Before start-up commences, pre-start checks are car­ried out to ensure that the necessary plant and equip­ment are available so that the start-up can safely pro­ceed. In addition to these the plant is prepared in a number of other ways. For example, if the reactor has been depressurised for access to the gas side dur­ing the shutdown, the reactor gas circuit may contain some air which is displaced by purging with fresh CO2. This is generally done at low reactor gas pres­sure, say 2 bar, then the required purity is achieved with the minimum consumption of CO2. The Operat­ing Rules state a minimum gas purity which must be achieved for operation at power, but operational con­venience and good operating practice often require a much higher gas purity than the Operating Rules’ minimum requirement.

In order to ensure that adequate reactor gas flow is available to remove reactor heat, then not only must the required number of boilers and gas circu­lators be in service, but also there must be adequate reactor gas pressure. On the steel pressure vessel sta­tions, however, the pressurisation of the reactor gas circuit is limited by the pressure vessel metal tempera­ture, i. e., to avoid brittle fracture the pressure vessel must be adequately warm before the reactor gas pres­sure is raised. A limit of maximum gas pressure against vessel minimum metal temperature is given in the Operating Rules, a typical curve is shown in Fig 3.24. Pressure vessel metal temperature is raised by heat from the gas.

The heat to raise reactor gas temperature may be supplied in a number of ways. Many of the mag — nox drum boilers have a facility to accept warming steam into the boiler drum from the running reac­tor. A common method, particularly on reactors with once-through boilers where the warming steam method

Fie. 3.24 Limn of maximum gas pressure against
vessel minimum metal temperature
Operating Rules on magnox stations with steel pressure
essels specify the maximum permissible gas pressure
for a given vessel minimum metal temperature.

The limit shown is taken from the Hinkley Point A
Operating Rules.

is not possible, is to run the gas circulators, because most of the energy input to a gas circulator appears as heat in the gas.

After a prolonged shutdown of any reactor it may be necessary to apply heat to raise reactor gas inlet temperature to the minimum value required for start­up. A minimum value is specified for several rea­sons. First, it may be a requirement of the plant, for example, boiler outlet components or reactor core support structure. Second, it is to ensure that when the reactor is operating at power there is a balance between the accumulation of Wigner energy in the graphite due to neutron bombardment, and its an­nealing out so that the level of stored energy is ac­ceptably low {graphite temperatures of greater than about 160°C will ensure adequate annealing). Third, it is to ensure that the reactor is operated within the range of conditions which have been studied in fault >tudies and for which adequate protection has there­fore been demonstrated.

There may be limitations on the rate at w’hich re­actor gas inlet temperature can be raised, for example, due to stresses set up by temperature differentials in below — core components or the lower layers of graphite, also on some concrete pressure vessels there are limitations on rate of change of concrete tempera­ture, At Hartlepool and Heysham I this latter point, combined with the need to maintain a trickle feed on і be boilers w hich remov es some of the heat put in b> the gas circulators, constitute a severe constraint on the time taken to reach the minimum reactor gas inlet temperature for start-up.