Class on 28th September

First of all, I will try to keep my post light and easy in accordance to my personal understandings and therefore I do apologize for any inaccurate information. My idea of presentation is to allow people from various backgrounds, and understanding levels to participate in an active discussion in order to trade knowledge on nuclear technology. For those who are craving for information or facts in depth could easily refer to other websites established by nuclear experts.

Today is my first duty to post a topic on this blog in accordance to what we have discussed in class dated 28^th September 2010. Basically what we learnt today, which is reactivity coefficient is continuity from previous lectures on reactor theory whereby each topic is closely related to one another.

Let’s recap and put it in simple words, neutron life cycle on average needs at least one neutron than cause fission of another nucleus expressed in terms of multiplication factor designated as k. Hence k is actually the ratio of neutron produced by fission in a generation over neutron absorbed in the preceding generation (neutron loss). Value of k = 1 means production of neutron is self sustaining (term used as critical), whereby k larger than 1 means that production of neutron is increasing (term used as supercritical), and k smaller than 1 means that production of neutron is decreasing (term used as subcritical).

For those who have a hard time understanding this, say we want to maintain the neutron population just nice. Too many neutron produced means more fission, more power, more heat and therefore could lead towards fuel pellet melting or even reactor meltdown. Decreasing number of neutron production means that the reactor will eventually die. No reactor can be constantly critical (neutron is self sustaining) due to fuel depletion, fission product build-up, and temperature changes and therefore controlling the value of k is essential to control the neutron population in a reactor. This could be achieved by adjusting the fuel concentration and size of the reactor.

After understanding how to control the neutron population, we move on to predict how it will change over time using a concept called reactivity. As discussed by my colleague in the previous post, reactivity is the fractional change in neutron population per generation. Means that reactivity can be expressed in terms of k and hence controlling the value of k directly affects the value of reactivity designated as ρ.

Up to this point, we have assumed that changes in reactivity were achieved by regulation of the system (k value) and could simply analyze the resulting power changes. Unfortunately there are inevitable reactivity changes which occur when the reactor is operating and therefore we need to introduce reactivity coefficients designated as α. Reactivity coefficients are useful in quantifying the reactivity change that will occur due to the change in physical properties inside the reactor.

There are 4 most important α but up to this point, we will look into moderator temperature coefficient of reactivity, α_Tmod. Due to expansion effect, the value could be either positive (over moderated, +α_Tmod) or negative (under moderated) whereby reactors are usually designed to operate in an under moderated condition (-α_Tmod) as it is stable with respect to changes in temperature. Skip all the physics, simply introducing negative reactivity feedback effect means that we can increase power and temperature by increasing the value of k or reduce power and temperature by reducing the value of k. Positive reactivity feedback effect will enhance the effect that produced it and destabilizing.