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368 J

Component Fabrication Development Heat Exchanger Tube-to-Tube-Sheet Attachment

The INOR-8 tube bundle of the primary heat exchanger of t h e MSRE, shown i n Fig. 25, contains 163 1/2-in.-diam X 0.42-in.-wall U-tubes welded t o a 1-1/2-in.-thick tube sheet 18 i n . i n diameter. Procedures were successfully developed for welding and back brazing t h e 326 closely spaced tube-to-tube sheet j o i n t s i n t h e unit, and t h e a c t u a l tube bundle w a s fabricated without incident. The welded and back-brazed design shown i n Fig. 26 provides a double seal between the f u e l and coolant salts and w a s used because of the necessity f o r high j o i n t i n t e g r i t y . Grooves were trepanned i n t h e weld side of the tube sheet so that low-restraint edge-type welds could be made. This technique greatly reduces the cracking problems associated with welding thin-walled tubes t o t h i c k tube sheets. Welding and assembly procedures were developed t h a t provided welds of high quality with a minimum of roll-over. Welding conditions a r e given i n Table 18. The 82 A w l 8 N i ( w t $) brazing alloy used i n t h i s application i s corrosion r e s i s t a n t , ductile, and produces j o i n t s exhibiting s a t i s f a c t o r y mechanical strength. A unique j o i n t design, incorporating a trepan groove and feeder holes i n the tube sheet, w a s used f o r t h e braze side. The purpose was t o prevent the brazing alloy from melting and flowing along the thin-walled tubes before the heavy tube sheet reached t h e brazing temperature. A f t e r the determination of optimum welding and brazing conditions on several subsize and f u l l - s i z e mockup samples, the a c t u a l u n i t was constructed. Nondestructive inspection of the welds revealed no defects. The general flow of the brazing alloy w a s good, and ultrasonic examination of brazed j o i n t s showed only minor, scattered porosity. m e completed u n i t passed both the helium leak t e s t and the 800-psi hydrostatic t e s t .

Development and Manufacture of MSRE Control Rod Elements A t o t a l of 160 E R E control rod elements were manufactured by Westinghouse Atomic Fuel Department. The elements a r e gadolinium o x i d e aluminum oxide bushings clad with Inconel, as shown i n Fig. 27. They a r e about 1-1/2 i n . long with an I D s l i g h t l y l a r g e r than 3/4 i n . and a w a l l thickness of about 3 / 8 i n . Each element contains three bushings which a r e made by pressing and s i n t e r i n g prereacted powder, s t a r t i n g as 70$ gadolinium oxide and 20% aluminum oxide. The use of prereacted powder was developed a t ORNL i n order t o minimize shrinkage and eliminate the possib i l i t y of hydrolysis, with subsequent deterioration of t h e s o l i d p e l l e t s .

The prereacted materials have densities as high as 6.0 g/cm3. This compares with a t h e o r e t i c a l maximum of 5.89 g/cm3 f o r a fully dense mixture. X-ray d i f f r a c t i o n analysis i d e n t i f i e d the reacted material t o be primarily a perovskite-type phase GdAlO3 with an excess of OLA1203.