10.1. Objectives

The objective of the core designer consists in obtaining with the minimum costs a reliable and safe reactor capable of satisfying given requirements.

In the case in which the reactor is used for electricity production these requirements are given by the maximum power-level and load-following capabilities, this second requirement becoming less important in the case of heat production for industrial purposes.

Safety and reliability can only be obtained if a certain number of constraints are satisfied concerning core materials, reactivity, etc.

Among all possible choices, the core designer has to find the solution of minimum cost under the given constraints. Such a process is called optimization. The possible choices are represented by a certain number of independent parameters which can be varied by the designer; their choice can be rather arbitrary, at the limit only the total reactor power and its load-following performances being fixed from the beginning. The optimization would then be extended from general decisions like the choice of the reactor type and its siting, down to the last technical details.

As this is obviously too complex a task, it is necessary to separate the general problem into a number of partial optimizations. In a general plant optimization one will, for example, limit the representation of the pressure vessel to an expression giving its cost as function of the core size, leaving any detailed design to a separate optimization. Whenever possible, different plant components are optimized separately. In this way the core optimization in a plant with steam turbine can often be performed without considering the secondary circuit, which is indepently optimized.

Previous experience is normally used to fix a high number of variables in the form of a reference design, in order to restrict the optimization to a limited number of parameters. Also the representation of the constraints depends on the amount of computational work one can invest in the optimization. On the fuel element side one can, for example, simply define a limit on the age factor (ratio between the maximum and the life average value of the macroscopic fission cross-section of the element) supposing that this parameter sufficiently defines the difference between the maximum fuel temperatures of various fuel cycles.

In a more detailed optimization simplified temperature calculations can be introduced in the procedure in order to limit temperatures instead of age factors.

An even better representation would set a limit on the probability of coated particle rupture and fission product release, calculating this probability as a function of the exposures to temperature, burn-up and fast flux the fuel has had during its life.

As the designer has to remain on the conservative side, every excessive simplification in the constraint representation tends to exclude as impossible some low cost cases which might be actually acceptable.