Environmental Engineering Reference
In-Depth Information
SA508 Class 3. Creep per se does not pose any safety related problem to
RPVs.
A major issue with the ferritic steels used for RPV applications, as
described earlier, lies in the increase in DBTT and decrease in the upper
shelf energy due to radiation exposure and these factors depend on the
concentration of alloying and/or impurity elements. Sensitivity of radiation
embrittlement of ferritic steels to the concentration of copper is illustrated
in Fig. 1.35a and 1.35b that depict the effects of copper composition on the
increase in transition temperature and decrease in the upper shelf energy,
respectively, with neutron radiation dose. 93 Welds and HAZ are relatively
more sensitive to radiation exposure than the base metal, the reasons being
variations in composition, microstructures, etc. Detailed knowledge gath-
ered on the effects of alloying compositions (in particular Cu, S, P and Ni)
on radiation embrittlement of RPV steels makes it possible to design mate-
rials for new systems to be devoid of these issues that are confronting the
currently operating reactors where the fracture characteristics in the weld
materials are degraded, mainly by impurities such as Cu. There have been
numerous studies to understand the underlying micro-mechanisms respon-
sible for the observed radiation embrittlement of RPV steels and the role of
alloying and impurity elements. 94 , 95 Recent emphasis has been on atomistic
modelling along with the characterization of defects using advanced micro-
structural evaluation techniques such as HRTEM, atom probe microscopy,
small angle neutron scattering (SANS), etc. 96
In the reactor vessel surveillance programmes (RVSPs), samples taken
from the base, weld and HAZ of the actual vessel material used during its
construction are included in a capsule that is placed closer to the reactor
core so that the samples withdrawn after different neutron dose levels can
be tested (tensile, Charpy and fracture toughness). The results from these
surveillance samples provide information regarding the degradation of the
real structure and corrective action can be taken before any major damage
occurs. While hardness and tensile tests are routinely performed to get an
idea on the effects of neutron irradiation, the effect of radiation dose on
RT NDT and upper and lower shelf energies through Charpy tests are sig-
nifi cant in evaluating the radiation embrittlement. The RVSP capsules are
taken out at intervals during reactor operation and changes in the proper-
ties of the samples are monitored to make sure that these changes are less
than those prescribed by the NRC regulation guide (10CFR50). In cases
where the results reveal degradation greater than the limit prescribed by
the regulation guide, the reactor vendor/utility needs to take appropriate
actions to demonstrate the safety of continued operation of the reactor
so that the RPV does not fail in a brittle mode. It is to be noted that the
Charpy tests do not yield fracture toughness ( K I ) data which are related to
the crack length (see, e.g. Equation [1.9]) and the specimen size required
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