January 12, 2011
Reactor vessel still
at risk?
by Terry Myers
The conditions which led to a 15-month shutdown of the NRU reactor at
Chalk River still exist.
But Atomic Energy of Canada is making progress in reducing the risk of
further corrosion to the reactor vessel, according to a report by
Canada's nuclear regulator.
Officials from AECL will appear next week before the Canadian
Nuclear Safety Commission for the first time since repairs to NRU were
completed and the reactor went back into service in August.
The focus of the meeting will be an update on the progress of AECL's
“corrective action plan” to address the “organizational (root) causes”
of the shutdown.
However, in a section on the status of the NRU reactor, CNSC staff note
that air and water are still leaking into the space around the reactor
vessel, meaning “the conditions that led to the corrosion of the vessel
would still be present.”
NRU was shut down in May 2009 after a small heavy water leak was
detected.
The leak was later traced to an area of corrosion near the base of the
reactor vessel.
In past presentations to the safety commission, former AECL
vice-president and chief nuclear officer Bill Pilkington said the
physical cause of the leak was the formation of nitric acid in the
“J-rod annulus,” a gap between the reactor vessel and the light water
reflector that surrounds the reactor core.
Pilkington explained that, knowing there was a light water leak from
the reflector into the annulus, AECL filled the space with carbon
dioxide.
The idea was that the water would combine with the CO2 to create weak
carbonic acid.
Instead, he said, air was getting into the annulus, and nitrogen from
the irradiated air combined with the water to create powerful nitric
acid, which eats away at the vessel wall.
In their report, CNSC staff note that AECL has taken steps to reduce
both the air and water leaking into the annulus.
The leak rate of light water into the annulus has decreased from 8,000
to about 1,000 litres a day, “as it was in the early 1970's,” the
report says.
And testing shows the concentration of CO2 has been around 90 per cent
in two of three sampling areas, with the third at about 60 per cent.
AECL plans to try to seal an opening that could be where air is still
getting into the annulus at an upcoming outage, the report says.
Other measures AECL is looking at to deal with the problem is the
development of an “aluminum cold spray” technique that could be used
during an extended outage later this spring to build up the vessel wall
from the annulus side.
Repairs to the corroded areas that caused the shutdown were done by
welding from inside the vessel.
In their comments, AECL says visual inspections of the J-rod annulus
since the reactor has restarted have shown that “while there are wet
areas at the bottom of the annulus, water is not accumulating.”
“New aluminum surfaces in the annulus remain 'shiny', indicating that
there is little corrosion occurring,” the company says.
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