To produce neutrons for scientific experiments, the proposed Spallation Neutron Source (SNS) must convert negative hydrogen ions into protons and slam them against a liquid metal target to knock loose (spall) and boil out the neutrons. That's how the largest neutron yield in the world will be produced for international research teams by 2006 at the SNS user facility at ORNL.
The technical components for the SNS project are being designed and will be built in Oak Ridge as a collaborative project among five DOE national laboratories—Argonne, Brookhaven, Lawrence Berkeley, Los Alamos, and ORNL. The facility will include an ion source, a linear accelerator to speed up the ions, a device to accelerate protons, a liquid-mercury target for the protons, and beam lines to carry neutrons produced in the target to experimental samples for measurements by scientific instruments.
The mercury target is being designed at ORNL under the leadership of Tony Gabriel. The SNS will be the first scientific facility to use pure mercury as a target for a proton beam. Research is being conducted at ORNL to determine which candidate materials for the target container are most compatible with mercury and how to design the target to shield workers from its radioactivity and to maximize the output of neutrons for research.
Because the SNS will be a DOE user facility, it must be available for experiments as often as possible. If a component in the mercury target wears out or fails, the component should be replaced rapidly to avoid a long shutdown. An ORNL team is developing methods to meet this need. The methods combine appropriate target-design features, remote-handling equipment, unique tools and fixtures, and operator training. To enable researchers to develop tools and procedures for the remote replacement of target system components, a full-scale replica of the SNS mercury target system called the Target Test Facility (TTF) is being assembled and installed at Building 7603 in the Robotics and Process Systems Division.
In 1998, the TTF components were purchased, and a contract was placed for fabrication and acceptance testing of the mercury system. An enclosure structure was built to contain the mercury system and protect operations personnel, and environmental, safety, and health issues of large-scale mercury handling were addressed.
The TTF will contain numerous sensors to measure the flow of mercury and variations in its temperature throughout the target module. These thermal-hydraulic measurements are needed to predict the locations of hot spots that must be eliminated to prevent overheating of either the mercury or its containment materials. The measurements will be compared with predictions of computer codes to help improve the codes.
The expected safe operation of this first, large-scale experimental system for handling mercury in the way envisioned for the SNS should increase the public's confidence that ORNL can safely manage large quantities of mercury at the SNS.