ML20147A355

Final ASP Analysis - Browns Ferry 3 (LER 296-80-024)

ML20147A355
Person / Time
Site:	Browns Ferry
Issue date:	05/26/2020
From:	Christopher Hunter NRC/RES/DRA/PRB
To:
Littlejohn J (301) 415-0428
References
LER 1980-024-01
Download: ML20147A355 (6)

Text

PRECURSOR DESCRIPTION AND DATA NSIC Accession Number:

163405 Date:

July 28, 1980

Title:

Failure of 76 Control Rods to Insert at Browns Ferry 3 The failure sequence was:

1. The reactor was being shut down for maintenance on the feedwater system following steady state operation at 400 MWe (37% power).

2.

Power was reduced to 390 MWe by decreasing recirculation flow and inserting 10 control rods.

3.

Normal operating procedures at Browns Ferry involve scramming the reactor from about 30% power, so the operator initiated a manual scram.

4.

All the west bank rods scrammed full-in except rod 30-23 which set-tled at position "02."

5.

However, due to excess water in the east bank scram discharge

headers, 75 out of 88 east bank withdrawn rods failed to fully in-sert, coming to rest at various positions from 46 to 02.

6.

This first scram was sufficient to reduce the reactor power from 36% to less than 2%,

but not to subcriticality.

Water level was maintained by the feedwater control system.

7.

The scram discharge volume was drained for periods of 93 and 53 s respectively and the reactor was manually scrammed two additional times.

On each occasion additional rod insertion occurred.

8.

The scram discharge volume was allowed to drain again for 160 s.

9.

Finally the reactor auto scrammed on high drain volume when the auto scram bypass was removed.

The remaining withdrawn control rods then inserted at normal speed to the fully inserted position.

10.

The total time that elapsed between the initial scram and final in-sertion of all the control rods was 14 min.

Corrective action

1. The operator manually scrammed all rods.

Scram discharge volumes were drained and the bypass was removed from the high-level-drain scram interlock.

2.

An investigation was initiated to identify the problem.

No problems were identified with the electrical/control system, hydraulic control unit valve lineup, vent and drain piping (i.e.,

no line blockage found),

or seal inleakage to the SDV.

The cause for the excess water in the east SDV was not identified.

3.

The following measures were to be incorporated promptly to ensure adequate drainage of the scram volume headers:

a.

Monitor the scram discharge headers daily.

b.

Increase the test frequency of the scram discharge volume level switches to once per month.

c.

Develop procedures to verify that the scram discharge volume is free of residual water following each scram and before startup.

d.

Visually inspect each hydraulic control unit daily to ensure that the manual valves are in their normal position.

e.

Verify daily the normal position indication of SDV vent valves and drain valves.

f.

Additional requirements of IE 80-17 will be implemented in accordance with that document.

Design purpose of failed system or component:

The scram system is designed to rapidly insert control rods for prompt reduction of the reactor power.

An orderly shutdown The operator initiated 76 control rods failed The operator reset Auto scram bypass from 37% power was a manual scram after to insert completely the scram system resumed and all under way to allow reducing the core after receiving scram and initiated two control rods then for maintenance power to 36%

signals due to water more scrams, but inserted upon on the feedwater in the east scram not all rods auto scram (the systems discharge volume fully inserted fourth scram)

Potential Severe Core Damage No No -

no requirement for scram, secondary side systems available to remove core heat No -

no requirement for scram, secondary side systems available to remove core heat No No NSIC 163405 - Actual Occurrence of Failure of 76 Control Rods to Insert at Browns Ferry 3

Loss of Reactor Standby RCIC/HPCI Automatic LPCI or Long Feedwater Scram Liquid

Response

Depressurization CS Response Term Flow Control Adequate System Operates Adequate Core Initiated Cooling Potential Severe Core Damage Sequence No.

No Yes 1

2 No Yes Yes Yes 3

4 S

6 No 7

Yes Yes Yes 8

9 10 NSIC 163405 -

Sequence of Interest for Failure of 76 Control Rods to Insert at Browns Ferry 3

CATEGORIZATION OF ACCIDENT SEQUENCE PRECURSORS NSIC ACCESSION NUMBER:

163405 LER NO.:

80-024 Rev.

1 DATE OF LER:

July 28, 1980 DATE OF EVENT:

June 28, 1980 SYSTEM INVOLVED:

Reactivity control COMPONENT INVOLVED:

Scram discharge volume CAUSE:

Insufficient drainage from east SDV SEQUENCE OF INTEREST:

Control rods fail to insert ACTUAL OCCURRENCE:

Failure of 76 control rods to insert REACTOR NAME:

Browns Ferry 3 DOCKET NUMBER:

50-296 REACTOR TYPE:

BWR DESIGN ELECTRICAL RATING:

1065 MWe REACTOR AGE:

3.9 years VENDOR:

General Electric ARCHITECT-ENGINEERS:

Tennessee Valley Authority OPERATORS:

Tennessee Valley Authority LOCATION:

10 miles NW of Decatur, Alabama DURATION:

1/2 of interval from last

scram, 240 h (estimated)

PLANT OPERATING CONDITION:

37% power TYPE OF FAILURE:

Inadequate performance DISCOVERY METHOD:

Operational event

COMMENT:

The analysis done on the control rod pattern that resulted from the scram showed that if the scram had occurred at 100%

power, the maximum power after the scram would have been 345 MW(t)

[see Nuclear Safety, 23(5), September-October 1982].

The after-scram power level would likely have been even higher if scram had been demanded at 100% power because some or all of the east side control rods (5) that were manually inserted as part of the power reduction procedure could have also failed to fully insert.