ML23349A206
| ML23349A206 | |
| Person / Time | |
|---|---|
| Site: | University of Missouri-Rolla |
| Issue date: | 12/15/2023 |
| From: | Graham J, Taber E Missouri Univ of Science & Technology |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| Download: ML23349A206 (1) | |
Text
Missouri Nuclear Reactor Facility 250 West 13th St, Rolla, MO 65409-0450 (573) 341-4236 l reactor@mst.edu l reactor.mst.edu An equal opportunity institution December 15, 2023 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001
SUBJECT:
FOLLOW-UP REPORT ON EN-56878, REPORTABLE OCCURRENCE UNDER MISSOURI S&T REACTOR (R-79) TECHNICAL SPECIFICATION 6.7.2.
Dear Sirs:
The Missouri University of Science and Technology Reactor ("MSTR" with License R-79, Docket No. 50-123) is hereby submitting a 14-day written report as a follow-up to our December 4, 2023 phone and email report. Both reports (this written report and the phone/email report) are in regard to the MSTR Technical Specification (TS) reportable occurrence identified as EN-56878. While facility review and opinion suggests that the event was not reportable, this cannot be conclusively shown, and the event will be conservatively assumed as Reportable under MSTR TS 6.7.2. The attached document details the event, surrounding circumstances and root causes, as well as efforts taken and planned to prevent recurrence.
If you have any questions regarding this report, please contact either me, at 573-341-4291; or Dr. Joseph Graham, Reactor Director, at 573-341-7759.
Sincerely, Ethan Taber Reactor Manager, MSTR Enclosure Joseph Graham, PhD Reactor Director, MSTR cc:
Radiation Safety Committee (S&T)
Dr. Joseph Newkirk, Chair of NERS (S&T)
Dr. Ayodeji Alajo, Deputy Reactor Director MSTR (S&T)
Ms. Paulette Torres, Project Manager (NRC)
Mr. Juan Arellano, Facility Inspector (NRC) 1/1
MSTR Licensee Event Report 23-01 On December 1, 2023, at 11:24 CST, with the Missouri University of Science and Technology Reactor (MSTR) operating at 180 kW, a 150% Full Power scram signal was received on one Safety Amplifier, and the reactor scrammed automatically. All shim rods fully inserted, and power dropped rapidly as expected following a scram. The console operator responded accordingly using MSTR Standard Operating Procedure (SOP) 150, Response to Alarms.
At the time of the event, the operator observed that the affected Safety Amplifier went blank, and following approximately a 0.5 s delay, returned to normal status tracking the decay power, with the scram indicator illuminated. This behavior was interpreted as the failure of a safety system during operations, and Reactor Director and Deputy Reactor Director were notified by, and discussed the incident with, the Reactor Manager by 12:41 CST.
MSTR Technical Specification (TS) 6.7.2.c)iii) specifies that [the licensee shall make a report for] a reactor safety system component malfunction that renders or could render the reactor safety system incapable of performing its intended safety function unless the malfunction or condition is discovered during maintenance tests or periods of reactor shutdowns.
During the event, the console operator and Senior Reactor Operator (SRO) on Duty observed that all other nuclear instrumentation (NI) channels behaved nominally and indicated that reactor power was stable at 180 kW until the scram took effect. At no time was reactor power in excess of licensed limits (200 kW) or the Limiting Safety System Setting (LSSS, 300 kW).
At no point was the health and safety of the public or MSTR in doubt. Due to ongoing reviews and replacement component sourcing, the MSTR has not operated since this event. As of the writing of this report, no surveillances or required facility activities are challenged by this inoperability.
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Background===
The MSTR utilizes two identical Imaging and Sensing Technologies (now a division of Mirion Technologies) NPA-2017 safety amplifiers to form the core of the Reactor Protection System (RPS), in combination with other scram signals (e.g., fast period, low pool water). The RPS consists of electromechanical relays and keyswitches, which in turn connect to the shim/control rod magnet power supply to energize the magnets when operating and to deenergize the magnets when securing the reactor or during a scram. The scram logic is wired in series, such that any scram signal (or projected failures of RPS components) will result in the circuit opening and causing a scram. The 150% Full Power scram trip of the NPA-2017 serves as the protective action for the MSTRs LSSS.
The NPA-2017 amplifier utilizes an uncompensated ion chamber (either BF3-filled or boron-lined), and measures and amplifies ion chamber current (0 nA-100 nA) in relation to MSTR-licensed power (200 kW). Internal to the amplifier, a 120 VAC to +/-15 VDC power supply provides operating voltage to the various components. For the failed amplifier, this power supply appears to be the OEM (original equipment manufacturer) component and has been in service for approximately 22 years. In the event of a failure of this power supply, a scram will occur.
For completeness, it is noted that a 150% Full Power scram was also received on the failed amplifier on 10/20/2023 and 11/13/2023. On both occasions, the observed behavior was different from the event of this report and were identified as non-failure, noise-related nuisance trips.
Follow-up testing of the safety amplifier was conducted, and no failures were observed.
However, triggering the shutdown of the internal +/-15 VDC power supply did replicate the behavior observed during the scram. An age-related, intermittent failure of the power supply is concluded to be the cause.
Root-Cause Analysis Two factors have been determined to root causes of this event:
- 1. Age-related failure of the safety amplifier power supply.
The existing amplifier power supply is approximately 22 years old. In reviewing the OEM documentation regarding reliability analysis, it was determined that the component chance of failure for the current lifetime is estimated at 23.1%.
- 2. Lack of redundancy in RPS components compared to TS requirements.
The existing configuration meets the number of required safety channels per MSTR TS 3.2.2. While this configuration ensures safe operations, it does not protect against exceeding licensing thresholds in the event of a system failure during operations.
Corrective Actions As required by MSTR TS 6.6.2, the reactor oversight Radiation Safety Committee reviewed this event at its most recent meeting (12/13/2023).
This event and the corrective actions described herein have been added to the MSTR Corrective Action Program (CAP) as CAP-2023-007.
To restore normal operations, the power supply within the safety amplifier will be replaced and the unit tested accordingly. This power supply is no longer manufactured, so alternative sourcing strategies or design changes are being evaluated with vendors.
If the facility establishes a need to operate (either due to encroaching surveillances or prior to the aforementioned repair, the following actions will be taken: the existing amplifier will be reinstalled as-is AND compensatory measures employed, as another limiting trip signal (conservatively below 150% Full Power) will be sourced from one of the other available NI channels to the series scram logic and annunciation. Alternatively, the facility may also evaluate replacing the amplifier with a different system. Either of these options would look to be performed/implemented under a 10 CFR 50.59 Screening and Evaluation, if applicable, or under a license amendment otherwise. The facility may also consider seeking a relief from potentially impacted TS surveillances until operations are restored.
Additionally, the following targeted corrective actions have been performed or planned by the facility to address the root-causes and prevent recurrence. These actions include an Estimated Time to Completion/Implementation (ETC). The NRC will be notified as soon as possible if the ETC implementation timelines are determined to be unworkable.
- 1. Age-related failure of the safety amplifier power supply A preventive maintenance item will be established to replace power supplies in the existing amplifiers (while in service) every ten years of operation. Another interval may be established by a Failure Mode and Effects Analysis (FMEA) or reliability assessment.
Future system modifications may eliminate this requirement. (Procedural ETC 1/31/2023, component replacement as soon as practical following receipt)
- 2. Lack of redundancy in RPS components compared to TS requirements.
Facility strategic planning will include considerations for modifications to RPS to improve redundancy and reliability. This may include a transition to a non-coincidence logic unit (NCLU) or two-out-of-four setup. Such modifications would be dependent upon external funding sources and potentially license amendments for the installation of a digital RPS. (Issue planning memo, ETC 1/31/2023)