Chernobyl DisasterChernobyl Disaster

ChernobylChernobyl

Built in Built in 1978-1978-19791979

ChernobylChernobyl

UkraineUkraine

BackgroundBackground Reacktor Bolshoy Moshehnosty Reacktor Bolshoy Moshehnosty

KipyashiyKipyashiy

RBMK, a Russian acronym translated RBMK, a Russian acronym translated roughly means “reactor cooled by water roughly means “reactor cooled by water and moderated by graphite”and moderated by graphite”

RBMK ReactorRBMK Reactor

Control RodsControl Rods The Control Rods: long metal rods The Control Rods: long metal rods

which, in the Chernobyl reactors, contain which, in the Chernobyl reactors, contain boron; they help control the chain boron; they help control the chain reactionreaction

Reactor Plant ScenarioReactor Plant Scenario1.1. As the reaction occurs, the uranium fuel As the reaction occurs, the uranium fuel

becomes hotbecomes hot2.2. The water pumped through the core in The water pumped through the core in

pressure tubes removes the heat from pressure tubes removes the heat from the fuelthe fuel

3.3. The water is then boiled into steamThe water is then boiled into steam4.4. The steam turns the turbinesThe steam turns the turbines5.5. The water is then cooledThe water is then cooled6.6. Then the process repeatsThen the process repeats

April 25, 1986April 25, 1986 01:00 The preparation for the test01:00 The preparation for the test 13:47 Lowering of the reactor 13:47 Lowering of the reactor

power halted at 1,600 MWtpower halted at 1,600 MWt 14:00 The ECCS was isolated14:00 The ECCS was isolated 23:10 The power reduction 23:10 The power reduction

resumedresumed

April 26, 1986April 26, 1986 24:00 Operation shift change24:00 Operation shift change 24:28 Power level is now 500 MWt and 24:28 Power level is now 500 MWt and

kept decreasing to 30 MWtkept decreasing to 30 MWt 24:40 The operator withdrew some control 24:40 The operator withdrew some control

rods rods 01:00 Power had risen to 200MWt01:00 Power had risen to 200MWt 01:03 Connecting the fourth main cooling 01:03 Connecting the fourth main cooling

pump to the left loop of the system pump to the left loop of the system 200MWt200MWt

01:07 Connecting the fourth main cooling 01:07 Connecting the fourth main cooling pump to the right of the loop system - pump to the right of the loop system - this this was a violation of NOPwas a violation of NOP

April 26, 1986 (cont.)April 26, 1986 (cont.) 01:19 Increased feed water flow to 01:19 Increased feed water flow to

the steam drums and removed more the steam drums and removed more control rods -control rods -violation of NOPviolation of NOP

01:23 The test was started01:23 The test was started 01:23:10 Automatic rods withdrawn 01:23:10 Automatic rods withdrawn

from the corefrom the core 01:23:21 Two groups of automated 01:23:21 Two groups of automated

control rods were back to the corecontrol rods were back to the core

April 26, 1986 (cont.)April 26, 1986 (cont.) 01:23:30 Power kept increasing01:23:30 Power kept increasing 01:23:40 Emergency button pushed01:23:40 Emergency button pushed 01:23:44 Power is at 300000 MWth01:23:44 Power is at 300000 MWth 01:23:48 101:23:48 1stst thermal explosion thermal explosion 01:23:55 201:23:55 2ndnd explosion explosion

Power ChartPower Chart

0

200

400

600

800

1000

1200

1400

1600

1800

3:47:00 0:28:00 0:40:00 1:00:00 1:03:00

Series1

0

50000

100000

150000

200000

250000

300000

350000

3:47:00 0:28:00 0:40:00 1:00:00 1:03:00 1:23:44

Series1

Direct CasualtiesDirect Casualties 31 employees 31 employees

and firefighters and firefighters dieddied

134 emergency 134 emergency workers workers suffered from suffered from acute radiation acute radiation sicknesssickness

Indirect CasualtiesIndirect Casualties By the year 2000 By the year 2000

there were 1800 there were 1800 case of thyroid case of thyroid cancer in children cancer in children and adolescentand adolescent

High number of High number of suicide and violent suicide and violent death among death among Firemen, policemen, Firemen, policemen, and other recovery and other recovery workersworkers

RadiatioRadiationn

Radioactive FalloutRadioactive Fallout

System AnalysisSystem Analysis Use of graphite as a moderatorUse of graphite as a moderator Lack of a well-built containment Lack of a well-built containment

structurestructure Inadequate instrumentation and alarms Inadequate instrumentation and alarms

for an emergency situationfor an emergency situation There were no physical controls that There were no physical controls that

prevented the operators from operating prevented the operators from operating the reactor in its unstable state the reactor in its unstable state

Human ErrorsHuman Errors Isolation of the emergency core cooling Isolation of the emergency core cooling

systemsystem Unsafe amount of control rods withdrawnUnsafe amount of control rods withdrawn Connection of the four main cooling Connection of the four main cooling

pumps to the right and left of the systempumps to the right and left of the system

Human Errors (cont.)Human Errors (cont.) Mental modelMental model

– The operator did not have a good mental The operator did not have a good mental model of the system itselfmodel of the system itself

OverconfidenceOverconfidence– By having an electrical engineer on site for an By having an electrical engineer on site for an

electrical testelectrical test– No confirmation of cues obtained from the No confirmation of cues obtained from the

systemsystem Beta too highBeta too high

– Many missed signals before the accident Many missed signals before the accident

Accident PreventionAccident Prevention

RecommendationsRecommendations Have proper Standard Operating Have proper Standard Operating

Procedures (SOP) for both normal Procedures (SOP) for both normal and emergency situationsand emergency situations

Have scheduled trainings and Have scheduled trainings and practices for normal and emergency practices for normal and emergency situationssituations

Recommendations (cont.)Recommendations (cont.) Always have a reactor expert on site Always have a reactor expert on site Have operators confirm any cues Have operators confirm any cues

from the system before making from the system before making hypothesis or take action hypothesis or take action

Have a team work kind of Have a team work kind of environment such that every body is environment such that every body is involved involved