Chernobyl

USSR - C hernobyl Location

RBMK Plant Schematic

RBMK - More Details

Plant Cross-sectional View

National Geographic, May 1987

RBMK-1000 Schematic

Why the Test

• Protect against Station blackout consequences

C h r onology of A c c i de nt

10 0 %

50%

6%

1%

A p r i l 2 5 A p r i l 2 6

0 1 : 00 O p e r at o r s B e g i n P o w e r D e sc en t

1 4 : 0 0 P o w e r D e scen t D e l a y e d f o r 9 H our s b y L o ad D i s p a t ch e r

E C C S D i sc o n n e ct ed

I n t e n d e d T e s t P o we r

2 3 : 10 P o w e r

D esc en t R e s u m e d

Xe n o n

Bu i l d s U p

O p e r a t or s S t a r t 2 A d di t i ona l

Re a c t o r Sc r a m s

0 1 : 23 T G

D i s c o nne c t e d

T i m e ( n o t t o s c a le )

C o ol a n t P u m p s . Fl o w L i m i t s V i o l a t e d

\Chernobyl Violations

 Test conducted at power level below that prescribed by test procedure

 Control rods positioned in unauthorized configuration

 Authorized coolant flow exceeded

 Reactor scram signals bypassed

 ECCS disabled

Chernobyl During Accident

Post Accident Construction

of Sarcophagus

Unit 4 Today

Pripyat

Typical Structure in

Chernobyl

Inside of Reactor Showing

Top Shield Plug

http://www.spaceman.ca/gallery/chernobyl/Chernopik

Schematic of Post Accident

Condition

Elephant’s Foot

Molten Fuel Solidified

Molten Fuel

Fuel Slag Piles

U n d ernea t h t h e reac t or

1 - F uel lava, 2 - poured concrete in 1986, 3 - the enclosure of passage to

otm. 3.00 m, 4 - d oor into pom. 214/2, 5 - cable of koroba.(Of ris.ya.e.y.).

Unit 4 Control Room

Prior Known Design Issues

 Positive Void Coefficient

 Slow, ineffective scram system

 Vulnerability to a "positive scram" phenomenon

 Slow complex monitoring and control system

 Inadequate separation of control and protection systems.

 Lack of full containment

 Overall lack of design consideration for the prevention and mitigation of reactivity excursion accidents.

Chernobyl Causes

 Overall management control not established

 Test procedure not reviewed for safety implications

 Operators felt sense of urgency

 Test delayed by load dispatcher

 Auto power level controller not adjusted

 Test procedure not followed

 Safety systems defeated

 Design dependent on adherence to admin and procedures for safe operation

Comparisons with TMI

• Both accidents occurred in early morning TMI-0400; Ch ernobyl 0100

• Both reactors w ere sensitive but:

TMI - u seful response cou ld have been taken over several hours but small inventory steam generator

Ch ern ob yl - response time was minutes to seconds due to positive void coefficiency

• General Complacency

• Warnings from sensors ignored

• Operators intentionally defeated the safety systems

• Poor training of operators

TMI - n ever trained for stuck-open PORV and no instructions in EOP Ch ern ob yl - n o simulator training

• Weakness in approval of operating proced ures

• Operators weak in understanding plant behavior

Some Cultural Insights---Human Behaviors Leading to Significant Events

 Lack of respect for the reactor core’s awesome energy

 Overemphasis on production or schedule with safety assumed

 Not using procedures or expected control room protocols

 Lack of rigor, discipline, high standards, oversight

 Non-conservative decisions made when faced with uncertainty (transient or unusual conditions)

 An insufficient questioning attitude – m aking assumptions

Consequences

 Health Concerns

 Contamination

 Economic

 Loss generation

30 Km Exclusion Zone

Zone of Contamination

Current Condition - Need to

Replace Sarcophagus

Homework

MIT OpenCourseWare http://ocw.mit.edu

22.091 Nuclear Reactor Safety

Spring 200 8

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