High Energy and Nuclear Physics
Spallation Neutron Source
R. Samulyak and T. Lu
The Spallation Neutron Source (SNS) is an accelerator-based neutron
source being built in Oak Ridge, Tennessee, by the U.S. Department of
Energy (http://www.sns.gov). The SNS
will provide the most intense pulsed neutron beams in the world for
scientific research and industrial development.
The proposed liquid mercury target design for the Spallation Neutron
Source (see Figure 1) includes a main flow region inside a stainless
steel structure where mercury enters from th e sides, flows around a
baffle into the proton beam path, and exits out the center. A cooling
jacket that wraps from bottom to top around the target is used to cool
the target window through which the proton beam enters. The stainless
steel target structure is approximately 0.5 x 0.4 x 0.15 m3.
One of the most important issues associated with using liquid
metals as targets for pulsed proton beams is withstanding the loads
caused by the rapid pressure increase resulting from the intense heating
of the liquid metal from a single pulse of protons. This heating occurs
essentially instantaneously compared to acoustic time scales; therefore,
the mercury undergoes a large pressure increase. In addition to a set of difficult engineering problems associated, for instance, with the design
of windows able to withstand large thermal gradients and shocks, recent
experiments with an SNS target prototype uncovered yet another problem
critical to the target lifetime. They showed pitting of stainless steel
surfaces that were in contact with mercury subject to large pressure
pulses induced by the collapse of cavitation bubbles [1]. Due to the
cavitation-induced erosion, it will be necessary to replace the target
after two weeks of operation at frequency 60 Hz of a 1 MW proton pulse.
To extend the target lifetime, future research efforts will be
concentrated in two areas, each of which should lead to reduction of the
erosion damage:
- Evaluation of cavitation-resistant materials and coatings.
- Investigation of mitigation techniques such as introduction of
non-dissolvable bubbles into the system.
We have applied the direct numerical simulation technique for bubbly
fluids to the study of pressure mitigation through the injection of
non-dissolvable gas bubbles near the target front window. We have found that
while the bubbly layer indeed causes a significant reduction of pressure
during 200 microseconds, large transient pressure oscillations exist for a
short period of time (< 100 microseconds) after the proton beam energy
deposition (see Figure 2). We have studied the formation and evolution of
cavitation bubbles in mercury caused by the pressure distributions depicted
in Figure 2. The collapse pressure of cavitation bubbles was calculated by
solving the Keller equation. The mitigation efficiency, estimated by
performing statistical averages of pressure peaks, was found to be dependent
on the parameters of the bubbly layer such as the volume fraction and
average bubble size. For example, a bubbly layer with the average bubble
size R = 0.5 mm and a 0.53% volume fraction reduces the integral effect of
cavitation induced pressure peaks by 50 times.
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| Figure 2. Proton pulse induced pressure peaks on the
entrance window in the pure mercury (left) and mercury containing
gas bubbles (right). |
References
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[1] Status Report on Mercury
Target Related Issues, SNS-101060100-TR0006-R00, July 2002.
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[2] Lu, T., Samulyak, R., and Glimm, J. Direct numerical simulation of
bubbly flows and application to cavitation mitigation. J. Fluid Eng.
In press,
2006.
Last Modified: April 23, 2009
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Claire Lamberti
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