Hydraulic Safety Catchers Protect Spallation Neutron Source Shutter Operation
- Sunday, 01 April 2007
At full power, the Spallation Neutron Source (SNS), part of Oak Ridge National Laboratory (ORNL, Oak Ridge, TN), will produce the world’s most intense pulsed neutron beams for neutron scattering research methods. With a full-power capability of 1.4 megawatts, the SNS is used for scientific research and industrial development. While neutrons are abundant in the universe, for the detailed images researchers require, only a neutron of the right “brightness” can be used; SNS provides these brighter neutrons. The neutron delivery system consists of shutters composed of tungsten and steel, weighing 18 and 30 tons and two meters thick, and which are raised and lowered vertically by stainless steel hydraulic cylinders. The shutters are used to maintain position integrity and control the flow of neutrons of the SNS. Employed at each shutter are two specially designed hydraulic release stainless steel Sitema Safety Catchers designed by Advanced Machine & Engineering (AME, Rockford, IL).
In order to control the beam of neutrons that go out to the instruments and examination samples, ORNL turns the neutron flow on and off by way of shutters (which work much like camera shutters). A large steel gate system contains the shutters, and the shutters themselves swing 20" from the up/closed position to the down/open position. Neutron production is an almost constant event; SNS is due to run 5,000 hours per year (out of typical year’s 8,760 hours), producing pulses of neutrons every 17 milliseconds. Whenever high-energy particles hit the SNS’s target, neutrons are generated (however, if the SNS proton bean is shut off, neutron production ceases at once). The shutters block the neutron beam — instrumentation must be changed, maintenance performed, and examination samples installed by ORNL researchers without running the risk of being struck by the neutron flow. Along with the shutter system, the SNS is has several levels of containment to keep potentially hazardous material from escaping beyond ORNL.
The safety catchers, designed to prevent gravity fall of a vertical load by absorbing kinetic energy, are not used for the safety of personnel, but for that of the structure due to the weight of the shutters. The shutters raise and lower by way of a central hydraulic life cylinder with twin drive rods in parallel. SNS/ORNL turned to the AME safety catchers for their hydraulic nature; they develop holding force on the rod by self-intensification created by the load as the rod travels downward (they do not develop holding force when the rod travels upward). As part of their function, the shutters must be held in the up-and-locked position. An initial, lightweight demonstration facility used air cylinders that drove pins through a drive rod to lock to shutters into place. For the much heavier shutters eventually used, this option proved unreliable; the hole in the drive rob, even with the pin inserted, was an unacceptable stress concentration point in a critical part of the system. Additionally, the use of a pin system would create a stop sudden enough to generate a shock in the system. The safety catchers, using high-pressure (2,200 psi) filtered tap water, catch and hold the rod in the up position, and for a long period of time if needed or in the event of hydraulic pressure loss, cylinder failure, control valve failure, or other malfunction.
The neutron beam used by SNS/ORNL scientists for examination is the final stage of a complex harvesting process. When a fast particle, such as a high-energy proton, bombards a heavy atomic nucleus, some neutrons are knocked out, or “spalled,” in a nuclear reaction called spallation. For researchers to harvest neutrons needed for experiments, negatively charged hydrogen ions are first produced by an ion source. These ions are injected into a linear accelerator (linac), and are accelerated to very high energies (2.5 million electron volts).