Cavitation damage experiments for mercury spallation targets at the LANSCE – WNR in 2005

Abstract

In-beam experiments investigating cavitation damage in short pulse mercury spallation targets were performed at the Los Alamos Neutron Science Center -Weapons Neutron Research (LANSCE -WNR) facility in 2005. Two main areas were investigated. First, damage dependence on three mercury conditions -stagnant, flowing, and flowing with bubble injection -was investigated by employing a small mercury target loop with replaceable damage test specimens. One hundred beam pulses were passed through the loop mercury and specimen pair for each test condition. Damage with flowing mercury (V = 0.4 m/s) was less than half that which was incurred with stagnant mercury. Gas bubble injection added into the flow further reduced damage to about one-fourth that of stagnant mercury. Acoustic emissions from cavitation bubble collapse were concurrently measured on the exterior of the loop using a laser Doppler vibrometer and were correlated to the observed damage. The second area of experimentation was erosion rate dependence on proton beam intensity. Prior research had indicated that incubation-phase cavitation erosion rate is strongly dependent on beam intensity, by a power law with the exponent perhaps as large as 4. The 2005 results are inconsistent with earlier in-beam test results and do not support the power law dependence. This paper will provide a detailed description of the experiment, present results and discuss the findings. Published by Elsevier B.V.

Initial observations of cavitation-induced erosion of liquid metal spallation target vessels at the Spallation Neutron Source

“…18c. The similarity in the shape of the quasi-stagnation zone and the erosion damage pattern on Target 1/Disk 5 suggests that erosion damage is more substantial in areas of stagnation or decreased flow, which is consistent with previous findings [5,10,[18][19][20]. Another piece of evidence supporting the damage mitigating benefit of local fluid flow is the lack of damage to the window-flow facing side of Target 1/Disk 5.…”

“…A similar pattern was observed on the window channel-flow facing surface of Disk 1, the inner vessel disk cut from the off-set location, and all other disk surfaces facing the window flow. There is an ongoing debate about the role of flow in cavitation mitigation, with some results showing a decrease of cavitation-induced erosion on surfaces in contact with flowing mercury compared those exposed to static mercury under the same conditions [5], which will be discussed in the a later section. Though not shown here, the examinations of the disks removed from the water-cooled target shroud vessel showed no cavitationinduced erosion on the disk surfaces facing the water cooling channel.…”

“…Since the original discovery of cavitationinduced erosion in short-pulsed spallation systems a significant amount of effort has been devoted to determine the factors influencing and governing cavitation-induced erosion and the reader is referred to details described elsewhere [3][4][5][6][7][8][9][10][11][12][13]. Previous research efforts have revealed several possible characteristics of cavitationinduced erosion in liquid metal systems relevant to short-pulse spallation targets: (1) there is an incubation period during which surfaces incur pitting damage but little mass loss occurs, after which erosion progresses with additional pulse cycles; (2) for a given beam profile, the rate of erosion after the incubation period is heavily dependent on the power input to the system (proton beam power, P), and the rate of dependency may be as low as P 2 and as high as P 4 ; (3) the erosion rate of specimens in contact with flowing fluid is less than the erosion rate of specimens in contact with stagnant fluid; and (4) there appears to be a power threshold level below which no significant pitting or erosion damage will occur, which depends on several factors including material condition, mercury flow, and beam intensity.…”

Mitigation Technologies for Damage Induced by Pressure Waves in High-Power Mercury Spallation Neutron Sources (III)—Consideration of the Effect of Microbubbles on Pressure Wave Propagation through a Water Test—

“…22,24,26) Recently, a few indirect experimental lines of evidence for these numerical predictions have been reported with mercury and real proton beams. 28,29) However, in those experiments, the radii and void fraction of the injected microbubbles, on which the mitigation effects are predicted to sensitively depend, 21,22) were not measured because the in situ observation of microbubbles was difficult due to the opaqueness of mercury. Therefore, the dependence of the mitigation effects on the bubble radii and void fraction is still unclear and has to be examined in detail before applying this technique to actual neutron sources.…”

Influence of Heat Treatment on Mercury Cavitation Resistance of Surface Hardened 316LN Stainless Steel

“…However, annealed 316/316LN stainless steel has been shown to be quite susceptible to pitting and erosion damage in mercury when tested under cavitation conditions produced by a split Hopkinson pressure bar apparatus [4] or a drop-test variant [5], in-beam exposures [6][7][8][9], and a vibratory horn [10][11][12][13].…”