The Chernobyl nuclear power plant disaster occurred on April 26, 1986 at the Chernobyl Nuclear Power Plant in Ukraine. There were multiple procedural violations during a test of the plant that caused reactors to rapidly increase power and lead to explosions. The explosions exposed the graphite moderator of the reactor to air, causing a fire that released large quantities of radioactive contamination into the atmosphere. It was the worst nuclear power plant accident in history.

1. THE CHERNOBYL
DISASTER
The Timeline of Events
Sean Miller
NUC-495-OL009
November 4, 2012

2. REFERENCES
• Bethe, H. A. (1986). U.S. panel assesses Chernobyl. Bulletin of the Atomic
Scientists, 42(10), 45-46.
• Diatlov, A. (2006, April 9) Why INSAG has still got it wrong. Nuclear
Engineering International.
• Frontline. (2012). Nuclear reaction: Why do Americans fear nuclear power?.
• GreenFacts. (2012). Scientific Facts on the Chernobyl Accident.
Grishanin, International
• Atomic Energy Agency, International Nuclear Safety Advisory Group.
(1992). INSAG-7. The Chernobyl accident: Updating of INSAG-1 (Safety
Series No. 75-INSAG-7).
• Lorenzini, P. (2006, January 23). Chernobyl myths. American Spectator.
• Mikhail, M. (n.d.). The Chernobyl reactor: Design features and reasons for
accident..
• Rippon, S. (1986, September 11). Chernobyl: The Soviet report. Nuclear
News, 1-8.
• United States Nuclear Regulatory Commission. (2012). Background on Chernobyl
Nuclear Power Plant Accident.

3. The RBMK-1000 Reactor Plant
Image Source: : http://www.wentz.net/radiate/chernobyl2/chernobyl/rbmk.html

4. Key Plant Design Specifications
 Boiling Water Reactor with graphite moderator and water cooling
 Two independent loops with two main circulation pumps in each loop
 Water acts as a poison with positive temperature and void coefficients of
reactivity.
* Graphite also has a positive temperature coefficient of reactivity.
 Very complex rod control design/programming compensate for variable
positive temperature coefficients and exceptionally large reactor
 The containment does not cover the reactor for refueling purposes
Image Source: http://library.thinkquest.org/3426/data/cause/design.flaw.html

5. The Turbine Generator Coastdown Test
Test Problem: The ECCS DGs take too long to pick up electrical load
Proposed Solution: TGs are modified to slow their coastdown, allowing
the TGs to continue to power electrical loads as they coast down
Test Setup: 1)Reconfigure power to electrical loads (MCPs/MFPs);
2)Turn off ECCS to prevent inadvertent
activation
Basic Procedure: 1)Reduce reactor power to 22-32%, 2) Trip TG
*NOTE: The test had been run before (twice) with no major issues.

6. Test Planning Issues
1. Upcoming Maintenance Shutdown Period
2. Higher Authorities not Informed of
Test
3. Electrical Engineer Placed in Charge
4. No evolutionary briefs, walkthroughs, etc Image Source: http://planetwaves.net/pagetwo/daily-
astrology/radiation-bullshit-levels-increase/

7. Test Timeline: April
25, 1986
• Operators begin reducing power
0100
• Power reduced to 50% (~1600MW)
1305 • Half of the MCPs and MFPs configured to off-site power
• ECCS disabled
• Load dispatcher requests power be maintained for grid
1400 demands
• Downpower is resumed
2310 • Switched to low-power backup regulator

8. Test Timeline: April
26, 1986
• Power drops to 30MW due to faulty low power regulator
• Operators take manual control of rods to raise power to
0028 200MW
• Operators start a additional MCP in each loop
• Negative Reactivity transient on plant
0103 • Low power-flow, low subcooling; plant very sensitive
• Steam drum level and pressure oscillations
• Operators disable automatic protection for steam drum level and
0107 pressure to avoid test interference
• Operators increase feed flow to compensate
• Negative reactivity effect due to reduced coolant inlet
0119 temperature

9. Test Timeline: April 26,
1986
• Automatic control rods move to upper limit to compensate
• Operators move manual rods to further compensate
0119:3
• Manual rods move too far=>Automatic rods move back in
0
• Plant parameters appear to be stable
• Decision made to begin the TG Coastdown Test
0122 • Operators block emergency protection signals from turbine stop valve
• Feed flow sharply reduced to meet testing requirements
0122:1 • Large transient on plant; automatic control rods insert nearly to bottom
to compensate
0
• Printout of power density/control rod configuration received
0122:3 • Data indicates that operating reactivity margin requires immediate plant
shutdown
0

10. Test Timeline: April 26,
1986
• Turbine generator stop valve is shut
• MCPs and MFPs begin to coast down=>new transient on plant
• Steam voids collapse=>negative reactivity. Power stays constant due to disabling of
0123:04 TG stop valve protective action
• Automatic control rods withdraw in response to negative reactivity (void
collapse)
0123:10 • Graphite in rods is ~1m above bottom of core
• Automatic control rods now insert to compensate for positive reactivity from flow
reduction
• However, power continues to increase when rods cannot overcome positive reactivity
0123:21 • Power/Temperature Continue to Rise: RUNAWAY CONDITION
• Reactor Scram was ordered (reason unclear)
• A few seconds following the scram, shocks were felt in
0123:40 the control room

11. Test Timeline: April
• Operators notice26, had not reached lower stops=>de-
rods 1986
energize them, allowing them to fall by gravity
0123:4
1 • Rods not effective until very bottom of the core
• High power excursion rate alarm. Recorded power: 530MW on
0123:4 20 sec excursion
3
• Fuel fragments due to high power and rate of heat generation
• Fuel fragments explode through the cladding/canning and contact the
coolant
0123:4
• Heat transfer from fuel to water rapidly forms steam voids=>more
5 positive reactivity
• Reactor is Prompt Critical
• Rapid steam generation creates backpressure on check valves; steam dump valve
0123:4 opens to relieve pressure
6 • Circulation flow increases as fuel channels rupture
NOTE: This was the last recorded data.

12. Test Timeline: April
26, 1986
• Observers outside the plant report two separate
0124 explosions
• Firefighters dispatched to put out fires
• No training or equipment for radiation protection
0130 • Chernobyl Unit 3 is shut down
Image Source: http://www.chernobyl.org.uk/c_disaster.html Image Source: http://www.dauten.com/e-luminations/2011-10.html

13. Procedural Violations
Bypassed
Deactivated
Multiple
ECCS Protective Trips
Violated Countless Rod
Control Design
Assumptions
Image Source: http://noticiasdiarias10b.blogspot.com/p/radioactividad_03.html

14. Principle Errors Made
1.Operating Margin of Reactivity (ORM) reduced far
below allowable limit
2.Power levels lower than that specified for the test
program
3.Using all recirculation pumps, with some exceeding
their discharge limits
4.Blocking the shutdown signal from both Turbine
Generators
5.Blocking the water level and steam pressure trips
for the steam separators
6.Switching off the ECCS

15. Questions/Comments?
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