In common with all manufacturing processes, a pri­mary aim of the fuel cycle is to minimise production costs. In the case of the AGR this objective can be realised by reducing the refuelling rate, thereby re­laxing the demands on station fuel handling machinery and reducing the risk of expensive breakdowns. This is particularly important on the AGR since, in the absence of refuelling, reactor power will begin to decline rapidly after only two or three months.

The incorporation of a neutron ‘poison’ within a fuel element and arranging for it to ‘burn-out’ as ir­radiation proceeds can be used to advantage in AGRs in tw’o principal areas. Loading of fuel containing burnable poison within the initial charge can be used to delay the onset of regular refuelling, but the main area of interest lies in the use of burnable poison within feed fuel in order to assist in the attainment of higher discharge irradiations, and therefore reduced refuelling needs. Fuel which is required to last longer must be more highly enriched in U-235 in the interests of core reactivity, but high enrichment fuel would, if unchecked, produce very high channel powers at SOL, therefore poor form factors and related power penalties. The presence of poison in the fuel, however, suppresses high SOL powers and with suitable care the rate of destruction of the poison can be arranged such that the reactivity so released matches the reac­tivity lost through enrichment depletion. The overall result is that fuel reactivity, and therefore power, remains steady until the poison is completely used up (Fig 3.51).

This represents current AGR practice. For example, shortly after the beginning of loading the second replacement (feed) charge at Hinkley Point В (i. e., first feed discharged, second feed loaded), loadings began of fuel with higher feed enrichments, designed to reach 21 GWd/t at discharge and containing burn­able poison to assist in the maintenance of manage­able form factors. The poison used is gadolinium (which has a very high neutron absorption cross — section) in the form of gadolinium oxide, and is located in stainless steel cables or ‘toroids’ within the fuel element grid and brace support structure. The number of toroids within each element determines the reactivity taken up at SOL. In practice this means that the reactivity swing at refuelling is reduced by the presence of the poison, therefore assisting in the con­trol of channel powers. The rate of reactivity release from the poison burn-up is controlled by the mass of gadolinium within each toroid.

The choice of the most appropriate poison loading to achieve a given CAI at discharge is very compli­cated. In addition to the original enrichment increase, the presence of poisons in the fuel results in a lower average reactivity level over life when compared with the same enrichment unpoisoned, and accordingly the fuel needs further enriching in order to compensate. The use of burnable poisons therefore produces a reactivity ‘penalty’, resulting ultimately in more ex­pensive manufacturing costs, offsetting the original financial incentive to increase the discharge irradia­tion. However, there is usually a net gain because the reduced total fuel manufacturing costs more than compensates for the increased enrichment cost per stringer. At Hinkley Point В and Hunterston B, the 21 GWd/t fuel currently being loaded contains four toroids per fuel element, each with 25% gadolinium density so that the poison is completely burnt up after approximately 4.5 GWd/t. Eventually the re­fuelling rate will reduce in proportion to the dis­charge irradiations, i. e., in the ratio of 18 to 21. This represents a 14% reduction, equivalent to a saving of some 20 stringers per year for the station as a whole.