The closure weld and post-weld processing of Alloy 22 waste packages
are important to the Yucca Mountain Project (YMP) goal to ensure
that these containers will survive their design lifetime in the
emplacement environment. There are numerous welding processes that
can potentially be used to fabricate these containers and to make
the final closure weld.
A scoping study was recently completed which summarized the current
“state-of-the-art” of welding process technology in
the context of waste package fabrication alternatives. In consideration
of the optimum process (or processes) for completing the final closure
weld, reliability, efficiency, and long-term performance were weighed
more heavily than overall cost of developing or implementing the
technology.
Based on the evaluation of a number of candidate processes, friction
welding and electron beam welding were identified as potential technologies
for the final closure weld. These are both single-pass processes
that will allow the final closure weld to be made in minutes rather
than the hours required for arc welding processes. Inertia friction
welding (IFW) is an extremely robust process that can produce welds
of high integrity and low residual stress in Alloy 22. The main
disadvantage of this process is that the machine requirements in
terms of inertial mass and thrust forces far exceed even the largest
of existing IFW machines and the tooling and fixturing required
to restrain the waste package during the welding process will require
special engineering. The use of IFW represents a technological challenge,
but there are no apparent barriers.
Electron beam welding (EBW) will also result in reduced residual
stresses and increased process efficiency. Although not as robust
as IFW, EBW technology is mature and equipment is commercially available
to achieve the weld penetration characteristics required for the
closure weld. The evolution of reduced pressure EBW is particularly
attractive because it eliminates the need to achieve a vacuum within
the welding chamber.
The arc welding processes, gas tungsten arc welding (GTAW) and
gas metal arc welding (GMAW), represent lower cost alternatives
and are widely used for the fabrication of thick section stainless
steels and Ni-base alloys. The main disadvantages with these processes
is that total weld time for the closure weld will be extremely long
(several hours) and the residual stresses associated with these
processes will be high, requiring a post-weld stress relief.
Based on this analysis, the OCRWM Science and Technology Program
is preparing a competitive, multiple-award procurement. It is anticipated
that bidders will provide proposals for several of these technologies.
Initial awards will be to develop proof of principle, followed by
a feasibility phase, and a third stage for actual demonstration.
Depending on the confidence in successive development activities,
the cost and the schedule, further down-selections may be made at
each successive phase. A future decision will be made, whether to
change the waste package design to incorporate advanced welding
technology; any such baseline change will be subject to regulatory
review.
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