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Radiation Monitoring Systems Deficiencies at Aging Nuclear Utilities
Posted on August 29th, 2017

NRC Information Notice 2013-13, Rev. 1: “Deficiencies with Effluent Radiation Monitoring System Instrumentation” detailed trends in the malfunction of installed effluent radiation monitoring system instrumentation, also known as radiation monitoring systems (RMS), at aging nuclear power plants.  Many of these systems have been installed and in operation since the late 1980s in response to the Three Mile Island Accident in 1979, and are now susceptible to failures.  Twenty-seven examples at 20 aging utilities that had encountered RMS malfunctions or failures were provided in the Notice, detailing the specific root causes of the failures.  The most common root causes were improper system design modifications, incomplete or incorrect calibrations, inadequate representative sampling used for analysis and poor maintenance of the systems.

In many cases, the staff that implemented the RMS systems in the early 1980s no longer work as employees at the plant, creating a void of institutional knowledge on the nuances, operational considerations and maintenance needs of the systems. RSCS consultants have assisted with RMS issues at several plants as consultants.  Our staff was directly involved in the implementation of the accident monitoring requirements of NUREG 0737 in the 1980’s.  Implementation included monitor design and use for offsite dose projections using post-Accident source terms for all accident release pathways for BWRs and PWRs.  Also, our staff developed methodologies used at operating sites to support dose projection methods, techniques and tools.  Our key staff served as subject matter experts (SMEs) for the RMS system at a large operating PWR for well over 20 years.  One of our principals is a member of ANSI N13.1 committee on monitoring effluents from stacks and ducts and has been active in providing presentations to our industry on RMS issues.   

Some examples of RMS challenges that RSCS has supported include the following.

Isokinetic Sampling Evaluation.  A detailed evaluation of isokinetic sampling deficiencies at a large PWR was performed.  The deficiencies were related to the plant’s misinterpretation of a 1999 change to ANSI N13.1 and difficulties in the plant’s ability to maintain the isokinetic sampling skid.

Stack Flow Profiles.  3-dimensional flow profiles for NPP sites using EPA-approved methods were generated.  These evaluations were performed at sites having both rectangular and circular stack designs.  These studies showed compliance with the EPA criteria and allowed for quantification of the stack mass flow rates under nominal flow conditions.

Sample Line Loss Measurements and Calculations.  Several line loss studies for stack sampling were conducted for several sites.  One of these was an in-situ study using the plant’s continuous release of Cs-138.  Another study involved the assembly of a scale-model supported by calculations (“Deposition” code) to estimate sample line losses.

Development of an Alternate Stack Monitor.  An interim noble gas monitor was developed, designed, engineered, constructed, calibrated, and delivered to a large BWR.  This system was used by the site as an alternate monitor during a large-installed stack monitor replacement project thereby eliminating the need for compensatory sampling required by the plant’s Tech Specs.  Our design and engineering included the development of response functions for all normal operating and post-accident noble gases and we developed a conversion method that allowed the site to use the existing accident dose projection system.  Our calibration of the system used a combination of NIST-traceable sources and mathematical simulations using MCNP.  

Accident Personnel Dose Projection Analysis.  Dose projections for the collection of stack samples during accident conditions were performed.  This analysis involved a combination of site walk-downs and source-term and radiation transport calculations to demonstrate compliance with 10CFR50, Appendix A Criterion 19.

Calibration and Operational Deficiency Analysis.  The RMS system was reviewed and upgraded at a site in response to several deficiencies identified by the NRC.  This project included technical evaluations, and in some cases, monitor re-calibrations.  The deficiencies were part of several that prevented this shutdown plant from entering active decommissioning and our staff was able to reach successful resolution of all deficiencies.

Calibration.  NIST-traceable primary calibration of a turbine -building liquid sump monitor was performed for a large BWR.  The calibration included Cs-137 at several concentration levels, an energy response curve, and a linearity determination. RSCS also performs calibration of stack effluent monitors for cyclotron facilities using MCNP mathematical calibrations and transfer source calibrations.

Offsite Dose Projection.  URI (Unified Rascal Interface) has been implemented at over 40 US and 12 Canadian power plants for emergency dose projections under accident conditions.  This involves the customization of all release points and source terms for BWR, PWR, and CANDU reactors.

Training.  A 1-day RMS training session for regulators and stakeholders was developed and delivered.  This 5-module course covers RMS fundamentals, Principles of Operation, Standards and Calibrations, RMS Manufacturers, and NPP RMS lessons-learned. We can also customize training to your site’s needs and we can integrate this training into other topical areas that we have available such as internal/external dosimetry and more.

To learn more about our RMS support capabilities, please see here.

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