jef@unisoft.uucp (Jef Poskanzer) (07/09/87)
The 26 June 1987 issue of Science magazine contains an article by
Richard Wilson called "A Visit to Chernobyl". It's quite fascinating.
I've typed in the abstract and some excerpts, and I have a bit of
commentary at the end.
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Abstract
Details of the accident at the Chernobyl nuclear power plant
were given by Soviet experts at a special International Atomic
Energy Agency meeting in Vienna, Austria, in August 1986. Several
unanswered questions were made much clearer by a visit to the
decontaminated and operating power plant at Chernobyl and by
discussions with Soviet scientists. The visit gives us insights
into the way the Soviets design their technology, the consequences
of the accident, and the magnificent way they coped with the
disaster. Although there are general conclusions to be drawn
for the rest of the world, such as the realization that operators
of technological systems can and will deliberately cut out safety
systems, the primary specific conclusion is to be grateful that
the West did not follow the Soviet route in its development of
nuclear power.
[some excerpts]
The Accident
As is well known, at 0123:48 on Saturday, 26 April, unit 4 of the
four-reactor complex blew up as the core suffered a prompt critical
excursion. The steam pressure as the reactor went to between 1000
and 500 times full power lifted a 1000-ton cover plate, turned it
on its side, and ripped open the reactor, leaving the hot core
exposed to the environment.
In the initial burst, a large amount of radioactive material was
released, and more was released over the next 10 days. [...] These
releases led to radiation exposure of the local populace, evacuation
of more than 135,000 people, and an integrated radiation exposure
for the world comparable to that from a very large atmospheric
bomb blast.
Controlling the Accident
The first attempt to control the reactor after the accident was
made by local personnel before the Moscow experts, including
physicists Legasov and Velikhov, arrived. Their attempts to
flood the damaged reactor failed because water passed through
passages between the different reactors, threatening the integrity
of the adjacent units (this is a small but important design flaw).
Later that day, it was realized that the graphite in the reactor
was burning, and radioactivity releases were increasing. Then,
on 27 April and succeeding days, 5000 metric tons of material
was dropped by helicopter. This smothered the fire, but the heat
of the radioactivity still kept the core hot and continued to
evaporate fission products. Not until liquid nitrogen was
introduced into passages below the core, as suggested by Velikhov,
did the core cool and the releases stop.
Effects of the Radiation
[...] the 43 Rem received by persons most exposed adds for each
individual a 0.5% probability of dying of cancer. Since 1/6 of
all people die of cancer, this is 3% of the natural cancer death
rate. Undesireable though this increase is, perspective can be
gained by noting that a dose of 43 Rem causes less cancer, heart
disease, and genetic defects than a lifetime of cigarette smoking.
The integrated effect on the health of the world's population
can be described by adding up all the calculated cancers, leading
to a prediction of many thousand cancer deaths. But the effect
is probably less than that caused by burning fossil fuels for
1 year in the Soviet Union. If, therefore, the average public
health is the sole objective, and a Chernobyl accident happens
less than once a year, the RBMK reactors in the Soviet Union can
be considered less hazardous than coal-fired plants of similar
size.
Causes of the Accident
[...] My personal view is that the main cause was a bad reactor
design. Although many Soviet scientists agree with this privately,
it is hard for them to admit it publicly because the Soviet Union
is committed to operate the 15 existing RBMK reactors and others
now under construction, since there are few alternatives.
The Design Errors
The RBMK reactors are unique in the world. They have an instability
that is particularly dangerous at low power. As the water is
boiled in the reactor and replaced by steam, there is less neutron
absorption and the reactivity increases. Power then increases,
more water boils, and so on in a positive feedback. At high power
(greater than 20% of design), this "positive void coefficient"
is compensated by a negative coefficient as the neutron absorption
lines broaden [...] But these compensating mechanisms can only
work if the time constant of the reactor is long enough -- of the
order of a second. [...] At 0123:42 the operators noticed that the
time constant was less than a second. The reactor had gone prompt
critical and could only be stopped by disassembling and homogenizing
itself.
This design flaw was unnecessary. At the Hanford N reactor, less
graphite is used so that the neutrons are not completely slowed
down and the water in the channels is necessary to complete the
slowing down process. For the N reactor the "void coefficient"
is negative and the reactor is stable.
Management Errors
The instability problems of the RBMK design are so bad, and so
apparently unnecessary, that most Western designers did not
believe them as they perused the Russian reports before April
1986. But the Russian designers knew of these problems. They
specified a set of operating rules to be rigidly followed.
But they forgot that rules that are not understood are often
not complied with, and they seem to have made no attempt to
educate the plant operators. Six important safety devices
were deliberately disconnected on the night of 25 April. The
reactor was deliberately and improperly run below 20% power.
- - - - - - - - - -
Commentary: I found this article both enlightening and frightening.
This was the first place I had heard specifics about the reactor's
design instability. It is hard for me to imagine how anyone could
make such a STUPID mistake. And then to find out that there are
15 more of these guys, and still more in the works... Brrrr.
The comparison with coal-fired power is nothing new for me, but it
occurred to me that many people will mis-interpret it. Saying that
a Chernobyl each year would be less hazardous than coal-fired power
does not mean that Chernobyl was "ok". Chernobyl was a major disaster,
killing 31 people directly and thousands slowly. Coal is worse.
Coal-fired power has been a continuous major disaster for the last
few hundred years. The death toll at our current level of use is
in the tens of thousands per year.
Anyway, it's a good article.
---
Jef
Jef Poskanzer unisoft!jef@ucbvax.Berkeley.Edu ...ucbvax!unisoft!jef
Who is John Galt? And why do I keep getting his phone calls?js07@bunny.UUCP (Jack Shaio) (07/13/87)
In article <442@unisoft.UUCP>, jef@unisoft.uucp (Jef Poskanzer) writes: > The 26 June 1987 issue of Science magazine contains an article by > Richard Wilson called "A Visit to Chernobyl". It's quite fascinating. > I've typed in the abstract and some excerpts, and I have a bit of > .... > Commentary: I found this article both enlightening and frightening. > This was the first place I had heard specifics about the reactor's > design instability. It is hard for me to imagine how anyone could > make such a STUPID mistake. And then to find out that there are > 15 more of these guys, and still more in the works... Brrrr. perhaps it is not a question of someone making a stupid mistake, but of being able to report it, in a police state which was trying to export such reactors (to the Phillipines), and whose deputy foreign trade minister was Yuri Brezhnev (son of the other Brezhnev). The real lesson is that a free press plays a role in penalizing those who put such lemons on stream.