United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 93-71: Fire At Chernobyl Unit 2

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                            WASHINGTON, D.C.  20555

                              September 13, 1993


NRC INFORMATION NOTICE 93-71:  FIRE AT CHERNOBYL UNIT 2


Addressees

All holders of operating licenses or construction permits for nuclear power
reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to the fact that extensive fires may produce
unanticipated challenges such as happened at Unit 2 of the Chernobyl Nuclear
Power Station.  It is expected that recipients will review the information for
applicability to their facilities and consider actions, as appropriate, to
avoid similar problems.  However, suggestions contained in this information
notice are not NRC requirements; therefore, no specific action or written
response is required.

Description of Circumstances

On October 11, 1991, Unit 2 of the Chernobyl Nuclear Power Station was
operating at 70-percent power.  Each Chernobyl unit has two twin, independent
turbogenerators and all four units share a common turbine hall (see 
Attachment 1).  In order to perform minor repairs and adjustments on
turbogenerator 4 (TG-4), operators reduced Unit 2 reactor power and isolated
the steam supply to TG-4.  At 7:46 p.m., the operators switched off the
generator excitation field and opened disconnect breakers 1, 2, and 3 in the
330 kV switchyard to electrically isolate TG-4.

At 8:10 p.m., TG-4 had coasted down to about 150 rpm when disconnect breaker 2
spuriously closed and reconnected TG-4 to the electrical grid.  The generator
rotor motorized on the reverse-power condition and accelerated to synchronous
speed (3000 rpm) in under 30 seconds.  The influx of current to TG-4
overheated the conductor elements and caused a rapid degradation of the
mechanical end joints of the rotor and excitation windings.  A centrifugal
imbalance developed and damaged generator bearings 10 through 14 and the seal
oil system allowing hydrogen gas and seal oil to leak from the generator
enclosure.  Electrical arcing and frictional heat ignited the leaking hydrogen
and seal oil creating hydrogen flames 8 meters [27 feet] high and dense smoke
which obstructed the visibility of plant personnel.  

When the burning oil reached the busbar of the generator it caused a
three-phase 120,000 amp short circuit.  Within 0.07 seconds, the generator
fault protective circuits sensed the short and opened disconnect breaker 2, 


9309080104.

                                                            IN 93-71
                                                            September 13, 1993
                                                            Page 2 of 3


which spuriously closed again 0.25 seconds later.  The protection system
reopened the breaker 0.2 seconds after the closure and again the breaker
spuriously closed.  Finally, protection circuits in the 330 kV electrical grid
disconnected the generator 0.27 seconds after the closure by opening a remote
breaker located in the town of Korosten, about 120 kilometers [72 miles] away. 
Unit 2 control room operators shut down the reactor and initiated the maximum
allowable cooldown rate to achieve a safe reactor configuration.  

Fire fighting efforts focused on containing the fire and preventing it from
spreading to adjacent equipment.  There was little concern for the turbine
hall roof catching fire because the asphalt coating on the roof had been
removed after the 1986 accident at Chernobyl Unit 4.  However, the local
ventilation systems, which were the only method available for heat and smoke
removal, were unable to cope with the smoke and heat generated by the fire. 
The fire brigade was concerned for the structural integrity of the roof 
because the roof supports had no heat retardant coating and because sprinkler
systems in the turbine hall were not designed to cool the structural supports. 
The fire brigade attempted to cool the supports by spraying them with water
from below but were unsuccessful because the plant fire pumps could not
provide adequate flow to the area sprinkler systems and the local
fire-fighting efforts at the same time.  The metal trusses of the roof
structure reached a temperature greater than 900�C [1650�F] and collapsed at
8:35 p.m.

A 50 meter by 50 meter [165 feet by 165 feet] section of the roof collapsed
onto the turbine deck and also onto an adjacent pit that contained the main
feedwater pumps, the auxiliary feedwater pumps and their associated control
cabinets.  Damage to the pump systems and a fire in one control cabinet
disrupted makeup water flow to the reactor cooling system.  The operators cut
the electrical power to the pump motors and the control cabinets to remove
them as a potential ignition source.  Because of the significant amount of
damage, the operators believed that the main feedwater and auxiliary feedwater
pumps could not be readily restored.  Therefore, the operators added water to
the reactor primary coolant circuit by opening the steam relief valves to
reduce pressure and aligning a low-pressure nonsafety-grade condensate pump to
the auxiliary feedwater system piping.  This arrangement made controlling the
steam separator drum water levels difficult but allowed the operators to
provide core cooling throughout the event.  The fire was extinguished by 
11:30 p.m.

Discussion

An investigation of the event later determined that a short circuit in the
control wiring for generator disconnect breaker 2 caused the repeated failures
of the breaker to remain open.  This condition was critical because there was
no redundant on-site isolation breaker for the generator output such as is
included in the more recent design of Chernobyl Unit 3.
.

                                                            IN 93-71
                                                            September 13, 1993
                                                            Page 3 of 3


A notable safety significant aspect of the Chernobyl fire was the complete
loss of the main feedwater and auxiliary feedwater systems.  Because redundant
safety components were not truly independent (i.e., there were no separate and
protected cubicles for the auxiliary feedwater pumps), a single structural
component failure led to the common failure of all feedwater pumps.  The
proposed corrective actions include: (1) installation of three additional
emergency feed pumps outside of the turbine hall, (2) installation of an
automatic sprinkler system for the metal structure of the turbine hall and
roof, and (3) a review of the design of the smoke removal system in the
turbine hall and installation of a more effective system.  

This event illustrates that extensive fires may place unanticipated loads on
fire protection systems and that structural failures could cause failures of
multiple trains of safety-related systems and challenge the true separation,
independence, and redundancy of safety-related components.

This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
the technical contact listed below or the appropriate Office of Nuclear
Reactor Regulation (NRR) project manager.


                                    /S/'D BY BKGRIMES


                                    Brian K. Grimes, Director
                                    Division of Operating Reactor Support
                                    Office of Nuclear Reactor Regulation

Technical contact:  Eric J. Benner, NRR
                    (301) 504-1171

Attachments:
1.  Chernobyl Turbine Hall Layout
2.  List of Recently Issued NRC Information Notices.
Page Last Reviewed/Updated Tuesday, November 12, 2013