Aganpipe, doesn't the type of fuel rod also matter? Some are better than others, etc.
Honestly I do not know enough about the metallurgy of fission fuels to make a sensible comment, it does seem to be a highly specialised field.
That said I will make an attempt. Understand that I may be well wrong here. Also understand that this will be a somewhat disorganised answer. I shouldn't have to tell you not to try this at home.
In terms of the fissionable isotopes, my understanding is almost all use some form of uranium dioxide, either palletised, or sintered in place with a bunch of filler. This filler would be mostly for structural support, and would be chosen for a minimal influence on neutron velocity and path.
Different types of fuel rod, would be talking more about the different fillers being used, each with different thermal responses. ( aka, they can take different amounts of overheating before softening or melting).
I have read of sequences where control rod channels become blocked, either by distorted fuel rods, or by the control rod itself distorting and jamming the channel. This is more of the problem with solid moderator reactors than liquid moderator reactors, although it is still possible.
This failure mode is only relevant ( towards runaway) whilst the control rods are withdrawn, If it occurs whilst the control rods are fully inserted ( as in a SCRAM), the effect is merely to reduce the upper capacity limit of that plant, or to prevent restart.
That is the limit of what I can think of, in terms of different fuel rod 'types'.
Remember in the class of reactors that we are talking about, at least some active cooling *must* be used, even with the core in full shut down.
The *entire* time that there is the potential for a high energy decay to occur within the fuel in the reactor core, the reactor core is capable of creating heat. Over time, that potential to create heat is reduced, as the fuel mostly changes states to a more stable form ( more stable in terms of isotopes. the less potential for high energy emissions, the more stable). This is why in usual operation they change out the fuel rods from time to time, in order to hold the core at a given level of potential energy.
Also remember the entire job of a nuclear reactor is to turn water into high pressure ( and thus high temperature) steam. That is what it is designed to do. The rest of the power plant is *exactly* the same as you would find in a coal plant, a natural gas plant or a geothermal plant.
In short, talk of hem using " a different type of fuel rod", is IMO, a red herring. It might be true, but it is irrelevant. Understand that the press is not going to report on this dispassionately. This is a dangerous black magic to a majority of people's minds, and they will play on that discomfort.
They are not going to admit that ultimately what they are talking about
is just another kind of boiler, and that overpressure in any other kind of boiler is harmful too, as is running a boiler dry.
Ignore most of the voodoo sky is falling crap. Yes this can emit substances that are undetectable without specialised equipment and can kill you. So can granite.
