Re: "The Chernobyl disaster very probably a sabotage" ["no, just same procedure as last year"]


"Bill Ghrist" <notmyname@xxxxxxxxxxxx> wrote in message news:0MFHi.6148$A72.3702@xxxxxxxxxxx
Bill Ward wrote:
On Mon, 17 Sep 2007 18:45:57 -0400, daestrom wrote:

"Rolf Martens" <rolf.martens@xxxxxxxxx> wrote in message
news:2UmHi.8954$ZA.4705@xxxxxxxxxxxxxxxxxx
<snip>
What you're saying here is, that what took place at Chernobyl on
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.


The test was to determine how long the turbine-generator could continue 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of the test, more control rods had to be withdrawn further than normally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?

The point of no return was probably when they tripped the second turbine 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.

I'd agree. About the only way to have 'backed out' of their situation would have been to increase flow instead of tripping the turbine and gradually re-insert control rods. But the system was already very unstable at that point and it may not have been capable of shutting down even so.

daestrom

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