In the AGRs, the main factor which it is believed could impede the free entry of control rods into the core is distortion of the top dome (gas baffle). The articulation of control rods has been included to ca­ter for limited non-linearity of the control rod path. Checks are carried out from time to time to ensure that the rods can move freely; on some AGRs these checks are assisted by some of the coarse rods having fewer articulation joints (Hinkley Point B) or no arti­culation (Dungeness B), these rods are called ‘sensor rods’.

The secondary shutdown system in the AGRs is a nitrogen gas injection system. Nitrogen is a good absorber of neutrons, for example, it has been esti­mated that filling the Hinkley Point В reactor with nitrogen at a pressure approaching normal operating pressure would have a reactivity worth in excess of -20 Niles, twice the worth of the control rods. Nitro­gen at adequate pressure is capable of maintaining the reactor shut down for any reactivity state of the reactor core.

At Hinkley Point В and Dungeness В the nitrogen injection is initiated at the discretion of the operating staff. Plant abnormalities such as sensor rods jam­ming or failure of several control rods to fully enter the core on reactor trip will indicate the need for nitrogen injection. At Hartlepool and Heysham 1 the nitrogen injection may also be initiated automatically by tripping of the secondary shutdown (SSD) guard lines, the two tripping parameters being rate-of-change of top dome differential pressure (TDDP) and chan­nel gas outlet temperature (CGOT). The CGOT trip covers top dome failures which are insufficient to trip on TDDP. Tripping of the SSD guard lines also trips the main guard lines, thus releasing the control rods into the core. In all AGRs the displaced CO: rnav be vented through the blowdown system to ensure that the reactor gas pressure does not rise sufficient­ly high to lift the safety valves; at Hartlepool and Hevsham /, however, it has been calculated that the nitrogen injection will not cause the reactor gas pres­sure to exceed safety valve lift pressure if CO: is not vented off.

In the maenox reactors with steel pressure vessels, the main factor which it is believed could impede the free entry of control rods into the core is distortion of above-core components by the gas forces arising from failure of a top gas duct. Boron ball shutdown devices (BBSD) have been provided in these reactors to cover such an event. This is not normally known as a secondary shutdown system, probably because that name had already been taken b the secondary shutdown rods! (which were provided for a quite dif­ferent purpose). However BBSDs are included in this section because the reason for their existence is logi­cally related to the AGR SSD system.

A BBSD consists of a hopper filled with small boron steel balls and a release mechanism which will allow the balls to fall under gravity into a thimble in the reactor core when the rate of fall of reactor gas pressure exceeds a given value. There are typically 20 BBSDs in each reactor. The balls are recover­able. The boron balls are of sufficient worth to ensure that the reactor can be adequately shut down, their reactivity worth is typically about — 1 Nile. BBSDs are not normally claimed as first or second line pro­tection against particular faults, but it is usual to demonstrate their effectiveness by starting the reactor shutdown for biennial overhaul with the release of one or two BBSDs.

Tertiary shutdown systems

In AGRs the secondary shutdown system, nitrogen injection, is regarded as temporary. In the event of a more permanent shutdown being required, and if it is not possible to insert sufficient control rods into the core to aehieve this, the tertiary shutdown (TSD) system is used.

At Dungeness В and Hinkley Point В the TSD system is to fill the reactor internals with water. Clearly this is irreversible. The TSD pipework is not complete; at Dungeness В short closing lengths are provided to make the final connections on the day, at Hinkley Point В much of the pipework is tem­porary and must be installed to enable the operation to be carried out. The reactor will be cooled to about 50°C prior to filling to ensure that the fuel does not overheat between the cessation of normal cooling and the completion of filling, and to minimise the water flashing to steam on the hot components and displac­ing the neutron-absorbing nitrogen. Once commenced the filling must be completed because normal cooling cannot be restored.

At Hartlepool and Heysham / the TSD system is to fill a number of specially designated core chan­nels with boron-impregnated glass beads. The beads are blown in under CO: pressure. The beads should be recoverable, but it cannot be guaranteed that suf­ficient will be removed to take the reactor to power again. Unlike Hinkley Point В and Dungeness В the TSD plant is complete, so it could be in operation within a few hours of a possible need being identi­fied. Therefore if there is reason to believe that the control rod and SSD system together cannot maintain the reactor subcritical, the TSD system could be op­erated before the xenon has decayed.

In magnox reactors the ultimate shutdown system is boron dust injection, although it is not normally referred to as tertiary shutdown. A portable hopper and pump can be connected to the gas circuit near a blower inlet. Two powders, one a sticking agent and the other containing neutron-absorbing boron, will be mixed and pumped into the gas circuit. The blower will be running in order to distribute the mixture into the core. The boron dust, aided by the sticking agent, adheres to the hot surfaces within the reactor core and its reactivity worth ensures permanent shutdown.

Clearly the irreversible shutdown systems described above are under strict administrative control, requiring the authority of the Station Manager to proceed.