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Aims :

• This title deals with the reasons why the reactivity of a reactor core may change during power operations, long term effects due to changes in the isotopic composition of the fuel due to irradiation and variations in fission product concentrations due to the build up of 'poisons'.

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

• Have gained some understanding of the three elements of in-service effects on reactivity.
• Define the unit for measurement of irradiation (MWDTe-1) and explain why it is used.
• Recognise the four isotopic changes that occur in irradiated fuel and describe their individual effects on fuel reactivity.
• Explain the importance of the term 'conversion ratio' with reference to the fissile properties of U-235 and Pu-239. The student will also be able to recognise the effects of the conversion ratio and isotopic changes on irradiated fuel reactivity.
• Summarise the refuelling methods used in reactors.
• Describe how an equilibrium cycle containing the correct balance of irradiated fuel is stabilised in reactors.
• Explain the term 'poisoning' and list the two most important fission product poisons in a reactor core.
• Have a good understanding of the production and destruction processes of xenon-135.
• Describe how xenon levels vary on starting up a reactor and be able to state how the equilibrium levels of xenon in a working reactor vary with neutron flux.
• Describe how xenon levels during reactor power operation.
• Explain the operational significance of Xenon-135 as a reactor poison.
• Describe how Sm-149 levels vary following such power changes as initial start-up, reactor trip and subsequent start-up. The student will also be able to explain the operational significance of Sm-149 as a reactor poison.
• Define the temperature coefficients of reactivity and their units.
• State the mechanisms that determine and explain how its value varies through the life of a reactor.
• Compare the short and long term stability's of reactor systems.

Course Modules:

Introduction Effects of Irradiation Measurement of Irradiation Isotopic Changes in Fuel Conversion Ratio Refuelling Equilibrium Cycle Effects ofStart-Up and Equilibrium Levels Changes During Power Operation Operational Significance Samarium Temperature Coefficients Fuel Coefficient of Reactivity () Moderator Coefficient of Reactivity () Temperature Feedback Assessment