Spallation Neutron Source
SNS is entering a six-week maintenance period following a three and a half month operating period. During the latest operating period, machine availability averaged 85% of the planned beam time, meeting a major operating goal.
The first target module at SNS is being prepared for disposal, and one step in that preparation is to make sure all the mercury is drained from the target assembly. Two 2-inch-diameter holes were drilled through the front surface of the target module in order to make certain the draining was complete. As a side benefit, the coupons removed from the drainage holes will be examined to gain information about the extent and type of damage the beam produces on the front surface of the target. The 2-inch coupons were recovered from the drill assembly last week and were found to read up to 1,000 R/h on contact after a three-month decay period. Details of outloading the coupons from the target maintenance bay (a.k.a, hot cell) for transport to an examination facility will be carefully planned over the next few weeks.
Stack emission studies continue at SNS as operations ramp up in power and availability. A gamma spectroscopy examination of the stack during beam operations reveals that (as expected) the radiation emitted as the air exits the stack is dominated by various positron emitters. The only additional isotope identified in the spectrum was Ar-41, which seems to make up about 0.1% of the total emissions from the air inside the stack. The spectrum did not change from the last measurement a year ago, but statistics are better at the higher operating power. Since tritium is not detected by the gamma spec measurement, we are also making informal measurements of tritium-in-air in the various air streams feeding the stack.
The collective estimated dose for the SNS project in 2009 will be almost exactly 3 rem, as measured by the sum of electronic dosimeter measurements recorded for each individual during each entry into a radiological area. As was true in 2008, only four individuals received over 100 mrem estimated dose at SNS during 2009. An extensive study was recently performed comparing the estimated (electronic-dosimeter-based) doses for SNS workers to their (TLD-based) doses of record. Electronic estimates were fairly accurate for workers receiving the highest doses, but TLDs (exchanged quarterly) consistently recorded zero dose of record for workers receiving less than 30 mrem total 2008 estimated dose. A dose of less than 10 mrem on each TLD is recorded as zero dose of record, so a large number of workers with low estimated doses receive zero dose of record. The total collective dose of record for SNS workers in 2008 was about 1.1 rem compared to the 2.1 rem estimated from electronic dosimeter records. TLD dose is more accurate and is DOELAP certified, but is only available months after the exposure. The estimated dose results are available immediately (allowing daily review), and the estimated doses are consistently conservative. Therefore, we use estimated doses based on electronic dosimeter results for ALARA planning purposes during the year and to conservatively estimate collective-dose-to-date.
Submitted by Don Gregory