I'd use the normal Co-60 DAC for Rx disassembly with an 18 mo decayed mix and then I think your data would show you would want to use an effective DAC for Rx head lift, cavity decon, and reactor reassembly. PWRs with a fresh mix typically see gamma spec DAC a factor of 2-3 times less than assuming Co-60.
You can get the slow alarm uCi/cc off the monitor and count with gamma spec to get the DAC/uCi/cc for that unique mix. We recognize the efficiency of the monitor changes with mix and the gamma spec will give us the DAC for the reported uCi/cc for that mix with some Rn influence. That response will also vary slightly from unit to unit. If you compared with a GM frisker assuming 10% efficiency and gamma spec you'd expect a slightly diff response. If you only use the uCi/cc off of the gamma spec of the air sample, it will likely be more accurate than assuming Co-60, but it won't include the response of the instrument to the actual mix or Rn influence. I would also not run the monitor too long to minimize the contribution from Rn buildup. If you calculated an effective DAC of 5E-6, I'd probably put 1E-7 in the monitor to provide some margin reflecting variances in the field. That's still a significant improvement in the accuracy of the response. You could refine by checking several air samples in succession. You could very easily start building up data points each outage that would further support and refine what you're doing. Once you build some data points vs air samples, you can then keep an eye on air samples to validate your mix isn't changing significantly to alter your previous measurements. Ultimately you need to have this figured out before Rx head lift and cavity drain down, so you need to use the historical value from the previous outage or you'll miss the critical opportunity to have the setpoints in place before the outage starts. Rx head lift and cavity decon are the best time to capture these samples as these are the periods of interest. That will also be the mix for remaining reactor reassembly. Steam generator work is also interesting as the contamination levels on the platform start with a decayed mix that will change to fresh with Co-58 as the eddy current probes pull that mix out of the steam generators. I've found SG work to be more difficult as they are moving gamma sources around all the time to potentially cause false alarms, changes to the gamma subtract factor, etc. You also struggle with base gamma dose rates to maximize sensitivity of the unit. You may need to shield the detector head to improve sensitivity of the unit. If you put the unit on the elevation below the platform overhead gamma sources will have a greater influence on the bkg detector and suppress the DAC. If you put the unit on the grating above their head moving gamma sources will have a greater influence on the bottom beta detector and cause on over-response. You want to be on the same elevation of interest, but need to minimize gamma influence on the monitor. Good monitor, but difficult conditions. I never had an in-line head, but I think you could have a tygon tube nearer the work and the detector head in an area less impacted by varying gamma levels. Any time you apply an alternate mix and set points, you'd want to label them on the monitor kinda like a limited use label so people in the field would know this monitor had a diff response. You'll want a clear process known by all to ensure monitors are used properly. I have found the AMS-4 to be very predictable over the years and I don't miss carrying the old AMS-3 up the stairwell in containment. The challenge is making sure we understand the response characteristics and deploy it properly to get the best results. As you already know it takes a little thought before the outage to figure out how to best deploy them. Glen Vickers, CHP Exelon Corp RP Technical Lead 815-216-2723 (work/cell) From: Powernet <powernet-boun...@hpspowernet.org> On Behalf Of Diehl, John via Powernet Sent: Saturday, November 2, 2019 18:18 To: 'powernet@hpspowernet.org' <powernet@hpspowernet.org> Cc: Diehl, John <jadi...@stpegs.com> Subject: [EXTERNAL] [Powernet] DAC Values for AMS-4 alarm set points Hello, What DAC value to do you use for alarm set points on AMS-4 (or similar Continuous Air Monitor)? South Texas Project has been using 1E08 μCi/cc as it is the DAC for unknown βγ/60Co; however, it can be too conservative when the predominant nuclide is something with a less restrictive DAC, such as 51Cr or 58Co. Does anyone use a composite or effective DAC for areas such as around the reactor cavity where contamination is not likely to be aged? Thanks John Diehl, CHP South Texas Project Health Physics jadi...@stpegs.com<mailto:jadi...@stpegs.com> 361-972-8545 This Email message and any attachment may contain information that is proprietary, legally privileged, confidential and/or subject to copyright belonging to Exelon Corporation or its affiliates ("Exelon"). This Email is intended solely for the use of the person(s) to which it is addressed. If you are not an intended recipient, or the employee or agent responsible for delivery of this Email to the intended recipient(s), you are hereby notified that any dissemination, distribution or copying of this Email is strictly prohibited. If you have received this message in error, please immediately notify the sender and permanently delete this Email and any copies. Exelon policies expressly prohibit employees from making defamatory or offensive statements and infringing any copyright or any other legal right by Email communication. 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