Your analogy isn't accurate in this case. I've been trying to come up with a car analogy for this issue but can't really think of a good one. Maybe consider a car that has no brakes, and can only slow down via engine-braking. In this case if you want more braking power, you need a bigger engine which provides more resistance when at zero throttle. That's not quite right either but it's closer.@QuantumPion ,Isn't that like saying that in order to get better brakes on a car one must increase the total power of the engine?
Without the graphite tips the difference in reactivity between fully withdrawn and fully inserted would have been smaller but I think that would only impact the efficiency of the core at designed power rather than the very performance of the rods to kill the chain reaction, and closer to the end of the fuel lifetime you would end up with less burnup I believe. Maybe some other technical details would change probably, like asymmetric burn up of the fuel in the core etc.
What one wants from the control rods is to either decrease power or shut down the reactor completely, so the graphite tips in no way advance the main objective of the control rods (in Chernobyl case they indeed helped to sabotage the main objective of the control rods)
I think the rods themselves when fully inserted do the function of absorbing enough neutrons as to stop the chain reaction just fine , the graphite doesn't play any role in this it was just located there in the rod channels because the channels happen to be part of the reactor core and hence they also need to be designed such that they don't intervene negatively with the total efficiency of the reactor core.
PS. going back to the car analogy the control rods without graphite tips would be like driving a car at maximum designed speed down the highway and starving the engine of oxygen/neutrons at the same time, result is you still can drive but you have less power.
I discussed some modifications in previous posts in this thread. See the first page. I believe they shortened the graphite section and expanded the absorber section downward. However, I could not find complete details, in order to understand the full set of changes. I do mention that the designers decreased the drop time, but that is still too long by Western standards.I wonder how exactly were the rods modified after the Chernobyl accident in all of the other plants, maybe @Astronuc can say more on this.
I think the problem with the Graphite tips on the control rods was that the Graphite was less of a moderator than the water it was replacing. As the control rods were activated and returned into their control tubes to moderate a chaotically unstable reactor, the first action, as the leading graphite section of the control rods entered the boiling water column in the control rods steel tube, was to displace the water with an even LESS effective moderator (the graphite tip), it may also have instantly reduced the effective instantaneous coolant volume, increasing the meltdown thermal factors. So, in a nutshell, the first effect of lowering the control rods made the reactor more unstable, exactly the opposite of what you would wish to acheive in a SCRAM control event.You are asking good questions...
> 1. He says water reduces reactivity but steam increases it. How does this really work?
You're first sentence is really saying the same thing. In the case of the Chernobyl reactors, water acts as a neutron absorber. if you add more water, you absorb more neutrons and reactivity decreases. If the water boils, you have less water, so reactivity increases. Why this matters is that when the power increases, the reactivity increases, and this is a really bad design feature (called a positive reactivity coefficient).
Another thing to remember is that neutron moderation is performed with graphite blocks, which are stationary at all power levels.
Contrast this to light water reactors (LWR's) where the water is both the coolant and the moderator. When the power increases, the water density decreases, which decreases the moderation as well as the coolant. Therefore, LWR's have a negative power coefficient. When the power increases, the reactivity decreases.
> 2. He says heat reduces reactivity. (Completely hypothesizing here, is it something to do with phonons interacting with neutrons? I might be out of context though, if so please correct.)
This is something called the "Doppler effect". In the figure in comment:2, you can see "spikes" in the cross sections. These spikes are called resonances. When the temperature increases, the resonances increase in width, and absorption increases. This causes a decrease in reactivity.
So when the power goes up, the temperature goes up, and the reactivity decreases due to Doppler. This is also a negative reactivity coefficient and another very important safety feature.
The Doppler effect occurs in every reactor that has uranium or plutonium fuel (i.e. every reactor)
> 3. The deadly flaw about the rods: their graphite tips. He says graphite increases reactivity. But isn't it used for "moderating" purposes in the reactors all over the globe? An elaborate explanation I would appreciate.
Yes, you are correct. Graphite is a moderator. The key here is that the graphite is located on the tip of the control rod. Therefore, when you insert a control rod, you are inserting graphite and increasing reactivity. This is a bad thing, when you insert a control rod you expect that the reactivity will go down, not go up.
The graphite tips are complicated because the total reactivity depends on where the graphite tip is, and where the absorber regions are located relative the core. Almost always, the control rod is located where the neutron absorber has a much bigger affect than the tip, so the control rod is always negative reactivity. However, in the accident, they had the control rods withdrawn too much and the tip was located at the very top of the core and the absorber was completely withdrawn. Therefore, when the initially inserted the control rod back in the core, the tip was dominate and positive reactivity was inserted. This is what initiated the accident, but it was due to the reactor being in a very unsafe initial condition. The operators had to do a lot of things wrong to get the reactor into this condition.
Here's a new one:maybe someone in the net has given a summary which part of the series is true facts and false physics?