What Happened at That Time?
IMANAKA Tetsuji
Research Reactor Institute, Kyoto University
Kumatori-cho, Osaka, 590-0494 Japan The eve
On April 25, 1986 (Fri), at the 4th block of the Chernobyl Nuclear Power Plant (NPP) in Ukrainian
Republic of the former USSR, the procedure started to stop the reactor for maintenance for the first timesince its operation in December 1983. At the Chernobyl NPP, four RBMK-1000 type reactors (1 GWelectricity) were in operation and No. 5 and No. 6 blocks were under construction at that time.
It was in 1971 that a large project began to construct a nuclear power station on the bank of Pripyat
river, a branch of Dnepr river, located about 100 km north from the Ukrainian capital, Kyiv (Fig. 1). In
parallel with the power station, a new town named Pripyat city was constructed for station workers. The
first block became in operation in 1977.
“RBMK” is an abbreviation of the Russian term meaning “Channel-type Big Power Reactor”. From
its structure, it can be called “graphite-moderator, light-water boiling, channel-type reactor” (Fig. 2. Table1). RBMK was developed from the reactor originally constructed to produce plutonium for making Soviet
atomic bombs. Its merits are the followings: refueling is possible while the reactor is in operation,
power-upgrading is easy by attaching additional channels, inland construction is easy without difficulty oftransporting heavy structures such as pressure vessel of light water reactor and so on. Meanwhile, thefollowings are its weak points: reactor control is complicated because of a lot of power channels (1,661 in
the 4th block), vulnerable reactor characteristics that power will increase in a case of vapor increase at thereactor core (positive void reactivity coefficient), as well as power will surge in an extreme case that allcontrol rods move down together into the core (positive scram). The last two design defects are considereddirectly related with the Chernobyl accident, but operators did not know such defects [1].
A emergency generator test was planned at the time of the shutdown of the 4th block on April 25. It
was a test of a generator aimed to provide electricity to pumps at the time of blackout, using inert energy
of free-wheeling turbines [2]. At 01:00 on April 25, the process began to reduce the reactor power from thenominal value (3.2 GW thermal). At 13:05, when the power decreased to 1.6 GWt, one of two turbines
was isolated. Although it was planned to continue decreasing the reactor power, because of the order fromthe Kyiv grid center, the 4th block continued to operate at the power level of 50 %.

Fig. 1: Construction of No. 1 block at Chernobyl NPP.

-1-

Separator drum

C rod

Fuel
assembly

Water-steam two-phaseflow

Channeltube

Graphiteblock

Supply water

Primary
coolant

TurbineGenerator

Condenser
Secondarycoolant
Pump
Pump

Fig. 2: Basic scheme of RBMK
Table 1: Basic parameters of RBMK1000
Item
Power
TurbineReactor coresizeGraphite block

Reactor space
size

Fuel channel

Control rod
channel

Fuel

Fuel bundle

Coolant loop

Contents
1 GW electricity, 3.2 GW thermal (Efficiency 31.3％）
500 MW×2
Cylinder diameter: 11.8 m, height: 7.0 m
・ Primary structure is a pile of graphite moderator block.
・ Each block has a vertical hole in its center to penetrate a channel tube.
A square box of 25cm×25cm×60cm. Density:1.65 g/cm2.
・ Diameter of the hole for channel tube: 11.4cm
・ Total weight of graphite block: 1,700 ton
Cylindrical diameter: 14.52 m, height 9.75m.
・ Graphite reflector blocks are surrounding the reactor core. The metal shroud confines the reactor space,making it airtight.
・ Design pressure of the core space: 0.8kg/cm3．
・ Outside the shroud, annulus water tank of 2.4 m width, a layer of sand and then concrete shield.
・ The upper core plate (diameter: 17m, height: 3m) and the lower core plate (diameter: 14.5m, height: 2m)
have holes like a honeycomb for channels to penetrate.
Number of fuel channel: 1,661
・ Outer diameter: 88 mm, inner diameter: 80 mm.
・ Tube material: zirconium ally for the position corresponding to reactor core, welded with stainless stealtubes at upper and lower sides.
・ A fuel assembly is inserted in each fuel channel.
・ Coolant water goes up inside the fuel channel, boiling to steam.
・ Refueling is performed while operating reactor, by isolating channel tube one by one from the coolant loop.
211 channel
・ Neutron absorber: boron carbide
・ Automatic rod: 12, local automatic rod: 12, manual rod: 115, emergency rod: 24, local emergency rod: 24,
shortened rod: 24.
Uranium dioxide (UO2). Enrichment: 2 %.
・ Fuel pellet size: diameter 11.5 mm, height 15 mm.
・ Fuel rod: outer diameter 13.6 mm, length 3.5 m. Tube material: zirconium alloy, thickness 0.9 mm.
・ Amount of uranium in the core: 194 ton. Designed burnup: 20 MWD/kg
Bundle length 7 m.
・ Two sub-bundles (3.5 m) are connected up and down.
・ Sub-bundle: length 3.5 m, 18 fuel rods are fixed around the central supporting rod.
・ Uranium amount per fuel bundle: 114.7 kg
Two loops. Coolant: light water・Inlet temperature of fuel channel: 270°C.
・ Out let: 284°C, pressure 70kg/cm2, steam quality 14.5 ％.
・ 4 circulation pumps per loop (including one reserve). Totally 8 pumps.
・ Coolant flow rate: 37,600 ton/h. Steam supply: 5,800 ton/h.

The above information is mainly based on the 1986 USSR report [2].
-2-At 23:10 on April 25, theshutdown
．
．
．
．
．
．
．
． procedure was restarted. Then 00:00 on April 26, the shift of thecontrol room changed from Tregub’s team to Akymov’s team (each constitutes 4 staff). Soon after the shiftchange, when the power control system was replaced from local automatic control to automatic control,
the reactor power suddenly fell down almost to zero. The generator test was planned to be performed at
power level of 700 – 1,000 MW. If it could not be done, the next chance would be several years later.

 The 4th block exploded
Fourteen people were at the control room of the 4th block. Dyatlov, Deputy Chief Engineer was inresponsibility to carry out the test. By the order of Dyatlov, the operators tried to revive the reactor power,by pulling out almost all control rods from the reactor core. As is shown in Table 2, around 01:00, therector was somehow stabilized at the power level of 200 MWt. Then, it was decided to carry out thegenerator test at the power less than the planned.
At 01:23:04, by closing the steam valve to No.8 turbine, the generator test started using inert
energy of the free-rotating turbine. According to Dyatlov’s testimony, the reactor power was stable during
the test and there was no sign requiring operator’s attention or alarms.
The emergent event started just at 01:23:40 when the operator turned on the AZ-5 button to shut
down the reactor by inserting all control rods into the core. On the contrary to the intension of the operator,
a positive scram phenomenon happened, which led a power surge at the lower part of the core, damaging
several nuclear fuels and channel tubes. Then, following the rupture of channel tubes, a large amount ofvapor appeared at the core. A bigger-scale power surge was caused by the effect of positive void
Table 2: Event chronology (April 25 – 26, 1986)
01:00 April 25
03:47
04:13 -12:36
13:05
14:00
23:10
00:00 April 26
00:28
00:41-01:16
About 01:00
01:03 and 01:07
01:23

01:23:04

01:23:40
01:23:43
01:23:46-47
01:23:49
01:24

Shutdown procedures began by reducing the power from the nominal value (3.2 GWt).
Power decreased to 1.6 MW.
At power level of 1.6 GW, control systems and vibration characteristics were checked of No. 7 and No. 8
turbine-generators.
One (No. 7) of two turbines was detached.
ECCS system was detached. Owing to the request from the Kyiv electric center, the power was kept at 1.6
GWt.
Shutdown procedure restarted.
The control room shit was replaced from Tregub team to Akymov team.
At 500 MWt, power control system was switched from local automatic control to average automatic
control. When switching, abrupt power decrease occurred to almost zero power.
No. 8 turbine was detached. Its vibration characteristics were measured during inert rotation.
After the efforts to increase the power, the reactor somehow became stable at 200 MW. The decision wasmade to perform the emergency generator test at the level below the planned power.
Two main circular pumps (MCP) were added in operation. All eight MCP began operating.
According to the analysis after the accident, at this time the reactor was under the extremely unstablecondition because of the increase of positive void reactivity coefficient due to withdrawal of almost allcontrol rods as well as reactor operation at the low power level. The operators, however, did not knowsuch situation.
The test was started by closing the steam valve to No. 8 turbine. Coolant flow rate of 4 MCP that wereconnected to the test generator decreased to some extent, which in turn increased steam generation a little
at the core. The effect of this increase was compensated by a small increase of the pressure as well as bygradual insertion of automatic control rods. The reactor power was kept stable during the test, without any
extraordinary symptom prompting operator action or alarming signal.
Chief operator Akymov turned on the scram button (AZ-5).
Alarm of “rapid power increase” and “over power”.
Loss of electricity for MCP. Flow rate decreased. High pressure and high water level in steam separator
tank. “Control system failure” alarm.
“High pressure in the reactor space”, “Loss of electricity for control rod”, ”Failure of driving automatic
control rods” signals
The operator wrote in the log note, “01:24 strong explosion. Control rods stopped halfway before reaching
the bottom of the core. Loss of driving electricity for control rods.” According to eyewitnesses who wereoutside the reactor building, there were two sequential explosions, blowing up something like fireworks
into the night sky.

・The above sequences are mainly cited from the Steinberg report [3].

-3-

coefficient of reactivity, which led to explode the reactor and destroyed the building. According to theanalysis after the accident, the explosion occurred 6-7 seconds after pushing AZ-5. Eyewitnesses outside
the reactor building told that there were a series of explosions like fireworks up into the night sky.
Concerning the accident sequence, there are several versions. The above is based on the Steinberg reportin 1991 that reinvestigated the cause of the accident by the request of the USSR parliament [3].
It was 3 am when the first information on the accident reached the responsible person of the Soviet
Communist Party in Moscow. At 9 am in the morning, the first expert team flew from Moscow and arrived
at the scene of the accident in the afternoon. A special medical team from Moscow also arrived in theevening. They have checked patients of workers and firefighters being cared at the Pripyat hospital
because of acute radiation syndrome. They selected persons to be sent to Moscow for special treatment.
Mr. Shcherbina, Deputy Ministry of USSR, nominated as the chairman of the governmental committee for
the Chernobyl accident also arrive at Pripyat in the evening of April 26. The first tasks of the government
committee meeting held in the night of April 26 were the followings:
 To determine the method how to extinguish the graphite fire that continued in the destroyed core,discharging a large amount of radioactivity into the environment, Decision whether or not residents in Pripyat should be evacuated.
The committee decided to extinguish the fire by dropping the material such as sand, lead, boron etc.
onto the core using military helicopters. After the long discussion, by the decision of Shcherbina, theevacuation of Pripyat was scheduled on the next day.

 120 thousand evacuees
The weather was fine in Pripyat on April 26. Most residents (population 50,000) did know that something
serious event happened at the NPP. They spent, however, that day as usual Saturday. A lot of people wereat shops and even a wedding was celebrated. Some people watched the smoking 4th block from the roof of
the apartment while sunbathing (Fig. 3-4). Only few people were afraid of radiation and stayed inside theirflats closing windows.

Fig. 4. View of Sarcophagus from the roof ofan apartment in Pripyat. Photo by
Imanaka, October 2005.
Fig. 3. The 4-th reactor on the day of theaccident. Photo by Igor Kostine.

-4-

It was lucky for the people in Pripyat that the first “hot” radioactive plume released from thedestroyed 4th block did not hit directly on the city. It flowed to the west from the destroyed reactor, wherepine trees within 5 km died in several days because of strong radiation (Fig 5).
On April 27, because of the change of wind direction, radiation dose rate in Pripyat began to increase.
At 07:00, dose rate of 2 - 6 mSv/h was recorded inside Pripyat. Around noon, the following wasannounced through the local radio: “Dear citizens! Evacuation was ordered in relation with the accident at
NPP. Please take passport, indispensable materials and food for three days. Evacuation will begin at
14:00”. 1,200 buses were mobilized from Kyiv to evacuate Pripyat. 45,000 people were evacuated in twohours. Panic that the authority was afraid of did not happen. Many evacuees thought that they could comeback home in three days, but they could not restore their life in Pripyat again.
The region surrounding Chernobyl NPP was traditional rural area except Pripyat city. Although
Pripyat city were evacuated quickly on the second day, the people within the 30 km zone were left
uninformed for a while. It was on May 2, one week after the accident that the evacuation was decided ofthe people who were living in the 30 km zone other than Pripyat. Their evacuation began on May 3. In aweek about 71,000 people left their home. Compared with the case of Pripyat, the evacuation of rural
towns/villages was far more difficult. Several hundred thousands of livestock were evacuated together
with their owners. Many people reminded their experience at the time of the Nazi invasion. But, different
from the previous experience, they could not return home this time. In total 116,000 people were
evacuated from the 30 km zone around the Chernobyl NPP in two weeks after the accident.
Figure 6 indicates radiation monitoring data in settlements within the 30-km zone on May 1, 1986 [4].
The maximum of 3,306 µGy/h is seen in Krasnoe village about 6 km north from the ChNPP. The maincontamination in this direction was considered to occur on April 27-28 and Krasnoe village was evacuatedon May 3. This situation makes us easily suppose serious radiation exposure of residents staying there.
Imanaka previously estimated that some fraction of the residents in Krasnoe could receive more than 1 Gyof external dose, a criterion of acute radiation syndrome [5].
Meanwhile, according to the Chernobyl Forum report [6] that was released at a conference held by
IAEA and other organizations in September 2005 as a summary of 20-year investigation on theconsequences of the accident, the average dose of evacuees was evaluated to be about 30 mSv and theirmaximum was several hundreds of mSv.
○ Usov

131

Pripyat river○

Krasnoe○○

Shepelichi

Zimovishche

Pripyat city
Yanov St.

1

5

○

3

Krivaya gora

2○

Chistogolovka

4

5

Coolingpond
1 km

1.1st-4th blocks(1st is west side)
2. 5-6 blocks (under construction)
3. Water inlet canal
4. Water outlet canal
5. Railway for cargo
Solid line: road○○

Starocele

KopachiNorth-south distance from the Chernobyl NPP (km)

30

261
104
PogonnoeDernovichi
252
348
OrevichiRadin
696
Molochki
348

20

10

0

-10

-20

Lelev

157
Puchin104
Savichi

1044
Kulazhin
1914
Kryuki70
Grushnoe 44
70
Suvidi70

Kranosel 870
Pirki
435
Ulasi
Borshchevka
122
96
Doblyadi
96
957
3045
Denisovich 174
Mashevo
Usov
Zalese131 Khutor Les
3306
252
Khatki
Krasnoe
Benyovka
244
Rudki
261
139 148
Zimovishche
139
1053
Nov. Shepelichi
Chikalovichi
148
Chapaenka
165
Yanov
Krasnoe
870
3045
174 Nov. Krasnitsa
Krivaya Gora
1740 ChNPP
9174 Chistogolovka 1044 331
Starocele
653
Kopachi87 St. Krasnitsa
174
435
87
Lelev
Lubyanka
Koshovka
35
Stechaika
Ilintsi
26
78
61
87
174
Razezhnee
Parishev
Chernobyl
Korogod
122
35 Bichiki
96 Zalese
Lodizhichi
35
87
GlinkaCherevach78
122 Chipol
Rodinka87
44

Rudnya Veresnya

Rossokha

-30
-30

○

Fig. 5: Map just around Chernobyl NPP.

157
Vigrebnaya

-20
-10
0
10
20
East-west distance from the Chernobyl NPP (km)30

Fig 6. Dose rate in air at settlements within the
30-km zone around Chernobyl NPP on May 1,
1986 [4], unit: µGy/h.

-5- Accident liquidation and construction of Sarcophagus
Five minutes after the explosion at the 4th block, thefirst firefighter team arrived at the scene. It was firemenof the power station led by lieutenant Pravik. Five
minutes later another firemen team led by lieutenant
Kybenok arrived from Pripyat city. Pravik’s team started
extinguishing the fire on the roof of the turbine building
in order to prevent the spread of the fire. The Kybenok’s
team fought the fire at the central hall of the destroyed
4th reactor. Nobody hesitated at the fight being afraid ofFig. 7: Construction of Sarcophagus
radiation. Rather, they were not taught about danger ofradiation. They became feel sick one after another, and were carried to the hospital in Pripyat.
When the explosion happened, operators at the control room of the 4th block could not understandwhat happened at the reactor. As shown in Table 2, operator wrote in a log note, “At 1:24, a strong
explosion, control rods stopped halfway before reaching the bottom of the core”. The first priority for
operators was to prevent the reactor from destruction. Therefore, they tried to insert control rods to the fullposition and keep the core cooling by continuing pumping the coolant. In reality, however, the reactor core
was completely destroyed by the first explosions. The operators and other workers should quickly
evacuate and go to the shelter. On the contrary, as a result of useless efforts and unreasonable orders totackle the situation, radiation symptom also appeared in operators.
According to the reports from the USSR authorities, about 300 people was hospitalized, 28 out of
which died because of acute radiation syndrome. In addition, one worker became missing under the debris,
another died of severe burn on the day of the accident. Adding one more death of other reason, it wasreported that 31 people died in total by the Chernobyl accident.
The chemical detachment of the USSR army that was being trained to prepare nuclear wars arrived at
the scene on the second day, April 27. It is certain that the first step to liquidate the accident consequences
was carried out by this regular army. But, details of their works and radiation dose were yet unknown.
During three weeks of their stay, debris such as fuel rods, graphite block etc. thrown from the core andscattered on the ground around the 4th block building were cleared in order to begin the construction of“Sarcophagus” (Fig. 7).
In June, the construction began of “Sarcophagus” that would confine the whole destroyed block by
the concrete structure. “Patriotic workers” gathered to Chernobyl from all over USSR. They did heroic
works under the condition of strong radiation.

Fig. 8: Working scene of liquidators. From DVD movie “Sacrifice” [7].

-6-A large scale mobilization of reserves was also taken place for the works of decontamination of theterritory within the Chernobyl station as well as settlements within the 30 km zone. Their age was 30 – 40years old and called “liquidators”. The right photo in Fig. 8 shows a scene of clean-up work of radioactive
debris that had scattered on the roof of 3rd block. It was done by human-wave tactics at the final stage of
the Sarcophagus construction in September 1986. About 3,000 reserves were engaged in this work. The
exposure limit was set 25 Roentgen (about 250 mSv), but it is said there were a lot of cases of over
exposure.
Up to the end of 1990, the total number of liquidators amounted to 800,000 people, among which
200,000 worked in 1986 and 1987 under the condition of strong radiation.

 28 acute radiation deaths
In the evening of the day of the accident, a special medical team arrived from Moscow. Among thepatients accommodated at the hospital in Pripyat, serious ones were selected to be sent to No. 6 hospital in
Moscow. Dr. Gale, a US specialist of bone marrow transplantation came to Moscow to help the treatment.
He and his colleague did operation of bone marrow transplantation for 13 patients, but all cases could notsurvive. In spite of insensitive care at No. 6 hospital, 28 people died of radiation syndrome. Tabl 3 lists
these 28 people together with two persons who died on the day of the accident. It is noted that most of the
Name

Table 3. List of deaths by radiation syndrome
Date of
Working place
Age
death

Firefighters 6: persons
Lieutenant Pravik V.P.
NPP fire station
Lieutenant Kybenok V.M.
Pripyat fire station
Sergeant Vashchuk M.V.
do.
Staff sergeant Ihnatenko V.I.
do.
Staff sergeant Tytenok M.I.
do.
Sergeant Tyshchura V.I.
do.
Plant staff and business traveler: 24 persons
Akymov O.F.
Chief operator shift
Toptunov L.F.
Reactor operator
Kudryavtsev O.H.
Trainee operator
Proskuryakov V.V/
do.
Perevozchenko V.I.
Chief reactor engineer
Kurhuz A.K.
Reactor engineer
Khodemchuk V.I.
Machine engineer
Dehtyarenko V.M.
do.
Perchuk K.H.
Turbine engineer
Vershynun Y.A.
do.
Brazhnyk V.S.
do.
Novyk O.V.
do.
Lelechenko O.H.
Deputy of electric section
Baranov A.I.
Electric engineer
Lopachuk V.I.
do.
Shapovalov A.I.
do.
Konoval Y.I.
do.
Sytnykov V.I.
1･2 block deputy engineer
Orlov I.L.
Deputy chief of 1st block
Popov H.I.
Engineer
Savenkov V.I.
do.
Shashenok V.M.
Engineer of industrial meter
Luzhhanova K.I.
Guard
Iwanenko K.O.
do.

-7-

Remarks

23
23
27
25
26
26

May 11
May 11
May 14
May 13
May 16
May 10

33
25
28
31
39
28
35
31
33
27
29
24
47
32
25
45
44
46
41
46
28
45
59
53

May 11
15Gy
May 14
May 14
May 17
June 13
May 12
April 26
Missing inside debris
May 19
May 20> 10 Gy
July 21do.
May 14do.
July 26do.
May 7
25 Gy, dead in Kyiv
May 20
May 17
May 19
May 28
May 30
May 13June 13
Trip from Harkov
May 21
April 26 Dead by burns on the day
July 31
Entrance gate
May 26Spent fuel buildingdeaths occurred in the middle of May, 2 - 3 weeks after the accident, which indicates that the
blood-forming functions of their bone marrow were destroyed by radiation.
According to Gorbachev’s speech on May 14, about 300 firemen and plat staff were hospitalized
because of radiation syndrome. In November 1986, this number was reduced to 237 people. Then, after“reexamination of syndrome”, the current number is given to be 134 cases [8].
When the present author visited the Chernobyl museum in Kyiv, he found an exhibition of a medical
certificate that a serviceman (31 yr) of Ukrainian ministry of Internal Affairs was admitted at No.25
hospital in Kyiv on May 22 – August 12, 1986 and received bone marrow transplantation. He was again at
hospital December 1 – 31, 1986. His dose was estimated 3.2 – 3.7 Gy. Of course this case was notincluded in the official reports. We have to wonder how many such patients were who were not included
in official reports.

 Acute radiation syndrome among residents
According to official reports beginning from the USSR report [2] in 1986 up to the Chernobyl Forum
report [6] in 2005, acute radiation syndrome occurred only among station staff and firefighters who wereat the scene of the accident, but no radiation syndrome was observed among residents living around the
ChNPP. However, a number of descriptions about radiation syndrome among the residents were found in
the secret protocols of the communist party that was disclosed after the collapse of USSR [9]. In theformer USSR, The communist party was the core of its centralized power system, and its central
committee in Moscow was the summit of the power. When the Chernobyl accident occurred, a special
Table 4. Excerpts of descriptions of the health state of inhabitants from the secret protocols of theCommunist Party of the Soviet Union .
<Date of protocol><Description of the health state of people>
As of May 4, 1,882 people are hospitalized in total. Total number of examined people reached 38,000.
1986
May 4: Radiation disease of various degrees of seriousness appeared in 204 people, including 64 infants.
May 5: Total number of hospitalized people reached 2,757, including 569 children. Among them, 914 people
have symptoms of radiation disease. 18 people are in a very serious state and 32 people are in a serious
state.
May 6: As at 9:00 on May 6, the total number of hospitalized people reached 3,454. Among them, 2,609 people
are in hospital for treatment, including 471 infants. According to confirmed data, the number of radiationdisease cases is 367, including 19 children. Among them, 34 people are in a serious state. In the 6th
Hospital in Moscow, 179 people are in hospital, including two infants.
May 7: During the last day, an additional 1,821 people were hospitalized. At 10:00 May 7, the number of people
in hospital for treatment is 4,301, including 1,351 infants. Among them, diagnosis of radiation disease
was established in 520 people, including staff of the Ministry of Internal Affairs of the USSR. 34 people
are in a serious state.
May 8: During the last day, the number of hospitalized people increased by 2,245, including 730 children. 1,131
people left hospital. As at 10:00 May 8, a total of 5,415 people are in hospital for treatment, including
1,928 children. Diagnosis of radiation disease was confirmed for 315 people.
May 10: During the last two days, 4,019 people were hospitalized, including 2,630 children. 739 people left
hospital. In total 8,695 people are in hospital, including 238 cases with diagnosis of radiation disease,among which 26 are children.
May 11: During the last day, 495 people were hospitalized and 1,017 people left hospital. In total, 8,137 people
are in hospital for treatment and examination, among which 264 people with diagnosis of radiationdisease. 37 people are in serious state. During the last day 2 people died. Total number of death by theaccident amounted to 7 people.
May 12: During the last day, 2,703 people were hospitalized, most of which were in Belarus. 678 people left
hospital. 10,198 people are in hospital for treatment and examination, among which 345 people havesymptom of radiation disease, including 35 children. Since the time of the accident, 2 people perishedand 6 people died of diseases. 35 people in serious state.
remark: The total number of 40 protocols are contained in the secret document.

-8-

working group was formed at the central committee to decide basic policies to cope with the accident
consequences. The disclosed documents were protocols of the WG meetings.
Excerpts from the protocols are shown in Table 4. The numbers of deaths and serious patients seemto correspond to those of station staff and firefighters, but it is certain that a lot of radiation patients werealso among residents. For example, on May 12 when the evacuation of the 30-km zone almost finished, it
was written, “10,198 people are in hospital for treatment and examination, among which 345 people havesymptom of radiation disease, including 35 children”.
A noteworthy description is that two infants were at No. 6 hospital on May 6. In the protocol of thesame day, it was written “taking into consideration the situation that American doctors are working at No.
6 hospital, the proposal from Health Ministry was agreed that the number of patients and their condition
should be announced properly”. This means that the information would not have been released if Dr. Galeand his colleague were not at the hospital.
Meanwhile, Lupandin of Sociological Institute, Russia investigated in 1992 the remaining carte thatwas made at the time of the accident at the central district hospital, Khoyniki, Gomel region, Belarus. A
part of Khoiniki district was included in the 30 km zone (Fig. 9). Lupandin found 8 cases of radiationdiseases as well as 20 cases with some radiation symptoms. He described that more than 1,000 cases ofacute radiation diseases could have been in total at that period [10].

BelarusBorderAlienation Zone

60 km

30 km

Ch NPP

Zone Boundary

UkraineKiev reservoir

Fig. 9. Current alienation zone around Chernobyl. The areas on Ukrainian and Belarusian sides
are 2,000 and 1,700 km2, respectively. The map is made based on the figure in National
Geographic, April 2006. Basic photo is made using Google Earth.

-9- The blame was put on the operators
In August 1986 USSR government presented the report on the Chernobyl accident to IAEA [2]. Then thefirst expert meeting on the Chernobyl accident was held in August 25 – 29, 1986 at the IAEA headquarters
in Vienna. Specialists from the western countries were impressed with the frank presentation by Dr.
Legasov of the chief of the Soviet delegation, and accepted his explanation about the accident.
According to the 1986 USSR report, the reason for the Chernobyl accident was “a very rarecombination of a series of regulation violation by the operators”. Six violations pointed in the USSR
report are listed in Table 5. As a result of the combination of these violations, the reactor began runaway
during the test of emergency generator. The operator, having noticed sudden power increase, turned on thescram button, AZ-5, but it was too late to stop the reactor.
Table 5. Six violations by the operators pointed in the 1986 USSR report.
1. Reactor was operated in a condition that operational reactivity margin (ORM) was below thepermissible limit.
2. The generator test was conducted below the planned power level in the test program.

3. Coolant flow rate exceeded the limit because two additional pumps were operated.
4. Reactor trip signal for steam valve closure was bypassed.
5. Reactor trip signal for parameters in steam separator tank was bypassed.
6. ECCS signal was dispatched.
Dyatlov, Deputy Chief Engineer was responsible at the control room at the time of the accident. He
was sentenced to 10 years confinement in 1987. After being discharged from the prison earlier than theterm in 1990, he wrote an article [11] appealing, “There was noting extraordinary in the control room until
the operator switched on AZ-5. It was three seconds after pushing AZ-5 when the alarms of powerincrease appeared. Operators should not be blamed due to decrease of operational reactivity margin(ORM) because there was no instrument directly indicating ORM. Operating reactor at low power was notforbidden, but such regulation was made after the accident... The primary causes of the accident was
design defects of the reactor as well as the people who did not take countermeasures knowing such design
defects” and “The 1986 USSR report was full of lies. I can not understand why specialists of IAEA could
accept such explanation”.
It was already pointed out in May 1986 that the main causes of the Chernobyl accident likely were

Fig. 10 Control room of 4th block. October 2002.

Fig. 11 Control rod position panel of 1st block- 10 -

“positive void coefficient of reactivity” and “positive scram” [3]. It is a conventional trick by theauthorities to put the responsibility on personnel at the scene. If the cause of the accident was found to be
the design defects in RBMK, the responsibility should be asked of its designer of Academician
Alexsandrov, the president of USSR Academy of Sciences. In addition, it would become difficult tocontinue operation of 14 RBMK reactors other than the ChNPP. At the beginning of July, the official
conclusion on the cause of the accident was decided at the meeting of the central committee of the USSR
communist party where Gorbachev also participated [12].
With the progress of “Perestoroika” and “Glasnost” at the final period of USSR, intensive efforts
were made at the USSR parliament to reevaluate the consequences of the Chernobyl accident. In 1991 a
special committee nominated by the parliament released a report. It concluded that the real cause of theaccident was not the regulation violations by operators, but design defects and idleness of authorities
neglecting them. According to the report, the accident such as Chernobyl was inevitable [3].

 Positive void reactivity coefficient and positive scram
Neutrons produced by nuclear fission are with high energy (average 2 MeV), the speed of which is about
1/10 of light. Because of a small probability of high speed neurons to be caught by other uranium nuclei, it
is difficult to maintain fission chain reaction using high energy neutrons. So, in thermal reactors such as
RBMK, neutrons are designed to collide with light nuclei of “moderator” to slow down the speed. If the
distribution of neutron speed becomes equilibrium with the temperature inside the reactor, these neutrons
are called as “thermal neutrons”. The ability of thermal neutrons to cause fission reaction with uranium
nucleus is about 500 times larger than “fast neutrons”. Thermal neutrons are also captured easily by othernuclei than uranium. Therefore, thermal reactors are designed to slow down neutrons using moderator and,avoiding neutron capture by materials other than fuel, make thermal neutrons to be captured by uranium asmuch as possible. Good candidates of moderator are heavy water and graphite, while light water is worse
than them because light water itself will capture neutrons.

Full withdrawal

Full insertion
Wire

Control rod

Upper edge of the core
Wire

Graphite follower

Water column

Lower edge of the coreFig. 12. Control rod and graphite follower

Fig. 13: Power distribution before the- 11 -

Positive void coefficient of reactivity: “Reactivity” indicates a scale how fission rate is increasing or
decreasing. If reactivity = 0, the number of fission per unit time is constant. If it >0, the reactor power will
increase and if it<0, the reactor power will decrease. Reactivity depends on various factors such as
position of control rods, fuel enrichment, fuel temperature etc. “Void coefficient of reactivity” indicates
how reactivity will be effected by density change of coolant material. If it is positive, the reactor power
will increase when steam fraction at the core increases.
In the case of the Chernobyl accident, a positive value of void reactivity coefficient was exaggerated
by the facts operating the reactor at low power and pulling almost control rods out of the core. Afterseveral channel tubes were ruptured by “the first power excursion”, which made the steam fraction having
increased. Subsequently a more large power excursion was caused by the positive void coefficient ofreactivity”. It lifted up the upper structural plate tearing all channel tubes at a time, which led to anotherexcursion [1,3].
Positive scram: “Scram” means emergency shutdown of nuclear reactor. “Positive scram” is a word
made after the Chernobyl accident, reflecting an unbelievable event that the reactor power increased bypushing the scram button.
The structure of RBMK control rod is shown in Fig. 12. A graphite follower is pending under thecontrol rod that absorbs neutrons. The left part of the figure illustrates the complete pullout, making awater column under the graphite follower at the bottom of the core. The right part shows the full insertion.
Plotted data in Fig. 13 are power distribution it the core just before the accident [13]. An interesting
feature is seen that a large peak of power distribution was at the lower part of the core. The operator of the
4th block pushed the scram button (AZ-5) when almost control rods were fully pulled out. Water columns
in the left part of Fig. 12 were replaced with graphite followers, producing positive reactivity at the lower
part of the core, which was considered to cause the first power excursion [1].

 The amount of released radioactivity
Amount of radioactivity accumulated in the reactor core of 1 GWe NPP is about 1.5 × 1020 Bq, excluding
those of short half-lives. It is rather difficult task to estimate the amount of radioactivity released into theenvironment for such case as Chernobyl because radiation monitor was useless in the situation that thereactor core exploded and ruined together with the building. Several methods were elaborated to estimate
the amount of released radioactivity, for example, using ground contamination data of all over the worldfrom Chernobyl fallout. Table 6 is the radioactivity release estimated in the Chernobyl Forum report in2005 [6]. Rare gas elements such as 133Xe were released 100 % of the reactor core. Radioiodines to which
attention should be paid at the early stage were released 55 %. 137Cs, which is important at long-tem
contamination, was 30 %. Smaller fractions were seen for less volatile isotopes of 90Sr and 239Pu.
Compared with previous values given in the 1986 USSR report, these estimates were 2.8 and 2.3 times
Table 6. Estimates of radioactivity released by the Chernobyl accident（decay-corrected to 1986.4.26)
NuclideHalf life
Released activity, Bq
Ratio to core inventory
18
100 %Xenon-133
5.3 days
7 × 1018
Iodine-131
8.0 days55 %2 × 1016
Caesium-137
30 years
30 %9 × 10164.9%Strontium-90
29 years
1 × 1013
Plutonium-239
24,000 years
1.5 %2 × 1019
About 10 %＜Total release including others＞
1.4 × 10- 12 -

large for 131I and 137Cs, respectively. In total, about 10 % of the radioactivity in the core was released into
the environment beyond the territory of ChNPP.
Where is nuclear fuel?: The nuclear fuel mass loaded in the core of the 4th block was 190 ton.
According to the 1986 USSR report, about 3 % of nuclear fuel was released, while the rest remained in thecore where channel tubes, fuel tubes, graphite blocks etc were crowded together with materials of sands,
lead etc thrown down from helicopters to distinguish the fire (about 5,000 ton). Two year after the accident,
TV camera was inserted into the core through a hole bored in the side wall of the reactor cavity.
Surprisingly almost vacant space was found at the position of the reactor core (Fig. 14) [14]. 5,000 ton ofmaterial did not reach the core, but only piled up on the floor of the central hall.
Part of the reactor core was blown up and away by the explosion around the destroyed building. The
remaining fuel and channel tubes melted due to high temperature and formed materials like lava, which
moved along the floor and corridor to the lower compartments and dropped into pools (Fig.14). Most of
1,700 ton graphite in the core was considered to burn out during the fire that continued about 10 days.
Interestingly, as can be seen in Fig. 14, a concrete panel of the building wall was on the bottom of thereactor cavity. Considering the size of the concrete panel, it was supposed that this panel fell down into thecavity while the upper core structure plate (2,000 ton) was blown up in the air by the explosion.
Strong radiation as well as additional layers of concrete pored during the construction of Sarcophagus
is still preventing detailed investigation inside Sarcophagus. A current estimate for the amount of nuclear
fuel remaining inside Sarcophagus is about 60 % (± 20 %) of uranium in the core at the time of theaccident.
Separation tank
Upper structure
plate

Torn fuel channels

Outlet channel
pipes
Caughtconcrete plate

Water shield tank

Fallenconcrete panel

Lower structure
plate: 1/4 melted“Elephant leg”
Lava-likematerial
Pipes to steam
condensation poolFig. 14. Cross sectional view of the destroyed reactor.
- 13 -

 High level contamination at 200 km away was disclosed 3 years later
The Chernobyl accident happened in the midst of the cold war between USA and USSR. In USSR Mr.
Gorbachev appeared in March of the previous year as the General Secretary of Communist Party, andbegan to propose two slogans of Perestoroika (reconstruction) and Glasnost (openness), but innatecharacteristics of the communist acquired for the past 70 years hardly changed. Citizens, even scientists,
were forbidden to talk freely about the consequences the Chernobyl accident.
It was in spring of 1989, about three years after the accident that the situation seemed to changed
with enlargement of the movement seeking for democracy in USSR. A 137Cs contamination map around
Chernobyl was published in a newspaper in Belarus for the first time. Although previous contamination
maps reported by USSR specialists were limited to the area just near Chernobyl, the new map indicated ashockingly wide-scale of contamination. As can be seen in Fig. 15, there were spreading of strong
contamination detachedly at distances 200 – 500 km from Chernobyl [15].
Various kinds of radionuclides will be released by reactor accidents. Iodine-131 (half life: 8 days) is
important at the early stage after the accident, which irradiates specifically thyroid grand whenincorporated into the body. From the point of long-term contamination, 137Cs is the most importantbecause of its long half life (30 yr), high volatility and transportability, and accessibility to foodstuff. A
vast area was contaminated by 137Cs in Ukraine, Belarus and Russia. (Table 7) [16].
In July 1989 the Belarus parliament, which began to criticize the Moscow government requesting
countermeasures for the Chernobyl consequences, decided to relocate 110,000 residents from thecontaminated territories. At the end of 1991, however, the central USSR government that should takeprimary responsibility of the Chernobyl accident disappeared. Then, the responsibility of countermeasuresand compensation was transferred on the shoulder of newly independent governments. Each affected
republic independently established laws for countermeasures and compensation from the damages by the
Chernobyl accident.
Moscow

Belarus
RussiaMogilev

Minsk
Bryansk

Gomel

400km
Cs137 contamination
37〜185 kBq/m2

500km

100km
Kiev

185〜555 kBq/m2

200km

300km

Ukraine555 kBq/m2以上

Fig. 15. Cesium-137 contamination map around Chernobyl.

- 14 -

600km

Table 7. Areas contaminated with 137Cs in three affected countries, km2.

Level of 137Cs density（kBq/m2)
37 ∼ 175175 ∼ 555

555 ∼ 1,440> 1,440

RussiaBelarus
Ukraine48,80029,90037,2005,720
10,200
3,200

2,100
4,200
900

300
2,200
600

56,92046,50041,900Total

115,900

19,120

7,200

3,100

145,320>37 total

According to Chernobyl laws in these countries, contaminated territories are classified by the levelof caesium-137 contamination on the ground as follows:¾ > 1,440 kBq/m2 :zone of alienation,¾ 555〜1,440 kBq/m2 :zone of obligatory resettlement,¾ 175〜555 kBq/m2 :zone of guaranteed voluntary resettlement,¾ 37〜175 kBq/m2 :zone for radiation control.

 Almost all northern hemisphere was contaminatedIn the morning of April 28, 1986, an alarm of radiation monitor sounded at the Forsmark NPP located in
the southern part of Sweden, 1,200 km away from Chernobyl. Radioactivity leakage from the Forsmark
plant was suspected, but no extraordinary was founded. In this morning radiation level also increased atother nuclear facilities in Sweden. Radioactivity likely came from the USSR territory passing over theBaltic Sea. It was 9:00 pm on April 28, responding the query from the Swedish government, that TASS
News Agency in Moscow reported a short announcement about the accident at the Chernobyl NPP.
Table 8. Area of137

2

Cs contamination in European countries (excluding Ukraine, Belarus and Russia): km
Area
Level of 137Cs contamination, kBq/m2
Country
（km2）
10〜20
20〜37
37〜185
Sweden450,000
31,00033,00023,000Finland337,000
32,00059,00019,000Bulgaria
111,000
27,50040,4004,800
Austria
84,00028,00025,00011,000
Norway324,000
44,00023,0007,200
Rumania
238,000
54,00013,0001,200
Germany
366,000
29,00014,000320
Greece132,000
21,0008,300
1,200
Slovenia
20,000
8,100
8,700
610
Italy
301,000
15,0007,000
1,400
Moldova
34,00019,0001,900−
Switzerland
41,000
6,400
2,300
730
Poland313,000
10,0003,500
520
Hungary
93,000
5,200
230
−
U.K.
240,000
15,0001,700
160
Estonia
45,0001,700
280−
Litania
65,00050−−
Chex79,00013,0003,500
210
Slovakia
20,0006,800
800
20Croatia
56,0001,100
30
20France550,000
1,200−−

Remark: The level due to previous nuclear tests is 2 – 3 kBq/m2.

- 15 -

The fire at the Chernobyl 4th block continued about 10 days, releasing a large amount of radioactivity.
The first radioactive plume from ChNPP moved to the north-west direction, passing over the territories ofBelarus, Latonia and the Baltic Sea, and then arrived at Scandinavia. The second plume went to the west
direction over Belarus and Poland, which then reached Austria and Switzerland at the end of April. Table 8shows 137Cs contamination in European countries [17]. High level of the contamination was observed in
the Scandinavian countries and the Alpine countries, where rain occurred with the passage of theradioactive plumes. Simply saying, the average level of 137Cs contamination in European countries wasequal to the sum of the past fallout contamination by all atmospheric nuclear tests in 50s and 60s.

 Radioactive fallout in Japan
The present author clearly remembers that it was in the morning of April 29,1986, the holiday for theprevious Emperor’s birthday that for the first time he heard the unfamiliar name of “Chernobyl”. TV news
told that something serious NPP accident occurred at “Chernobyl”, while the details were unclear. The
Chernobyl news became bigger and bigger with time, reporting radioactive contamination in various
European countries. Japanese meteorologists told in TV rather negative opinions about whether or notradioactivity would come to Japan, traveling 8,000 km of the distance from Chernobyl.
Hearing the news of radioactive contamination in European countries, Imanaka and his colleague,
which were used to radioactivity monitoring around nuclear facilities in Japan, half in doubt prepared toobserve radioactive fallout from Chernobyl at their institute in Osaka. The first radioactivity was observedin rainwater that was sampled in the evening of May 3. A clear gamma-ray peak of 361-keV specific to 131I
was seen by gamma-ray spectrometry using Ge detector. Figure 16 indicates gamma-ray spectrum of air
filter sampled on May 5. A series of fission products can be seen: 131I, 132I, 132Te, 134Cs, 136Cs, 137Cs,
103
Ru… Seeing this spectrum, Imanaka was surprised, asking himself, “Can we breathe this air?” He
quickly calculated concentrations and, comparing them with permissible levels, said to himself, “It’sirritating, but we can not live without breathing air”. Iodine-131 concentration at that time was 0.8 Bq/m3[18]. If an infant breathed this air for one day, thyroid dose from inhaled 131I would be 0.01 mSv using a

Fig. 16. Gamma-ray spectrum of air filter sampled May 5, 1986 at Research Reactor
Institute, Kyoto University in Kumatori, Osaka.

- 16 -

Fig. 17. Monthly deposition of 137Cs and 90Sr observed at Meteorological ResearchInstitute, 1955 - 2005．

breathing rate of 3 m3/day and a dose conversion coefficient of 3.7×10-3 mSv/Bq. This value seemed not
so high to be nervous and not so low to be negligible.
The same level of fallout contamination was observed through the whole territory of Japan. The
maximum 131I concentration was 500 Bq/l in rainwater and 25 Bq/l in cow milk. The average deposited
amount of 137Cs was 200 Bq/m2. Figure 17 is the deposition trend of 137Cs and 90Sr observed at themeteorological research institute for the past 50 years in Japan [19]. The 137Cs deposition from Chernobyl
was about 3 % of the total deposition from nuclear tests. Estimates of the average dose for the first year
after the accident in Japan is shown in
Table 9. Average radiation dose in Japan by Chernobyl
fallout during the first year after the accident. mSv
Table 9 [20]. Compared with annual
AdultsChildrennatural background dose of 1 mSv, theExternal exposure: whole body
0.003
0.003
whole body dose from Chernobyl could beInternal exposure: whole body
0.001
0.006
negligible, but some attention should beInternal exposure: thyroid
0.15
0.5
put on thyroid doses.

 Chernobyl sufferers
Almost all people who were on the northern hemisphere at the time of the accident received someradiation from Chernobyl. Of course the contamination around Chernobyl was predominant. Chernobyl
sufferers can be classified as listed in Table 10. Other than the total body dose, evacuees and inhabitantsreceived 10 – 100 times lager dose to thyroid from incorporation of 131I.
Table 10 Category of Chernobyl sufferers

Category
Population
A. Staff of NPP and firefighters who were at the scene.
1,000∼2,000
B. Liquidators (military, construction workers etc)
600,000∼800,000
C. Evacuees from the 30km zone
120,000
D. Inhabitants of highly contaminated areas and resettlers
250,000∼300,000
2E. Inhabitants of contaminated areas (>37 kBq/m )6 million*; The present author considers that this value is underestimated.
- 17 -

Total body dose1∼20 Sv
0.1∼1 Sv
Average 30 mSv*
Average 50 mSv
Average 10 mSv

 Cancer deaths and indirect effects
As a result of 20-years of investigations on the consequences of the Chernobyl accident, Chernobyl Forumconcluded that the total number of deaths due to the accident was 4,000 people, including the future
cancer deaths [6]. Following this conclusion, mass media in the world announced “The true effects of theChernobyl accident was found to be far smaller than those previously considered.” The breakdown of
4,000 deaths is as follows: 60 deaths so far confirmed and 3,940 cancer deaths estimated by a model
calculation among 200,000 people of liquidators in 1986 and 1987, 120,000 evacuees from the 30-km
zone and 270,000 inhabitants in heavily contaminated areas.
This conclusion of the Chernobyl Forum was criticized by specialists from Ukraine and Belarus aswell as by the Belarusian government. In addition, WHO [21] and IARC (International Agency for
Research on Cancer) [22] published their own estimates that were several times larger than that of theChernobyl Forum.
Table 11 summarizes various estimates of total cancer deaths due to the Chernobyl accident. The
Chernobyl Forum gives the lowest estimate, while the highest estimate by Greenpeace [24] is more than
20 times larger than that by Chernobyl Forum. This difference reflects the fact that the number of cancer
deaths largely depends on the risk model and the size of population used by the evaluator. Considering
uncertainty of estimates, the present author considers that a total of 20,000 – 60,000 cancer deaths seem tobe a reasonable one.
Table 11. Various estimates of cancer deaths due to the Chernobyl accident
Cancer death
Evaluator
Cancer deaths
Population
risk per 1 Sv
Chernobyl Forum (2005) [6]3,940
600,000
0.11
WHO (2006) [21]9,000

7.4 million in three countries

0.11

IARC (2006) [22]16,000570 million in Europe

0.1

NGO Kiev conference(2006) [23]30,000〜60,000

Whole world

0.05〜0.1

Greenpeace (2006) [24]93,000Whole world−

One comment should be added about the health effects of the Chernobyl accident. Through theexperience of the present author who has been involved in the study of the Chernobyl consequences formore than twenty years, it became clear that radiation effect is merely one aspect of the huge catastrophethat the Chernobyl accident brought upon. He thinks that more attention should be paid to the effects notdirectly related to radiation exposure. For example, it can be easily imagined what an adverse change oflife would arise when the old people who had been quietly living in rural areas were suddenly obliged toevacuate to a big city such as Kyiv. Some evacuees who lost their jobs might become alcoholics in despairof their future. These cases should be recognized as indirect effects of the Chernobyl accident.
According to Scherbak Yu who gave a lecture about Chernobyl at the institute of the present author
in April 2006, there were 17,000 families in Ukraine, the death of whose householder were admitted to becaused by the Chernobyl accident and receiving special privileges for it. This number suggests thatindirect deaths could be far more than the direct deaths from radiation exposure.
The present author is sure that scientific approach is effective to reveal what happened. However,considering the areas is limited where the scientific approach is effective, he is also sure that ourimagination should be trained in order to understand the whole picture of the Chernobyl accident.

- 18 -

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the Chernobyl accident”, Gijutsu-to-Ningen, May 2005 (in Japanese).
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чернобыльской АЭС 26 апреля 1986г”, 17.01.1991.
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http://www.dissident-media.org/infonucleaire/sacrifice.html．
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RBMK-1000”, Nuclear Science and Engineering, 103:289-293, 1989.
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UN Chernobyl Forum Expert Group "Health", 2006
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Future’ Conference”, Kyiv, April 23-26, 2006.
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- 19 -