This is my 3rd official duty, sure you can distinguish my writing but did it bore you till death? If yes, then at least I know there are people out there reading my essay. I did mention my posts would be light and easy in accordance to my personal understandings but apparently my posts had been lengthy. Hope it could be enough to patch up our team’s weakness. Where to start...
As mentioned before, we would like to balance a nuclear reactor core at its critical state (k =1 , ρ = 0) all the time. Unfortunately it is impossible to achieve that due to fuel depletion, fission product build-up, and temperature changes.
Disturbance 1 – Temperature changes
In the previous posts we have understood the term reactivity (ρ) and 4 most important reactivity coefficients (α) needed to be considered in order to maintain a reactor at critical state. Recall 1) Moderator temperature coefficient of reactivity which we can control, 2) Fuel temperature coefficient of reactivity which we can’t control, 3) Pressure coefficient of reactivity which is negligible in PWR, and 4) Void coefficient of reactivity which is negligible in PWR but crucial in BWR. As fuel temperature coefficient is always positive (thus providing positive reactivity feedback), moderator temperature coefficient is controlled to provide negative reactivity feedback most of the time. Hence we have dealt with temperature changes effect.
Disturbance 2 – Fuel depletion
Fuel depletes constantly during operation whereby after a long run, we don’t have enough fuel to run the core at its critical state. How to avoid this? We have to add on fuel but unfortunately we can’t add fuel in an operational reactor. It’s a onetime go for 60 years. So we have to place in excessive fuel in the reactor before it is started (when the reactor is built).
Still can’t get the idea? You work on Monday to Thursday, 4 days a week. Suppose you need ¼ tank of fuel to drive your car to work in a day but your boss allows you to claim for the fuel only in a single receipt per week. So the best option is to fill up the whole tank on Monday before work. By doing this, there is some inevitable collateral damage - increasing the car’s weight and increase fuel consumption. Storing fuel for long would lower its combustible energy. Fuel is volatile and hence it evaporates and leaks out.
Back to our main topic, similarly placing excessive fuel causes some inevitable collateral damage. It creates an excessive positive reactivity and need to be compensate with negative reactivity from neutron absorbing material such as Boron (remember that positive reactivity promotes to supercritical state [k>1, ρ>0] where the neutron population is increasing). Among the most popular neutron absorption device is the control rods but relying on it alone is undesirable or impractical for several reasons that would be discussed later. So what do we do if we have too many monkeys around our neighbourhood? Poisson them to death right? Similar here, we introduce neutron poison. Well it actually absorbs neutron, not killing it but as long as we manage to get rid of the excessive neutron means problem solved.
There are 2 classification of neutron poisons, 1) Burnable poisons 2) Non-Burnable Poisons. Difference? Burnable doesn’t actually mean caught fire and burn ya. Burnable poison absorbs neutron and converted into low neutron absorption cross section material. It is like hungry living things that eats only once then die. Non-burnable in the other hand has relatively constant neutron absorption characteristics over core life, example Hafnium (Hf), used to shape power and to prevent excessive flux power peaking near moderator regions.
Fixed burnable poisons are generally used in the form of compounds of Boron or Gd that are shaped into separate lattice pins or plates, or introduced as additives to the fuel whereby it is distributed more uniformly than control rods which results in less disruptive to the core power distribution. Advantages are that it can better shape or control core flux profile and does not affect the moderator temperature coefficient. Meanwhile soluble poison or chemical shim is a soluble neutron poison that is circulated in the coolant during normal operation, e.g. PWR: boric acid also known as solbor (solubleboron). Advantages are it has a spatially uniform effect and it is possible to regulate the amount of poison in the core during operation.
Disturbance 3 – fission product build-up
Fission fragments generated at the time of fission decay to produce a variety of fission products. Fission products are of concern in reactors primarily because they become parasitic absorbers of neutrons and result in long term sources of heat. So as the fission product that absorbs neutron is also called as neutron poison. The most substantial impact on reactor design and operation is the most powerful neutron poison: Xenon-135, a fission product (yield 6.3333%) produced 95% by the decay of Iodine-135. Recall that we actually introduce neutron poison to control reactivity, so fission products of neutron poisoning such as Xenon should be useful right?
Put it this way, you have 3 friends... All 3 of them are useful and you can’t live without either one of them. The problem is one of them had a pretty annoying and irritating attitude. What you normally do is get to understand his/her attitude, monitor closely, and adapt to it.
In the neutron poison clique, Xenon-135 is the one having an annoying and irritating attitude. It absorbs neutron and became Xenon-136 which is a non poisonous stable isotope as it won’t absorb neutron. Remember burnable poison? This is one of it. But only 10% to 50% of Xenon-135 produced in a reactor during operational undergo this neutron capture while the rest undergoes beta decay. Here is where the problem begins.
The rest of Xenon-135 is removed is by beta decay where it has a half life of about 9.1 hours. Iodine -135 has a half life of about 6.5 hours. So the parent live for 6.5 hours the while the daughter survive for 9.1 hours. This time differential is one of the factors that make Xenon such a problem for nuclear reactors. Since Xenon takes longer to decay than the Iodine takes to build in the Xenon, then there is a natural tendency for Xenon levels to increase in a reactor when not at equilibrium. Equilibrium refers to when the rate that Iodine decays into Xenon-135 (build in) is equal to the rate Xenon-135 decays plus the rate of Xenon burn out. The key is to keep it equilibrium.
Doesn’t sound much troublesome right? Fortunately my colleague found a comprehensive article on the web explaining in a pretty similar to my style about Xenon and therefore I could easily paste it here to share. I felt disgrace if I rob this article and claim it as mines so I’ll leave it in its original state. Written by Jack Gamble on 6/5/2010.
When Enrico Fermi fired up the first nuclear reactor at Hanford in 1944, he was in for quite a surprise. Shortly after the reactor went critical, power stalled and the reactor shut down. A few hours later, the reactor unexpectedly started up again all by itself. This was the result of poisoning brought on by Xenon-135 (Xe).
Xenon and Reactor Power Levels
The real problem with Xenon comes into play when power levels in the reactor change. When power rapidly decreases in the reactor, the rate of Xenon burn out drops. However, the existing Iodine-135 continues to decay and produce more Xe-135. This causes Xenon levels to increase, bringing the available neutrons down and lowering power. A few hours later, as Iodine-135 production slows, the Xe-135 levels off and power rises again. So you haven’t touched anything, but power is now higher than you left it.
The converse is also true. When power is rapidly increased, the rate of Xenon-135 burn out rises sharply but the Iodine-135 decay remains unchanged. This causes a lowering of Xe-135 concentration and increase in power. Eventually, the rate of Iodine-135 production and decay along with the rate of Xe-135 production and burnout reach equilibrium. Now power is lower than you left it because Xenon has built back in.
The end result of this Xenon-135 is a major nuisance to nuclear reactor operators and core engineers. The solution is placing limits on the rate at which a plant rises and lowers power. This enables operator to keep a close eye on Xenon and make sure the reactor is running in safe manner.
Xenon-135 contributes Xenon precluded startup (Inability of a reactor to be started due to the effects of Xe-135) and Xenon dead time (The period of time where the reactor is unable to override the effects of Xe-135). I think I’ll stop here... I’m exhausted and running out of time. Please visit link below to read on Xenon precluded startup and Xenon dead time.
http://www.chemie.de/lexikon/e/Xenon-135/
