Oarai Research and Development Center ■Development of Experimental Techniques / Facilities at JAEA R&D Centers13-9 Development of a Dissimilar Metal Joint of Zircaloy-2 and 316L Stainless Steel=Application to Power Ramping Test of LWR Fuel in JMTR=

Joint technique of fuel sample-Development of dissimilar metal joint-Pressurevessel

In-pile tube㩷

Fuel sample

Instrumentation

BOCA

Tensile strength (MPa)(a) Relationship between friction time and tensile strength
600
Rotation speed N:1500 rpm
Friction Pressure P1:147 MPa
Upset puressue P2:245 MPa
Upset time t 2:6.0 s

500

Test temp:294 K400
Zircaloy2316 L SS110°

Test temp:573 K

300
Pressure direction

20001234Friction time (s)(b) Photographs of dissimilar metal joint
Zircaloy2316L SSZircaloy2316L SSFuel sample
CoreHAZ

Inner structure of BOCA

Welding part
(Friction time: 3 s)

Cross section of welding partFig.13-20 Power ramping test of LWR fuel in JMTR
The fuel center temperature and pressure in the fuel sample
are measured by the instrumentation attached to the fuelsample. The sample and instrumentation cannot be welded
directly because they are made of dissimilar materials.
Therefore, a dissimilar joint is first fabricated, and then, thejoint is attached to the tip of the instrumentation. In this way, it
is possible to form a weld in the same metal in the hot cell.

Fig.13-21 Joint test of the dissimilar metal joint(a) Relationship between friction time and tensile strength wasrevealed by tensile tests using a dissimilar metal joint of
Zircaloy-2 and 316L SS.
(b) The heat-affected zone and white discoloration due toetching were observed near the weld interface, and the weld
flash was also observed. Moreover, no cracks or holes wereobserved in the weld interface.

A power ramping test facility using a boiling water capsule(BOCA) in JMTR (Fig.13-20) has been prepared for integrity
tests of high burn-up light water reactor (LWR) fuels under
the power transient condition. In the BOCA, different
material bonding has been applied to the fuel sample andinstrumentation components, which are made of Zircaloy-4
and 304 SS, respectively. However, each metal hasdisadvantages: For Zircaloy-4, it is difficult to obtain thematerial for manufacturing a tube having the same diameteras the fuel sample. Moreover, 304 SS is prone to stresscorrosion cracking under neutron irradiation, and its use
increases the amount of radioactive waste owing to the highdose after irradiation.
To overcome these disadvantages, two materials, Zircaloy-2and 316L SS, were selected, and a new dissimilar metal joint
was developed. In the development process, the condition of
Zircaloy-2 and 316L SS joints obtained by friction welding
was investigated. Joint tests were conducted to examine
parameters such as the rotation speed, friction pressure, andfriction time at room temperature. A suitable rotation speed
and friction pressure were first determined, and then, thefriction time was determined.
Tensile tests at room temperature showed that the tensile
strength increased with increasing fraction time and ranged
from 529 to 536 MPa at friction times greater than 3 s.
Therefore, it is possible to obtain a minimum tensile strengthof more than 480 MPa as regulated for 316L SS by thestandard of the Japan Society of Mechanical Engineers(Fig.13-21(a)); note that 316L SS has a smaller strength thanZircaloy-2 at room temperature. Furthermore, a tensile test at573 K (the maximum temperature during the power ramping
test in JMTR) showed a tensile strength ranging from 277 to281 MPa, which is almost the same as that of Zircaloy-2, 286 MPa
at 573 K. Note that Zircaloy-2 has a smaller strength than
316L SS at 573 K.
Fig.13-21(b) shows an optical observation of the weldinterface of the dissimilar metal joint. The heat-affected zone(HAZ) near the weld interface was observed in the crosssection of the joint. However, the outer part of the HAZ wassmaller than the center part, and the HAZ was forced out withthe weld flash by friction welding. In addition, no cracks orholes were observed in the weld surface, thus confirming theintegrity of the welding joint.
The above results clarify the relationship between thefriction time and tensile strength for a dissimilar metal jointof Zircaloy-2 and 316L SS, and the joint condition wasdetermined. In future, the technology developed for obtaining
the dissimilar metal joint by friction welding will be applied
to the fabrication of the BOCA.
This study was sponsored by The Secretariat of the Nuclear
Regulation Authority (former Nuclear and Industrial Safety
Agency, the Ministry of Economy, Trade and Industry of Japan).

Reference
Kitagishi, S. et al., Performance Evaluation of Friction Welded Joint of Zircaloy-2 to Type 316L Stainless Steel, Nippon Kikai Gakkai Rombunshu, A Hen,vol.78, no.788, 2012, p.564-570 (in Japanese).

118 JAEA R&D Review 2012