The final leaktest of subassemblies uncovered only two 
detectable leaks, one in a 2-in.-pipe elbow, the other in a 
section of 2-in. pipe. A preliminary vacuum test of the 
completed system indicated it met desired tightness. No 
leaks occurred during the first six months operation. 
Other factors contributing to system tightness were 
design and welding. The primary coolant system is 
designed to contain relatively few small pipes or fittings. 
The system was amenable to subdivision into a relatively 
small number of subassemblies and components. For 
strength as well as leaktightness, all welds were subjected 
to a dye-penetrant test of first and last pass, followed by 
radiographic examination. A mass-spectrometer test 
also indicates soundness of a weld. 
In addition to fabricating techniques, choice of mate- 
rials is important to minimize leaks because of the possi- 
bility of flaws. For example, in using nickel-bearing 
stainless steels, the unstabilized types give ‘“‘cleaner”’ 
welds; the stabilizers, sueh as Cb and Ti, tend to cause 
carbide inclusions in the welds. Where a stabilized type 
is required, tight specifications on carbon and stabilizer 
content may be desirable. Commercial specifications on 
type-347 stainless call for stabilizer not less than ten 
times the carbon content and not more than 1.0%. 
Where needed, 347 stainless has been procured with 
0.06% carbon, max., and columbium eight times carbon. 
To minimize fissures and cracks in welds, the composi- 
tion of the weld should be such that it contains about 4% 
the weld composition can be made nearly as desired. 
For example, in welding tube-mill products of 347 stain- 
less, which commonly runs well on the austenitic side 
(no ferrite), a filler rod of 8-10% ferrite produced welds 
containing the desired 3-4 % ferrite. 
However, the type of weld, preparation, and weld 
dimensions, as well as material composition, must be 
considered in estimating the final weld composition. 
Trial welds are the best method of obtaining the necessary 
ferrite composition data. Another indirect means of 
controlling welds through material specifications is ob- 
tained by specifying weld electrode coatings. For exam- 
ple, where ease and rapidity of welding is important, a 
titania-bearing coat is used on electrodes. The titania 
increases the fluidity of the flux; however, the rapid weld- 
ing and reduced puddling resulting from the use of such a 
coating sometimes is considered to contribute to slag 
inclusions in welds. If weld quality is paramount, a 
straight lime-coated rod should be considered. 
Since the skill of the welder is most important in pro- 
ducing good welds, attention should be given to details 
in welder qualification tests. Tests should be used that 
are representative of the work to be done. The use of 
dye-penetrant tests to assure soundness of first and last 
pass of a weld is good practice and reduces the amount of 
repair time. Radiographic inspection of the completed 
weld is highly desirable. In thick material requiring 
many weld passes, more frequent testing should be 
ferrite. 
1 micron of mercury per hour due to 
atmospheric in-leakage). To obtain 
this ultimate sensitivty, the surface to 
be tested must be scrupulously clean 
and extreme care must be exercised in 
the test procedure. These require- 
ments may be met under special condi- 
tions, but for most practical field work, 
0.005 mcfh is considered the limit. 
Testing Procedure 
A progressive type of leak-detection 
procedure making use of several tests 
gives the best results. 
Hydrostatic procedure with water 
generally is used for testing components 
and subassemblies. The soap-bubble 
test is attractive for finding the large 
leaks in the assembled system before 
employing more sensitive methods— 
large leaks mask the smaller ones. 
In one large pressurized-water nuclear 
system, ~63% of the pre-operational 
leaks were discovered by this method 
in 15 days. After repairing the leaks 
found by the bubble test, a hydrostatic 
water test should be performed on high- 
pressure systems; exposure of the sys- 
tem to 1.5 times the design pressure 
may open additional leaks. 
The system is now ready for a high- 
By specifying the proper filler-rod composition, 
done. 
sensitivity leak test, e.g., halogen or 
helium sniffing or standard mass-spec- 
trometer testing. The choice largely 
depends on the degree of tightness 
required. 
The most satisfactory procedure has 
been to localize leaks by standard 
mass-spectrometer testing of various 
portions of the system under vacuum. 
Then the suspect areas can be worked 
over using the spectrometer as a helium 
sniffer. Ifa higher degree of tightness 
is required, the boot technique should 
be employed. 
Omitting the standard mass-spec- 
trometer testing for leak localization 
appears to result in prolongation of the 
helium-sniffng test. Providing the 
operator with a known leak to search 
out markedly increases his diligence. 
Although this type of rigorous leak- 
tightness program increases the cost of 
individual components and_ subas- 
semblies, it offsets a comparable ex- 
pense in rework time after the system 
is completed. The specific advantages 
gained in a program of this type, rather 
than relying on corrective measures 
after assembly, are: 
1. The responsibility for achiev- 
ing a particular degree of leaktight- 
ness is properly divided between 
designer, manufacturer, fabricator, and 
constructor. 
2. Unpredictable delays in placing 
the system in operation after assembly 
are avoided. 
3. A specific leaktightness goal can 
be systematically approached. 
Rather rigid test requirements were 
of necessity placed on the first nuclear 
power-plant prototypes. However,.as 
methods and materials improve and 
experience is gained, considerable sim- 
plification of the leaktightness proce- 
dures should result. 
* *x* * 
The authors thank R. A. Koehler of the 
General Electric Co., General Engineering 
Laboratory, and G. E. Halm of Knolls Atomic 
Power Laboratory, who were associated with 
the authors on leaktightness problems and 
who graciously checked the manuscript for 
accuracy. 
BIBLIOGRAPHY 
1. Radiological Handbook, U.S. Dept. of Health, 
Education and Welfare (1954) oes 
2. A. O. Nier, C. M. Stevens, A. Hustralid, T. A. 
Abbott, J. Appl. Phys. 18, 30 (1947) 
3. R. B. Jacobs, H. F. Zuhr, J. Appl. Phys. 18, 
34 (1947) 
4. W. G. Worcester, E. G. Doughty. High 
vacuum leak testing with the mass spectrom- 
eter, presented at AIEE convention, Detroit, 
June, 1946 (AIEE Tech. Paper 26-142) 
§. Liquid-Metals Handbook, NAVEXOS P-733 
(1952) 
117 
