The reactor core is situated in the pressure vessel between the upper and lower core plates. It comprises 193 vertical fuel assemblies each containing 264 fuel rods, 24 guide thimbles and one instrumentation tube arranged in a 17 x 17 square array. The fuel assem­blies are constrained in the vertical direction by hold — down springs (which are attached to their top nozzles) impinging on the core upper plate and, in the hori­zontal direction, by the core baffle plates at the core periphery. A general view of the pressure vessel in­ternal components, including the core, appears in Fig 2.129.

During power operation, the natural water cool­ant flowing upwards through the core is kept under a pressure of about 155 bar which is sufficiently high to prevent fuel dryout with a core inlet tem­perature of 292.4°C and a coolant flow rate of 17.9 kg/s.

The fuel assembly design is shown in Fig 2.130. The 264 fuel rods are each 3.85 m long and 9.5 mm in diameter and consist of a stack of pellets of low enrichment uranium dioxide, approximately cylindri­cal in shape, clad in 0.57 mm thick zirconium alloy (Zircaioy). The fuel rods are backfilled with helium during manufacture to a pressure of about 25 bar to improve the pellet-to-clad conductance and keep fuel temperatures down. The pellet stack is held in place by a spring located in the upper plenum of the fuel rod. The fuel rods are retained by springs in the eight stainless steel grids. Six of the grids have vanes which serve to mix the coolant as it flows through the core, thereby reducing the potential for hotspots.

Each assembly contains 24 guide thimbles which are also constructed of Zircaioy and are joined to the grids and the top and bottom nozzles. They can accommodate control rods, burnable poison rods, neu­tron sources, or simply thimble plugs when they are not required for anything else. In addition, there is a central thimble which can permit the periodic in­sertion of in-core instrumentation which is used to monitor neutron flux distributions. The bulk of the structural material in the active core is constructed of Zircaioy to reduce the parasitic absorption of neutrons. This results in a reduction in the level of U-235 enrichment that would otherwise be needed to keep the reactor critical.

The reactor is designed for batch reloading at ap­proximately one yearly intervals when about one-third of the most highly irradiated fuel is discharged and replaced with fresh fuel. The neutron spectrum is such that the reduction in reactivity due to fission of the U-235 atoms and the build up of absorbing fission products are not compensated for by a fissile plutonium build-up due to U-238 neutron capture. This leads to a continuous reduction in reactivity throughout the cycle.

To enable the core to remain critical throughout the cycle, sufficient reactivity must be invested in the fuel by enrichment to, typically, 3.1% U-235 from a natural uranium enrichment of 0.71% U-235. This is mainly compensated for at the beginning of the cycle by boric acid, which is a strong neutron ab­sorber, dissolved in the coolant. It is only possible to change this boric acid concentration slowly during operation, and so rapid reactivity control is provided by control rods made up of a mixture of silver, indium and cadmium, and clad in stainless steel.

Control rods are designed to travel vertically in the guide thimbles of fuel assemblies and a cluster of 24 such rods is called a rod cluster control as­sembly (RCCA). There are 53 such RCCAs grouped together in either control or shutdown banks. Each control bank is constrained to move as a unit with a fixed overlap with respect to other control banks.

The RCCAs are designed to fall freely under gravity to insert negative reactivity very rapidly and render the core sub-critical in the event of a fault or incor­rect operation.

For reasons of safety, the core and fuel design are such as to have negative temperature feedback characteristics. In the fuel, this occurs naturally be­cause the dominant U-238 isotope of uranium has
very strong resonance absorption bands at epithermal neutron energies. These widen as the fuel is heated, due to the Doppler effect, thus increasing the neutron absorption and bringing about a reduction in reac­tivity. In the moderator, a negative coefficient is brought about by designing the core such that it is always slightly undermoderated. An increase in mod­erator temperature reduces the moderator density and

lakes the uranium-to-moderator ratio even further away from the optimum. At very high boric acid concentrations, however, it is possible for the reduc­tion in the moderator boric acid density to more than offset the reduction in moderation, resulting in a positive moderator temperature coefficient. Thus there is an upper limit to the negative reactivity which is permitted to be held by the boric acid.

Excessively high boric acid concentrations are avoid­ed by introducing burnable poison rods, which can be situated in any of the 24 thimble locations in assemblies not associated with control rods. These burnable poison rods consist of hollow borosilicate glass rods containing 4% by weight of boron (as B1O3) sheathed in stainless steel. These are designed in such a way that at the end of the fuel cycle the
boron is almost completely burnt out, thus minimising the U-235 enrichment penalty. Similarly the fuel U-235 enrichment selected is such that, at the end of the cvcle, the reactor coolant boric acid concentration is virtually zero.

The distribution of fuel assemblies in the core is arranged such that the resulting power distribution peakina factors are minimised. This necessitates the most reactive fuel being loaded in the outer annulus of the core where the neutron leakage is greatest, leading to a flat power distribution.

The first core is arranged to have 3.1% enriched fuel assemblies loaded in the outer annulus, and 2,6% and 2.1% enriched fuel assemblies arranged in a chequerboard fashion in the core interior.

After the first cycle, the fuel with the lowest re­activity (the 2.1% enriched fuel) is discharged. The 3.1% enriched fuel is moved inboard and shuffled with the 2.6% enriched fuel and fresh 3.1% enriched fuel is again loaded in the periphery. After cycle 2, and each subsequent cycle, the 2.6% enriched fuel is discharged and, again, the fuel in the periphery is shuffled inboard and replaced with fresh 3.1% en­riched fuel.