ML25135A434
| ML25135A434 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon  |
| Issue date: | 05/15/2025 |
| From: | Samson Lee Plant Licensing Branch IV |
| To: | Richardson M, Schrader K Pacific Gas & Electric Co |
| Lee S, 301-415-3158 | |
| References | |
| EPID L-2024-LLA-0179 | |
| Download: ML25135A434 (1) | |
Text
From:
Samson Lee To:
Schrader, Kenneth Cc:
Richardson, Michael
Subject:
Diablo Canyon online monitoring audit questions (EPID: L-2024-LLA-0179)
Date:
Thursday, May 15, 2025 5:12:00 PM By letter dated December 31, 2024 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML24366A169), Pacific Gas and Electric Company (the licensee) submitted a license amendment request (LAR) for Diablo Canyon Nuclear Power Plant, Units 1 and 2 (Diablo Canyon). The proposed amendments would revise Technical Specification (TS) 1.1, Definitions, and add TS 5.5.21, Online Monitoring Program, to use online monitoring methodology.
The licensee proposes to use online monitoring (OLM) methodology as the technical basis to switch from time-based surveillance frequency for channel calibrations to a condition-based calibration frequency based on OLM results. The proposed amendments are based on the U.S. Nuclear Regulatory Commission (NRC)-approved Analysis and Measurement Services Corporation (AMS) topical report (TR) AMS-TR-0720R2-A, Online Monitoring Technology to Extend Calibration Intervals of Nuclear Plant Pressure Transmitters, August 2021 (ML21235A493).
The NRC staff has identified the need for a regulatory audit to examine the licensees non-docketed information with the intent to gain understanding, to verify information, or to identify information that will require docketing to support the basis of the licensing or regulatory decision. The NRC staff issued an audit plan dated February 25, 2025 (ML25051A270).
The NRC staff has formulated initial audit discussion questions below. If time allows, please prepare responses to these questions in advance. It would facilitate the audit discussions, especially if responses can be posted in the online portal as they become available.
Audit Questions:
- 1. The proposed method of monitoring instrument channel drift performance requires the use of a chain of equipment that is external to the instrument channel being monitored. This external equipment chain (Plant Process Computer, Data Historian, and Data Acquisition Equipment) is used to flag instrument channels for calibration that have drifted beyond the weighted average of the group of similar function instrument channels. The amount of drift that is to be detected is only fractions of a percentage of calibrated span of the instrument channel. However, each piece of external monitoring equipment in the chain of monitoring equipment between the monitored instrument channel and the final string of data that is analyzed to detect excessive drift in the instrument channel has its own uncertainties. It is not clear from the license amendment whether the sum of the uncertainties in the chain of monitoring equipment may actually be capable of measuring with a high enough precision to be able to detect such small drift values or whether this chain of equipment may in fact be masking the fractions of a percent of calibrated span worth of drift that is intended to be detected.Please provide a calculation of the uncertainty
that is propagated in the chain of monitoring equipment (i.e., Plant Process Computer, Data Historian, and Data Acquisition Equipment) that includes all the uncertainties that are included within the chain, such as analog to digital (A/D) quantization conversions, anti-aliasing and sampling effects, numerical analysis effects due to rounding and truncating, algorithm-specific uncertainty, data communication error effects, and other effects that are typically associated with the use of digital equipment for processing live data.Please show a comparison of the resulting uncertainty of the monitoring equipment measurement uncertainty against the magnitude of drift error, in percent of calibrated instrument channel span such that it can be determined that the magnitude of drift to be detected is not being masked by the uncertainty in use of this monitoring equipment.
- 2. For the provided uncertainty terms found in the Analysis and Measurement Services Corporation (AMS) Report DBC2411R0, OLM [Online Monitoring] Analysis Methods and Limits Report for Diablo Canyon, for each of the included uncertainty terms (for example, Computer Published Accuracy (CPA) and Computer Input Accuracy (CIA),
these differ between some and are the same for other transmitter groups) how were each of these determined? What components went into this determination if they contain multiple uncertainty terms for that loop portion? If there are any assumptions being made, please delineate these and justify why these assumptions are deemed reasonable.
- 3. Is the plant data historian uncertainty and OLM data acquisition equipment uncertainty being captured as part of the safety related loop uncertainty determination for the OLM calculation? If so, since none of this equipment is part of the instrument loop being monitored, why isnt the uncertainty of this OLM monitoring equipment not added algebraically to the total loop uncertainty calculation rather than represented within the square-root-of-the-sum-of-the-squares calculation for the safety-related instrument channel? Provide a justification as to why the uncertainty of the plant process computer should be considered as part of the loop uncertainty calculation.
Since the output of the plant process computer is not part of the safety related instrument channel being monitored, then why is the uncertainty of the process computer not added algebraically to the uncertainty of the channel being monitored?