Msc PHDE, Pokhara University, School of engineering, Cse study on USSR cernobly disaster

1. Chernobyl Disaster , USSR
ER.ISHWAR TIMILSINA &ER. HIMAL
BASNET
M.Sc in Public Health and Disaster Engineering
School of Engineering
Pokhara University

2. • USSR is called as Union of Soviet Socialist Republics. It is a country which is
compare with USA in all ways. It is a big area country which now divided into
many country like Russia , Ukraine Belarus etc.
• For the development of the country USSR plan of dream project of making a big
cities where all facilities are found. Since the country's many parts are village or
very remote or under developed. So they choice a place called Chernobyl located
near the Ukraine -Belarus boarder. It is Pripyat in Ukraine.
• This project were very important for USSR because USA were moving very fast in
developing as well as other sector. The dream project is named as state of a arch so
that it can increase the economic development.
• For any development electricity is need so for that project also electricity is needed
and USSR is decided to make a nuclear plant for generating electricity.
Introduction

3. Continued..
• Is was soviet union 9th atomic city which was in Pripyat.
• The project was express purpose of housing , school , hospital etc for the
people.
• 1980 ‘s early economic down for USSR and increase of UAS so that this city
have a major role in economic status for USSR.
• Then USSR make a security strong like early warning radar system(OTH)(over
the horizon radar system).this system help find the attached or any problem for
this city .
• After complete of the city people have all facilities and population slowly
stared to increase.

4. Now about the nuclear plant
• Consist of 4 unit
• Unit I – complete in 26th nov 1977
• Unit II – complete in 1978
• Unit III– complete in 1981
• Unit IV – complete in 1983
• Nuclear plant stated to produce electricity for the city
• 25-26th April 1986
• Test is done for emergency function and make sure they were still working

5. Continued..
• The Chernobyl Nuclear Power Plant (officially, the Vladimir Ilyich Lenin
Nuclear Power Plant) is a closed nuclear power plant near the abandoned city
of Pripyat in northern Ukraine, 16.5 kilometers (10 mi) northwest of the city
of Chernobyl, 16 kilometers (10 mi) from the Belarus–Ukraine border, and about
100 kilometers (62 mi) north of Kiev.
• Reactor No. 4 was the site of the Chernobyl disaster in 1986, and the power plant is
now within a large restricted area known as the Chernobyl Exclusion Zone.The
three other reactors remained operational after the accident but were eventually shut
down by 2000, although the plant remains in the process of decommissioning as of
2020. Nuclear clean-up is scheduled for completion in 2065.

6. Construction
• Aerial view of all 4 units prior to the accident viewed from NW
• The nuclear power plant consisted of four RBMK-1000 reactors, each capable of
producing 1,000 megawatts (MW) of electric power (3,200 MW of thermal power), and
the four together produced about 10% of Ukraine's electricity at the time of the
disaster. Construction of the plant and the nearby city of Pripyat to house workers and
their families began in 1970, with reactor No. 1 commissioned in 1977. It was the third
Soviet RBMK nuclear power plant, and the first plant on Ukrainian soil.
• The completion of the first reactor in 1977 was followed by reactor No. 2 in 1978, No. 3
in 1981, and No. 4 in 1983. Two more blocks, numbered five and six, of more or less the
same reactor design, were planned at a site roughly a kilometer from the contiguous
buildings of the four older blocks. Reactor No. 5 was around 70% complete at the time
of block 4's explosion and was scheduled to come online approximately six months later,
on November 7, 1986. In the aftermath of the disaster, the construction on No. 5 and
No. 6 were suspended, and eventually canceled in April 1989, just days before the third
anniversary of the 1986 explosion.Six other reactors were planned on the other side of
the river. All 12 reactors would be planned to be running in 2010.
• Reactors No. 3 and 4 were second generation units, whereas No. 1 and 2 were first-
generation units

7. About disaster event.
• The Chernobyl disaster was a nuclear accident that occurred on Saturday 26 April
1986, at the No. 4 reactor in the Chernobyl Nuclear Power Plant, near the city
of Pripyat in the north of the Ukrainian SSR. It is considered the worst nuclear
disaster in history and is one of only two nuclear energy disasters rated at seven—
the maximum severity—on the International Nuclear Event Scale, the other being
the 2011 Fukushima Daiichi nuclear disaster in Japan.
• The accident started during a safety test on an RBMK-type nuclear reactor, which
was commonly used throughout the Soviet Union.
• The test was a simulation of an electrical power outage to aid the development of a
safety procedure for maintaining reactor cooling water circulation until the back-
up electrical generators could provide power.
• This gap was about one minute and had been identified as a potential safety
problem that could cause the nuclear reactor core to overheat.
• It was hoped to prove that the residual rotational energy in a turbine generator
could provide enough power to cover the gap.

8. Continued..
• Three such tests had been conducted since 1982, but they had failed to provide a
solution.
• On this fourth attempt, an unexpected 10-hour delay meant that an unprepared
operating shift was on duty. During the planned decrease of reactor power in
preparation for the electrical test, the power unexpectedly dropped to a near-zero
level.
• The operators were able to only partially restore the specified test power, which put
the reactor in a potentially unstable condition.
• This risk was not made evident in the operating instructions, so the operators
proceeded with the electrical test.
• Upon test completion, the operators triggered a reactor shutdown, but a
combination of unstable conditions and reactor design flaws caused an
uncontrolled nuclear chain reaction instead.

9. Design
• Electrical systems
• Turbo generators
• Reactor fleet
• Computer systems

10. Continued..
• A large amount of energy was suddenly released, vaporising superheated cooling
water and rupturing the reactor core in a highly destructive steam explosion.
• This was immediately followed by an open-air reactor core fire that released
considerable airborne radioactive contamination for about nine days that precipitated
onto parts of the USSR and western Europe, before being finally contained on 4 May
1986.
• The fire gradually released about the same amount of contamination as the initial
explosion. As a result of rising ambient radiation levels off-site, a 10-kilometre
(6.2 mi) radius exclusion zone was created 36 hours after the accident.
• About 49,000 people were evacuated from the area, primarily from Pripyat. The
exclusion zone was later increased to 30 kilometres (19 mi) radius when a further
68,000 people were evacuated from the wider area.
• The reactor explosion killed two of the reactor operating staff.
• In the emergency response that followed, 134 station staff and firemen were
hospitalized with acute radiation syndrome due to absorbing high doses of ionizing
radiation.
•

11. Continued..
• Of these 134 people, 28 died in the days to months afterward and approximately 14
suspected radiation-induced cancer deaths followed within the next 10 years.
• Among the wider population, an excess of 15 childhood thyroid cancer deaths were
documented as of 2011.
• The United Nations Scientific Committee on the Effects of Atomic
Radiation (UNSCEAR) has, at multiple times, reviewed all the published research
on the incident and found that at present, fewer than 100 documented deaths are
likely to be attributable to increased exposure to radiation.
• Determining the total eventual number of exposure related deaths is uncertain based
on the linear no-threshold model, a contested statistical model, which has also been
used in estimates of low level radon and air pollution exposure.
• Model predictions with the greatest confidence values of the eventual total death
toll in the decades ahead from Chernobyl releases vary, from 4,000 fatalities when
solely assessing the three most contaminated former Soviet states, to about 9,000 to
16,000 fatalities when assessing the total continent of Europe.

12. Response
• To reduce the spread of radioactive contamination from the wreckage and protect it
from weathering, the protective Chernobyl Nuclear Power Plant sarcophagus was
built by December 1986.
• It also provided radiological protection for the crews of the undamaged reactors at
the site, which continued operating.
• Due to the continued deterioration of the sarcophagus, it was further enclosed in
2017 by the Chernobyl New Safe Confinement, a larger enclosure that allows the
removal of both the sarcophagus and the reactor debris, while containing the
radioactive hazard.
• Nuclear clean-up is scheduled for completion in 2065.
• The Chernobyl disaster is considered the worst nuclear power plant accident in
history, both in terms of cost and casualties.
• The initial emergency response, together with later decontamination of the
environment, ultimately involved more than 500,000 personnel and cost an
estimated 18 billion Soviet rubles—roughly US$68 billion in 2019, adjusted for
inflation.
• The accident resulted in safety upgrades on all remaining Soviet-designed RBMK
reactors, of which 10 continue to be operational as of 2019.

13. Photo: The aerial view of the disaster site after explosion

14. Reactor cooling during power outage
• In steady-state operation, a significant fraction (over 6%) of the power from a
nuclear reactor is derived not from fission but from the decay heat of its
accumulated fission products.
• This heating continues for some time after the chain reaction has been stopped
(e.g. following an emergency scram) and active cooling is required to prevent core
meltdown. RBMK reactors like those at Chernobyl use water as a coolant.
• Reactor No. 4 at Chernobyl included about 1,600 individual fuel channels, each of
which required coolant flow of 28 tonnes (28,000 litres or 7,400 US gallons) per
hour.
• Since cooling pumps still require electricity and must run for some time after an
emergency shutdown in the event of a power grid failure, each of Chernobyl's
reactors had three backup diesel generators.
• The backup generators could start up in 15 seconds, but took 60–75 seconds to
attain full speed and generate the 5.5-megawatt output required to run one main
pump.

15. Safety test
• This capability still needed to be confirmed experimentally, and previous tests had
ended unsuccessfully. An initial test carried out in 1982 indicated that
the excitation voltage of the turbine-generator was insufficient; it did not maintain
the desired magnetic field after the turbine trip.
• The system was modified, and the test was repeated in 1984 but again proved
unsuccessful. In 1985, a test was conducted a third time but also yielded negative
results. The test procedure was to be run again in 1986, and scheduled to take place
during a maintenance shutdown of reactor No. 4.
• The test program was not problematic, even though its documentation of safety
measures would not hold up to modern standards. However, the test program
developers were not aware of the unusual RBMK-1000 reactor behavior under the
planned operating conditions.
• It was regarded as purely an electrical test, not a complex unit test, even though it
involved critical unit systems. According to the regulations in place at the time,
such a test did not require approval by either the chief designer of the reactor
(NIKIET, the scientific manager or the Soviet nuclear oversight regulator.
• The test required disabling of some safety systems (in particular, the emergency
core cooling system, a passive/active system of core cooling intended to provide
water to the core in a loss-of-coolant accident), and a special approval from the
chief engineer had been obtained according to regulations.

16. Continued..
• The experimental procedure was intended to run as follows:
• The reactor was to be running at a low power level, between 700 MW and 800 MW
• The steam-turbine generator was to be run up to full speed
• When these conditions were achieved, the steam supply for the turbine generator
was to be closed off
• Turbine generator performance was to be recorded to determine whether it could
provide the bridging power for coolant pumps until the emergency diesel generators
were sequenced to start and provide power to the cooling pumps automatically
• After the emergency generators reached normal operating speed and voltage, the
turbine generator would be allowed to continue to freewheel down
• The maintenance shut down of the reactor was to be completed

17. Test delay and shift change

18. Nuclear Reactor vessel

19. Test delay and shift change
• The test was to be conducted during the day-shift of 25 April 1986 as part of a
scheduled reactor shut down. The day shift crew had been instructed in advance on
the reactor operating conditions to run the test and in addition, a special team
of electrical engineers was present to conduct the one-minute test of the new
voltage regulating system once the correct conditions had been reached. As
planned, a gradual reduction in the output of the power unit began at 01:06 on 25
April, and the power level had reached 50% of its nominal 3,200 MW thermal level
by the beginning of the day shift.
• The day shift performed many unrelated maintenance tasks, and was scheduled to
perform the test at 14:15 and preparations for the test were carried out, including
the disabling of the emergency core cooling system. Meanwhile, another regional
power station unexpectedly went offline and at 14:00 the Kiev electrical grid
controller requested that the further reduction of Chernobyl's output be postponed,
as power was needed to satisfy the peak evening demand. The Chernobyl plant
director agreed, and postponed the test.

20. Continued..
• Soon, the day shift was replaced by the evening shift. Despite the delay,
the emergency core cooling system was left disabled – it was disconnected by a
manual isolating slide valve which in practice meant that two or three people spent
the whole shift manually turning sailboat-helm sized valve wheels.The system
would have no influence on the events that unfolded next, but allowing to run the
reactor for 11 hours outside of the test without emergency protection indicated a
generally low level of safety culture.
• At 23:04, the Kiev grid controller allowed the reactor shutdown to resume. This
delay had some serious consequences: the day shift had long since departed, the
evening shift was also preparing to leave, and the night shift would not take over
until midnight, well into the job. According to plan, the test should have been
finished during the day shift, and the night shift would only have had to maintain
decay heat cooling systems in an otherwise shut-down plant.
• The night shift had very limited time to prepare for and carry out the
experiment. Anatoly Dyatlov, deputy chief-engineer of the entire Chernobyl
Nuclear Power Plant, was present to supervise and direct the experiment; as he out-
ranked all other supervisory personnel present, his orders and instructions overrode
any objections of other senior personnel present during the test and its preparation.
Serving under Dyatlov, Aleksandr Akimov was chief of the night shift, and Leonid
Toptunov was the operator responsible for the reactor's operational regimen,
including the movement of the control rods. Toptunov was a young engineer who
had worked independently as a senior engineer for approximately three months.

21. Unexpected drop of the reactor power
• The test plan called for a gradual decrease in power output from reactor No. 4 to a
thermal level of 700–1000 MWand an output of 720 MW was reached at 00:05 on
26 April.
• Due to the reactor's production of a fission byproduct, xenon-135, which is a
reaction-inhibiting neutron absorber, core power continued to decrease in the
absence of further operator action—a process known as reactor poisoning.
• In steady-state operation, this is avoided because xenon-135 is "burned off" as
quickly as it is created from decaying iodine-135 by the absorption of neutrons
from the ongoing chain reaction, becoming highly stable xenon-136.
• With the reactor power reduced, previously produced high quantities of iodine-135
were decaying into the neutron-absorbing xenon-135 faster than the now
reduced neutron flux could burn it off.
• When the reactor power dropped to approximately 500 MW, the reactor control had
been switched to a different mode in order to manually maintain the power
level.Around that moment, the power suddenly fell into an unintended near-
shutdown state, with a power output of 30 MW thermal or less.
• The exact circumstances that caused the power fall are unknown because Akimov
died in hospital on 10 May and Toptunov on 14 May; early reports attributed it to
Toptunov's mistake, but it has also been suggested it was due to an equipment
failure.

22. Continued..
• The reactor was now producing 5% of the minimum initial power level prescribed
for the test. This low reactivity inhibited the burn-off of xenon-135 within
the reactor core and hindered the rise of reactor power.
• Control-room personnel had to raise power by disconnecting most of the reactor
control rods from the automatic control rod regulation system and manually
extracted the majority of rods to their upper limits in order to promote reactivity
and counteract the effect of the poisoning.
• Several minutes elapsed between their extraction and the point at which the power
output began to increase and subsequently stabilized at 160–200 MW (thermal).
• The operation of the reactor at the low power level (and high poisoning level) was
accompanied by unstable core temperatures and coolant flow, and possibly by
instability of neutron flux, which triggered alarms.
• The control room received repeated emergency signals regarding the levels in the
steam/water separator drums, and large excursions or variations in the flow rate of
feed water, as well as from relief valves opened to relieve excess steam into
a turbine condenser, and from the neutron power controller.
• Between 00:35 and 00:45, emergency alarm signals concerning thermal-
hydraulic parameters were ignored, apparently to preserve the reactor power level.

23. Reactor conditions prior to accident
• When a power level of 200 MW was reattained, preparation for the experiment
continued, although the power level was much lower than the prescribed 700 MW.
As part of the test plan, extra water pumps were activated at 01:05, increasing the
water flow. The increased coolant flow rate through the reactor produced an
increase in the inlet coolant temperature of the reactor core (the coolant no longer
having sufficient time to release its heat in the turbine and cooling towers), which
now more closely approached the nucleate boiling temperature of water, reducing
the safety margin.
• The flow exceeded the allowed limit at 01:19, triggering an alarm of low steam
pressure in the steam separators. At the same time, the extra water flow lowered the
overall core temperature and reduced the existing steam voids in the core and the
steam separators.Since water weakly absorbs neutrons (and the higher density of
liquid water makes it a better absorber than steam), the activation of the additional
pumps decreased the reactor power. The crew responded by turning off two of the
circulation pumps to reduce feedwater flow, in an effort to increase steam pressure,
and by removing more manual control rods to maintain power.

24. Continued..
• The combined effect of these various actions was an extremely unstable reactor
configuration.
• Nearly all of the 211 control rods had been extracted manually, including all but 18
of the "fail-safe" manually operated rods of the minimum 28 that were supposed to
remain fully inserted to control the reactor even in the event of a loss of coolant.
• While the emergency scram system that would insert all control rods to shut down
the reactor could still be activated manually (through the "AZ-5" switch), the
automated system that would ordinarily do the same had been mostly disabled to
maintain the power level, and many other automated and even passive safety
features of the reactor had been bypassed.
• The reduction of reactor coolant pumping left little safety margin; any power
excursion could produce boiling, thereby reducing neutron absorption by the water.
The reactor configuration was outside the safe operating envelope prescribed by the
designers. If anything pushed it into super criticality, it would be unable to recover
automatically.

25. Test execution
• At 01:23:04, the test began. Four of the eight main circulating pumps (MCP) were
active, versus six under regular operation. The steam to the turbines was shut off,
beginning a run-down of the turbine generator. The diesel generators started and
sequentially picked up loads; the generators were to have completely picked up the
MCPs' power needs by 01:23:43. In the interim, the power for the MCPs was to be
supplied by the turbine generator as it coasted down. As the momentum of the
turbine generator decreased, so did the power it produced for the pumps. The water
flow rate decreased, leading to increased formation of steam voids in the coolant
flowing up through the fuel pressure tubes.
• Unlike western light-water reactors, the RBMK has a positive void coefficient of
reactivity at low power levels. This means that the formation of bubbles (voids)
from water boiling intensifies the nuclear chain reaction. The consequent power
increase then produces more voids which further intensifies the chain reaction, and
so on. Given this characteristic, reactor No. 4 was now at risk of a runaway increase
in its core power with nothing to restrain it.
• Throughout most of the experiment the local automatic control system (LAC)
successfully counteracted this positive feedback, by inserting control rods into the
reactor core to limit the power rise. However, this system had control of only 12
rods, as nearly all the others had been manually retracted by the reactor operators.

26. Reactor shutdown and power excursion
• At 01:23:40, as recorded by the SKALA centralized control system,
a scram (emergency shutdown) of the reactor was initiated as the experiment was
wrapping up.The scram was started when the AZ-5 button (also known as the EPS-
5 button) of the reactor emergency protection system was pressed: this engaged the
drive mechanism on all control rods to fully insert them, including the manual
control rods that had been withdrawn earlier.
• The mechanism would be used even to routinely shut down the reactor after the
experiment for the planned maintenance[and the scram likely preceded the sharp
increase in power. However, the precise reason why the button was pressed when it
was is not certain, as only the deceased Akimov and Toptunov partook in that
decision, though the atmosphere in the control room was calm at that
moment.Meanwhile, the RBMK designers claim that the button had to have been
pressed only after the reactor already began to self-destruct.

27. Steam plumes continued to be generated days after the initial explosion

28. Steam explosions
• As the scram was starting, the reactor output jumped to around 30,000 MW
thermal, 10 times its normal operational output, the indicated last reading on the
power meter on the control panel. Some estimate the power spike may have gone
10 times higher than that. It was not possible to reconstruct the precise sequence of
the processes that led to the destruction of the reactor and the power unit building,
but a steam explosion, like the explosion of a steam boiler from excess vapour
pressure, appears to have been the next event. There is a general understanding that
it was explosive steam pressure from the damaged fuel channels escaping into the
reactor's exterior cooling structure that caused the explosion that destroyed the
reactor casing, tearing off and blasting the upper plate called the upper biological
shield,[ to which the entire reactor assembly is fastened, through the roof of the
reactor building. This is believed to be the first explosion that many heard.
• This explosion ruptured further fuel channels, as well as severing most of the
coolant lines feeding the reactor chamber, and as a result, the remaining coolant
flashed to steam and escaped the reactor core. The total water loss in combination
with a high positive void coefficient further increased the reactor's thermal power.

29. Continued..
• A second, more powerful explosion occurred about two or three seconds
after the first; this explosion dispersed the damaged core and effectively
terminated the nuclear chain reaction. This explosion also compromised
more of the reactor containment vessel and ejected hot lumps of graphite
moderator. The ejected graphite and the demolished channels still in the
remains of the reactor vessel caught fire on exposure to air, greatly
contributing to the spread of radioactive fallout and the contamination of
outlying areas.
• According to observers outside Unit 4, burning lumps of material and
sparks shot into the air above the reactor. Some of them fell onto the roof of
the machine hall and started a fire. About 25% of the red-hot graphite
blocks and overheated material from the fuel channels was ejected. Parts of
the graphite blocks and fuel channels were out of the reactor building. As a
result of the damage to the building an airflow through the core was
established by the high temperature of the core. The air ignited the hot
graphite and started a graphite fire.

30. Continued..
• After the larger explosion, a number of employees at the power station went outside
to get a clearer view of the extent of the damage. One such survivor, Alexander
Yuvchenko, recounts that once he stepped outside and looked up towards the
reactor hall, he saw a "very beautiful" laser-like beam of blue light caused by
the ionized-air glow that appeared to "flood up into infinity".
• There were initially several hypotheses about the nature of the second explosion.
One view was that the second explosion was caused by the combustion
of hydrogen, which had been produced either by the overheated steam-
zirconium reaction or by the reaction of red-hot graphite with steam that produced
hydrogen and carbon monoxide. Another hypothesis, by Konstantin Checherov,
published in 1998, was that the second explosion was a thermal explosion of the
reactor as a result of the uncontrollable escape of fast neutrons caused by the
complete water loss in the reactor core.A third hypothesis was that the second
explosion was another steam explosion. According to this version, the first
explosion was a more minor steam explosion in the circulating loop, causing a loss
of coolant flow and pressure that in turn caused the water still in the core to flash to
steam; this second explosion then caused the majority of the damage to the reactor
and containment building.

31. Investigations and the evolution of identified causes
• According to the original IAEA's 1986 analysis INSAG-1, the main cause of the
accident was the operators' actions. But according to the IAEA's 1993 revised and
final analysis INSAG-7 the main cause was the reactor's design.
• Both views were heavily lobbied by different groups, including the reactor's
designers, power plant personnel, and the Soviet and Ukrainian governments. One
reason there were such contradictory viewpoints and so much debate about the
causes of the Chernobyl accident was that the primary data covering the disaster, as
registered by the instruments and sensors, were not completely published in the
official sources.Also, Soviet authorities identified a multitude of operator actions as
regulation violations for the original 1986 report while no such regulations were in
fact in place.
• The primary cause of the accident was a major deficiency in safety features of the
reactor design, in particular the "positive scram" effect that caused reactivity to
increase when control rods were inserted to the reactor in certain conditions.This
was caused by an overly positive void coefficient of the reactor and a deficiency in
control rod design.To avoid such conditions, it was necessary for the operators to
track the value of the reactor ORM (operational reactivity margin) during operation,
but this value was not readily available to the operators in the control room and they
were not aware of the safety significance of ORM.

32. Continued..
• Even in the revised analysis, the human factor remains identified as a major
element in causing the accident, particularly the operating crew's deviation from the
test programme. "Most reprehensibly, unapproved changes in the test procedure
were deliberately made on the spot, although the plant was known to be in a very
different condition from that intended for the test."
• Both IAEA reports identify inadequate "culture of safety" at all levels as the third
major factor of the accident. Deficiency in the safety culture was inherent not only
at the operational stage ("both the operating regulations and staff handled the
disabling of the reactor protection easily enough: witness the length of time for
which the emergency core cooling system was out of service while the reactor was
operated at half power"). The culture of safety was lacking also, and to no lesser
extent, during activities at other stages in the lifetime of nuclear power plants
(including design, engineering, construction, manufacture, and regulation). The
poor quality of operating procedures and instructions, and their conflicting
character, put a heavy burden on the operating crew, including the chief engineer.
"The accident can be said to have flowed from a deficient safety culture, not only at
the Chernobyl plant, but throughout the Soviet design, operating and regulatory
organizations for nuclear power that existed at that time."

33. Impact
• Environmental
• Water bodies
• Flora and fauna
• Human

34. Before and Now

35. Power Plant

37. Some picture of Chernobyl Disaster

38. The debris and the persons wearing PPE

39. Impact on ecosystem
Cleaning the environment

40. No.4 plant after explosion

41. Impact on Human and Animals

42. Reference
• https://en.wikipedia.org/wiki/Chernobyl_disa
ster
• https://www.youtube.com/watch?v=VxwKsXG
G2fM
• https://www.youtube.com/watch?v=as2oGJKv
0EA

43. Thank You !