Overview of ageing and degradation issues in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 395

for obtaining valid K IC tests, as in the case of low strength RPV steels, is
too large to be practical to investigate radiation effects. Thus, efforts are
being put in to correlating the C V values obtained from standard Charpy
tests to fracture toughness evaluated using compact tension (CT) and/or
elastic-plastic toughness tests ( J IC , etc.). 97 The master curve approach pro-
vides an alternative transition temperature index parameter to the RT NDT
data measured from Charpy tests. This new parameter, defi ned as RT T 0 , 98
is based on a simple addition of 19.4°C (35°F) to the value of T 0 evalu-
ated according to ASTM E 1921. The advantage of this approach is that
RT T 0 can be measured directly on irradiated samples rather than having to
measure initial properties and then add the transition temperature shift. 99
It is also worthwhile considering 'dynamic' values such as dynamic frac-
ture toughness ( K Id ) which are sensitive to applied strain-rate and which
are of importance during accidents such as loss of coolant (LOCA); K Id is
generally determined using pre-cracked samples by instrumented Charpy
impact tests 100 though these are not routinely considered in RVSP sched-
ules. Further details on the reactor vessel integrity are included in a later
chapter in Part II.

Creep of RPV and internals

For RPV steels which undergo a damage of about 0.1 dpa, deterioration
due to irradiation creep is much less in comparison to toughness loss. But
creep crack growth studies indicate that the HAZ, with a different micro-
structure and coarser grain size than the base metal, can lead to lower life
after prolonged neutron exposure in the temperature range 320-420°C. 101
Many of the components of PWR internals (screws, core barrel and baf-
fl e assembly) are made of austenitic stainless steels and undergo an aver-
age damage rate of about 1 dpa/year (

10 13 n/cm 2 s) at a temperature
which may reach a maximum of 400°C due to gamma heating. They undergo
irradiation-induced creep and stress relaxation. 102
In some PWRs the core baffl e consists of sheets and formers. The sheets
are separated by small gaps (0.2-0.4 mm). The connection between the
core baffl e sheets and the formers, and between the formers and core bar-
rel is completed by a large number of bolts (about 900). During the core
baffl e manufacturing process the bolts are tightened with well defi ned
pre-stress to guarantee the geometrical and mechanical stability of the
structure. During operation the pre-stress of the bolts becomes reduced
as a consequence of thermal and mechanical loads aided by neutron irra-
diation which can possibly affect the fl ow induced vibrations of fuel rods
in the outer fuel elements. 103 Biaxial creep rates measured in solution
annealed (SA) 304L (used as baffl e plates) and cold-worked 316 (used
as bolts) in the temperature range 280°C to ≈380°C and irradiated to a

= 5 ×

￿ ￿ ￿ ￿ ￿ ￿