IR 05000457/2024090
| ML24107B089 | |
| Person / Time | |
|---|---|
| Site: | Braidwood  |
| Issue date: | 04/19/2024 |
| From: | Jason Kozal NRC/RGN-III/DORS/EB1 |
| To: | Rhoades D Constellation Energy Generation, Constellation Nuclear |
| References | |
| EA-24-003 IR 2024090 | |
| Download: ML24107B089 (1) | |
Text
SUBJECT:
BRAIDWOOD STATION, UNIT 2 - FINAL SIGNIFICANCE DETERMINATION FOR THE 2B AUXILIARY FEEDWATER PUMP DIESEL ENGINE FUEL OIL DILUTION ISSUE - NRC INSPECTION REPORT 05000457/2024090
Dear David Rhoades:
This letter provides you the final significance determination of the preliminary Greater-than-Green finding discussed in the U.S. Nuclear Regulatory Commissions (NRC)
inspection report 05000457/2023011 dated January 24, 2024 (Agency Documents Access and Management System (ADAMS) Accession Number ML24022A100). The finding was associated with an apparent violation of Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Appendix B, Criterion XVI, Corrective Actions, for failure to identify and correct a condition adverse to quality resulting in inoperability of the 2B auxiliary feedwater (AF) pump due to fuel oil dilution of the lubricating oil above the fault range for the pumps diesel engine. The final significance results were discussed with Adam Schuerman, Braidwood Site Vice President on April 19, 2024.
On February 26, 2024, Constellation Energy Generation, LLC (Constellation) provided supporting documentation (ML24057A302) to the NRC discussing the results of the full-scale testing performed on a similar diesel engine in December 2023. Additionally, at your request, a regulatory conference was held on March 7, 2024, to discuss your views on this issue. During the conference, your staff described the results of your full-scale testing, assessment of the significance of the finding, and the corrective actions taken to resolve the issue, including details regarding root cause evaluation of the finding. On March 18, 2024, Constellation provided additional information to address the open questions from the regulatory conference (ML24078A361). The NRCs independent review of the information you provided is detailed in the report.
Based on the NRCs observation of the full-scale testing, and independent review of the information provided at the regulatory conference and in your letters dated February 26, 2024, and March 18, 2024, the NRC has concluded that the test diesel engine was sufficiently similar to the 2B AF pump diesel engine, and therefore, the 2B AF pump diesel engine was likely functional for its 24-hour mission time at fuel oil dilution levels that would have been April 19, 2024 experienced due to this degraded condition. As such, the NRC has documented one finding of very low safety significance (Green) related to this issue in this report. This finding involved a violation of NRC requirements. We are treating this violation as a non-cited violation (NCV)
consistent with Section 2.3.2 of the Enforcement Policy.
If you contest the violation or the significance or severity of the violation documented in this inspection report, you should provide a response within 30 days of the date of this inspection report, with the basis for your denial, to the U.S. Nuclear Regulatory Commission, ATTN:
Document Control Desk, Washington, DC 2055-0001; with copies to the Regional Administrator, Region III; the Director, Office of Enforcement; and the NRC Resident Inspector at Braidwood Station.
If you disagree with a cross-cutting aspect assignment in this report, you should provide a response within 30 days of the date of this inspection report, with the basis for your disagreement, to the U.S. Nuclear Regulatory Commission, ATTN: Document Control Desk, Washington, DC 20555-0001; with copies to the Regional Administrator, Region III; and the NRC Resident Inspector at Braidwood Station.
For administrative purposes, this letter is issued as NRC Inspection Report 05000457/2024090.
Accordingly, the apparent violation documented in NRC Inspection Report 05000457/2023011 (ML24022A100) is now designated as NCV 05000457/2023011-01.
This letter, its enclosure, and your response (if any) will be made available for public inspection and copying at http://www.nrc.gov/reading-rm/adams.html and at the NRC Public Document Room in accordance with Title 10 of the Code of Federal Regulations 2.390, Public Inspections, Exemptions, Requests for Withholding.
Sincerely, Jason W. Kozal, Director Division of Operation Reactor Safety Docket No. 05000457 License No. NPF-77
Enclosure:
As stated
Inspection Report
Docket Number:
05000457
License Number:
Report Number:
Enterprise Identifier:
I-2024-090-0000
Licensee:
Constellation Energy Generation, LLC
Facility:
Braidwood Station
Location:
Braceville, IL
Inspection Dates:
January 21, 2024 to April 19, 2024
Inspectors:
K. Barclay, Senior Reactor Inspector
C. Bickett, Senior Reactor Analyst
M. Gangewere, Reactor Inspector
J. Havertape, Senior Reactor Analyst
D. Kimble, Senior Project Engineer
L. Rodriguez, Senior Reactor Inspector
Approved By:
Karla K. Stoedter, Chief,
Engineering Branch 1
Division of Operating Reactor Safety
SUMMARY
The U.S. Nuclear Regulatory Commission (NRC) continued monitoring the licensees
performance by conducting a Final Determination Report at Braidwood Station, in accordance
with the Reactor Oversight Process. The Reactor Oversight Process is the NRCs program for
overseeing the safe operation of commercial nuclear power reactors. Refer to
https://www.nrc.gov/reactors/operating/oversight.html for more information.
List of Findings and Violations
Contamination of Auxiliary Feedwater (AF) Pump Diesel Engine Lubricating Oil
Cornerstone
Significance
Cross-Cutting
Aspect
Report
Section
Mitigating
Systems
Green
Closed
[H.11] -
Challenge the
Unknown
A self-revealed Green finding and associated non-cited violation (NCV) of 10 CFR 50,
Appendix B, Criterion XVI, Corrective Action, were identified for the licensees failure to
assure that conditions adverse to quality are promptly identified and corrected. Specifically, no
action was taken to identify and correct a deficiency associated with the 2B AF pumps diesel
engine after the licensee received the results from a May 19, 2023, lubricating oil sample on
June 1, 2023, and these results showed viscosity levels in the alert range. These facts were
later revealed on September 21, 2023, when the next regularly scheduled periodic oil sample
results for the 2B AF pumps diesel engine were received by the licensee from their oil
analysis vendor and showed lubricating oil viscosity values and the value for fuel oil
contamination within the lubricating oil to be beyond acceptable fault limits.
Additional Tracking Items
None.
INSPECTION SCOPES
Inspections were conducted using the appropriate portions of the inspection procedures (IPs) in
effect at the beginning of the inspection unless otherwise noted. Currently approved IPs with
their attached revision histories are located on the public website at http://www.nrc.gov/reading-
rm/doc-collections/insp-manual/inspection-procedure/index.html. Samples were declared
complete when the IP requirements most appropriate to the inspection activity were met
consistent with Inspection Manual Chapter (IMC) 2515, Light-Water Reactor Inspection
Program - Operations Phase. The inspectors reviewed selected procedures and records,
observed activities, and interviewed personnel to assess licensee performance and compliance
with Commission rules and regulations, license conditions, site procedures, and standards.
OTHER ACTIVITIES - BASELINE
71153 - Follow Up of Events and Notices of Enforcement Discretion
Event Follow up (IP Section 03.01) (1 Sample)
(1)
The inspectors reviewed and evaluated information provided by the licensee during a
March 7, 2024, Regulatory Conference to determine the final safety significance
associated with the following event:
The identification of fuel oil contamination in the 2B Auxiliary Feedwater (AF) Pump
diesel engine oil system as documented in NRC Integrated Inspection Report 05000456/2023004; 05000457/2023004 and NRC Inspection Report 05000457/2023011.
INSPECTION RESULTS
Contamination of Auxiliary Feedwater (AF) Pump Diesel Engine Lubricating Oil
Cornerstone
Significance
Cross-Cutting
Aspect
Report
Section
Mitigating
Systems
Green
Closed
[H.11] -
Challenge the
Unknown
A self-revealed Green finding and associated non-cited violation (NCV) of 10 CFR 50,
Appendix B, Criterion XVI, Corrective Action, were identified for the licensees failure to
assure that conditions adverse to quality are promptly identified and corrected. Specifically,
no action was taken to identify and correct a deficiency associated with the 2B AF pumps
diesel engine after the licensee received the results from a May 19, 2023, lubricating oil
sample on June 1, 2023, and these results showed viscosity levels in the alert range. These
facts were later revealed on September 21, 2023, when the next regularly scheduled periodic
oil sample results for the 2B AF pumps diesel engine were received by the licensee from
their oil analysis vendor and showed lubricating oil viscosity values and the value for fuel oil
contamination within the lubricating oil to be beyond acceptable fault limits.
Description:
During Braidwood Unit 2 Refueling Outage A2R23, which took place from April 24, 2023,
through May 13, 2023, the 2B AF pump underwent a substantial preventative maintenance
work window that included the replacement of all sixteen (16) fuel injectors on the pumps
diesel engine (prime mover). The fuel lines associated with all 16 fuel injectors were also
replaced with new lines. While the engine design has connections that support pressure
testing the fuel system following fuel line replacement, pressure testing was neither required
by the licensees maintenance instructions/procedures nor performed prior to return of the 2B
AF pump to service.
Following the completion of Braidwood Unit 2 Refueling Outage A2R23, the 2B AF pump was
run on May 19, 2023, for its normal quarterly surveillance in accordance with the stations
Inservice Testing (IST) Program. Oil sampling for analysis was performed as a normal part of
this surveillance routine. On June 1, 2023, the station received the results of the 2B AF
pumps diesel engine lubricating oil analysis from their offsite vendor. The oil sample results
indicated a viscosity of 12.6 centistokes (cSt) at 100 degC and 114.6 cSt at 40
degC. Licensee procedure MA-AA-716-230-1001, Oil Analysis Interpretation Guideline,
Attachment 49 for diesel engines like the 2B AF pump using Mobil Delvac 1640 lubricating oil,
placed both of these viscosity parameters outside of acceptable limits and within the Alert
Range. In addition, the fuel content parameter indicating the percentage of fuel oil
contamination measured within the lubricating oil was listed as N/R (Not Recorded) on the
report. In spite of these anomalies, the licensees review of the oil analysis results failed to
generate any additional actions or reviews. No corrective action program (CAP) issue report
(IR) was written to document the irregular results nor were any follow-up actions performed.
On September 1, 2023, the next quarterly IST surveillance on the 2B AF pump was
conducted. Again, oil sampling for analysis was performed as a normal part of this
surveillance routine. On September 21, 2023, the station received the results of the 2B AF
pumps diesel engine lubricating oil analysis from their offsite vendor. Viscosities of 5.7 cSt at
100 degC and 53.4 cSt at 40 degC were reported by the oil analysis vendor. In addition, a
fuel concentration of 17.0 percent was indicated as present within the lubricating oil. All of
these values were well outside of the fault range for the respective parameters. Per the
licensees MA-AA-716-230-1001 procedure, any oil analysis parameter in the fault range
would indicate an immediate need to protect the component from damage or possible failure
with continued operation.
Following receipt of the vendors oil analysis results on September 21, 2023, a confirmatory
sample was immediately taken from the 2B AF pumps diesel engine and sent off for analysis
with an expedited priority. On September 22, 2023, the station received the results of this
confirmatory oil sample from their vendor. Viscosities of 8.3 cSt at 100 degC and 52.4 cSt at
40 degC were reported, along with a fuel concentration of 18.2 percent indicated within the
lubricating oil. At this point, with the results of the confirmatory oil sample also well outside of
the fault range for all of these parameters, the licensee immediately declared the 2B AF pump
inoperable and entered an emergent repair work window to drain the 2B AF pumps
lubricating oil from the diesel engine, check the fuel system for leakage, perform necessary
repairs, and refill the diesel engine crank case with new oil. On September 23, 2023, the
licensee returned the 2B AF pump to service after changing the lubricating oil and repairing
leaking fuel line connections on two fuel injectors.
Corrective Actions: As part of the corrective actions for this issue, the licensee replaced the
engine's lubricating oil and identified fuel leaks on the 5L return line at the fuel injector
connection point and at both ends of the 2R return line (injector connection and fuel rail
connection points) were repaired. The 2B AF Pump was returned to service on
September 23, 2023, following completion of post-maintenance testing activities. A formal
root cause evaluation was chartered by station management on October 5, 2023.
Corrective Action References: IR 4703982; Degraded Oil in 2B Diesel-Driven Auxiliary
Feedwater (2AF01PB) Engine Crank Case; 09/21/2023
Performance Assessment:
Performance Deficiency: The licensee's failure to identify and correct conditions adverse to
quality, such as nonconformances and deficiencies, in accordance with 10 CFR 50,
Appendix B, Criterion XVI was a performance deficiency. Specifically, the licensee failed to
identify and correct a condition adverse to quality from June 1, 2023, to September 23, 2023,
when they received the results for a 2B AF pump diesel engine lubricating oil sample which
showed the viscosity levels in the alert range provided in licensee procedure
MA-AA-716-230-1001, Oil Analysis Interpretation Guideline. As a result, the 2B AF pumps
diesel engine lubricating oil continued to degrade due to fuel oil dilution until September 22,
2023, when the licensee declared the 2B AF pump inoperable due to receiving lubricating oil
sample results for the diesel engine which showed viscosity and fuel oil percentage levels in
the fault range specified in MA-AA-716-230-1001.
Screening: The inspectors determined the performance deficiency was more than minor
because it was associated with the Equipment Performance attribute of the Mitigating
Systems cornerstone and adversely affected the cornerstone objective to ensure the
availability, reliability, and capability of systems that respond to initiating events to prevent
undesirable consequences. Specifically, the inspectors concluded that licensees failure to
have taken any actions in response to the anomalies contained within the May 19, 2023,
lubricating oil sample results for the 2B AF Pumps diesel engine represented a significant
missed opportunity that resulted in adverse impact to the 2B AF Pumps availability and
reliability. The inspectors also compared the finding with the examples listed in IMC 0612,
Power Reactor Inspection Reports, Appendix E, Examples of Minor Issues. Example 4.f was
found to be similar in that a missed opportunity to have properly identified and/or corrected a
condition adverse to quality resulted in an adverse impact to the availability, reliability, and/or
capability of a mitigating system.
Significance: The inspectors assessed the significance of the finding using IMC 0609
Appendix AProperty "Inspection Manual Chapter" (as page type) with input value "NRC Inspection Manual 0609</br></br>Appendix A" contains invalid characters or is incomplete and therefore can cause unexpected results during a query or annotation process., The Significance Determination Process (SDP) for Findings At-Power.
The inspectors subsequently concluded that because the initial finding documented in
Inspection Report 05000457/2023011 could not be easily screened to be of very low safety
significance, a detailed risk evaluation was required. The time from the May 19, 2023,
lubricating oil sample that indicated that fuel oil was leaking into the 2B AF pumps diesel
engine lubricating oil to the point where the licensee completed their corrective actions on
September 23, 2023, led the inspectors to answer yes to Mitigating Systems Screening
Question A.3, Does the degraded condition represent a loss of the PRA function of one train
of a multi-train technical specification (TS) system for greater than its TS allowed outage
time?
In their letter dated February 26, 2024 (ADAMS Accession Number ML24057A302),
Constellation provided new information that they believed supported a final significance
determination of Green (very low safety significance) for the preliminary Greater-than-Green
finding discussed in NRC Inspection Report 05000457/2023011 (ML24022A100). During the
regulatory conference with the NRC on March 7, 2024, Constellation presented the
information described in their February 2024 response and acknowledged the violation as
documented in Inspection Report 05000457/2023011. In their letter dated March 18, 2024,
Constellation provided additional information to address the open questions from the
regulatory conference (ML24078A361). The NRCs assessment of the information provided
by Constellation is included below.
Full-Scale Testing Overview
To determine the effect of fuel oil dilution on performance of the 2B auxiliary feedwater (AF)
pump diesel engine, Constellation contracted a third-party vendor to perform full-scale testing
on a similar engine. The testing was performed at Southwest Research Institute in December
2023.
The contractor conducted a baseline run on the test diesel engine to demonstrate that the
test engine could operate reliably at the required loads for a cumulative 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> under
normal engine lube oil conditions. Data recorded during this test were used to generate a
baseline for each test parameter. Following the baseline test, the contractor performed a
diluted lube oil test to determine the impact of an elevated and increasing fuel oil
concentration in the engine lube oil on the test engines operation and performance. The
diluted lube oil test started at a concentration of approximately 14.2 percent fuel oil in the
lubrication oil. Fuel oil dilution was then slowly increased over 24.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> of runtime to a final
concentration of approximately 45.6 percent fuel oil, and lube oil samples were taken hourly
for analysis. The results of the diluted lube oil test were directly compared to the baseline
data to determine how the fuel oil contamination of the engine lube oil affected engine
performance. Additionally, following the diluted lube oil test, the test diesel engine was
shipped to Braidwood Station for disassembly and post-test inspections.
NRC inspectors were present at the facility to observe test setup and performance. The intent
of the NRC observation was to perform an independent assessment of the testing to inform
the final detailed risk assessment for the 2B AF pump fuel oil dilution issue.
Test Diesel Engine
Constellation obtained a similar diesel engine and provided the lube oil and fuel oil used for
the testing, which were identical to that used in the 2B AF pump engine. Both the test engine
and the 2B AF pump engine were the same make and model (Detroit Diesel Series
149). Though the test engine had 12 cylinders, and the 2B AF pump engine was a 16-cylinder
design, the individual cylinder packages were identical between the models, so both engines
had the same displacement per cylinder, cylinder bore, and stroke length. Both engines also
had a similar continuous rating per cylinder. The lubrication systems of both engines provided
nearly identical per-cylinder lube oil flow rates and had identical lube oil pressure regulator
settings. The following components, which could be affected by lube oil viscosity, were the
same on both engines: main bearings; crankshaft thrust bearings; connecting rod bearings;
camshaft bearings; cylinder heads; valve and injector operating mechanisms; piston skirts,
rings, and pins; cylinder liners; and front and rear crankshaft oil seals. Both the test engine
and the 2B AF pump engine had four turbochargers. Though they were different models
supplied by the same manufacturer, the turbochargers on both engines were of similar
construction. The turbochargers on both engines were supplied with lube oil, and at their
respective rated loads, operated at similar speeds.
While on-site at Southwest Research Institute, the NRC observed differences in
environmental conditions between the test engine and the 2B AF pump engine. Specifically,
the NRC noted differences in turbocharger insulation (test engine had none), ambient
temperature (lower for the test engine), and cooling water temperature (lower for the test
engine). These factors would result in increased lube oil and jacket water temperatures on
the 2B AF pump engine, which would increase jacket water cooler demand. During the
regulatory conference, Constellation stated that the 2B AF pump engine jacket water cooler
had sufficient margin to mitigate the heat loads resulting from higher ambient and air intake
temperatures. The NRC verified the condition of the 2B AF pump engine jacket water cooler
and noted that the cooler had two of 188 tubes plugged, which is below the limit for the
maximum cooling water temperature experienced during the exposure period of this
degraded condition. The NRC also noted that the 2B AF pump engine is equipped with an
intercooler, which would reduce air intake temperature. The test engine did not have an
intercooler.
Diesel Engine Failure Mechanisms
In reviewing the testing and supplementary information provided by Constellation, the NRC
evaluated the likelihood of the following three primary failure mechanisms related to fuel oil
dilution of the 2B AF pump diesel engine lube oil system:
Reduction of lube oil viscosity that reduces the load carrying capability of the
lubricated engine components.
High level in the lube oil sump, which could contact rotating engine components,
causing heating and aeration of the diluted lube oil.
Possible engine fire due to decreased flashpoint of the diluted lube oil.
Reduction in Lube Oil Viscosity
The NRC concluded that the 2B AF pump diesel engine was likely to function for its 24-hour
mission time even with reduced viscosity of the lube oil. The key considerations that
supported this conclusion were as follows:
The NRC reviewed the load profile for the test and verified that loading on the test engine
components during the diluted lube oil test exceeded the expected design basis loading on
the 2B AF pump engine during its 24-hour probabilistic risk assessment mission time. The
test engine was operated at loads approximately one to five percent higher than the
per-cylinder loads expected for the 2B AF pump engine. The NRC also noted that diesel
engine loading would be highest early in an event, when fuel oil dilution is lower (i.e., higher
lube oil viscosity) and would decrease as auxiliary feedwater flow was reduced later in the
event, with engine load being the lowest at the higher fuel oil dilution levels (i.e., lower lube oil
viscosity).
The NRC reviewed trend graphs of engine parameters and compared the results of the
diluted lube oil test to the baseline test. Of note, due to the increasing fuel oil concentration,
lube oil pressure decreased slowly over the duration of the diluted lube oil test to
approximately 47 psig, which was still within the manufacturer's normal range (45 - 70 psig)
and remained above the 2B AF pump engine trip point of 10 psig. Test engine lube oil
temperature during the diluted test slowly decreased throughout the test, likely due to
increased flow of the lube oil through the lube oil cooler due to the lower viscosity of the
mixture resulting from increased fuel oil concentration. Other parameters aligned with those
measured during the baseline testing.
The NRC evaluated the test diesel engine bearing inspections following the fuel oil dilution
test that revealed no signs of abnormal bearing wear. The NRC reviewed the lube oil sample
analysis results and verified that there was no increase in wear metal concentrations.
Additionally, there was no damage to other lubricated components, including the
The NRC noted several concerns regarding fuel oil dilution levels that could have reduced the
applicability of the full-scale testing to the degraded condition associated with the 2B AF
pump diesel engine. Specifically, actual fuel oil dilution levels were lower than what was
specified in the test plan; American Society for Testing and Materials (ASTM) standards were
inappropriately applied in determining fuel oil concentration in the lube oil samples from the
testing; and inconsistent results from a September 1, 2023 oil sample from the 2B AF pump
engine indicating that as-found fuel oil dilution levels may have been significantly higher than
the initial fuel oil dilution level used for the testing.
Due to underestimation of residual oil in the engine, the fuel oil concentration started
about 4.5 percent lower than outlined in the test plan and reached an average of
about 45.6 percent versus the planned fuel oil concentration of 52.5 percent.
Constellation estimated that an additional six hours of test engine runtime would have
been needed to achieve the planned end fuel oil dilution of 52.5 percent. Given the
condition of the bearings post-test, as well as trended engine parameters, the NRC
concluded that this did not have a significant impact on the outcome of the test.
Regarding analysis of the oil samples from the diluted lube oil test, the standard that
the contractor initially used to determine fuel oil concentration was not accurate for the
grade of lube oil used during the test or for fuel oil concentrations greater than
12 percent. As a result, the contractor developed an alternate methodology for
determining accurate fuel oil concentrations using additive elemental concentrations.
The NRC determined this alternate methodology was reasonable.
Bureau Veritas, an oil analysis vendor, documented the viscosity of the September 1,
2023, 2B AF pump diesel engine oil sample as 5.7 centistokes (cSt) at 100 degrees
Celsius, with an associated fuel oil concentration of 17 percent. Based on a plot of
viscosity versus fuel oil concentration, the NRC was concerned that this viscosity
could correspond to a fuel oil concentration of about 30 percent, and therefore,
inquired as to whether the test results provided by Bureau Veritas were accurate since
it was the starting fuel oil dilution level for the full-scale testing. Following the
regulatory conference, Constellation provided the methods used by Bureau Veritas to
analyze the oil samples and concluded that the viscosity reading of 5.7 cSt was
anomalous. This conclusion is further supported by a confirmatory oil sample taken
from the 2B AF pump diesel engine on September 21, 2023, which showed a viscosity
of 8.3 cSt and 18.2 percent fuel oil dilution. Based on this information, the NRC
determined that Constellations conclusion was reasonable.
The NRC also noted that during the diluted lube oil test, the test diesel engine was started on
residual, undiluted lube oil from the baseline test (i.e., the contractor did not rotate the engine
to ensure that diluted lube oil was present in the lubricated components prior to the start of
the diluted lube oil run). In their response, Constellation noted that the 2B AF pump diesel
engine was started at least five times with diluted lube oil without any sign of degradation,
abnormal bearing wear, or increase in wear metals in the lube oil analyses. Constellation also
stated that this would not impact the results of the diluted lube oil test because hydrodynamic
conditions would be reached within a few seconds of engine start and well below the full
engine operating speed regardless of whether the start occurred on undiluted lube oil or lube
oil diluted to 18.2 percent fuel oil. Constellation also stated that turbocharger speeds and
bearing loads would still be relatively low and well below their peak values by the time the
diluted lube oil reached the applicable bearings; and diesel fuel oil is used as a lubricant in
other applications, albeit with a lower viscosity than lube oil. The NRC determined that
Constellations response to this question was reasonable.
High Level in the Engine Lube Oil Sump
Approximately 3.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> into the diluted lube oil test, a mixture of fuel oil and lube oil began to
leak from the crankshaft rear main lube oil seal. Constellation stated that leakage past the
test engine rear crankshaft seal was not due to seal damage or failure based on inspections
following the diluted lube oil testing. Leakage occurred due to both increasing level in the lube
oil sump as well as decreased viscosity of the lube oil due to fuel oil dilution. Constellation
also noted that this was an expected condition per the vendor manual.
The NRC questioned the design of the seal of the test engine versus the 2B AF pump engine,
and whether seal leakage would be affected by the different oil sump configurations. In their
response, Constellation stated that all Detroit Diesel Series 149 engines shared a common
block design, but with different block lengths to accommodate the required number of
cylinders. Constellation stated that the rear main seals on the plant and test engines are the
same design and share the same location on the engine block. Based on this information, the
NRC concluded that leakage from the crankshaft rear seal likely would have occurred on the
2B AF pump engine, even with the differences in oil sump configuration. The NRC
determined that this leakage would mitigate level in the lube oil sump sufficiently to prevent
significant heating and aeration/foaming of the lube oil. Aeration/foaming of the lube oil would
also be mitigated by anti-foaming additives in the lube oil. As such, the NRC concluded that
high level in the lube oil sump would likely have not impacted the functionality of the 2B AF
pump diesel engine during its 24-hour mission time.
Engine Fire
The NRC determined that Constellations test report did not address potential engine fire
concerns due to fuel oil dilution of the lube oil. Specifically, a review of operating experience
by the NRC team revealed a fire in an emergency diesel generator (EDG) turbocharger
following a fuel oil dilution event in 1984 at Surry Power Station.
Based on information presented at the regulatory conference, the NRC determined that there
are significant design and operational differences between the Surry EDG and the 2B AF
pump diesel engine that would limit the applicability of this operating experience to this
degraded condition. Most notably, the Surry EDG was an Electro-Motive Diesel (EMD) Series
645 engine that had a unique turbocharger design that was significantly different than that of
the 2B AF pump Detroit Diesel engine. The EMD turbocharger design exhibited lower margin
with respect to loss of lubrication, a vulnerability that was subsequently addressed by adding
a separate dedicated oil pump to pre-lubricate the turbocharger prior to startup and prevent
the turbocharger from seizing upon shutdown. The NRC also determined that the Surry EDG
experienced high fuel oil dilution concentration (about 40 percent) early in the run while under
heavy load (about 90 percent loading), whereas both the Braidwood test and 2B AF pump
diesel engines would experience the highest loads under the lowest fuel oil dilution
levels. EMD engines generally have a higher power per cylinder and as a result, have higher
exhaust temperatures which approach the reduced ignition temperature of the lube oil and
contribute to decreased viscosity of the oil.
Given the observed leakage of the fuel oil/lube oil mixture from the crankshaft rear seal, the
NRC also questioned the possibility of a consequential fire in the 2B AF pump room which
could impact the functionality of the pump during its 24-hour mission time. In their
supplemental response, Constellation stated that the risk of a fire caused by the fuel oil/lube
oil mixture leaking out of the rear main seal of the 2B AF pump diesel during a diluted lube oil
event was extremely low. Unlike the test diesel engine, the 2B AF pump diesel engine had a
rear housing with an opening that would direct any leakage from the rear main seal
downward and thus prevent contact with the hot surfaces of the engine, which were
insulated. Constellation identified one potential ignition source associated with the jacket
water heater and thermostat switch. This source was in solid conduit and the jacket water
heater was only energized during standby conditions. The NRC reviewed the supplemental
information provided by Constellation and determined that their assessment was
reasonable. Additionally, the NRC observed the rear main seal leakage during the diluted
lube oil testing and noted that even though the fuel oil/lube oil mixture spray contacted hot
surfaces of the engine, which were not insulated, no flashing or fire occurred.
Final Significance Determination
The NRC recognized there were significant uncertainties associated with the assumptions
used to support the preliminary detailed risk evaluation. Most notably, based on
best-available information available at the time of the preliminary evaluation, the NRC
assumed that fuel oil dilution of the diesel engine lube oil greater than 17 percent would fail
the function of the 2B AF pump.
Based on the full-scale testing and supplemental information provided by Constellation, as
discussed above, the NRC determined that the test diesel engine was sufficiently similar to
the 2B AF pump diesel engine. The NRC also determined that the testing provided
reasonable assurance that the 2B AF pump diesel engine would function for its 24-hour
mission time at the fuel oil dilution levels that would have been experienced due to this
degraded condition. The diesel engine selected for the full-scale testing was the same make
and model (Detroit Diesel Series 149) as the 2B AF pump diesel engine. Though the test
engine had 12 cylinders, and the 2B AF pump engine was a 16-cylinder design, the individual
cylinder packages were identical between the models, so both engines had the same
displacement per cylinder, cylinder bore, and stroke length. Major engine components which
could be affected by lube oil viscosity were the same on both engines and during the diluted
lube oil test, the test engine was operated at loads approximately one to five percent higher
than the per-cylinder loads expected for the 2B AF pump engine. As previously discussed,
the NRC also determined that the three failure mechanisms of concern, which were reduction
of lube oil viscosity, high level in the lube oil sump, and engine fire, were reasonably
addressed by Constellation. The NRC noted that these test results may not be applicable to
fuel oil dilution levels that fall outside of the ranges examined during this testing or to other
diesel engines given differences in design and operating parameters.
In the preliminary detailed risk evaluation, the NRC documented a second influential
assumption related to crediting the medium head FLEX pumps as a source of feedwater to
the steam generators during loss of secondary heat sink events. Specifically, the NRC did not
credit this strategy due to concerns with procedural adequacy, feasibility of the required
actions, and alternate strategies the operators may consider following declaration of
10 CFR 50.54(x). At the regulatory conference, Constellation stated that the full-scale testing
demonstrated that the 2B AF pump diesel engine would likely run longer than was initially
assumed, and that longer runtimes would provide significant benefits in operator response,
citing factors of lower decay heat, which would slow steam generator dryout, and availability
of additional resources to support FLEX deployment. While the NRC acknowledges that
these factors would aid operator response during an event, and that operators would continue
to take action to feed the steam generators in the event of a 2B AF pump failure, the NRC
determined that it would still be inappropriate to quantify the medium head FLEX strategy for
the same reasons documented in the preliminary detailed risk evaluation.
The NRC determined that evaluating the degraded condition of the 2B AF pump diesel
engine by failing the function of the pump in the Braidwood SPAR model (i.e., setting the
fail-to-run basic event to TRUE) or increasing the nominal failure probability of the 2B AF
pump over its baseline value is no longer a best estimate given the new information provided
by Constellation. As such, the NRC determined that the increase in core damage frequency
(CDF) due to this finding would be limited to the duration when the 2B AF pump diesel
engine was inoperable for repairs to correct the fuel oil dilution, which was 27 hours3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br />
(September 22, 2023, at 1157 to September 23, 2023, at 1501).
Contributions from Internal Events
The NRC evaluated this issue using Systems Analysis Programs for Hands-On Evaluation
(SAPHIRE) version 8.2.9, and a test and limited use (TLU) version of the Braidwood SPAR
model created by Idaho National Laboratories (TLU3). SPAR model changes are the same as
those discussed in the preliminary detailed risk assessment. Using the Events and Conditions
Assessment (ECA) module of SAPHIRE, the NRC estimated the CDF from internal events
for an exposure period of 27 hours3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br /> to be 3.8E-8/year. The dominant core damage sequence
for internal events was a dual loss of essential service water initiating event with failure of AF,
main feedwater, and failure to recover essential service water.
Contributions from External Events
Using the ECA module of SAPHIRE, the NRC estimated the CDF from seismic, high winds,
and tornado events to be 6.9E-9/year. Because the Braidwood SPAR model does not
evaluate fire risk, the NRC reviewed Constellations fire risk results for this issue. For an
exposure period of 27 hours3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br />, the NRC estimated the CDF from fire to be approximately
8.9E-8/year.
Total CDF from Internal and External Events
Using an exposure period of 27 hours3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br />, the NRC determined the total increase in risk from
both internal and external events due to this degraded condition was approximately
1.4E-7/year, or of very low safety significance (Green). The NRC did not provide credit for
feeding the steam generators from the refueling water storage tank via the medium head
FLEX pumps in this evaluation.
Sensitivity Evaluation
For this detailed risk assessment, the NRC assumed that the full-scale testing provided
reasonable assurance that the 2B AF pump diesel engine would function for its 24-hour
mission time. The NRC conducted a sensitivity analysis by setting the fail-to-run probability at
twice the nominal value. Though the exposure time in the preliminary detailed risk evaluation
was 127 days, the NRC used 114 days for this sensitivity evaluation. The NRC determined
this was more appropriate given the performance deficiency was related to 10 CFR 50
Appendix B, Criterion XVI, Corrective Action, and thus the start of deficient performance by
Constellation was on June 1, 2023, when lube oil sample results were received, and
Constellation first had the opportunity to identify the degraded condition. The NRC did not
credit steam generator feed from the refueling water storage tanks via the medium head
FLEX pumps in this analysis. The NRC determined that even at double the failure probability,
the total risk from both internal and external events was of very low safety significance
(Green).
Total Estimate of CDF from Internal and External Events
Best Estimate
Sensitivity
Internal Events
3.8E-8/year
2.9E-7/year
Seismic, High Winds, and Tornadoes
6.9E-9/year
1.1E-7/year
Fire Events
8.9E-8/year
3.1E-7/year
Total
1.4E-7/year
7.1E-7/year
Contributions from Large Early Release Frequency (LERF)
The SRAs evaluated LERF using the SPAR model and determined that the impact to LERF
was bounded by the increase in CDF.
Cross-Cutting Aspect: H.11 - Challenge the Unknown: Individuals stop when faced with
uncertain conditions. Risks are evaluated and managed before proceeding. Specifically, this
finding involved licensee individual(s) responsible for reviewing oil analysis results deciding to
accept unusual and anomalous test results, rather than stopping and alerting other applicable
parties and/or station management to the condition before proceeding on.
Enforcement:
Violation: Title 10 CFR Part 50, Appendix B, Criterion XVI, Corrective Actions, requires, in
part, that measures shall be established to assure that conditions adverse to quality, such as
failures, malfunctions, deficiencies, deviations, defective material and equipment, and
nonconformances are promptly identified and corrected.
Contrary to the above, from June 1, 2023, to September 23, 2023, the licensee failed to
establish measures to assure that conditions adverse to quality are promptly identified and
corrected. Specifically, the licensee failed to identify and correct a deficiency associated with
the 2B AF Pumps diesel engine after they received the results from a May 19, 2023,
lubricating oil sample on June 1, 2023, indicating viscosity levels in the alert range.
Enforcement Action: This violation is being treated as a non-cited violation, consistent with
Section 2.3.2 of the Enforcement Policy.
EXIT MEETINGS AND DEBRIEFS
The inspectors verified no proprietary information was retained or documented in this report.
On June 5, 2024, the inspectors presented the Final Determination Report results to
Adam Schuerman and other members of the licensee staff.