Lecture Summary ( Monday, 4 October 2010 )

Today, we learned about the Control Rods(CR).
1 ) Selection of CR materials
2) Types of CR
3) CR effectiveness
4) Integral CR worth
5) Differential CR worth
6 ) Rod control mechanism
7 ) CR for different Nuclear Power Plant (PWR and BWR)


Selection of CR materials : 

These are the materials of the CR,  Silver(Ag), Indium(In) Cadmium( Cd), Boron (B) Hafnium(Hf)

Most reactors contain CRs made of n absorbing materials that are used to provide precise and adjustable control of the core (move in and out).
A “black” neutron-absorbing material  absorbs essentially all incident neutrons while A “grey” neutron-absorbing materia labsorbs only part of the incident neutrons.

The grey absorber causes smaller depressions in n flux and power in the vicinity of the rod like a flat tern flux profile and more even power distribution in the core.
Resonance absorber sometimes preferred to thermal absorber.


Types of CR :


There are three types of CR.
1) Shim rods :coarse control and /or to remove in relatively large amounts.
2) Regulating rods : fine control/adjustment and to maintain desired power or temperature.
3 )Safety rods : provide a means for very fast shutdown in the event of an unsafe condition. Addition of a large amount of –ve by rapidly inserting the safety rods is referred to as a "scram" or " trip“– fast shutdown.

'SCRAM' stand for Safety Control Rod Axe Man.

CR effectiveness :


It depends primarily upon the value of the ratio of the at the location of the rod to the avarage flux in the reactor.





Integral CR worth :
The total reactivity worth of the CR at a particular degree of withdrawal from the core – greatest when the CR is fully withdrawn. Integral CR curve has an 'S' shape.





Differential CR worth :
Differential CR worth is the change per unit movement of a rod (a plot of the slope of integral CR worth curve) and is normally expressed as ρ/inch, Δk/k perinch, or pcm/inch. Typical integral CR curve has an bell shape.





Rod control mechanism :


The CR insertion rates on a scram are designed to be sufficient to protect the reactor against damage in all transients that are expected to occur during he life of the reactor.

Normal rod motion : the CR must be able to move rapidly enough to compensate for the most rapid rate at which +ive p is expected to build within the reactor in order to provide +ive control.

Minimum rod speed setting : the transient that is normally considered is the burnout of maximum peak Xe while at full power . Xenon burnout is usually the most rapid, non-accident transient expected.

Maximum rod speed setting : normally limited in order to reduce the severity of an accident involving the continuous withdrawal of control rods.

CR for different Nuclear Power Plant (PWR and BWR) :


PWR : control rod at the top






http://www.mnf.co.jp/pages2/pwr2.htm


BWR : control rods at the bottom








http://www.thefullwiki.org/Boiling_water_reactor