Environmental Engineering Reference
In-Depth Information
5.5.1 Poolside examinations
Sipping
If a core is suspected or known to contain leaking fuel, the fuel is almost
universally checked for leaks during the subsequent refuelling (or some-
times forced) outage. Various techniques are used to identify leaking fuel
assemblies and are collectively referred to as 'sipping' methods. They rely
on changes in the concentration of fi ssion products that are released from
the FA or rod being tested to signify the presence of a leak; such as, changes
in gamma activity, beta activity, isotopic composition of fi ssion products or
combinations of such measurements. The technique used to identify a leak-
ing assembly can vary depending on the size of the leak, the background
activity from tramp uranium and on the time of sipping relative to shut
down. Changes in the gross (total) gamma activity of water or noble gas sam-
ples representative of the fuel being tested are sometimes used to identify a
leaker, particularly in cases of low background activity. In general, however,
changes in the gamma or beta activity of nuclides with moderate-to-long
half lives are typically used to minimize the effects of background activity
and decay time; examples include Xe-133 (5.25 d), I-131 (8.04 d) or Kr-85
(10.72 y). In addition, many of the current sipping methods also involve the
collection and measurement of noble gases to enable the detection of leaks
that are too small to allow the release of soluble fi ssion products in quanti-
ties clearly detectable in the sipping process.
Historically, sipping methods have been classifi ed as wet, dry or vacuum,
based on the manner in which fi ssion products are collected for measure-
ment (Lin, 1996). In practice, however, sipping techniques can better be clas-
sifi ed as 'open' or 'closed' methods based on the manner in which the fuel
being tested is isolated from other assemblies during the sipping process.
The wet, in-reactor methods represented by the TELESCOPE, INMAST
and various hood systems are open methods. They have become the pri-
mary means of leak detection because of the small amount of time needed
to inspect a BWR, PWR or VVER core; for example, ~16 h for a large BWR
with an in-core sipping hood versus close to a week for vacuum sipping
(Knecht
et al
., 2001). Wet sipping is also used in storage pools to test indi-
vidual assemblies or fuel rods. The dry method makes use of a canister to
isolate the fuel being tested and is a closed method. Dry sipping is not used
today in power reactors because of issues related to handling and test time
and to decay heat and cladding temperature. Vacuum sipping also makes
use of a canister to isolate individual fuel assemblies and is frequently used
to supplement the in-reactor methods because of its higher resolution and
detection capabilities.
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