Materials-related problems faced by light water reactor (LWR) operators and corresponding research needs - Materials' Ageing and Degradation in Light Water Reactors - page 321

Upper plug

Plenum spring

Plenum is the space for fission gas released
from fuel pellets in the rod

Fuel pellet

Cladding

φ 10 mm × h 10 mm
BWR

Zircaloy
Zr based alloy
BWR Zry-2 (Zr-1.5Sn-0.12Fe-0.05Ni-0.1Cr)
PWR Zry-4 (Zr-1.5Sn-0.15Fe-0.00Ni-0.1Cr)

φ 8 mm × h 12 mm

PWR

Fuel rod

Lower plug

9.1 Nuclear fuel pellets and rods (Kazuya Idemitsu, Genshiryoku
Zumenn Syuu, JAERO, p. 4, used by permission of the author).

both the fuel itself which contains most of the fi ssion products, and then
the fuel cladding which normally contains the rest of the fi ssion products.
Occasionally, the cladding leaks and the second boundary comes into play -
the primary system of the pressurized water reactor (PWR) (Fig. 9.2) and
the steam system of the boiling water reactor (BWR) (Fig. 9.3). After that,
the boundary is the secondary system and the containment of the PWR or
BWR. During postulated accident conditions, the innermost system (fuel
and cladding) fails and then all that remains to avoid exposure to the public
is the concrete and steel containment structure. Thus, a good understanding
of all the containment systems is also needed.

￿ ￿ ￿ ￿ ￿ ￿

9.2 Fuel and cladding materials - the first

fission barrier

The fi rst boundary for fi ssion products is the fuel and the cladding (Fig. 9.1).
The fuel normally holds up about 90% of the fi ssion products during the
normal operating cycle of an LWR. The main exceptions are elements that
are gaseous at normal fuel operating conditions (~400°C to ~1200°C) such
as I 2 , Kr, and Xe which are held up by the cladding. Even under normal