FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

To gain primary statistical information/ tests were initiated soon after
the failure to determine spring locations in uncommissioned units at
Bruce and Pickering. It was soon evident that numerous springs had moved
from their design locations in a seemingly random manner.
Tests were conducted in a pressure tube mock-up to simulate actual
reactor conditions. It was found that the springs/ which are compressed
and fitted in the calandria tubes in the vertical position, had relieved
the spring compression by assuming a slanted orientation (see Figure 2).
The spring would then "walk" away from the design location in small
incremental steps each time the pressure tube moved in relation to the
calandria tube.
By experimentation it was determined that mechanical work around the
reactor accounted for a large portion of spring movement. It was also
discovered that devices such as soil compactors working up to 500 meters
away could also cause motion. Inspections conducted before and after
commissioning indicated that circulation of water through pressure tubes
and around calandria tubes would also contribute a significant amount of
movement.
Investigations in uncommissioned reactors indicated that by the time a
reactor was ready for fuelling, the cumulative out of tolerance positions
of springs varied from 200 to 500 meters and that springs would be
randomly spaced. In some cases the four springs were clumped together at
one end of a pressure tube which provided no effective support for that
tube.
There were three difficult choices that could be made: leave as is and
accept lower efficiency and the risk of shortened pressure tube life; cut
and replace using new or shortened tubes with redesigned spacers; or
reposition existing spacers and somehow immobilize them in their design
location.
After an in-depth study, Ontario Hydro decided to correct spring
locations in all uncommissioned units before going critical.
Repositioning appeared to be the most attractive of the remaining two
options as replacement would be costly in terms of time and materials.
The repositioning option boiled down to; "How is it possible to
selectively move springs in a 6 meter annulus without being able to
either touch or see them?"
A number of methods of moving springs were tabled during the numerous
brainstorming sessions that ensued. Interest spread and ideas flowed
in. The policy was to fully review any suggestion no matter how
unconventional it sounded. Psychokinesis, exploding hydrogen and filling
the annulus with urea formaldehyde were just a few that left the option
list quietly and quickly. After sifting, the short list of options
centered around electrical, vibratory and simple mechanical levering
methods. Development programs were started in all three areas.