, resulting from the reactors being charged from beneath rather than the conventional layout of pile-cap refuelling.

The low melting point of the metallic fuel and cladding resulted in low irradiation temperatures. The maximum cladding temperature was nominally in the range 400 °C to 470 °C, with maximum fuel temperatures in the range 500 — 600 °C. Maximum thermal ratings varied from approximately 3.5 to 5 MW/te(U).

The mass of Magnox fuel elements was also highly variable according to design, with uranium metal content varying between approximately 5 kg and 12 kg, and gross element weights in the range 7 kg to 20 kg.