A STRUCTURAL INTEGRITY ASSESSMENT OF UNDERGROUND PIPING ASSOCIATED WITH THE TRANSFER OF RADIOACTIVE WASTE Page: 4 of 9
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core piping range between 1-3 inches in diameter. Core piping
that was installed prior to 1988 was Schedule 40 wall
thickness, while since then Schedule 10 has been allowed.
The piping was designed according to ASME B31.3. The
design pressure for the piping was 150 psig, although the
operating pressure for the line is typically in the 100 psig range.
Underground
Corrosion C Stl Pipe
Protection (Welded Joints)
Waste Transfer
Line (St Sti)
Figure 1. Cut-away drawing of SRS waste transfer line
system.
The jacket pipe is constructed of carbon steel. The size of
the jacket piping usually depends on the number of core pipes
that are contained (between 1 and 3) within. Thus the pipe
diameter may range from 4 to 10 inches. Schedule 20 or
Schedule 40 piping was typically utilized.
The outside of the jacket piping is usually protected by one
of several coating methods: a fusion bonded powder coating
system, a coal-tar system, a polyethylene coating, a
polyethylene tape, or a bitumastic coating. The first four
coatings were utilized on the older pipes (circa 1950-1970);
while the last coating has been utilized for the more recently
constructed or repaired transfer lines. The thickness of the
outer protection for the jacket depended on the type of coating
utilized.
Loose granular or powdered thermal insulation was also
placed around the coated carbon steel jackets. The layer of
thermal insulation was 6 to 8 inches in depth. In general, the
insulation materials are hydrophobic and thus prevent water
penetration. Therefore, if the insulation was properly placed
and the temperature near the insulation has been relatively low,
minimal corrosion of the exterior of the steel jacket is expected.
Service Conditions
Core Pipe
Under normal service conditions the interior of the core
pipe is exposed to alkaline nitrate solutions (pH > 13). Three
examples SRS waste compositions are shown in Table 1. Note
that the chloride concentration in the waste is relatively low in
comparison to both the nitrate and hydroxide concentrations.
The temperature of the waste is generally less than 60 C. An
exception is the line associated with an evaporator which mayon occasions be exposed to waste stream temperatures of
approximately 135 C.
Table 1. Molar Anion Concentrations for Simulated Waste
Solutions
Waste Stream Fresh Evaporator Dilute
Waste Concentrate Waste
OH- (M) 1.5 11.8 0.34
CO3 (M) 0.01 0.2 -
NO2 (M) 1.1 1.5 0.24
NO3 (M) 2.9 1.1 0.07
Cr-(M) 0.01 0.024 0.0007
F-(M) 0.01 0.013 -
S04 (M) 0.06 0.004 0.0004
Al(OH)4(M) 0.34 0.38 -
Jacket Pipe
Although the jacket pipe is coated and surrounded by
thermal insulation, there is a potential that the jacket may be
exposed to the soil. The soil characteristics at SRS are
summarized below:
a) Soil moisture content ranges from approximately 9% to
38%.
b) Soil texture over a large part of the site is described as
loamy and composed of particles that are clay, silt, and
sand. The texture contributes to the well drained and
excessively drained soil characteristics in most areas
at SRS.
c) pH ranges from acidic to slightly alkaline (pH of 3.6 to
a high of 8), but typical values are almost neutral pH.d)
Specific conductance measurements indicate very low
soluble salt levels in the soils at SRS, which is
probably the result of continued leaching by
percolating rain water (chloride and sulfate leachate
levels are relatively low).e) Soil resistivity measurements taken at depths of 5 feet
to 20 feet range from 2300 ohm-cm to 149,000 ohm-
cm.
These soil characteristics at SRS, which include well
drained soils, low total dissolved solids and neutral pH,
indicate that the potential for underground corrosion is
relatively low. However, underground corrosion of piping at
the Savannah River Site is well documented. Soil surveys inCopyright 2006 by ASME
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Wiersma, B. A STRUCTURAL INTEGRITY ASSESSMENT OF UNDERGROUND PIPING ASSOCIATED WITH THE TRANSFER OF RADIOACTIVE WASTE, article, April 25, 2006; [Aiken, South Carolina]. (https://digital.library.unt.edu/ark:/67531/metadc881876/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.