MODULE 11

Section 1:

Criticality and Neutron Multiplication

Reactivity

Section 3: 

The Effect of Reactivity on Neutron Multiplication

Section 4: 

The Effect of Reactivity on Neutron Flux and Reactor Power

Section 5: 

Exercises

A reactor is critical when the effective multiplication factor (k_eff) is equal to 1. The factor that determines how subcritical or supercritical a reactor may be is the following quantity:

 


This is called reactivity.

r @ Dkef = k_eff - 1

UNITS

The changes implemented in a functioning reactor as a means of control are always very small and are expressed in %, ‰, mk (milik), pcm (per 100000).

Let's take a reactor with k_eff = 1.002. In this case we say that the reactor is 2 mk supercritical. And if it were k_eff = 0.997, we would say that it was 3 mk subcritical, since:

Thus:

k_eff = 1.002 represents a reactivity of 0.2%, 2‰, 2mk or 200pcm.

k_eff = 0.997 represents a reactivity of -0.3%, -3‰, -3mk or -300pcm.

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