mmt@dciem.UUCP (Martin Taylor) (12/03/83)
This may not be the right newsgroup, but nuclear reactor reliability
has become a political issue, so I decided to put it here. Furthermore,
I think the issue has implications beyond Canada, so the distribution
is net.all. Some of the material should interest net.physics readers.
On Aug. 1 1983, one of the 5(?) CANDU reactors at the Pickering Power
station outside Toronto developed a leak in its heavy water cooling
system, and had to be shut down for repairs. The press made a great
play of this incident, and suggested that Ontario nearly had a major
nuclear disaster. They also suggested that the accident was very
costly to consumers. Now the professional journal of the Association of
Professional Engineers of Ontario has published a detailed inside
account of what happened, which I want to abstract here, because
the implications are very different from those that might be gained
from the papers and TV discussions.
CANDU reactors are based on a principle different from those of other
countries. They use D2O (heavy water) both as moderator and as coolant.
Most reactors use H2O (ordinary water) as coolant, perhaps boiling
or perhaps under high pressure, and a different material as moderator.
CANDU reactors have rather higher neutron efficiency than other reactors
(except breeders), and use either natural uranium or uranium only
slightly enriched (I am not sure which). The supply of uranium available
at economic rates using CANDU for all the world's reactors would last
centuries rather than the decades that the supply is expected to last
with the reactors that now exist. CANDU reactors also top the world's
reliability list year after year. These are the reasons why it is
important what happens to CANDU.
[The rest is abstracted or quoted from "Engineering Dimensions" Nov/Dec 1983]
The break in pressure tube G-16 of Pickering Unit 2 was 1.92 metres
long, causing a large leak of heavy water. About 13% of the total
heavy water supply had to be replenished in total, none of it from
emergency supplies. No fuel failures occurred, and no radiation
escaped from the reactor building.
The type and size of the rupture was one failure mode anticipated
in the original design of the reactor. Such a failure had been
experimentally generated in laboratory studies in 1961; if a defect
larger than 75mm long were to occur, then the rupture should be
expected to follow. Pressure-tube failures do occur. The design
target was one failure in 100 reactor-years. In practice the rate
has been one failure in 106 reactor-years. Most result in small leaks.
The Aug. 1 rupture was the first one of substantial size.
During the emergency, the initial leak was observed at 11:10. At 11:30
it was decided to shut down the reactor, and by 12:00 the escaping heavy
water was being recycled into the cooling system from the sump.
The biggest problem was the fact that two fuel pencils had been dislodged
and were eventually found in the rupture itself, after a video camera
was sent down the fuel channel. Chemical analysis later determined
that the metal had been embrittled by heavy hydrogen that had built
up in the tube. Other nearby tubes were therefore checked. High
deuterium levels were found at the outlet of at least one tube, but
no "blisters" were present.
The CANDU reactors were designed so that tube failures would have
no serious safety consequences [in 1958]; the operators had rehearsed
a similar accident on a simulator shortly before it occurred.
The target for lifetime reliability of the reactor remains at its
original 80%. "This compares with the average 74% capacity factor
recorded for 500-megawatt coal fired units in N. America."
The main cost to the consumer of the shut-down has been in the
replacement of the power by more expensive power gained from coal-fired
plants.
Even if the damaged reactor stays out of service for the rest of the
year, the Pickering station will still maintain a service factor of
between 80% and 82%. Last year the service factor for Pickering and Bruce
(the other major Ontario nuclear power station) was 87%. "The lifetime
performance of CANDU rectors has exceeded that of any other type of
nuclear-electric station." More trouble has come from the generator
turbines than from the reactors themselves. Overall, the Pickering
nuclear station has saved Ontario taxpayers about $800 million since
1971, as compared with coal generation.
--
Martin Taylor
{allegra,linus,ihnp4,uw-beaver,floyd,ubc-vision}!utzoo!dciem!mmt