Spallation Neutron Source

Oak Ridge National Laboratory

SNS shipped the first used target module to its final resting place at the Nevada Test Site in May 2010.  The circulating mercury target material meets the megawatt beam of protons traveling at 1 GeV inside the stainless steel target module, creating a shower of neutrons that are moderated and then used by scientific experiments on 18 SNS beamlines.  After 2.3 years and about 3,000 MW-hrs of beam the target module became brittle as well as highly activated, especially the “nose” section where the proton beam passes through the target module on its way into the mercury.  After removal from service, all mercury was drained from the target and eventually the spent target module was ready to be shipped away for disposal.  After at least four months of decay, the empty target module is estimated to contain about 6,000 Curies of activity, with a long list of isotopes produced by spallation of the stainless material.  The dose rate of the drained target module was measured to read approximately 1,500 R/h at a meter.  In order to make sure the target module was completely drained of mercury, two holes were drilled in the nose section.  The metal coupons resulting from the drilling were about 3 mm thick and 2 ½” in diameter.  Each of these coupons measured 25 to 40 R/h at a foot.  The coupons will be examined in order to learn more about damage to the target module material.

Planning and preparation for the disposal of the used target module began in the earliest days of the SNS project.  The target load-out operation affected many aspects of the design of the Target-handling  portion of the Experiment Hall, and detailed procedure preparation took place over a period of more than a year.  Many details must be pinned down the first time such an operation is planned, and some parts of the job will not work out exactly as envisioned in the quiet of a planner’s office.  The tenth load-out will probably be routine, but the first one was quite exciting considering the potential for exposures/contamination and the fact that every action was being performed for the first time.   In the end, the disposal activity went very well, and the total dose to personnel involved in the activity was only 25 mrem.  Lessons learned are being incorporated into the procedures, some hardware is being modified to work more effectively, and the planning begins for disposal of the next target module – which will be removed from service next month.

Submitted by Don Gregory, Radiation Safety Officer