J-PARC Center ■Development of Experimental Techniques / Facilities at JAEA R&D Centers14-5 Development of Damage Reduction Technique on
Mercury Target for the Spallation Neutron Source–Visualization of the Bubble Formation Behavior in Mercury–

H2O

Hg
Hg

H2O
（a）
Measurement of contact angle
Heliumbubbleozzle
OutN
er:200 m
Inne
r:100m

Bubble formation
in mercury
visualized by using X-rays

Hg

HeliumbubbleNozzleOuter:200Ǵm
Inner:100Ǵm

H2O
HeliumbubbleNozzleOuter:200Ǵm
Inner:100Ǵm

HeliumbubbleBubble formation in water（b）
Results of numerical analyses

Fig.14-9 Visualization of Bubble Formation Behavior
The bubble formation behavior in opaque mercury wasvisualized by using high intensity X-rays. The bubble formed
around the tip of the nozzle in mercury because of the poor
wettability of the mercury while the bubble formed from thetip of the nozzle in water.

Fig.14-10 Measurement of Wettability and Prediction ofBubble Formation Behavior by Numerical analyses
Contact angles, θc, which are a parameter to express
the wettability between the liquid and nozzle material weremeasured. The results of the numerical analyses clearlyrepresented the visualizing test results by using the measured
contact angles in the analyses.

In the materials and life science experimental facility of“J-PARC”, innovative sciences in the materials and life sciencefields will be developed by using high intensity neutrons froma MW-class spallation neutron source. Liquid mercury is usedas the target material to produce neutrons by the spallation
reaction from a viewpoint of heat removal. However, pitting
damage degrades the target structural material, which is caused
by pressure waves generated in rapid heating while the proton
beam bombards the mercury. To improve the endurance of themercury target and to use it under a high power proton beam,
it is essential to reduce the pressure waves which cause thepitting damage. We aimed for a technique that would reduce
the pressure wave by injecting helium gas bubbles into themercury, because the gas bubbles act as a cushion.
It was predicted that the pressure wave would be reducedby half by injecting 0.1 Vol.% of tiny bubbles of 100 μmin diameter (microbubbles). The reduction mechanism ofthe pitting damage by the injection of microbubbles is being
considered in corporation with CCSE.
To understand the bubble formation behavior in mercury
and to develop the technique for injecting the microbubbles
into mercury, we tried visualizing bubble formation behavior

in opaque mercury by using X-rays. Although it was difficult
to visualize the behavior clearly because the absorption rateof the X-rays is high in mercury, the visualization of thebubble formation behavior was successful by increasing thetransmission of X-rays using high intensity X-rays in “SPring-8”,
and optimizing the thickness of the mercury. Fig.14-9 shows
the bubble formation behavior while helium gas was injected
into stagnant mercury from a narrow nozzle (inner diameter:100 μm, outer diameter: 200 μm) when compared with thecase in water. In water, the bubble formed from the tip of thenozzle, but in mercury, the helium gas flows to the boundary
between the mercury and the nozzle wall because of the poor
wettability of the mercury as shown in Fig.14-10 (a), and the
bubble formed around the tip of the nozzle.
The numerical analysis became a useful tool to predict thebubble formation behavior in opaque mercury. It was confirmedthat the numerical analysis represented the visualized resultsby considering contact angle, which is a parameter to express
the wettability as shown in Fig.14-10.
Hereafter, a bubble injecting device to apply the practicalmercury target will be considered to improve the endurance ofthe mercury target.

Reference
Kogawa, H. et al., Effect of Wettabitlity on Bubble Formation at Gas Nozzle under Stagnant Condition, Journal of Nuclear Materials, vol.377, issue 1, 2008,
p.189-194.

106

JAEA R&D Review 2008