05000325/LER-2016-005
| Brunswick Steam Electric Plant (Bsep) Unit 1 | |
| Event date: | 10-03-2016 |
|---|---|
| Report date: | 12-05-2016 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3252016005R00 - NRC Website | |
comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
Brunswick Steam Electric Plant (BSEP) 005 - 000 05000-325 2016 Energy Industry Identification System (EIIS) codes are identified in the text as [XX].
Background
Initial Conditions On October 3, 2016, Unit 1 and Unit 2 were both in Mode 1 (i.e., Run mode) at 100 percent of rated thermal power. No out-of-service equipment contributed to, or affected the course of, this event.
Reportability Criteria This event is being reported in accordance with 10 CFR 50.73(a)(2)(i)(B) because Units 1 and 2 were operated in a condition prohibited by the Technical Specifications (TS). Specifically, both units' Drywell High Range Radiation Monitors (DWHRRMs) [IP] were declared inoperable due to signal cables which can degrade the accuracy of the DWHRRMs. With the DWHRRMs inoperable, a Special Report should have been submitted to the NRC within 14 days per TS 3.3.3.1, Condition F. The requirement to submit a Special Report has been in effect since 1984 following issuance of License Amendments 73 and 99 for Units 1 and 2, respectively. The Special Report was not submitted because the DWHRRMs were not previously recognized as inoperable. Therefore, the plant was operated in a condition prohibited by the TS. In accordance with 10 CFR 50.4(e), this event is being reported by LER only, and a Special Report is not required.
Event Description
On October 3, 2016, Engineering personnel were re-assessing signal cables for the DWHRRMs. These cables are susceptible to a phenomenon known as thermally induced current (TIC). During transient conditions, TIC can result in connected radiation monitors temporarily not meeting the accuracy requirements of Regulatory Guide (RG) 1.97, Revision 2, "Instrumentation for Light-Water-Cooled Nuclear Power Plants to Assess Plant and Environs Conditions During and Following an Accident." Per RG 1.97, Table 1, note 7, DWHRRMs must be accurate to within a factor of 2. The cables installed in both units at BSEP were previously assumed to be bounded by analysis of Rockbestos cables addressed in Electric Power Research Institute (EPRI) Technical Report 110379, "High Range Radiation Monitor Cable Study:
Phase 1." It was assumed that the materials in use at BSEP have essentially the same dielectric properties and thermal response as those tested by EPRI. Based on the reported behavior of those materials, calculations predicted that TIC-induced inaccuracy in the DWHRRMs at BSEP would not result in significant impact.
Re-assessment of the previous analysis at BSEP cast doubt on the earlier conclusion. The BSEP cables use an insulation material that was not tested by EPRI, and no rigorous basis supported the assumption that the dielectric properties and thermal response would be the same. Engineering personnel, therefore, determined that the previous conclusion about the acceptability of cables used at BSEP lacked sufficiently rigorous justification. This was reported to Operations personnel via Condition Report 2066681. Both BSEP Unit and Unit 2 DWHRRMs were conservatively declared inoperable.
comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
Brunswick Steam Electric Plant (BSEP) 005 05000-325 2016 - 000 Event Causes During accident conditions, cables exposed to rapidly changing temperatures can produce electrical currents large enough to cause false indications of high radiation. The TIC phenomenon was made known to the nuclear power industry via NRC Information Notice 97-45, Supplement 1, "Environmental Qualification Deficiency for Cables and Containment Penetration Pigtails." BSEP's initial assessment concluded that the effect of TIC upon the DWHRRMs was too small to appreciably affect their performance in a design basis accident. A new review cast doubt on that conclusion because documentation of the initial assessment contained no rigorous basis for the assumption that BSEP's cables were bounded by those tested by EPRI. Therefore, the DWHRRMs were conservatively declared inoperable.
Safety Assessment The purpose of the DWHRRMs is to provide operators with indication of radiation conditions inside the primary containment following an accident. Radiation conditions, in turn, are used to assess the integrity of the fuel cladding and provide input for emergency action levels (EALs) to classify a plant emergency.
The affected monitors would continue to function despite their susceptibility to TIC, but could indicate radiation levels higher than actual if cables were exposed to increasing temperatures, or lower than actual if cables were exposed to decreasing temperatures. Radiation indications would be inaccurate only during times when temperature in the drywell was rapidly changing. When temperature changes in the drywell grew smaller or stopped, inaccuracy in the DWHRRMs would diminish or vanish entirely. During times when the accuracy of the DWHRRMs might appear questionable, alternative methods of assessing the condition of the fuel are available to operators as provided in existing plant procedure OPLP-37, "Equipment Important to Emergency Preparedness and ERO Response." The DWHRRMs do not control any other structures, systems, or components.
Based on this analysis, this event had no adverse impact on the health and safety of the public.
Corrective Actions
Any changes to the corrective actions and schedules noted below will be made in accordance with the site's corrective action program.
An already-existing site procedure, OPLP-37 (referenced above), provides alternate indications for the DWHRRMs for assessment of drywell or fuel cladding conditions. This procedure will remain in place as a preplanned alternate method until a permanent resolution is implemented.
The subject cables will be replaced with cables less susceptible to TIC. The replacement schedule will be developed following assessment of available options.
comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
Brunswick Steam Electric Plant (BSEP) 005 - 000 05000-325 2016
Previous Similar Events
No events have occurred in the past three years in which instrumentation was found to be inoperable due to a unique physics phenomenon such as TIC.
Commitments This report contains no new regulatory commitments.