Tuesday Class re-cap!!!

OOOOpppppsss....i did it again...i know we are late with our posts but we will make it up for you addicts!
So, what have I learn during my Tuesday class? Frankly, the topics covered is kinda hard to digest but here is what I understand from my class that day.The focus of that day was on the moderator of a nuclear reactor.

Let me explain to you what is a moderator..

• part of a nuclear reactor

• reduces the speed of fast neutron; thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235

• Commonly used moderators include regular (light) water (roughly 75% of the world's reactors), solid graphite heavy water (5% of reactors). Beryllium has also been used in some experimental types, and hydrocarbons have been suggested as another possibility.

Why do we need to reduce the speed of fast neutron?
Slower moving neutrons are more effective at keeping the chain reaction going by causing more uranium nuclei to break apart.

So what have I learned?
A moderator reduces the speed of fast neutrons. What will happen if over moderated happens??Reactors are usually designed to operate in  an under moderated condition so that the moderator temperature coefficient of reactivity is negative.

• over-moderated reactors will suffer under a simple thermal expansion.
• number of density of atoms is reduced due to moderator expansion.
• neutron mean free path and leakage will increase.
• thermal absorption goes down. 
• the additional heat raises the temperature in the cooling circuit and more steam is produced.
• more steam means less cooling and less neutron absorption, and the problem gets worse.

Thus,there must be something to override moderator expansion effects, right??So what can be done??
Mr.Syukri and Mr.Shamsul will kill me if I am wrong with this.....here we go....

• The process of Doppler broadening of resonance provides the needed feedback.
• The process is "prompt"....so it responds to the fuel temperature. 
• Moderator effects is a delayed effect.....because heat is transferred from fuel to the coolant.

There you go...I will stop here and let you addicts digest the informations above.For more info, you can always email us or leave a comment...Insya-allah, we can learn more.....