Lecture Summary ( Monday, 27 September 2010 )

Do you know what the Reactivity is????

After going through today lecture, we have gained knowledge about the reactivity and the important of the reactivity in the reactor of the nuclear power plant. 

Reactivity is a measure of the departure of a reactor from criticality. A nuclear reactor is an engineering device, in which nuclear fuel and structural materials are arranged such that a self-sustained fission chain reaction can occur in a controlled manner.

The reactivity is related to the value of keff . Reactivity is a useful concept to predict how the neutron population of a reactor will change overtime.

A rector is critical when keff = 1.

Figure 1

If keff <1, the reactor is subcritical and the number of neutrons is going down.

Figure 2

 On the other hand, if keff >1, the system is supercritical.

Figure 3

All the figure credit to nuclear_chain_reaction.pdf

For better understanding about reactivity, we look through some example :

·         The amount of reactivity, ρ= (  keff – 1 )/ keff = 1 – 1/ keff = Δ keff/ keff

In practice, keff is very close to 1 or unity. Therefore,

P~ keff – 1= Δ keff

The changes implemented in a functioning reactor as a means of control are always very small and are expressed in %, mk (milik) or pcm ( per 100,000).

Case 1:

Reactor with keff = 1.002

ρ= (  keff – 1 )/ keff = ( 1.002 – 1 )/1.002 = 0.002

Or simply

P~ keff – 1= Δ keff = 1.002- 1= 0.002

Therefore, we can say that the reactor is 2 mk supercritical.