will comprise a simple continuous 180 mm reinforced concrete slab with adequate expansion joints. Con­ struction joints should be carefully detailed and con­ structed to avoid excess leakage and hence deteriora­ tion of the underlying formation. On poor ground the pond floor will require simple pattern piling which has to be fully compatible and probably integral with the packing support piles.

Since operational requirements necessitate the emptying of the ponds on occasions, due attention must be paid in the design to flotation where deeper ponds are built below basic ground level.

6.7.4 The distribution culvert

The culvert that carries water from outside the tower to a distribution system above the packing is a relatively heavy reinforced concrete box culvert of several pos­

its length will produce lateral forces on the support columns as the structure expands at full operating temperature from a cold start. Hcncc bending moments will rcsull. Isolation of this foundation from the sur­ rounding pack support foundation is usual.

6.8 Reactor foundations

The foundations for all nuclear reactors arc subject to particularly stringent design and construction require­ ments. The advanced gas-cooled reactors (AGRs) in particular represent exceptionally high concentrated loadings when contained within prestressed concrete

pressure vessels and typically weigh 40 000 to 60 000 tonnes per reactor. Such massive single loads have nearly all been founded on deep concrete foundations bearing directly over a large area onto competent soil or rock. This simple foundation concept is relatively easy to analyse and monitor and minimises the effect that hazard factors, such as seismic loading, have on the competence of the structure.

its foundations. Here the total weight of the loaded reactor vessel is transmitted to the ground through a mass concrete base of 25.9 m diameter. The gross pressure on the underside of the mass concrete base is approaching 1300 kN/m2, although the net pressure is over 200 kN/m2 less due to overburden pressure relief as the base is founded over 11 metres below ground level. Long term settlement records are kept by means of precise levelling techniques as onerous settlement criteria are imposed on such critical structures to ensure that critical clearances between reactor components are not prejudiced.

In this case the 3.35 m thick concrete base was poured in three equal height lifts, each lift being subdivided into bays so that the plan locations of all vertical joints were staggered. The pouring of adjacent bays was programmed for a minimum period of four days to allow the major proportion of thermal shrink­ age to occur prior to pouring the adjacent concrete. This reactor (as most others) has a surrounding rein­

portions of the circumferential reactor building. The