Chernobyl Accident: Lessons Learned
N. Steinberg
Nicolai Steinberg
CNSC, Ottawa, August 4, 2015
Validity of the conclusions that was made for myself many years
ago still in force, and was confirmed by Fukushima:
 the severe accidents could not be excluded in principle;
 probability and effects of severe accidents could be minimized;
 achievement of mentioned goals require a sustained effort of
both, Operators and Regulators;
 all severe accidents have resulted from a series of mistakes and
events in routine design and operation;
 the accidents at TMI, Chernobyl and Fukushima occurred due to
lapses in national systems and cannot be considered as being
inherent to nuclear technology.
Scientific, technical and managerial aspects of the accident at
Chernobyl and Fukushima are quite different, but the scenario of
accumulation of deviations from safety requirements that have
reached a critical value is similar.
Huge resources and time, in terms of scope and technical
solutions, are being implemented to improve nuclear safety in the
World after TMI, Chernobyl and Fukushima, but ...
The question remains:
 Can there be a new accident anywhere in the World?
 Could the slogan "Never Again” be realized?
 What should be done to exclude consequences of severe accidents,
tragic for people, environment and economy?
It is very problematic to give either
positive or negative answers due to
suddenness and unique nature and scale of
the possible severe accidents.
In 1986 we, for the first time, faced the
radiation accident of the global scale when
it was necessary to undertake measures for
protection of millions of people.
In 2011 we, for the first time, faced the
accident connected to simultaneous severe
damage of several nuclear installations.
What could be the next?
Physical and thermo-hydraulic instabilities of the reactor was the main cause of the
accident. These effects were well known for many years. Combination of design
deficiencies and actions of personnel on April 26, 1986 resulted in severe accident.
During the test unit #4 was in very unstable thermo-hydraulic condition and after
completion of the test a manual switch-on the reactor protection system (“Push red
button!”) initiated the accident. Due to imperfections in design, insertion of safety and
manual control rods in the core increased reactivity and lead to catastrophic run-out of the
reactor power on instantaneous neutrons due to enormous positive void coefficient.
Circumstances contributed to the accident:
 operators were not properly informed on RBMK safety deficiencies;
 operational procedures contained ambiguous requirements and,
 I&C due to imperfect design did not provide up to date safety important information to
operators.
GENERAL REASONS AND CIRCUMSTANCES OF ACCIDENT:
crucial importance of the NPP deficiencies, its influence on the safety and its possible
consequences were not properly evaluated for the long period of time. Safety
measures to prevent the catastrophe were not implemented.
5
Main periods of radioactive release:
 April, 26. The first peak of release due to explosions at 01.23 am as fine dispersed
fuel, volatile radionuclides, noble gases, etc. and the second peak of release was
connected to quasi explosion after passing the iodine well at 10.30 -11.00 pm (quasi
secondary criticality);
 April 27 – May 2. Release mainly associated with graphite burning;
 May, 3 - 5. Release due to graphite burning and fuel overheating as a result of
radioactive decay (release of refractory nuclides including 90-Sr, Pu, etc.);
 May 6 and later. Release reduction due to decrease temperature of fuel contained
mass (FCM) but with sporadic volley emissions of radioactivity.
Radioactive release at Chernobyl
was carried directly into the atmosphere - the NPP was not equipped with
containment (the release was 10 time more than in Fukushima).
Total release resulted in about 5300 – 5600 PBq (with noble gases
(appraisal) close to12,000 PBq). Release included: 1,800 PBq – 131I; 85
PBq – 137Cs. These two radionuclides gave main input in professional
exposure.
NPP site area (around 1 km2) was mainly contaminated with dispersed
nuclear fuel, debris of construction structures and structures of reactor,
including graphite.
Basement of the main building and cable channels and tranches were
flooded by contaminated water with radioactivity up to 10-1 k/m3.
Typical exposure rates at the NPP site were: 50 mGy/hr (East side) to
5000 mGy/hr (West side) with many spots where exposure rates reached
10000 mGy/hr and more. Exposure rates at the site at the end of April 26
exceeded the “natural” background by 106 to 108 times.
Neutron exposure input had minor impact on the total dose (of up to 12% of gamma exposure in vicinity of Unit - 4, June-July 1986)
Chernobyl catastrophe raised wide range of problems (1):

How to isolate the destroyed Unit - 4? How to decrease radioactive
releases? How to predict distribution of radioactive contamination
(irregular fallout)? What to do with the dust containing «hot particles»
(fine dispersed nuclear fuel)? How to fight the dust transfer? How to
assess and forecast radiation consequences of the accident?

Where to put the radioactive waste, how to isolate it reliably from the
environment? What as a whole the radioactive waste management
system shall be in contaminated zone?

How to save (protect) the water bodies both underground and surface?
How to protect the rivers from ingress and further migration of the
radionuclides?

How to cultivate farmlands? How to organize the pasturage and
feeding of the domestic animals? What shall be done with agricultural
products contaminated with the radionuclides?

How to decontaminate NPP and surrounding areas?
Chernobyl catastrophe raised wide circle of problems (2):

What necessary to do with «Red Forest» (forest in vicinity of
ChNPP suffered by radionuclides fallout)?

What is the best way to organize the work of involved military and
civil divisions and organizations? How to arrange their living
conditions, housing, remuneration?

What to do with the Units 1-3 of ChNPP? How to return them into
operation? When? And what shall be done for this? How to be
with the NPP staffing, where the personnel will live, where will
the families of the NPP personnel live?

How to arrange the living conditions for the evacuated population,
how to provide the population with the housing, food stuff,
medical services, how to compensate them the material losses?

What is the future of the town of Pripyat and other evacuated
settlements? Is the return of population possible within the nearest
future?
By the morning of 26.04.1986 it became clear that the magnitude of the
accident exceeded the imaginable limits and it’s impact is beyond the
single region (province).
It was also evident that the authority and resources of the NPP and
Ministry of Power were not sufficient to solve the complex problems
associated with the overcoming of the accident consequences.
Government Commission (GC) was set up in the morning of April, 26
to investigate the causes of the accident. By the evening, it was decided
to entrust this commission with the management of liquidation of the
accident consequences.
In the evening of the same day GC met directly in town of Pripyat (3
km from NPP), and at 2 a.m. of April, 27 the decision on evacuation of
the town was made.
April, 27 it became apparent that consequences of the accident had an impact on the all
European territory of the USSR and, moreover, beyond the State*. It was required:
 to mobilize all the necessary resources of the State for emergency response and
 to manage the emergency response at the highest level of the State power.
Taking into account the role of ruling party, Task Force Group (TFG) of the Politburo
(bureau) of the Central Committee of Communist Party was created. Decisions made
by the TFG were mandatory for all of the State, as well as armed forces.
TFG began operation at April 28, 1986 and last meeting of the TFG took place in
January 1988 (there were 44 meetings). The situation stabilized, necessity of TFG
operation had ceased and all functions were transferred to the GC.
*The overall area of Western Europe countries, where levels of 137-Cs contamination
exceeded 20 kBq/m2 (almost 10-fold higher that the global background) due to the Chernobyl
disaster, amounted to approximately 280 000 km2.
It was created a rigid centralized system for management of Chernobyl
accident and its consequences. Headquarters of the GC were located in
Chernobyl (18 km from the site).
Special headquarters (departments) connected to Chernobyl matters
were established in the main location of the Governmental and Local
Authorities. Their representatives are located in Chernobyl. This allowed
maximum increase in the promptness of decision making and in their
implementation. Headquarters of the Soviet Army connected to accident
were also located in Chernobyl.
H. Blix, IAEA Director General, though he was not an adherent of the
command and administration system, noted: “… exactly this system
managed to promptly and efficiently solve the tasks related to the
liquidation of the consequences of the accident. Not many countries of
democratic doctrine could cope with them in such a successful manner”.
CPSU Task Force Group
(TFG)
Governmental Commission (GC)
Ministries & Local
Authorities
Soviet Army (SA)
Chernobyl Task
Force
Target
setting
Chernobyl NPP
Contractors not
connected to
ChNPP
Construction
Administration
(US-605)
Contractors
connected to
ChNPP
SA Troops at the
ChNPP site
Three Strategic Goals for Emergency Response:
1.
Protection of the population against negative influence of ionizing
radiation (evacuation, life support of the population of the suffered areas,
compensation of the losses, building of new settlements, medical
treatment, kids health improvement, etc.).
2.
Recovery of the Chernobyl NPP and return of Units 1 - 3 in operation
(decontamination of the site and facilities of NPP, dust suppression,
protection of the underground and surface water, units overhaul, safety
upgrading, solution of the personnel housing problems, including
construction of the new town, etc.).
3.
Isolation of destroyed Unit 4 from the environment before start – up of
Unit 1 (building of the Shelter Object (Sarcophagus)).
The goals and schedule for implementation of Strategic Goals were not
supported by technical and economical substantiation, and radiation
conditions were not practically taken into account.
Management of Chernobyl NPP (1)
April - May 1986 all activity was that of active defence of nature and works have been
executed according to daily formulated tasks. The middle – and long term programs were
developed at the end of May.
Management system of Chernobyl NPP was shaken to its foundation on April 26 – many
of the managers and leading specialists already by the morning were overexposed and
taken out of the NPP.
The departed personnel were replaced by the specialists arriving from other NPPs and
institutions. At the middle of May, more than half top managers of Chernobyl NPP were
substituted by the persons from the other organizations.
Workplaces for the NPP, exclude shift and limited maintenans personnel, were organized
in Chernobyl town. Rear base and residence of the NPP and contractors staff were created
in the kid’s summer camp (35 km from the NPP). The work schedule was transformed
into 12-hour shifts.
May – middle of July, prior to completion of the first round of decontamination and start
of restoration and overhaul of Units 1 & 2, the number of the NPP staff was reduced to
the minimum (550 – 650 persons (number of staff before the accident - 4000)). The rest
of the employees were sent to medical treatment and for arrangement of the families after
evacuation
The atmosphere of the relationship GC – NPP resulted in successful
implementation of 1986 Strategic Goals:

The GC makes decisions and sets major tasks and implemented harsh
monitoring of tasks fulfilment;
 The NPP was free to develop specific solutions to fulfill the tasks set by the
Commission to achieve specific goals;
 NPP carried on full responsibility for the implementation of own solutions.
Such relationship was supported by maximum concentration of material and
human resources and exclusion of collective irresponsibility
Such co-operation (it is the best wording for characterization of relationship of
the Government and Operator) gave the possibility for creative work. Never
before and never after there was an opportunity to work in such atmosphere.
In order to prevent the spreading of contamination the principal
and very important decisions related to zoning and the schemes
of the freights and people delivery to the emergency area and
directly to NPP site were implemented - a system of the sanitary
treatment facilities (STF, sanitary locks) was established to
organize three barriers on the way from the site to personnel
settlements:
 the NPP site boundary (number of boundaries were
implemented inside of the NPP site taking in to account
radiation condition),
 boundary of 10 km, and
 Boundary of 30 km zones;
Three sanitary barriers established between Chernobyl NPP and
settlements: “Pioneer Camp”, “White Ship” and “Green Cape”,
allowed avoid contamination of the NPP personnel housing.
The exposure limits for all participated in emergency response:
1986 – 250 mSv; 1987 – 100 mSv; 1988 and later– 50 mSv.
Results (average doses of the NPP personnel): 1986 – 150-170 mSv (very high
uncertainties of April – June); 1987 – 130 mSv; 1988 – 30 mSv; 1989 – 15
mSv.
Lessons learned showed that the main inputs in dose reduction are:
 personnel qualification and experience in the execution the work in hard
radiological conditions and psychological behaviors of the staff;
 execution of the works by the same qualified team managed by strong leader
(it could lead to minor increase of individual doses but significantly decrease
the collective dose);
 exact knowledge of exposure sources including spatial (angular) distribution
of exposure before planning and execution any works.
01.10.1986 – Unit 1 returned to operation
02.11.1986 – Unit 2 returned to operation
30.11.1986 – “Sarcophagus” of destroyed
Unit 4 completed
Absence of Safety Culture is the Principal Source
of All Severe Accidents. Great number of words is spent in
discussion about high / low level of safety culture or about
deficiencies in safety culture, but …
SAFETY CULTURE IS EITHER THERE OR ITS NOT.
The lack of safety culture at all levels in the Government,
Operators and Regulators at different stages of Chernobyl
(and Fukushima) NPPs live cycle led to the disaster.
Following list of lessons learned finally connected to action
should be implemented by persons of the Government,
Operators and Regulators (list of lessons learned is presented
not taking in to account their priorities)
 State of shock of the Operator management and staff in the face of
severe accident required their temporary or permanent replacement,
both for psychological and health reasons. Managers under great stress
and in conditions of significant uncertainty tend to make wrong
decisions and/or do not possess enough character to implement the
decision.
 Range of alternative decisions in unexpected situation with significant
uncertainties is very wide: to do all available actions or to do nothing
except actions directed towards reduction of uncertainties. To take and
implement step-by-step simple, clear and transparent decisions are the
optimal way in a case of total or high uncertainty.
 Conditions of severe accident could lead (or require?) a breach in
management hierarchy - informal leaders with great qualification,
authority and will could come (or should?) to the first line.
It is absolutely necessary to search for and train the key leaders
to make the appropriate decisions in response to challenging
events.
 Design of NPPs did not (or could not?) include number of measures necessary
for accident management and emergency response (for example, waste
management; unmanned techniques (robotics); design of NPP auxiliary
installations, etc.).
 The hierarchy of radioactive harmful factors in severe accident:
 external gamma
 external beta
 internal exposure.
Clear and reasonable long-term and every day stepping of the well substantiated objectives,
and the well organized and executed works lead to reduction of personnel exposure.
 High level of qualification and experience to act in the complex radiation
environment, as well as psychological training – are the foundation of success
(execution works by the team experienced in the specific work significantly
reduce the duration of works and reduce the doses);
 Reliable system of sanitary locks and sanitary barriers are absolutely necessary
for protection against proliferation of contamination and against secondary
contamination.
Absence of specific safety regulations adequate to conditions of the damaged
NPP can result in significant waste of time and resources. In many cases it is not
possible to implement “routine” regulations. For example:


How to implement “defence in depth” principle?
Are the damaged power units the permanent radioactive waste storage
facility, or are they to be used as interim radioactive waste storage facilities?
Is it storages of high level waste? Is it installations for fuel reprocessing?
 Should we substantiate compliance of destroyed installations with the safety
regulations, or describe present status and propose the ways to enhance the
safety? Do we need a SAR of the damaged units? If yes, it should be
formalized requirements to format and content of SAR (for example, some
time it is absolutely impossible to rely on results of investigation or results of
tests, or on proven practice).
To have the predictable relation Operator-Regulator and to have opportunity for
scheduling of licensing process it is essentially to develop specific safety
regulations or amendments to “routine” regulations.
 Psychological readiness of operators to act in unforeseen circumstances raises doubts.
NPP have reached such a level of automation, where the role of the operator significantly
reduced, essentially, to monitor the processes. Training at the simulators has not formed
operator’s confidence and self-assurance in implementation of the decision, especially in
a severe accident. This is an important problem.
 In compliance with EP&ER regulations emergency units have to be created. In fact these
are the same units that operate NPP in the normal modes, but renamed for emergency
conditions (for example, “reactor shop” is renamed in “emergency reactor brigade”). This
purely bureaucratic procedure causes confusion in the conditions of the accident and
nothing but the harm does not bring.
 All personnel of the NPP should be prepared for to use the means of radiation monitoring
and in a severe radiation environment. Otherwise it will lead to a significant increase in
the number of employees on radiation protection and totally unjustified additional
exposure.
 It is important to train plant managers (at all levels) to the organization of efficient
interaction with external organizations (police, army, security services, health care, etc.)
in an accident condition. They must be able to define distinct the scope of work,
distribution of responsibilities and to maintain work safety.
 Objectives of accident overcoming should be reasonable and well
substantiated taking in to account socio-economic, socio-political and
humanitarian aspects and consider radiation conditions.
 Necessary to define Strategy - final goals and acceptance criteria. Develop
draft Roadmap (Program) taking into account preliminary calculated
resources and time schedule. Living monitoring and correction of the
Strategy taking into account results of it implementation (feedback).
 Decommissioning. Conversion of the damaged installations. Goals of
decommissioning and acceptance criteria. Necessary works to be
implemented on-site, and those that must be performed throughout the State.
Decommissioning of damaged installations requires considerable time and
resources.
 Scientific and engineering support should be provided to support everyday
activity in both, acute and successive phases of severe accident overcoming
 It is important:
 to avoid excessive influence of politicians on work programming and
every day activities;
 to formulate and justify criteria for establishing the long-term programs;
 Management of emergency radioactive waste require distinct vision
of National Radwaste Management System, including management
of high level and long – lived wastes.
 Important point in emergency response - trusting relationship with
the public. Populations demonstrated extremely low knowledge of
nuclear and radiation issues. Lack of knowledge causes lack of trust.
Significant influence on psychological conditions of inhabitants
belongs to medical doctors, school teachers and clergymen. It is
expedient to attract the mentioned persons and constantly collaborate
with them to maintain opportunity for communicating to the public
current status and objectives of emergency response.