Re: "The Chernobyl disaster very probably a sabotage" ["no, just same procedure as last year"]
- From: Bill Ward <bward@xxxxxxxxxxxxxxxxxxxxxxx>
- Date: Tue, 18 Sep 2007 10:12:10 -0700
On Tue, 18 Sep 2007 01:23:08 +0000, Bill Ghrist wrote:
Bill Ward wrote:
On Mon, 17 Sep 2007 18:45:57 -0400, daestrom wrote:The point of no return was probably when they tripped the second turbine
"Rolf Martens" <rolf.martens@xxxxxxxxx> wrote in message<snip>
news:2UmHi.8954$ZA.4705@xxxxxxxxxxxxxxxxxx
What you're saying here is, that what took place at Chernobyl onThe test was to determine how long the turbine-generator could continue
26.04.1986 was just "the same procedure as last year". "Such tests,
with disabling of several safety systems, was the rule in the Soviet
Union at that time."
No, very clearly it was not "just the same procedure" - which would
have been a pretty risky thing too, even considering the well-known
sloppy procedures in that social-imperialist state at the time.
to generate power after the reactor was shutdown. It had been
performed at several RBMK sites. This is an easily verifiable fact.
The test results would have been used to determine how long
turbine-generator power would be available until emergency power
sources came on line to power cooling pumps.
The difference was that at Chernobyl the test was delayed for several
hours after setting up the conditions. This meant that power was
reduced to a low level and then the test was delayed. This means that
Xe-135 built up within the core and reduced power further than what the
test required. See any good reactor physics text to understand why Xe
concentration in the reactor goes *up* when you reduce power.
In order to complete the test, the reactor power level had to be at a
low (but not zero) level to start the turbine-generator 'coast-down'.
Because power continued to decrease below the target point for the
start of the test, more control rods had to be withdrawn further than
normally allowed by procedure. Also in an attempt to maintain the
power level, the flow of coolant through the core was adjusted downward
several times. This too was outside the normal test methodology.
Because the water coolant has a positive void coefficient in the RBMK
design, this meant that the boiling in the coolant channels was
becoming very unstable. Normally the flow is maintained at a higher
level and the formation of steam bubbles (i.e. 'voids') is a stable
phenomenon. But with very low flow rates, boiling water shifts from a
stable 'bubbly/froth' flow mode to one of 'chugging/slug' flow. This
leads to large local power oscillations as the voids travel up the
coolant channels. Overall power level instruments don't register this
very well as they average the power readings from several areas at
once.
The control rod design of RBMK's require that a certain number *not* be
fully withdrawn at all times so that a scram will immediately insert
negative reactivity and shutdown the fission process. By withdrawing
more rods and withdrawing them further than allowed, the zirc-alloy
'follower' below the control rod was the only part in a high neutron
flux region. This meant that if/when a scram occurs, the rapid
insertion of the control rods would actually *increase* power initially
instead of reducing it. This critical design flaw would come into play
when the operators, noting a rapid rise in power level, scrammed the
reactor. The sudden insertion of all those zircalloy rod followers
into the active region of the core caused power to *rapidly* rise
off-scale instead of shut down the chain reaction.
This delay in starting the test, resulting in the operators reducing
flow too far and the withdrawing of the extra control rods is the key
difference between the test at Chernobyl and the previous performance
of the test at several RBMK's.
Basically, the test had been performed before without incident so long
as there is no prolonged delays between establishing the conditions and
initiating the test itself. But the delay that day meant that
operators had to start 'winging it' a bit to maintain power level for
the next stage of the test. They put the reactor into a state that it
had *not* been on previous tests (very low coolant flow and excessive
rod withdrawal).
The design was flawed (positive void coefficient, zircalloy
rod-followers that actually raise power), the operators were doing
things they shouldn't have in an attempt to maintain the power level,
and the test didn't have clear guidelines of when to abort the test
(such as if power can't be maintained within certain guidelines).
There was no conspiracy, it was just that a number of bad things, when
taken individually they wouldn't have had the disasterous consequences.
But the 'planets aligned' and all the bad things lined up that day and
boom. Any number of single items, if done differently, would have
prevented it from happening.
Thanks, daestrom, for another incredibly interesting and informative
post.
When do you think was the last point of safe return? It seems like once
the rods were pulled outside the design limits, there was no way out.
Was there any way to shut down the reactor other than suddenly
reinserting the control rods? Could a slower insertion rate (before the
final power transient) have recovered from the situation?
for the test (the unit had two turbine generators). At that point the
steam pressure started rising and the water flow started decreasing
(because half of the water pumps operated off of the turbine that just
tripped). The displacement of water in the reactor by steam caused a
rapid rise in reactivity due to the positive void coefficient (steam
absorbs fewer neutrons than liquid water). The power increased from 200
Mw to 530 Mw in less than three seconds. The operator responded by
scramming the reactor, but that just initiated a greater excursion for the
reasons explained by daestrom.
Thanks. It reminds me of the events that usually precede an aircraft
crashing. Reactions to a chain of apparently minor anomalies combine and
lead to an unexpected outcome, obvious after the fact.
.
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