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Aims :

• To introduce the concept of spatial variations in neutron flux within a reactor.
• To cover the methods and reasons for operational variations of these neutron flux shapes.

Objectives :
By the end of this course the student will be able to:

• Describe the general spatial variation of neutron flux across a finite reactor.
• Explain the term 'relative flux shape' and sketch the radial and axial flux shapes for a homogeneous cylindrical reactor of constant K throughout, naming their mathematical functions.
• Indicate how the basic radial neutron flux shape is modulated by the periodic lattice structure of a heterogeneous reactor.
• Relate the radial variation in channel power to the relative radial flux shape of a reactor, stating the ideal shape for maximum reactor power output.
• Indicate how radial variations in K will distort the relative radial flux shape and indicate the best practicable radial flux shape for increasing reactor output.
• Briefly outline the schemes used to vary K in order to achieve radial flux flattening in AGR, Magnox and PWR.
• Define the term 'reflector', as applied to nuclear reactors and state the effect of a reflector on the non-leakage probability (PNL) and on channel power at the reactor edge and hence indicate the reason for using reflectors on large power reactors.
• Differentiate between flux (power) flattening and temperature flattening.
• Identify the effect on core K of adding neutron absorbing material and describe how inserting control rods can cause an axial step change in K.
• Describe how the effect of an axial step change in K will distort the axial flux shape, noting in particular the effect on the position and magnitude of the peak neutron flux, and the condition for the worst distortion.
• State the implications of a distorted relative axial flux shape on the peak can temperature in a reactor.

Course Modules:

Distribution in Homogeneous Reactors Radial Flux Shapes Radial Flux, Channel\Reactor Power Methods of Altering K Neutron Reflection Flux & Temp. Flattening Axial Flux Distortion Assessment