[ARC5] OD3 VR Tube - Not At All Like A Nuclear Reactor

[Mike Morrow]

John wrote:

> The math is very similar to a nuclear chain reaction, where a reactor is
> balanced on a knife edge between running away and shutting down. Control
> rods adjust the 'loop gain' of the reactor to exactly 1. Reactors do not
> work w/o active control.

Well, er...nuclear reactors do in fact *often* operate without active controls
of the type you imply.  Especially naval propulsion pressurized water reactors.
For those, the operator's opening of throttles to a steam turbine increases
the heat demand on the steam generator, which lowers the temperature of
the water coming out of the primary side of the steam generator that
then goes into the reactor core.  This lower temperature water is denser,
which moderates or slows down neutrons to the point that more nuclear
reactions occur as a result, which increases reactor fission thermal power
generation, which raises the temperature of the water coming out of the
reactor that is going back to the steam generator, to the point where
in a matter of about a minute or so the thermal power generated by the
nuclear core has matched the increased thermal power demand that was
caused when the operator opened the throttles to the turbine.  Everything
then stabilizes.  Had the throttles to the turbine been closed, the
response would be identical, but in the opposite direction of parameter
change.  This response is automatic and controlled by the physics of
the process, not by **any** external automatic human-designed control
loop.  It's rather beautiful to watch, especially when one understands
what is taking place!

Plus...it's all rather slow and easily controlled, once again, by the physics
of the process, not by a man-made external control system.  About 94 percent
of the neutrons that initiate one fission process cycle were generated
"prompt" one-hundredth of one-trillionth of a second earlier.  But the other
6 percent of neutrons taking part in one fission process cycle come from
neutrons generated by the decay of radioactive products of fission cycles
that occurred as long as several **minutes** earlier.  It's the effects of
these "delayed" neutrons that control the rate at which the fission cycles
take place, to the point that slow-moving control rods or thermal changes
as described above or any other man- or physics-made effect will very
easily control the fission and thermal power generation process.  There's
no "knife edge" anywhere in the process!

Commercial pressurized water reactors and boiling water reactors are
not quite as simple in practice as the naval reactors, but are still
fundamentally similar.  I spent thousands of hours at the controls
of naval propulsion reactors and commercial boiling water reactors in
the past 38 years, before retiring.

Returning to the subject of gas ionization avalanche, that is somewhat
analogous in nuclear fission terminology to a reactor operating from
one fission cycle to the next on prompt neutrons only.  That's called
a nuclear weapon...not a nuclear reactor!!

73,
Mike / KK5F
Former Senior Reactor Operator