Spallation Neutron Source
On September 18, 2009, SNS for the first time operated at its design power of 1 MW on target. The ramp-up from initial beam on target took almost 3 ˝ years, which was only slightly longer than was predicted during the design phase of the project. The normal operating power is 850 kW at present, with availability being the primary focus of the operational efforts. Following the initial start-up from the latest maintenance period the availability has been approximately 85%, and the objective is to maintain or improve that level during long-term operations.
Following the first change of mercury target vessels at SNS, the “target service bay” (which looks a lot like a hot cell) has become contaminated with activated mercury and the spallation products contained in that mercury. The contamination is the result of mercury spilled as connections to the old target vessel were broken and remade on the new vessel. As the mercury evaporates and is drawn out of the cell with the filtered exhaust air, it leaves behind an assortment of spallation products. The first smears of this residue from inside the cell have been analyzed by gamma spectroscopy and show the predicted mix of isotopes, dominated by some exotic radionuclides such as Re-183, Lu-173, and Os-185. The rhenium isotope is of particular concern because no DAC is listed for it in Appendix A of 10CFR835. Under the 10CFR835 version which must be implemented by next summer, a very conservative default DAC must be used to determine airborne postings and respiratory protection for all unlisted gamma-emitting isotopes. ORNL is preparing paperwork to request an exemption to allow use of an appropriate DAC for a number of unlisted isotopes which are of interest at SNS.
During the SNS target change-out, radiation protection personnel took the opportunity to test the shielding properties of the target service bay walls and windows. The walls are made of poured high-density concrete, and were among the first major high-density concrete pours made at SNS. There was some concern that there might be voids inside the cell walls and that the shielding might be less effective than designed. As the activated used target was transported by crane from its operating position to its shielded container at the opposite end of the cell, RCTs made radiation measurements along the external surfaces of the cell walls. The highest radiation level observed outside the cell was about 10 microrem/h (0.1 microSv/h) at a penetration, versus about 700 rad/h (7 Gy/h) measured (remotely!) at six feet from the target vessel inside the cell. The cell shielding more than meets the design objectives.
Submitted by Don Gregory