Accidental Reactivity Changes

The ultimate application of the Doppler reactivity coefficients is a calculation of the reactivity change when the temperature of the reactor changes in either a controlled or accidental manner but in either case usually in a nonuniform manner. There are at least three important causes of non-uniform temperature changes. Two are the nonuniform neutron flux and coolant temperature distribution, slowly varying nonuniformities that can be accounted for by the procedures developed by Foussoul (72A). 

The maximum reactivity change rate for accidental withdrawal of two control banks is set such that peak linear heat rate and the departure from nucleate boiling ratio limitations are not challenged (Section 4.3.1.4.1 of Reference 6.1). Reactivity addition associated willi an accidental withdrawal of a control bank or banks is limited by the maximum rod speed (set at 1.14 m (45 inches) per minute for both rod cluster control assemblies and grey rod cluster assemblies) and by the worth of each bank (Section 4.3.1.4.2 of Reference 6.1). 

Hanna, S.R., Chang, J.C., and Zhang, S.J. (1997). Modeling accidental release to the atmosphere of a dense reactive chemical (uranium hexafluoride), Atmos. Environ. 31,901-908. 

The maximum reactivity insertion rate due to withdrawal of rod cluster control assemblies or grey rod cluster assemblies must be limited by design, so that the rate of change of reactivity for the accidental withdrawal of a control bank can cause neither fuel melting nor departure from nucleate boiling. 

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