Modelling fuel behaviour under irradiation

The modelling of fuel behaviour under irradiation is described in this section. The requirements are first addressed in Section 14.3.1. The modelling approaches, and the commonly used computer programs which implement these approaches, are then discussed in Sections 14.3.2 and 14.3.3. Finally, the advantages and limitations of fuel behaviour modelling, and the future trends in such modelling, are described in Sections 14.3.4 and 14.3.5, respectively.

14.3.1 Requirements

The design and licensing of nuclear fuel require the fuel behaviour under irradiation to be predicted. This includes the behaviour of individual fuel pins and the behaviour of the fuel assembly as a whole (excluding Magnox fuel, where the concept of a fuel assembly is not applicable). The aim is to ensure that the fuel will operate safely and within design constraints, even under accident conditions.

The behaviour of a given fuel pin is governed by the evolution with time of: (a) the pin power distribution; (b) the pin boundary conditions (primarily the axial distribution of coolant temperature and pressure); and (c) the thermo-mechanical response of the fuel pin to the imposed powers and boundary conditions. (b) is in turn dependent upon (d): the evolution with time of the thermal-hydraulic behaviour of the coolant in the primary circuit, commonly termed the ‘system thermal-hydraulics’. With respect to the fuel assembly as a whole, it is generally only (e), the mechanical behaviour, that is of interest, including the stresses imposed by the loads applied to the various assembly components (during normal operation, anticipated operational occurrences and accidents).

Since the fuel behaviour in its entirety is inherently complex, and due to historical restrictions in computing power, (a) to (e) are generally evaluated separately (notable exceptions are analysis of PWR steamline break and BWR power-flow oscillation events, where (a), (b) and (d) are strongly coupled). This is achieved using a suite of computer programs, or codes, with: (i) neutronics codes; (ii) core thermal-hydraulics codes; (iii) fuel performance codes; (iv) system thermal-hydraulics codes; and (v) mechanical design codes used to evaluate (a) to (e), respectively. The codes and their interactions are summarised in Fig. 14.4 . Other types of code are used for ad hoc or specialised analysis, including computational fluid dynamics (CFD) codes for detailed thermal — hydraulic assessments, and coolant chemistry codes to evaluate the complex coolant chemistry in the primary circuit (including the dissolution of metals in the heat exchanger piping, the reactions of the resulting chemical species with the coolant and its additives, and the deposition of the reaction products on the fuel pins in the form of crud).

In general terms, the design and licensing assessment involves comparing calculated parameters with design limits according to a number of design criteria. The design criteria ensure that the functional requirements of the fuel pins and assembly structural components are met. The effects of manufacturing tolerances, model uncertainties, etc., are incorporated into either the calculations of the relevant parameters or the design limits, or both. In the case of each design criterion, the limiting pin is that for which there is the minimum margin between the parameter of interest and the corresponding design limit. Different functional requirements and design criteria generally apply in normal operation, anticipated operational occurrences (AOOs) and accidents. Further information on the


14.4 Schematic of computer codes used for modelling fuel behaviour under irradiation and their interactions.

generalities of design and licensing assessments can be found elsewhere (IAEA, 2003a).

Design criteria and functional requirements vary from country to country due to the differences in regulatory regimes. There are also variations due to differences in reactor types and, to some extent, fuel types. However, as an example: a typical functional requirement would be that the fuel cladding integrity is maintained during normal operation and high probability faults; a typical design criterion which would, together with other design criteria, ensure that this functional requirement is met would be that the maximum effective (or generalised) stress in the cladding shall not exceed the yield stress of the clad material. The parameter of interest here is the maximum effective stress in the cladding, and the design limit is the cladding yield stress.

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