Nuclear power plants cannot explode like nuclear bombs due to their design and the nature of the fuel used. The uranium in reactors, primarily U-235, is not enriched to the levels required for a nuclear explosion, which necessitates a critical mass of nearly pure fissile material. The worst-case scenario for a reactor would involve a steam explosion, as seen in the Chernobyl disaster, but even that is mitigated by robust containment structures. The introduction of reactivity sufficient to cause a nuclear explosion is physically impossible within the operational parameters of commercial reactors.
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Homer,Homer Simpson said:Have you ever heard of the Graphite tipped absorbers used at Chernobyl?? that seemed a bit of design gaffaw. Upon pulling out the absorbers to far in order to remain critical, they eventually dropped the absorbers to trip the reactor. The thing is that the graphite displaced water which led to yet another positive reactivity insertion (along with the voiding)
Morbius said:Homer,
Yes - I have heard that.
Remember, the Chernobyl RBMK reactor is NOT SUPPOSED to be run with the rods
completely out for exactly the reason you point out.
By pulling the rods completely out, when the operators called for a SCRAM when
the reactor power spiked, the very first part of the rod's insertion was actually
inserting moderator - not neutron absorber.
So the first several inches of rod insertion is actually a reactivity insertion not a
neutron poison insertion. So with the rods completely out - you have to ADD
MORE reactivity to the reactor before you can DECREASE reactivity.
Yes - that's yet another BAD design aspect of the Soviet RBMK reactor.
Grogs,Grogs said:I'm curious. It couldn't be an accident they put one of the best moderating materials known to man on the end of the control rods. Do you have any idea what the intended purpose of the tips was? Some sort of oscillation dampening perhaps?
Do you have any idea what the intended purpose of the tips was?
Homer,Homer Simpson said:The Shutoff rods were also used as Control Rods.
Since these rods were cooled by light water in a tube, the boron rod (a good absorber) would displace light water (also a good absorber)
so the difference in reactivity would not be that great. The graphite was called a 'displacer'. It made the effect of rod movement greater, because as the boron rod moves down it displaces graphite, not light water.
In the fully inserted position, the graphite section was out of core on the bottom. When the rods were fully out of core the control rod tubes were part graphite at the top and light water at the bottom.
During the accident, when the rods began to move in the bottom of the core became very supercritical.
Relative to graphite it is.Light water is NOT a good absorber relative to Boron.
There is not an equal amount of graphite entering the core as there is boron. The graphite exits towards the lower core, replaced by boron.Only to the extent that the graphite ADDS reactivity - that the subsequent Boron counteracts. That is relative to an LWR, this increase in control rod effect merely serves to cancel the positive contribution of the graphite.
Homer Simpson said:There is not an equal amount of graphite entering the core as there is boron. The graphite exits towards the lower core, replaced by boron.