[ RadSafe ] Chernobyl

[Dan McCarn]

Joe & Sandy:

A channel reactor can be gas-cooled or fluid-cooled with either deuterium,
light water or graphite as a moderator. The D2O moderated reactors can burn
natural uranium such as the CANDU. Being a channel-type reactor, they can
be fueled while operating.

The Fort Saint Vrain reactor in Colorado was a uranium / thorium fueled,
graphite moderated, high-temperature CO2 gas-cooled reactor with
significantly higher thermal efficiency than Light Water Reactors.

As I recall, the Chernobyl-class reactor was an enriched-U-235 reactor,
graphite moderated and light-water cooled which had the ability to load
follow.

The channels provide pressure containment.

>From my old memory of the IAEA Chernobyl meetings while in the Division of
Nuclear Fuel Cycle (please remember that I'm a geologist!):

As I mentioned, 6 safety systems were turned-off for a test following a
high-power run to supply Kiev. The neutron poisons build-up in the reactor
creating a condition by which the reactor cannot be shut down and
immediately restarted until the neutron poisons (fission products) decay.

The sequence of events (Features, Events, Processes)  that led to the
Chernobyl failure included a feature - a sensor which measured the average
neutron flux in the reactor. This sensor did not indicate a reaction was
proceeding due to the central placement of the sensor. This gave the
operators a false sense that the reactor did not have excess reactivity so
they continued to remove moderator rods. The event was that outlying
portions of the reactor still had sufficient reactivity to begin to heat
causing water to boil in a channel. Since the reactor had another feature,
a positive void coefficient, the reaction escalated and propagated. The
event leading to the reactor failure was that part of the reactor became
deformed, possibly from a local pipe bursting from super-heated steam. The
operator tried to put the control rods back in the reactor but was
unsuccessful. A few seconds later the reactor had an extreme power
excursion, melting the fuel and causing the graphite to burn (5,000C or
so). The temperature that graphite begins to burn is given in the following
paper:
http://www.hindawi.com/journals/stni/2011/589747/

Following design analysis, the Chernobyl-class and later models were
designed not to have a positive void coefficient, additional neutron
sensors were placed throughout the reactor to better assess reactivity, and
the safety system was enhanced to prevent idiots from defeating the safety
systems. The permanent moderator rods can not be removed as was the case
with the original Chernobyl design.

I don't think it would have mattered if the reactor had a dome-type
containment. With such an extreme power excursion and destruction of the
reactor, a massive release was inevitable.



Dan ii

Dan W McCarn, Geologist
108 Sherwood Blvd
Los Alamos, NM 87544-3425
+1-505-672-2014 (Home – New Mexico)
+1-505-670-8123 (Mobile - New Mexico)
HotGreenChile at gmail.com (Private email) HotGreenChile at gmail dot com

On Sun, Oct 4, 2015 at 9:41 PM, Sander Perle <sandyfl at cox.net> wrote:

> But a Channel Reactor is a graphite moderated reactor.
>
> Regards,
>
> Sandy
> Sent from my iPhone
>
> > On Oct 4, 2015, at 20:18, Dan McCarn <hotgreenchile at gmail.com> wrote:
> >
> > The Chernobyl-class reactor is a channel reactor.  So is the Canadian
> CanDu reactor.
> >
> > Six safety features were intentionally overridden causing the reactor
> failure.
> >
> > Safety analysis and redesign corrected most of those issues.
> >
> > Dan W McCarn
> > 108 Sherwood Blvd
> > Los Alamos, NM 87544 USA
> > +1-505-670-8123
> > Sent from my iPhone
> >
> >> On Oct 4, 2015, at 18:16, Ted de Castro <tdc at xrayted.com> wrote:
> >>
> >> You mean if it had NOT been essentially a pile of charcoal with a chunk
> of white hot metal in the middle and NOT running at 120% for accelerated
> fuel production (at least that's how I heard it) --- would there still have
> been a problem???
> >>
> >> There's a good reason we don't make graphite reactors.
> >>
> >>> On 10/3/2015 7:50 PM, Joseph Preisig wrote:
> >>> Radsafe,
> >>>
> >>>      So, if the Chernobyl reactor(s) had properly designed containment
> >>> structures, would the releases and dose rates have been much more
> limited,
> >>> perhaps comparable to those at Three Mile Island???  Of course, the
> answer
> >>> would be design dependent....
> >>>
> >>>     Joe Preisig
> >>> _______________________________________________
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