Regional Basic Professional Training Course in Korea

Cladding failure
❙ 181 ❙
3. Basic Principles Of Nuclear Safety
Failure of the cladding barrier can occur due to defects in end‐cap welds or in the
cladding tube itself. Such failures are relatively rare compared to the number of fuel rods
in a reactor. Other potential failure mechanisms include pellet‐cladding mechanical
interaction or high pressure due to fission gas release in transients, flow‐induced or
mechanical vibrations, or excessive cladding corrosion. Cladding failures can be detected
promptly by detection of fission product radioactivity or delayed neutrons in the coolant.
While plant technical specifications may allow operation with up to a specified fraction
of defective fuel, prudent operating practice and ALARA principles dictate that failed
fuel be removed at the earliest practical time to minimize contamination of the primary
coolant system with accompanying radiation exposure to the plant operating and
maintenance staff.
3.5.2.3. The primary coolant system
Description
In a PWR, the primary coolant system pressure boundary consists of the reactor pressure
vessel, the coolant piping, the steam generators, and main coolant pumps, along with
some auxiliary systems including the pressurizer, chemical and volume control systems
(CVCS), and parts of the emergency core cooling and residual heat removal systems,
depending on the design and operating details.
The primary coolant system is intended to be leak‐tight, except for controlled outflow, for
example, through the main coolant pump seals, or the CVCS. Under normal conditions,
the radioactive inventory of the primary coolant system consists of radionuclides that
have leaked from defective fuel, plus activated corrosion products. An important class of
design basis accidents involves loss of integrity of the pressure boundary. This class
includes the large‐break loss‐of‐coolant accident (LBLOCA), and the small‐break
loss‐of‐coolant accident (SBLOCA). So long as these accidents do not lead to core