a reply to: Bluntone22
Nuclear reactors don't reach critical mass though.
That would be explosive.
Their fuel is not refined pure enough for that.
Actually, they do. If they never reached critical mass, they would produce no power.
The fuel is not enriched to weapons grade, true. But it is enriched enough to create a chain reaction. That is the definition of "critical mass"...
when the chain reaction is self-sustaining due to enough U-235 atoms in close enough proximity to each other so that the decayed alpha particles from
one can initiate decay in others. In a bomb, the enrichment is such that the decay becomes extremely rapid and the resultant heat causes an
explosion... in a power plant, the lower enrichment and the presence of control rods to absorb alpha particles (neutrons, mostly) keeps the
It's the same principle as with any other explosive. if you have a tank of gasoline enclosed and ignite it, it will explode violently. Put it into a
car and have it injected in smaller amounts and the explosion is controlled. Two things control the decay of U-235 in a power plant: enrichment levels
and control rods. The enrichment levels keep it from exploding like a bomb, even under the worst conditions, but can still result in a runaway decay
chain. the control rods are used to control it further to keep it at operating temperature.
When a reactor melts down the chain reaction has already stoped due to the loss of the water moderator.
The isotopes move too fast too start a chain reaction so the water slows the isotopes enough for them to collide and split.
The control rods do the moderating by absorbing the neutrons before they interact with adjacent U-235 atoms. They do not need to be slowed down to
instigate decay in adjacent atoms; if they are too slow, they are not powerful enough to do so. Water is typically kept on site (with added boron) as
a safety protocol to help cool a reactor before it melts down, but it is the boron, not the water, that really does the trick. Boron has the ability
to absorb a neutron without becoming radioactive itself.
The reactors do contain water. The water absorbs the heat from the reaction and transfers it to the main steam lines to drive the generators. The
water never comes in direct contact with the uranium... it surrounds the containers that hold the fuel pellets. In a PWR reactor (the type I worked
on), that water never leaves the primary containment. It simply passes through a heat exchanger which transfers the heat to the steam lines. In a BWR
design (like Fukushima), it directly drives the generators. In both cases, external water is used to cool it further before returning it to the source
of heat (my plant used cooling towers; Fukushima used ocean water).
The absence of control rods or boron water means the U-235 reacts faster, not slower. Unlike most other generation methods we use, nuclear power is
always on until it is turned off by control rods, not off until the control rods are inserted. To start a nuclear reactor, the control rods are fully
inserted, the fuel pellets are loaded, and the control rods are removed
until the reaction reaches the desired point. That was one of the
things that caused the panic at Fukushima: those rods are highly machined to fit precisely between the fuel pellets, and when they became overheated
and warped, they wouldn't re-engage.
When the heat boils off the water the reaction stops.
No. The water that was used to flood the reactors was used to cool
them to prevent overheating the rods and to try to slow the reaction just a
little. The attempt was futile.
To be fair, I went looking for the exact specs on Chernobyl and found some websites that (incorrectly) state what you just did. So I don't hold the
misinformation against you. Please understand, however, that this is incorrect information... a lack of moderation causes the chain reaction to
continue unabated. Only the lack of enrichment keeps it from becoming a bomb in that case.
Chernobyl didn't use this design and was a crime against humanity.
Chernobyl had advantages in the design. it wasn't that the design was terrible, but that the construction was sub-par compared to today. There weren't
as many safety checks as we have at present. I suppose that is inevitable to some degree with any new technology; the reactor that sits 5 miles from
me (in the plant I helped build as my first real job out of high school) is the same reactor design as the one at Three Mile Island. Safety measures
have been implemented to prevent the same kind of situation, but the design itself is not flawed.
As for irrelevance...
It's all irrelevant without the levels of contamination being reported.
Only so far as human precautions go. Remember, my interest is scientific curiosity: what will happen during a complete core meltdown? The relative
levels do offer some insight on that, because they indicate something about where the cores are now in relation to the readings.
But as to precautions being taken for the area of the readings? Certainly the absolute levels are the most relevant. Of course, what's the difference
between enough radiation to kill a person in 60 seconds and enough radiation to kill a person in 5 minutes? 4 minutes of agony.