ML25273A161
| ML25273A161 | |
| Person / Time | |
|---|---|
| Site: | Armed Forces Radiobiology Research Institute |
| Issue date: | 09/25/2025 |
| From: | Burke G US Dept of Defense, Armed Forces Radiobiology Research Institute |
| To: | Office of Nuclear Reactor Regulation |
| ckm2 | |
| References | |
| IR 2025001 | |
| Download: ML25273A161 (3) | |
Text
UNIFORMED SERVICES UNIVERSITY OF THE HEALTH SCIENCES ARMED FORCES RADIOBIOLOGY RESEARCH INSTITUTE 4301 JONES BRIDGE ROAD, BUILDING 42 BETHESDA, MARYLAND 20889-5648 www.usuhs.edu/afrri Learning to Care for Those in Harms Way September 25, 2025 U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 Attention: Document Control Desk
Subject:
Reply to a Notice of Violation Dated September 4, 2025, Docket No. 50-170, License R-84.
References:
(a) AFRRI USNRC Safety Inspection Report 05000170-2025201 and Notice of Violation, dated September 4, 2025 (b) Letter to Ms. Cindy Montgomery USNRC, Follow-up Report for Event Notification EN57730, dated 04 June 2025 (ML25155A009, ML25155A008)
(c) Initial Event Report (EN57730), 22 May 2025 Reason for the violation: As described in references (a) through (c), on 22 May 2025, the AFRRI TRIGA Mark F (License: R-84, Docket: 50-170) experienced a high-power SCRAM (automatic shutdown) during startup in steady-state mode under two (2) control rod automatic control. During power escalation, the Logarithmic Nuclear Instrument Channel (NLW) malfunctioned, failing to provide a period signal (rate of change of power) to the automatic control system. The apparent cause of the event was high frequency noise induced in the log channel detector input, causing the channel to fail to a fixed high frequency of constant amplitude. The flat signal produced a negligible period signal for input to the automatic control algorithm. With no significant period input, the automatic control algorithm produced an output that withdrew the two control rods continuously for 5-7 seconds. Due to the rate of power increase, reactor power momentarily exceeded the steady-state license limit of 1.1MW for approximately 100 msec. Peak power level recorded by the digital chart recorders was 1.2MW. All reactor protection systems functioned as designed.
The event was reported to NRC Headquarters Operations Office at 1504 on 22 May 2025 (EN 57730), as documented in reference (c).
Technical specification 3.2.1.a - Log Power Channel requires one (1) channel be operable in steady-state mode. Contrary to this requirement, the Log Power Channel malfunctioned and failed to provide an accurate power and period signal. Due to the rate of power increase, the reactor power momentarily exceeded the steady-state limit of 1.1 megawatts, resulting in a power transient. This has been determined to be a Severity Level IV violation, Section 6.1.d of the Enforcement Policy as documented in reference (a).
Corrective steps that have been taken: As documented in reference (b), subsequent troubleshooting revealed that the source of the noise is related to AC-grounds. The largest sources that cause a change in signal were identified:
Switching the pool lighting, Manipulating the transient rod drive AC/DC motor circuits, Movement of the core, particularly at the region transition points, Manipulating the NLW detector flexible conduit.
2 Operators were briefed on the event with increased emphasis to be aware of these activities, indications, and failure modes during operation. Based on results of troubleshooting, minor flexing of the NLW flex conduit away from the transient rod drive wiring appeared to restore the indications to normal.
Corrective actions (CAs) were initiated to track like-for-like replacement of the NLW detector cable (CA037), which is a long-lead procurement item, and investigation and repair of the 120VAC electrical distribution system (CA038).
The following short-term restrictions were put in place:
Minimize use of 2-rod automatic for routine automatic startups until auto mode drive speeds can be tuned properly, which will be performed with software modifications (FSM-003 & FSM-005). Since the period interlock is bypassed during square-wave ramp-up, use of 2-rod auto for square wave operations is still authorized.
If indications of NLW noise become apparent during an automatic startup, immediately place the system in manual, and dispatch an operator to the bay to immediately correct the issue in accordance with Tech Spec 3.2.1.a.(2).
Corrective steps that will be taken:
The following longer-term actions are being pursued:
Fission chamber, cabling, jacket, and connector replacement on NLW once materials can be procured.
A quote and lead time for procurement of like-for-like replacement cable for the NLW was received from General Atomics with a procurement lead time of 44-46 weeks for the engineered radiation-resistant cable. The contract has been awarded to initiate the procurement. In the meantime, a different cable style that is more readily available is being identified and will be evaluated under 10 CFR 50.59 that can serve as a temporary replacement for the intervening period. The temporary replacement is scheduled during the upcoming annual maintenance outage in November/December 2025, but no later than 31 Dec 2025.
Transient rod drive motor modification to use a stepper motor like other drives rather than AC circuits switching a DC motor (FM-031 & FSM-005). This modification was awarded to General Atomics on 20 Sep 2024 with an anticipated completion date during the upcoming outage, anticipated completion is no later than 31 Dec 2025.
Drive speed tuning accounting for various modes of operation and number of drives being controlled automatically to reduce the reactivity insertion rate (FSM-003 & FSM-005). This modification in part has been awarded to Plantation Productions (OEM software vendor) on 12 Mar 2025 and will be implemented in conjunction with the transient rod drive motor modification with an anticipated completion date no later than 31 Dec 2025.
Investigation of the condition of building AC ground network revealed the presence of shared neutrals from circuits originating in different electrical panels. These electrical panels and specific circuits are associated with equipment that have been shown to impact the NLW signal. Modification of the electrical distribution system based on results of the investigation is being prepared by facilities engineering (CA038).
Date when full compliance will be achieved:
It is anticipated that most of the modifications and temporary repairs will be completed at the conclusion of the annual maintenance outage, scheduled to be completed on 31 Dec 2025. The final installation of the NLW detector cable is dependent on successful delivery. Task Order 4 under IDIQ contract No.
3 HU000122D0003 to General Atomics has been issued for procurement, with period of performance ending September 30, 2026.
If you need any further information, please contact Mr. Andrew Smolinski, Chief, Radiation Sources, Radiation Sciences Department, AFRRI via andrew.smolinski@usuhs.edu or (301) 295-1288.
Gerald F. Burke CAPT, MSC, USN Institute Director, AFRRI BURKE.GERALD.F RANCIS.10279493 27 Digitally signed by BURKE.GERALD.FRANCIS.1027 949327 Date: 2025.09.25 09:49:26
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