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R U S S I A N R E S E A R C H C E N T R E

"KURCHATOV INSTITUTE"

Chernobyl Accident

Technical Lessons

Moscow,1996


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The report represents the opinion of RRC KI

Russian Research Centre "Kurchatov Institute"

Kurchatov Square, 1

123182 Moscow

Russia

CopyrightRRC KI 1996


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Contents

1 Causes of the accident 1

2 Safety improvement of NPPs with the RBMK reactors after the

Chernobyl accident

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2.1 Reduction of the reactivity void effect 5

2.2 Improvement of the control and protection system efficiency 5

2.3 Metal inspection of headers and large diameter pipes 7

2.4 Capacity improvement of steam discharge pipes 7

2.5 Correction of the design documentation 8

3 Chernobyl "Sarcophagus" or "Shelter" 10


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1 Causes of the Accident

10 years have passed since the Chernobyl NPP Unit No. 4 accident. During that time

many specialists in Russia and abroad studied the causes of the accident.

The accident occurred when rundown tests of the Chernobyl NPP Unit No. 4

turbogenerator at auxiliary power were being performed. The objective of the tests

was to check for the opportunity to extend the reactor forced cooldown upon NPP

blackout.

Peculiar features of the test mode included low power level, increased coolant flow

through the reactor, and insignificant core inlet subcooling up to the boiling

temperature. It turned out that these factors had a direct bearing on the scale of the

emerged effects.

The first discussion of the accident data reported at the meeting of the Expert

Commission of the USSR Minenergo and the Minsredmash set up on April 27, 1986

can be considered as the initial investigation of causes resulted in the Chernobyl NPP

Unit No. 4 accident (04.26.86) and a detailed study of individual factors and their

different combinations that led to destruction of a large nuclear reactor.

An accurate determination of the accident causes was very important for deciding on

the possibility of further operating other units with the RBMK reactors. Apart from the

Chernobyl NPP Unit No. 4, fourteen similar units with a total power of 14.5 GW were

in operation at that time.

The conclusions of the Commission were used to formulate the first list of urgent

measures to be undertaken unconditionally at the existing NPPs with the RBMK

reactors. On May 5, 1986 this instruction was circulated to all NPPs in operation

demanding strict observance of technological regulations. Each director of the

respective NPP has been personally responsible.

The Commissions Conclusions were formulated in the early May. They served as the

basis for development of a large complex of measures to improve safety of the

existing NPPs with the RBMK reactors.

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Late May - early June, 1986 the Kurchatov Institute obtained new results on the

reactivity void effect for the RBMK reactors. They confirmed the assumption at which

many specialists had arrived on a large positive void effect that, in its turn, always

gave rise to a fast positive power effect of reactivity under a varying coolant density ata low power level. The danger of further operation of the RBMK reactors featuring

those neutronics characteristics became obvious.

A special test program was developed to verify the actual quantitative value of the

reactivity void effect by measuring reactivity during the reactor core drainage that was

in operation long enough and had a core isotope composition close to that of the

reactor operated in the mode of continuous refuelling overloads. The first experiment

was conducted at the Chernobyl NPP Unit No. 1 during the period of its start-up

preparation in September 1986. In the fall of that year, similar programs were realized

in other power units. The results of these experiments put an end to all doubts

concerning the sign and magnitude of the reactor reactivity void effect. Thus, by using

in investigations the most high-performance computers available in the USSR at that

time and conducting a series of experiments on drainage of the cooled-down reactor

cores with full-scale loads, the value of the reactivity void coefficient was found to be

not less than "plus" 4

.

The second problem of the RBMK type reactor safety providing that was in the focus

of attention after the Chernobyl accident is associated with the necessity to make the

control and protection systems (CPS) much more efficient both in normal operation

and under the conditions of those violated regulations that provided for the adequacy

of the reactor shutdown systems efficiency and the reactor neutronics state. Still

before the accident during some RBMK reactors start-ups, a strong dependence ofefficiency of a CPS actuator (CPSA) single control rod or a small group of control rods

(up to five in number) on the particular initial point of its movement, on the ad hoc

reactivity margin, and neutron field distribution along the height.

Computation studies showed a sharp reduction of the reactor protection system

efficiency and even the opportunity of positive reactivity insertion by CPS rods under

conditions arose at the Chernobyl NPP Unit No. 4 by the moment of the emergency

protection system actuation. Distinctions in the results obtained by various specialists

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lay in determination of the quantitative value of positive reactivity released during

control rods downward movement. In this case, the very potentiality of such

phenomenon must be of course eliminated proceeding from neutronics calculations

and experimental tests.

Besides, it was assumed necessary to increase CPSA rods rate in the modes of the

reactor emergency shutdown and to equip reactors with an advanced system of the

reactor scram.

It should be noted that in 1986, at the first stage, the accident analysis was performed

on the base of rather simple reactor models in the point and one dimensional

approximations of reactor kinetics. The core thermohydraulics was based on

single-channel models.

Nevertheless, the principal causes of the accident were clear. It was the reactor power

excursion caused by a positive power coefficient of reactivity. Insertion of rods by a

signal from the emergency protection system could lead to reactivity growth at the

initial stage of their movement. Both factors can be attributed to the reactor state

characterized by the following:

- Low ad hoc reactivity margin;

- Low power level;

- Low subcooling with at a high coolant rate.

At the second stage of the accident investigations, say up to 1991, three-dimensional

models were used. That enabled a more detailed analysis of the CPS rods function.

Three independent sets of codes were used in the accident analysis allowing the

non-steady-state integral calculation of a three-dimensional neutron field and

channel-by-channel distribution of the RBMK reactor thermohydraulic parameters.

These programs were developed in the RRC "Kurchatov Institute", Research and

Design Institute of Power Engineering (RDIPE), Research Institute of Nuclear Power

Engineering (VNIIAES) jointly with the Institute of Nuclear Research of the Ukrainian

Academy of Sciences (Kiev).

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The second stage investigations showed that quantitatively the accident process was

described correctly but quantitative results were far from being in agreement with the

factual evidence.

Among recommendations made at the second stage of investigations there were the

following ones:

- Necessity of model upgrading;

- More detailed analysis of the pre-accident reactor facility operation.

The works along these lines were continued at the third stage, conventionally up to

late 1995.

Despite differences in the models and some results, the investigations at the third

stageproved that the major factors contributing to the accident development were a

great positive steam effect of reactivity and positive reactivity insertion during insertion

of control rods.

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2 Safety Improvement of NPPs with the RBMK Reactors after

the Chernobyl Accident

2.1 Reduction of the reactivity void effect

Development of measures on reduction of the reactivity void effect was initiated

practically immediately after obtaining new data on its magnitude. Now, it is assumed

permissible if the reactivity void coefficient measured at regular intervals does not

exceed 1.0.

For the operational RBMK reactors, the following two compromise ways of the

reactivity void effect reduction were validated.

The first one is to increase the amount of additional absorbing materials in the core

and the operative reactivity margin determined by computation in the number of

manual control rods with regard to actual height fields of energy release.

The second one is to vary the uranium fuel to moderator ratio in the core, thus,

increasing uranium-235 enrichment of fuel.

For loading the RBMK-1000 reactors, 2.4% U-235 enrichment of fuel was used. These

measures were realized in all 15 operating RBMK units.

Further investigations showed that application of new fuel with the erbium burnt-out

absorber allowed to eliminate additional absorbing materials and to make the NPP

operation more economically efficient. These works are now at the stage of pilot -

full-scale introduction at the existing NPP.

2.2 Improvement of the control and protection system efficiency

Design and manufacture of new control and protection system actuators eliminating

water columns under supplanters and simultaneously ensuring high rate of absorber

rods insertion into the core. Therefore, the options were initially computed of such

rods arrangement where up to 50% of manually controlled rods were set 1.2 - 1.4 m

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lower than the core upper level. Besides, shortened absorber rods were left in the

core lower part. This solution was not the optimum one for maintaining uniform power

density height fields. Besides, the prerequisites were produced of positive reactivity

release in the core upper part in the considered situations of the emergency protectionsystem actuation.

In 1986, during zero-power start-ups of the Chernobyl NPP Unit No. 1 and 2,

numerous measurements were made of the emergency protection system rate

efficiency vs. the initial depth of rods insertion. These experiments resulted in the

reduction of the insertion depth up to 0.7 m to ensure acceptable rate characteristics

of the emergency protection system with negligible variations of power density height

fields.

In the first half of 1987, a pilot set of new control and protection systems was

developed and manufactured. The design of manually operated rods eliminating water

columns under the supplanter was modified and servodrives were upgraded. Tests of

new control and protection systems in operating reactors confirmed that time of rod

movement from the upper terminal to the lower one was reduced from 18 to 12 - 14

seconds and the rate efficiency of the emergency protection system at first seconds

was increased up to 0.9 /sec.

A new independent reactor shutdown system was developed practically

simultaneously with the regular reactor shutdown system upgrading. Initial

requirements for providing for 2.5 negative reactivity insertion within 2 second were

adopted for a new fast-acting emergency protection system. Based on neutronics and

thermal-hydraulic calculations, a new design of the fast-acting emergency protection

system, an independent water cooling loop and gas feeding loop were developed.

After the bench tests at RDIPE, the full-scale fast-acting emergency protection

systems were first tested at the Leningrad and Ignalina NPPs.

These tests completed a series of R&D works that were applied at all operating power

units in 1987 - 1989 with the industry and the NPP operating personnel participation.

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2.3 Metal inspection of headers and large diameter pipes

The problem of eliminating large leaks of coolant that resulted in the reactor core

drainage is not of less importance for the RBMK reactor safety than the problems of

neutronics characteristics and actuation efficiency of the reactor shutdown system.

Today, instantaneous full-section rupture of headers and large diameter (900, 800

mm) pipelines, distributing group headers and steam pipelines are taken as the

design basis accidents for NPPs of the second generation equipped with the

emergency cooldown systems, steam release localization systems, including

reinforced leaktight compartments and pressure suppression pools.

At the NPPs of the first generation (6 power units) having no safety facilities to cope

with extensive ruptures of large-sized equipment, safety justification is based on

extensive control of the equipment metal state. Therefore, after the Chernobyl

accident, control of metal and, first of all, of welded joints became more strict and

extensive. Proceeding from the results of determination of the stress-strain metal state

and experimental studies to determine a critical size of cracks causing extensive

rupture of headers and pipelines, the frequency of a complete (100%) examination of

welded joints and water pressure tests was set up to be once every four years.

During the decade since the Chernobyl accident, practically two cycles of testing were

accomplished in each power unit.

2.4 Capacity improvement of the steam discharge systems

Investigations have shown that the capacity of existed systems for steam discharge

from the reactor cavity during accidents can ensure the reactor integrity with two

pressure tubes rupture.

Despite the low probability of a simultaneous rupture of several pressure tubes, two

design options were developed to increase the capacity of steam and gas discharge

systems.

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The first option provides for the reactor cavity integrity in the case of a simultaneous

rupture of 4 pressure tubes. If ruptures occur at the interval of several seconds steam

can be removed with ruptures of a much greater number of tubes. This option does

not require a large scope of assembly works and can be realized during a unitshutdown for a medium-scale repair. This activity includes laying of an additional

pipeline of 400 mm diameter. and installation of a protection valve without any

additional reactor cavity penetrations.

The second design option ensures the reactor cavity integrity at a simultaneous

rupture of 9 pressure tubes. The option is more complicated for realization. It

demands additional vertical laying of a pipeline through the upper biological shield into

the reactor cavity and as well as laying of new pipelines outside the reactor cavity and

installation of protection valves.

This option was realized at the Smolensk NPP Unit No. 3 during its assembling and

start-up after the Chernobyl accident.

At the operating NPPs, mounting works are performed during the long idling periods

associated with the replacement of pressure tubes and other modification works.

2.5 Correction of the design documentation

The principal design documents defining the rules and basic procedures of the safe

NPP operation, a general procedure of the NPP safe operation include the "Standard

Technological Regulations for the NPP Operation with the RBMK-1000 Reactors" and

"Technological Regulations for the Ignalina NPP Operation with the RBMK-1500Reactors".

After the Chernobyl accident, standard regulations for the RBMK reactors were

developed by the reactor General Designer enterprise. The last revision of this

document was approved on December 27, 1991. Using this document, the operational

personnel works out relevant regulations for each specified power unit.

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Technological operation regulations for the Ignalina NPP was also developed

proceeding from the Standard Regulations for the RBMK-1000 but taking into account

the specific operation modes and equipment composition. The last revision of the

regulations for the Ignalina NPP was approved on June 13, 1994.

These regulations and other design documentation introduced after the Chernobyl

accident include all technical and organizational measures to improve safety of the

NPP power units with the RBMK reactors.

Measures realized in the post-accident period to improve safety of NPPs with the

RBMK reactors eliminated all design drawbacks revealed during the accident.

Besides, there are still problems of the existing NPPs reconstruction aiming to make

their technological status meet the requirements of the current normative

documentation of the RF Gosatomnadzor. All power units of the first generations are

to be subjected to the reconstruction.

The developed reconstruction concept provides first of all the following:

- Updating of control and protection systems aiming to replace the worn out

equipment, to create the multiregional control and protection systems;

- Development of fast-acting and long-operating systems of emergency water

feeding into the reactor ensuring three alternate channels;

- Creation of new systems for reliable power supply;

- Creation of new systems for emergency releases localization including additional

sealing of compartments and bubble condenser systems.

The most progress has been achieved in the reconstruction of the Leningrad NPP

Units No. 1 and 2, the first stage of reconstruction is underway at the Kursk NPP Unit

No.1.

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3 The Chernobyl "Sarcophagus" or "Shelter"

The necessity of entombment for the Unit No. 4 became clear in the first days after

the Chernobyl accident. Among 18 considered proposals, some projects suggested

the construction of a fully sealed huge building around the Unit, others suggested the

maximum use of the remained walls and other elements of the destroyed Unit to

construct a protection containment.

The second approach was selected. It gave essential saving of money and time for

the construction (design and construction were completed within 6 months that is an

unparalleled case in the world practice). However, there were still some negative

aspects: lack of full information on strength of the existing frames supporting the new

ones, the necessity to apply remote methods of concrete placement, impossibility, in

some cases, to use welding, etc. These difficulties arose due to the extensive

radiation fields nearby the destructed unit. In the final run, they led to two significant

drawbacks of the design:

- Uncertain strength of supports on which the main "Shelter" beams were laid;

- Inadequate sealing (the overall area of slots in the roof and walls is 1000 sq. m).

In November 1986, the "Shelter" above the destructed Unit No. 4 was constructed. It

marked a completion of the essential stage of works on elimination of the accident

consequences that prevented the propagation of radioactive products and enabled

the protection of the environment from a direct exposure to ionizing radiation.

Behaviour of nuclear fuel contained in the "Shelter" caused no apprehensions.

However, more information was required to ensure its long-term safe staying under

the conditions of the damaged unit and to work out plans of its ultimate storage.

By the moment of the "Shelter" construction completion, the information on the

location and state of nuclear fuel was limited by data that were obtained mostly while

investigating peripheral rooms of the Unit No. 4. The access to those rooms close to

the damaged reactor was impeded due to huge radiation fields, ruined structures and

concrete that got inside the buildings in the course of the "Shelter" construction (the

so-called "fresh concrete").

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Meanwhile, fuel in the "Shelter" could be a source of hazards of several types.

The nuclear hazard represents the occurrence of self-sustaining chain reaction as a

result of nuclear materials relocations inside the objective. According to the estimates,

the parts of the core survived after the accident and uranium-graphite assemblies

were especially hazardous. Self-sustaining chain reactions would result in additional

activity releases into the environment.

The thermal hazards lies in the fact that heated fuel can gradually melt through the

concrete roofing in the building, move down, and finally, land into the environment.

Finally, the radiation hazardrepresents radioactive dust release through the "Shelter"

slots formed after the collapse of internal destructed and displaced frames.

At the end of 1987 in order to conduct works that would ensure safety of the "Shelter",

the Complex Expedition of the Kurchatov Institute of Atomic Energy was set up in

Chernobyl by the Minsredmash. It consisted of a relatively small division of scientists

(30 to 50 persons), an efficient supplying and constructing-mounting subdivisions.

During the "peak" periods, the Expedition numbered 3,000 members.

Though the "Shelter" still remained an element of the Chernobyl NPP, principal works

on safety improvement in 1988-1991 were carried out by the above mentioned

Complex Expedition.

Following the Kurchatov Institute program, western and southern compartments of the

unit were cleaned and decontaminated. Drilling rigs were installed there to sink

boreholes through the concrete to the assumed places of nuclear fuel buildup.

Extensive investigations were performed by visual observations (periscopes, TV

cameras) and specially developed thermal and radiation detectors. At the same time,

selected samples of materials were analyzed. In the course of those works, the nature

of destructions inside the Unit No. 4 was realized and a series of structures being in a

menacing state was reinforced.

The above investigations showed that irradiated fuel inside the "Shelter" was of the

following modifications:

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- The core fragments a large part of which is supposed to be thrown up by the

explosion to the upper stories of the unit, in particular, into the central hall and is

still there under ruins of materials disposed there in 1986. High radiation fields still

prevent investigations in the central hall and adjacent compartments.

- The fine-dispersed fuel (dust) represents hot fuel particles. The sizes of those

particles vary from fractions to hundreds of microns. They are observed practically

in all compartments and soil samples taken in the near and far regions. The total

amount of fuel dust in the plant is estimated at 10 t and the amount of dust under

the entombment roofing - at 1 t (both amounts are known not better than to an

order of magnitude).

- The solidified lava-like fuel-containing masses. These masses were formed in the

active stage of the accident (04.26.86 - 05.06.86) in the course of

high-temperature interaction of fuel and the unit structural materials and spread

over subreactor compartments. Much information was acquired about lava

accumulations in the unit lower storages including their location and

physics-chemical properties. Accurate determination of the overall amount of fuel

on these storages presents difficulty due to intensive fields and concrete that filled

many compartments of the unit in the course of the "Shelter" construction. Present

estimations put it at 70 - 150 t. Affected by external factors, especially water, the

lava is subjected to rapid destruction.

- The last modification represents water soluble forms of uranium, plutonium,

americium (mostly in trace amounts, uranium of about 1 mg per liter).

Starting from 1991, the Chernobyl NPP and the "Shelter" were turned over under the

Ukraine jurisdiction. Now, the "Shelter" is a structural subdivision of the Chernobyl

NPP that is responsible for its safety as an operating organization.

On February 4, 1992 the Cabinet of Ministers of the Ukraine issued the Decree on the

establishment of the Institute of Nuclear Research of the Ukrainian Academy of

Sciences and the All-Russian Research and Design Institute of Complex Energy

Technology (VNIPIET) of the Interdisciplinary Scientific and Technical Center "Shelter"

(ISTC) on the base of the Complex Expedition the main task of which was to conductscientific and design works to transform the "Shelter" into the ecologically safe system.

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The above Center incorporated the Department of Nuclear and Radiation Safety

under scientific supervision of the Russian Research Center "Kurchatov Institute".

What are potential menaces of the "Shelter" ?

Radioactive dust release after collapse of building frames, release of radioactive water

from the "Shelter" into the natural environment; initiation of a self-sustaining chain

reaction in one of the fuel-containing accumulations and radioactivity release through

the slots.

What is the probability of these processes and what are their consequences?

The upper estimated level of radioactivity release through the object slots was 0.03 Ci

in 1990, 0.08 Ci in 1991, 0.09 Ci in 1992, 0.04 Ci in 1993, 0,03 Ci in 1994, and 0.03

Ci in 1995. The share of Pu in the total activity is within 0.4 to 1.2%.

The works conducted in 1987 - 1989 to strengthen accessible inner structures that

were severely damaged during the accident prevented so far further destruction.

No anomalies were observed in buildings subsidence. The seismic waves of 3.5 - 4

magnitude that arrived to the Chernobyl NPP region from the Rumanian earthquake

source on May 30 - 31, 1990 caused no observable destructions and displacements.

More cracks appeared in the walls inside the "Shelter".

All major supporting frames of the "Shelter" were designed and constructed in full

conformity with the building norms and rules, thus, their strength does not cause

anxiety. Their life-time is limited due to impossibility to inspect periodically and restore

anti-corrosion coatings (their service life was estimated at 30 years).

The situation with supports of basic structures is quite different. In recent years (1993

- 1995), the Ukrainian builders have done much to inspect the "Shelter" and determine

reliability of its frames.

Most anxiety is caused by the state of the supports of beams on which 27 large steel

tubes covering the central hall of the destructed unit rest. They are also carrying the

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side steel shields. Under normal climatic effects (snow, wind, temperature), a service

life of old frames supporting beams is estimated at ten years.

Roofing collapse will cause an upward motion of large air masses which will carry up a

mixture of dust and fuel particles. Estimations have shown that under such conditions

the turbulent trail can draw in about 5 t of dust containing some 50 kg of fine

dispersed fuel. The cloud of 20 m in dia. can rise as high as 100 m above the ground

surface (the building height is 60 m). According to estimates, about 20% of the

release will fall out in an aerodynamic "shadow" behind the building. "The shadow" is

about 200 m long.

Water is one more menace to the "Shelter" safety. It can

- Destruct fuel-containing masses thereby increasing the amount of radioactive dust

in the plant;

- Facilitate destruction of building frames;

- Lead to the increase of the fuel-containing mass to the critical level with time on

as it cools down and destructs, to formation of nuclear dangerous compositions.

Therefore, one of the most important tasks for the "Shelter" safety is to take measures

to reduce water access into compartments and to organize continuous monitoring of

its location, radionuclides composition, presence of dissolved fission materials and, if

necessary, taking active countermeasures.

The danger of repeated self-sustaining chain reaction in the Unit No. 4 reactor core

was a constant headache for specialists until mid-1988. It is explained by the fact that

a relatively small part of the core of the RBMK-1000 reactor (over 150 fuel channels

with graphite moderators and without absorber rods) with fuel burnt up to a level of the

Chernobyl NPP can easily "start burning" again. And it is quite probable that part

could survive the accident. However, in May 1988, after sinking wells drilled from the

decontaminated compartments via the concrete walls and other obstacles, specialists

got an opportunity to look into the interior of the cavity to prove that there was no core

stack as such.

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Then, the last question to be answered concerned the opportunity of self-sustaining

chain reactions in the fuel-containing masses. The answer was given in the "Technical

validation of the "Shelter" nuclear safety" issued in 1990 by the Kurchatov Institute.

The work summarized the experimental and estimated data available by mid-1994 onthe "Shelter" fuel-containing masses and offered conclusions on nuclear safety. A

number of scientific organizations of Russia, Ukraine and Byelorussia took an active

part in that work.

The final conclusion of the "Technical validation of the "Shelter" nuclear safety" was

as follows: "... It can be assumed that now the "Shelter" is nuclear safe". Besides,

proceeding from investigations of the dynamics of fuel-containing materials behaviour,

likely unfavourable tendencies were shown that could in the future lead to increased

fuel criticality and to the revision of the conclusions on "Shelter" safety. In the recent

five years, extensive experiments and estimations have been undertaken including

those relating to the "Shelter" nuclear safety.

Now the most dangerous scenario of the accident development is associated with fast

water flooding of fuel-containing compositions. In the absence of protective barriers,

the consequences of such accident will be irradiation of the personnel in direct vicinity

from the "Shelter" with the doses of several rem. Consideration of the protective

barriers of the "Shelter" indicates likely reduction of expected doses by one to three

orders.

In spring 1995, the Kurchatov Institute proposed the "Concept of works on the

"Shelter" in 1995 - 2000" showing the immediate realization of stabilization measures

as the main task.

It is an ungrounded optimism to plan a complete transformation of the "Shelter" or

isolation from the environment by means of the "Shelter" within a short period of 3,5

and 7 years. Therefore, the solutions of the "Shelter" problems shall include in 1995 -

2000 the following basic tasks:

- Provision of current safety;

- Provision of long-term safety ("stabilization");

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- Preparation for transformation of the "Shelter" ("Shelter"-two).

At the "stabilization" stage, measures are taken for sufficiently long-term, over 15

years, stabilization of the "Shelter" and minimization of its environmental effects. Such

measures will enable safe and thorough preparation for the "Shelter" transformation.

The "Concept of works on the "Shelter" in 1995 - 2000" contained a large-scale

program of stabilization:

- Creation of fast-acting alarm system of radioactive releases and the system to

suppress them by special devices installed around the "Shelter" and spraying the

dust suppressing composition. The system shall completely suppress the dust

released from the "Shelter" slots in the case of internal structures collapse;

- Reinforcement of building structures;

- Realization of effective measures against the ingress and harmful impact of water

in the "Shelter" compartments, increasing "Shelter" leaktightness, impregnation of

fuel-containing masses with special compositions to prevent destruction thereof,

water condensation, etc.

In doing so, the investigations inside the "Shelter" will be continued to duly detect any

unexpected events associated with the "Shelter".

The concept was adopted in general by the Ukrainian organizations, extended and

revised to issue the document "Main directions of activity to assure safety of the

"Shelter" for 1995 - 2000".

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