6.7.1 Control rods

As described earlier in the magnox section, control rods are provided for two purposes. Firstly, as their name implies, they control the rate and distribution of heat aeneration in the fuel by absorbing more or less neu­trons as required. Secondly they are connected to safety circuitry that enables them to trip (this is the fast in­sertion of a large quantity of absorber) when required to prevent a fault situation from developing.

As in magnox stations, control rods are of two ‘worths’. Some are opaque to neutrons (black) and are generally only inserted when it is intended to shut down the reactor, though they can be used for flux shaping and power level control if necessary. The others are translucent to neutrons (grey) and are intended to be inserted about halfway into the core, i. e., they are nor­mally in a position where, by inserting them further or by withdrawing to some extent, the total or local power level can be adjusted. The potential reactivity of the core and the size and number of grey rods are generally designed to achieve a nominal 50% insertion of the grey rods. As local and general reactivity changes occur or when power changes are required in the core, the grey rods can be expected to move in the range 25% to 75% insertion.

Grey rods are made from stainless steel which is a moderate neutron absorber. Black rods contain stain­less steel tubular inserts with about 4% boron added to enhance the absorber worth. Materials are selected for their resistance to long term oxidation, since their operating temperature when inserted to maximum flux position may be up to 600°C. A further material con­sideration when designing rods is that boron steel swells under irradiation and clearances of boron stainless steel inserts must be sufficient to ensure that control rod sheaths do not split. Clearance however must not be excessive since this tends to reduce heat transfer and raise temperatures.

Black and grey rods are geometrically similar (and therefore have to carry clear external type identification) and their insertion routes (standpipe, control rod guide tube and core channel) are identical. Both types are designed to articulate, i. e., they are made up of six, seven or eight hinged segments, so as to reduce any possibility of failure to insert in the event of an exces­sive or unexpected distortion of the charge path. Each rod is attached to the chain of a control rod plug unit which contains the rod operating mechanisms. Since the chain passes through the hot above-dome region ol the guide tube, cooling reactor inlet gas is allowed to pass up the guide tube from the below-dome region via ports which also supply a down-flow to cool the rod itself.

The general arrangement of a control rod stringer is shown in Fig 2.91. Hartlepool and Heysham / control rods have a different articulation feature to that used in most AGRs, being built on a tie rod; but for both arrangements adhesion of the contacting surfaces at the articulating joints is prevented by applying hard coatings of chromium carbide.

In the event ot an accidental drop into the reactor, for example, due to breaking the control rod chain, a shock absorber is installed at the bottom of each con­trol rod channel (Fig 2.92).

The shock absorbers are designed and tested to ab­sorb the energy of a full rod drop from the highest operating position, plus a further drop from the highest recovery position. The highest operating position is with the rod nose joint above the top of the active core and the highest recovery position (using an emergency re­trieval grab) is with the nose of the rod just within the fuelling machine. The stiffness of the shock absorber is low enough to prevent damaging reactions on the graphite bricks or shock absorber supports and to pre­vent rod damage that might impede recovery. Allow­ance is made for irradiation-induced hardening of the shock absorber material.