Initial implementation of helium Gas into the SNS mercury target for mitigation of fatigue and cavitation damage

The short-pulse, 60 Hz beam used at the Spallation Neutron Source (SNS) initiates pressure waves in the mercury target with each pulse that 1) lead to huge numbers of fatigue stress cycles in the target vessel, and 2) pitting / erosion damage to the target vessel caused by mercury cavitation. Both phenomena can lead to target leaks and interruption of neutron production. The number of beam pulses expected over a successful target life is on the order of 109; the number of response stress cycles at a vessel position can exceed 1 per pulse. This number is even beyond the regime typical of high-cycle fatigue. It is important to be aware that contemporary knowledge fatigue damage includes probabilistic behavior for many materials, and that for many materials a true endurance limit does not exist. Obtaining data in the giga-cycle regime is difficult, particularly repeatable data. In this regime, the effects of very small material defects, inclusions and inhomogeneity become relevant as their size approaches that of the material microstructure. Detecting and controlling such defect sizes is problematic in the SNS mercury target module weld assembly. While considerable efforts are being made to improve the robustness of the SNS target, one thing that can be assuredly stated is that reducing high-cycle fatigue stress magnitude will add margin to its fatigue life. Injecting gas into the target mercury is one way to accomplish this. The degree of pulse stress reduction to be expected from a given gas condition is difficult to predict. There are SNS experimental data and JPARC target gas injection experience to expect stress reduction to 1/2 similar to 1/4 of that without gas throughout much of the target mercury vessel. As a first step towards reduction of cyclic stresses and mitigation of cavitation damage, SNS will implement gas injection in its mercury target module beginning in the fall of 2018. The project is known as Gas Injection Initial Implementation, or GI3. GI3 will comprise of a once-through system. That is, the injected helium will be processed through the Mercury Off-gas Treatment System (MOTS) rather than recycled for re-injection. GI3 will employ relatively low flow rate of up to 1 slpm of helium gas through a total of sixty orifices, each with a diameter of 8 microns. The upstream supply pressure will be approximately 6.9 atm. GI3 in envisioned as a first step towards a more complete and complex gas injection system that would include much higher flow rates, gas re-circulation, and the potential combination of small gas bubble and gas wall supplies. Despite this reduced scope, there are significant design, installation, operational, and safety related challenges associated with the project. This paper and presentation will describe these challenges and the strategies employed to overcome them.