RRFM 2009 Transactions - European Nuclear Society

Fig.2. Reflector element of JRR-4.
The crack was found on the weld of the aluminum cladding on December 28th, 2007. A
photograph of the cracked reflector element is shown in Fig.3. We investigated the reflector
elements by visual examination, dimensional examination, fractography examination, etc. As
the result, the main cause of the crack was concluded by growth of graphite reflector due to
fast neutron irradiation. The growth was about 7 mm (dimensional change: about 1 %) in
longitudinal direction. An excessive stress broke the weld of aluminum cladding, since the
dimensional change exceeded the gap of 4 mm between top of the graphite reflector and the
joint part. In the next phase, we carried out a radiographic testing of verifying other reflector
element. This result showed that the most of the graphite reflector were grown in the aluminum
cladding.
When the cracked reflector element was designed, the gap size of graphite reflector was
designed based on the irradiation data of nuclear grade graphite material of which irradiation
temperature is above 350 ˚C. These irradiation data show that irradiation growth of about
0.05 % happens only on the early stage of irradiation, while irradiation shrinkage proceeds with
fast neutron irradiation by 10 25 n/m 2 after the early stage. The irradiation data of the IG-110
graphite material indicates that the irradiation shrinkage is caused due to the fast neutron
irradiation at high irradiation temperatures above 600 ˚C [1]. Irradiation behaviour of
anisotropic graphite was reported that the growth was observed in perpendicular to the
direction of extrusion at 225 ˚C and 250 ˚C [2].
Reflector body
Handle
Crack
Fig.3. Crack of the reflector element.
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