T. Matsuoka et al., CRACK INITIATING LIMIT FOR CLADDING TUBES OF PWR RCCA RODLETS SUBJECTTO ABSORBER SWELLING, JSME international journal. Series A, mechanics and material engineering, 39(4), 1996, pp. 555-564
On-site measurement of the outer diameter of rodlets and hot cell exam
ination of typical rodlet tips were carried out for the rod cluster co
ntrol assembly (RCCA) of a PWR plant after 10 cycles. The main objecti
ves of the examination are to investigate the extent of degradation of
control rods and to determine the cracking mechanism involved since c
racks are observed in a number of cladding tubes near the tips. In thi
s study, the crack initiating limit for cladding tubes of PWR RCCA rod
lets subject to absorber swelling is investigated. Although the claddi
ng tube material consists of a ductile material, 304 S. S., it should
be treated as low ductile material that is intermediate between a duct
ile and a brittle material because its ductility is reduced by neutron
irradiation. The crack initiating strain limit for the low ductile ma
terial is investigated and is obtained as follows. epsilon(cr)=epsilon
(ue) + 1/2 epsilon(up) where epsilon(cr): crack initiating strain, eps
ilon(ue): elastic component of uniform strain, epsilon(up): plastic co
mponent of uniform strain The proposed crack initiating strain limit s
hows good agreement with the results of on-site examination. Axial cra
cking in RCCA rodlets can be prevented if RCCAs are replaced when the
total amount of neutron fluence reaches approximately 8 x 10(21) n/cm(
2) (E > 1 MeV). Moreover, it is possible to increase the lifespan of a
RCCA by increasing the gap between the cladding tube and the absorber
.