Nuclear Meltdown at Chernobyl

Austin Conn

Pripyat, Chernobyl, and Kiev Ukraine

Chernobyl Nuclear Power Plant

• 4 RBMK-1000 Nuclear Reactors capable of providing 1,000 Megawatts per reactor.• Reactor 1 was commissioned in 1977, with Reactors 2, 3, and 4

commissioned in 1978, 1981, and 1983, respectively.• Two additional reactors, 5 and 6, were planned an reactor 5 was 70%

completed at the time of the accident.• Reactors 1 and 2 were first generation reactors, while 3 and 4 were

second generation, equipped with more secure accident localization systems. • These additional systems actually thwarted what could have been much more

detrimental in a generation 1 reactor.

Pripyat

• City built to house the workers for the Chernobyl Nuclear Power Plant.• At the time of the accident:• Population: 49,400• 15 Schools• 1 Hospital

Plant and Reactor Design

• Under normal operation, the plant is fed by the power grid.• In the event of a loss of power, such as an emergency shut down or

external loss of power, each reactor had 3 diesel backup generators to power the cooling pumps and other essential systems.• These generators were to be started within 15 seconds of power loss.• However, they took an additional 60-75 seconds to reach operating speed.

• To bridge the 1 minute gap without power, it was theorized that the 220 ton turbine would have enough rotational energy to “coast” through the gap, providing power for the cooling pump.

Reactor Diagram

Testing Procedure

• Before beginning the test, the reactor’s thermal power output should be no less than 700MW.• Upon starting the test, the steam turbine, weighing 220 tons, was to be run

up to full speed, around 3000 RPM.• Upon reaching full speed, the steam supply from the reactor was to be cut

off.• The emergency generators were to be started and run up to full power,

about 5MW.• The turbine’s performance was also to be recorded, albeit for testing.

• Upon reaching operating speed, the generators were to power the cooling pumps, and the turbine would be allowed to “freewheel” down.

Preparations

Scheduled to take place during the day shift on April 25, 1986:• Workers were to gradually reduce the reactor’s thermal output

power, beginning at 1:06am.• By the beginning of the day shift, the reactor was at 50% capacity.• At some point in the beginning of the day shift, the power plant in

Kiev unexpectedly went offline, and in order to maintain power for the coming evening demand, the test was postponed.• At 11:04pm, the Kiev power plant went back online and the test was

permitted to proceed.

Timeline of Events

• 12:05am – The thermal output of Reactor 4 reached 700MW.• Sometime between 12:05am and 1:05 am:• The reactor’s output, unknown to the operator, fell below 700MW due to a

phenomenon known as reactor poisoning. Reactor poisoning is the build up of xenon-135, a neutron absorber.• For an unknown reason, the operators inserted the control rods too far,

further reducing the reactor’s output to approximately 500MW.• This rendered the reactor in an unintended near-shutdown state.

• In this state, the reactor went “prompt supercritical,” a condition where power spikes exponentially in seconds. To prevent supercritical instances, 28 control rods were to remain inserted in the reactor as a means of control over the reactor’s output power.

Timeline of Events (Cont.)

• Upon reaching only 5% of the minimum initial power laid out in the testing procedure, the control room operators attempted to restore power to the reactor by means of disabling the automatic system responsible for keeping the reactor stable.• This system was also in charge of the 28 control rods intended to remain inserted

to prevent supercritical situations.• Within several minutes of extracting the majority of the control rods, the

reactor’s output rose and stabilized between 160 and 200MW.

• Due to operating at such low power, the poisoning of the reactor core restricted any further rise in reactor power.• To counteract the poisoning of the reactor, the operators extracted additional

control rods.

Timeline of Events (Cont.)

• 1:05am – A power level of 200MW was achieved.• At this time, preparations for the test resumed and as per the test, additional

coolant pumps were to be activated.

• 1:19am – The flow rate of coolant exceeds the limit, lowering the overall core temperature and raised alarms in the control room.• To counter the core cooling, two coolant pumps were turned off and

additional control rods were extracted.

Reactor State at Beginning of Test

• The reactor was in a highly unstable state, with numerous automated and passive safety features disabled.• Of the 211 total control rods, including the 28 “fail-safe” rods, 193 controls

rods were extracted, leaving only 18 left in the reactor.• Of the eight total cooling pumps, six of which are to be operating under

normal conditions, only four are operating.

The Emergency SCRAM System

• In the event of the reactor going supercritical, an Emergency SCRAM, or Emergency Shutoff, system is in place which manually inserts all 211 control rods into the reactor and is activated via the “AZ-5” protocol (Pronounced “A Zed Five”).• To prevent the necessity of utilizing the AZ-5 protocol, the automated system

was designed to control all 211 control rods and regulate the reactor from ever reaching such a state. However, the operators disabled this system.

• The two systems essentially do the same thing, however, the automated system shuts down the reactor in a more controlled and safe manner, where the AZ-5 protocol does not regulate the shutdown procedure.

The Test

• At 1:23:04am, the test began.• The steam supply from the reactor to the turbine was shut off, initiating the

run-down of the turbine generator.• The diesel generators started and sequentially picked up loads.

• By design, the generator was to have satisfied the MCP’s power needs by 1:23:42am. • However, as the momentum in the turbine decreased, so did the amount of

power it supplied. This reduction in power caused the coolant flow to decrease and caused further poisoning in the reactor. • One of the automated systems that was not disabled was able to control this poisoning

by inserting control rods. This system, however, only had control of 12 of the 211 control rods, the majority of which were still fully extracted.

1:23:40 am, April 26th, 1986

• The SKALA Central Control System registered a manual emergency shutdown command (the AZ-5 protocol) and began the insertion of all control rods.• The control rods are 7 meters tall, and move at 0.4 m/s. Within 18 to 20

seconds, all control rods should have been inserted.• The control rods themselves are flawed, being constructed with graphite tips, the control

rods displace the neutron-absorbing coolant with graphite, a material which does not absorb neutrons. This process actually increased the power at the top of the reactor.

• As the rods entered the reactor core, a power spike was recorded, and the core overheated, causing fuel rods to fracture and jammed the control rods at only 1/3 insertion, with the non-neutron absorbing graphite in the middle of the core.

1:23:40 am, April 26th, 1986 (Cont.)

• Within three seconds, the reactor’s power output rose above 530MW.• The last reading registered on any instrumentation shows the reactor’s output

reaching 33,000MW, more than ten times its normal operational output.• This surge in power caused any remaining coolant in the core to flash to steam,

increasing the pressure within the reactor.• The pressure generated was enough to life the 2000 ton upper plate off the reactor,

which the entire reactor was fastened to.• This is generally thought to be the first explosion..

• The second explosion, occurring two to three seconds after the first is what terminated the nuclear reactor and exposed the core to air.• Upon exposure to air, the graphite and other materials ignited, spewing radioactive

material into the atmosphere.

The Aftermath

• As a result of the disaster, the nearby town of Pripyat was evacuated and an exclusion zone was established. The town of Pripyat, still to this day, is a moment, frozen in time. Teachings still written in classrooms, tables set for meals, toys left in the beds of children in orphanages. The area is expected to be uninhabitable until

Works Cited

• https://tribfox40.files.wordpress.com/2014/07/planecrash.jpg• https://en.wikipedia.org/wiki/Pripyat• https://upload.wikimedia.org/wikipedia/commons/d/d5/Chernobylre

actor.jpg• https://en.wikipedia.org/wiki/Chernobyl_Nuclear_Power_Plant• https://en.wikipedia.org/wiki/Chernobyl_disaster