January 19, 2011
NRU vessel to last
"indefinitely"?
by Terry Myers
There is no risk of severe corrosion to the vessel at the core of the
NRU reactor.
In fact, repairs and improvements to the reactor, and new techniques to
treat the surface of the vessel, mean the aluminum container will last,
if not forever, then longer than the reactor itself is expected to last.
That was the response last week from officials at Atomic Energy Canada
Ltd to a story in the North Renfrew Times headlined, “NRU: reactor
vessel still at risk?”
The story quoted a report prepared by staff of the Canadian Nuclear
Safety Commission for a meeting of the commission tribunal tomorrow.
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 15-month shutdown of NRU that ended last August.
However, in a section on the status of the NRU reactor, CNSC staff
noted 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.”
But AECL vice-president, general manager of operations and chief
nuclear officer Hank Drumhiller said that in fact, improvements made to
NRU mean the life of the reactor vessel could be extended as long as
it's needed.
“I hesitate to say indefinitely, but I would say we are well down the
path to making the life of the vessel not a limiting condition for
NRU,” he said.
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.
AECL found the 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.
Knowing there was a longstanding leak of light water from the reflector
into the annulus, AECL had tried to counteract the risk of corrosion by
filling the space with carbon dioxide.
The idea was that the water would combine with the CO2 to create weak
carbonic acid.
Instead, air leaked 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.
AECL is also looking at using an “aluminum cold spray” technique to
build up the vessel wall from the annulus side.
Drumhiller said the work AECL has done has led to a “significant
improvement” in the conditions in the annulus.
As well as reducing the influx of air and water, the company has
cleared the drains in the bottom of the annulus so that any nitric acid
that does form simply drains away instead of pooling against the vessel
wall.
“We feel quite good about the effects of the work we've done,” he said.
Drumhiller said that although the company will work to reduce the air
leaking into the annulus, “we will probably always have some” getting
in.
“We do believe we can get it low enough that it certainly minimizes if
not eliminates the (formation of) nitric acid,” he said.
Combined with the “cold spray” tooling, which would allow AECL to add
thickness to the vessel wall, Drumhiller said the life of the vessel
could be extended for years to come.
AECL is making plans to relicence NRU to at least 2021.
Drumhiller said that with the repairs that have been made and the “cold
spray” technique available, AECL could extend the life of the reactor
vessel “out beyond (2021), for whatever period we need.”
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