Thursday, April 25, 2013

The Chernobyl Disaster : The World's dam surges may have caused the unprecedented power surges

THE CUMULATIVE DYNAMIC NUCLEAR EFFECT OF DAMS MAY HAVE BEEN THE INITIATOR OF THE CHERNOBYL DISASTER LIKE FUKUSHIMA
Read this is conjunction with the wikipedia report:
http://en.wikipedia.org/wiki/Chernobyl_disaster
© 2016 Ramaswami Ashok Kumar

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"They scare us, that even our water you can't drink. But how can you do without water? Every person has water inside her. There's no one without water. Even rocks have water in them. So, maybe, water is eternal? All life comes from water. Who can you ask? No one will say. People pray to God, but they don't ask him. You just have to live.
Anna Petrovna Eadaeva, re-settler  

In SVETLANA ALEXIEVICH . 1996. VOICES FROM CHERNOBYL p51.



On 26 April 1986, at 01:23 (UTC+3)/wiki/Moscow_Time), reactor four
suffered a catastrophic power increase, leading to explosions in its
core. This dispersed large quantities of radioactive fuel and core
materials into the atmosphere and
ignited the combustible graphite moderator. The
burning graphite moderator increased the emission of radioactive
particles, carried by the smoke, as the reactor had
not been encased by any kind of hard containment vessel. 

The accident occurred during an experiment
scheduled to test a potential safety emergency core cooling
 feature, which took place during a normal shutdown procedure.



      Steam turbine tests

An inactive nuclear reactor continues to generate a significant amount
of residual decay heat. In an initial shut-down state
(for example, following an emergency SCRAM) the reactor
produces around 7 percent of its total thermal output and requires
cooling to avoid core damage. RBMK reactors, like those at Chernobyl, use water as coolant. Reactor 4 at Chernobyl consisted
of about 1,600 individual fuel channels; each required a coolant flow of
28 metric tons (28,000 liters or 7,400 US gallons) per hour.
Since cooling pumps require electricity to cool a reactor after a SCRAM,
in the event of a power grid failure, Chernobyl's reactors had three
backup diesel generators; these could start up
in 15 seconds, but took 60–75 seconds to attain full speed and reach the 5.5‑megawatt(MW) output required to run one main pump.
To solve this one-minute gap, considered an unacceptable safety risk, it
had been theorised that rotational energy (as it wound down under residual
steam pressure) could be used to generate the required electrical power.
Analysis indicated that this residual momentum and steam pressure might
be sufficient to run the coolant pumps for 45 seconds, bridging the gap between an external
power failure and the full availability of the emergency
generators.
This capability still needed to be confirmed experimentally, and
previous tests had ended unsuccessfully. An initial test carried out in
1982 showed that the excitation </wiki/Excitation_(magnetic)> voltage of
the turbine-generator was insufficient; it did not maintain the desired
magnetic field </wiki/Magnetic_field> after the turbine trip. The system
was modified, and the test was repeated in 1984 but again proved
unsuccessful. In 1985, the tests were attempted a third time but also
yielded negative results. The test procedure was to be repeated again in
1986, and it was scheduled to take place during the maintenance shutdown
of Reactor Four.^[22] <#cite_note-NV_Karpan:_312.E2.80.9313-23>

The test focused on the switching sequences of the electrical supplies
for the reactor. The test procedure was to begin with an automatic
emergency shutdown. No detrimental effect on the safety of the reactor
was anticipated, so the test program was not formally coordinated with
either the chief designer of the reactor (NIKIET) or the scientific
manager. Instead, it was approved only by the director of the plant (and
even this approval was not consistent with established procedures).^[23]
<#cite_note-insag7-24>

According to the test parameters, the thermal output of the reactor
should have been /no lower/ than 700 MW at the start of the experiment.
If test conditions had been as planned, the procedure would almost
certainly have been carried out safely; the eventual disaster resulted
from attempts to boost the reactor output once the experiment had been
started, which was inconsistent with approved procedure.^[23]
<#cite_note-insag7-24>


The Chernobyl power plant had been in operation for two years without
the capability to ride through the first 60–75 seconds of a total loss
of electric power, and thus lacked an important safety feature. The
station managers presumably wished to correct this at the first
opportunity, which may explain why they continued the test even when
serious problems arose, and why the requisite approval for the test had
not been sought from the Soviet nuclear oversight regulator (even though
there was a representative at the complex of 4 reactors).
The experimental procedure was intended to run as follows:

 1. The reactor was to be running at a low thermal power level, between 700 MW
    and 800 MW.
 2. The steam-turbine generator was to be run up to full speed.
 3. When these conditions were achieved, the steam supply for the
    turbine generator was to be closed off.
 4. Turbine generator performance was to be recorded to determine
    whether it could provide the bridging power for coolant pumps until
    the emergency diesel generators were sequenced to start and provide
    power to the cooling pumps automatically.
 5. After the emergency generators reached normal operating speed and
    voltage, the turbine generator would be allowed to freewheel down.


      Conditions prior to the accident



The conditions to run the test were established before the day shift of
25 April 1986. The day shift workers had been instructed in advance and
were familiar with the established procedures. A special team of
electrical engineers was present to test the
new voltage regulating system. As planned, a
gradual reduction in the output of the power unit was begun at 01:06 on
25 April(22:06 UTC on 24 April 1986), and the power level had reached 50% of its nominal 3200 MW
thermal level by the beginning of the day shift(10:06 hrs on 25 April 1986, UTC or 13:06 hrs local time).

At this point, another regional power station unexpectedly(Perhaps because of dam surge like at Chernobyl) went offline,
and the Kiev  electrical grid controller requested that the further reduction of Chernobyl's output be
postponed, as power was needed to satisfy the peak evening demand. The
Chernobyl plant director agreed, and postponed the test at 11:00 hrs UTC or 14:00 hrs local time for nine hours.

Herein lies another effect, that due to dams that may have played a sinister plot. 


See Table CHDams for the derivation and details. The earthquakes are caused by dams and reveal data about dam content changes and the forces they apply. The analysis is in terms of the acceleration caused by the dam caused forces acting on a free standing nuclear unit, Unit No.4. This gives a ball park in which to view the cause after despecialising one's vision to see the holistic truth, applying the precautionary Principle.




At 23:04, the Kiev grid controller allowed the reactor shut-down to
resume. This delay had some serious consequences: the day shift had long
since departed, the evening shift was also preparing to leave, and the
night shift would not take over until midnight, well into the job.
According to plan, the test should have been finished during the day
shift, and the night shift would only have had to maintain decay heat
cooling systems in an otherwise shut down plant.
The night shift had very limited time to prepare for and carry out the
experiment. A further rapid reduction in the power level from 50% was
executed during the shift change-over. Alexander Akimov
</wiki/Alexander_Akimov> was chief of the night shift, and Leonid
Toptunov was the operator responsible for the reactor's operational
regimen, including the movement of the control rods. Toptunov was a
young engineer who had worked independently as a senior engineer for
approximately three months.
The test plan called for a gradual reduction in power output from
reactor 4 to a thermal level of 700–1000 MW.^[25] <#cite_note-27> An
output of 700 MW was reached at 00:05 on 26 April. However, due to the
natural production of xenon-135 </wiki/Xenon-135>, a neutron absorber
</wiki/Neutron_absorber>, core power continued to decrease without
further operator action—a process known as reactor poisoning
</wiki/Iodine_pit>. As the reactor power output dropped further, to
approximately 500 MW, Toptunov mistakenly inserted the control rods too
far—the exact circumstances leading to this are unknown because Akimov
and Toptunov died in the hospital on May 10 and 14, respectively. This
combination of factors rendered the reactor in an unintended
near-shutdown </wiki/Shutdown_(nuclear_reactor)> state, with a power
output of 30 MW thermal or less. See Figure 1 above: The world's dams may have already inserted the rods too far down!


The reactor was now only producing around 5 percent of the minimum
initial power level established as safe for the test.^[23]
<#cite_note-insag7-24> ^:73 Control-room personnel consequently made the
decision to restore power by extracting the majority of the reactor
control rods to their upper limits.^[26] <#cite_note-28> Several minutes
elapsed between their extraction and the point that the power output
began to increase and subsequently stabilize at 160–200 MW (thermal), a
much smaller value than the planned 700 MW. The rapid reduction in the
power during the initial shutdown, and the subsequent operation at a
level of less than 200 MW led to increased poisoning
</wiki/Reactor_poisoning> of the reactor core </wiki/Reactor_core> by
the accumulation of xenon-135.^[27] <#cite_note-nf-29> ^[28]
<#cite_note-30> This restricted any further rise of reactor power, and
made it necessary to extract additional control rods from the reactor
core in order to counteract the poisoning.




The operation of the reactor at the low power level and high poisoning
level, was accompanied by unstable core temperature and coolant flow,
and possibly by instability of neutron flux. Various alarms started
going off at this point. The control room received repeated emergency
signals regarding the levels in the steam/water separator drums, and
large excursions or variations in the flow rate of feed water, as well
as from relief valves </wiki/Relief_valve> opened to relieve excess
steam into a turbine condenser </wiki/Condenser_(heat_transfer)>, and
from the neutron power controller. In the period between 00:35 and
00:45, emergency alarm signals concerning thermal-hydraulic
</wiki/Thermal-hydraulic> parameters were ignored, apparently to
preserve the reactor power level. Emergency signals from the reactor
emergency protection system (EPS-5) triggered a trip that turned off
both turbine-generators.^[29];#cite_note-31;


After a while, a more or less stable state at a power level of 200 MW
was achieved, and preparation for the experiment continued. As part of
the test plan, extra water pumps were activated at 01:05, Chernobyl local time on 26 April,
increasing the water flow. The increased coolant flow rate through the
reactor produced an increase in the inlet coolant temperature of the
reactor core, which now more closely approached the nucleate boiling
</wiki/Nucleate_boiling> temperature of water, reducing the safety
margin/wiki/Factor_of_safety.

AT 1:05+ local Chernobyl time on 26 April 1986 or 22 hrs 05 mins UTC on 25 April 1986, 

a dam initiated surge wave passed the Chernobyl power plant
aggravating the instability of Reactor No.4 where the test was to start(TABLE CHDS1986).
This surge wave may have heated the steel shroud housing the reactor core
greatly increasing the reactor temperature and also severely stressing the entire structure. See nuclear effect in causing
earthquakes:
http://glaringlacuna.blogspot.com/2012/01/nuclear-effect-in-causing-earthquakes.html

In Tables below, A is date, B is Type of Occurrence, C,D,E are UTC time: Hr,Min,Sec;
F,G ,H are latitude, Longitude and depth of quake;I is magnitude,J is type of magnitude;
K is USGS ID of quake; the rest are defined in the Tables except for column P which is explained below.
In between the time at which the Damquake at Sr. No 1 occurred and the time at which the damquake at Sr. No 2 occurred, the Chernobyl catastrophe started(and is still ongoing). The time elapsed between these two quakes is given in col. P at Sr. No 3 as 2.116 hrs. To get the time at which the dam dynamic disequilibrium surge wave hit Chernobyl Reactor No. 4 , this time duration is multiplied by the ratio (distance between the first damquake and Chernobyl/distance between the two damquakes) or 2.116*(3150/6567) = 1.015 which appears in Col. P at row Sr. No 2. This is added to the time at which the first damquake occurred to get the time at which the dam surge hit Chernobyl  as 22.089 hrs UTC and is shown in col. R at row Sr. No. 2. This works out to 22 hrs 5.37 minutes UTC on 25th April 1986. See similar dam related occurrences also at Collaterals of Climate Change by Google search.
 

                                  
Earthquakes around Chernobyl as around the World dance in synchronism with one another as they are caused by the surge waves of water moment unleashed from instant to instant by the instantaneous simultaneous sum of water content changes behind the World's dams. These earthquakes around Chernobyl as well as in the World correlate significantly with World dam capacity and the changes in reservoir contents as a study for the hydrological years 1973-74 to 2013-14  proves. For the 41 years from 1973-74 to 2013-14, the correlation coefficient for annual mean number of earthquakes 650 km around Chernobyl with the sum of annual dam content changes is 0.51, df 39, p 0.00067 and with the World Annual Dam Capacity is 0.59,39,0.00004. The figures for the world earthquakes during this period: 0.949,df 39, p 1.0x10^-21  and 0.962,39, p  3 x 10^-23.
Now have a look at the dirge for the dammed played out by the dams of the world for Chernobyl and the World:






The magnitude of the dam surge that  probably aided  this Chernobyl disaster is a 1 second power surge of 700000 terawatts! See Table below:


The calendar years 1985 and 1986 each was a year of severe dam surges caused by copious rainfall in the India-China region and the World's dams exerted nuclear effect water moment surges and Chernobyl was passing through such a severe phase of stress build up and breakdown into damquakes: Extracts from USGS Significant Earthquakes 1985 and 1986 Reports
http://earthquake.usgs.gov/earthquakes/eqarchives/significant/sig_1985.php and
http://earthquake.usgs.gov/earthquakes/eqarchives/significant/sig_1986.php

August 23 1985 saw a 7.3 MM magnitude major damquake in Southern Xinjiang,China(71 people killed) followed by great 8.1 MM magnitude damquake in Mexico:
SEP 19   13 17 47.3  18.190 N  102.533 W   28    6.8  8.1  1.3  311  MICHOACAN, MEXICO.  Ms 7.9 (BRK), 7.9 (PAS).  
             Mo=1.1*10**21 Nm (HRV).  At least 9,500 people were 
  killed, about 30,000 were injured, more than 100,000 people were left homeless, and severe damage was caused in 
  parts of Mexico City and in several states of central Mexico.  According to some sources, the death toll from 
  this earthquake may be as high as 35,000. It is estimated that the quake seriously affected an area of 
  approximately 825,000 square kilometers, caused between 3 and 4 billion U.S. dollars of damage, and was felt by 
  almost 20 million people.  Four hundred twelve buildings collapsed and another 3,124 were seriously damaged in 
  Mexico City.  About 60 percent of the buildings were destroyed at Ciudad Guzman, Jalisco.  Damage also occurred 
  in the states of Colima, Guerrero, Mexico, Michoacan, Morelos, parts of Veracruz and in other areas of Jalisco.  
The maximum Modified Mercalli intensity was IX at Mexico City, Ciudad Guzman and the Pacific Coast towns of 
  Lazaro Cardenas, Ixtapa and La Union.  Felt reports were received from Mazatlan, Sinaloa to Tuxtla Gutierrez, 
  Chiapas, and as far away as Guatemala City, Guatemala and Houston, Texas.  The quake was also felt at 
  Brownsville, McAllen, Corpus Christi, Ingram and El Paso, Texas.  It was felt very strongly by people on 
  board the ship "Nedlloyd Kyoto" located at 17 35.4' North, 102 36.9' West.  

         Landslides caused damage at Atenquique, Jalisco and near Jala, Colima.  Rockslides were reported along the 
  highways in the Ixtapa area and sandblows and ground cracks were observed at Lazaro Cardenas.

         A tsunami was generated which caused some damage at Lazaro Cardenas, Zihuatenejo and Manzanillo.  Estimated 
  wave heights were 3 meters at Zihuatenejo and 2.8 meters at Lazaro Cardenas.  Tide stations recorded maximum 
  wave heights (peak-to-trough) of 1.4 meters at Acapulco, Mexico; 60 cm at La Libertad, Ecuador; 58 cm at 
  Acajutla, El Salvador; 24 cm at Kahului, Hawaii and at Pago Pago, American Samoa; 22 cm at Hilo, Hawaii; 21 
  cm at Baltra Island, Galapagos; 14 cm at Apia, Samoa; 7 cm at Rikitea, Gambier Islands; and 5 cm at Papeete, 
  Tahiti.  There were some reports, still unconfirmed, that some ships off the Pacific coast of Mexico observed 
  unusually heavy seas up to 30 meters high near the time of the earthquake.  

         Seiches were observed in East Galveston Bay, Texas and in swimming pools in Texas, New Mexico, Colorado and 
  Idaho.  Water well fluctuations were recorded at Ingleside, Texas; Santa Fe, New Mexico; Rolla, Missouri; 
  Hillsborough County, Florida; and Smithsburg, Maryland.

         A large percentage of the buildings which were damaged in Mexico City were between 8 and 18 stories high, 
  indicating possible resonance effects with dominant two-second period horizontal ground accelerations which 
  were recorded in the area.
Sept 21 and Sept 26 1985 also saw major earthquakes(damquakes):

SEP 21   01 37 13.4  17.802 N  101.647 W   31    6.3  7.6  1.2  344  NEAR COAST OF GUERRERO, MEXICO.  Ms 7.2 (BRK), 
             7.5 (PAL).  Mo=2.4*10**20 Nm (GS). Mo=2.5*10**20 NM 
             (HRV).  Additional casualties and damage (VI) in 
             the Mexico City area.  Felt in many parts of central 
             Mexico.  Local tsunami recorded at Acapulco with 
             maximum amplitude (peak-to-trough) of 1.4 meters.  
             Water well fluctuations recorded at Santa Fe, 
             New Mexico.  The focal mechanism is poorly 
             controlled and corresponds to reverse faulting.

SEP 26   07 27 51.1  34.693 S  178.656 W   52 D  6.3  7.0  1.1  448  SOUTH OF KERMADEC ISLANDS.  Ms 7.0 (BRK), 6.8 (PAS), 
             6.8 (PAL).  Mo=2.5*10**19 Nm (GS).  Mo=2.4*10**19 Nm
             (HRV).  Felt on Raoul Island.  Also felt in the 
             eastern and southern parts of North Island and at 
             Christchurch and Dunedin, South Island, New Zealand.

Further major damquakes occurred in November:
NOV 17   09 40 21.2   1.639 S  134.911 E   10 G  6.0  7.1  1.4  179  WEST IRIAN REGION.  Ms 6.9 (PAS), 6.8 (BRK).
             Mo=5.1*10**19 Nm (GS).  Mo=4.9*10**19 Nm (HRV).
             Damage at Manokwari.  Felt strongly in many parts 
             of West Irian.  The focal mechanism is moderately
             well controlled and corresponds to strike-slip
             faulting.

NOV 28   02 25 42.3  14.043 S  166.240 E   33 N  6.0  7.0  1.0  336  VANUATU ISLANDS.  Ms 7.2 (BRK), 6.5 (PAS).
             Mo=2.6*10**19 Nm (GS).  Mo=3.0*10**19 Nm (HRV).
             The focal mechanism is poorly controlled and
             corresponds to normal faulting.

NOV 28   03 49 54.1  13.987 S  166.185 E   33 N  6.3  7.1  1.1  302  VANUATU ISLANDS.  Ms 7.6 (BRK), 6.1 (PAS).
             Mo=3.7*10**19 Nm (GS).  Mo=3.6*10**19 Nm (HRV)
 In December a major damquake:

DEC 21   02 46 33.2  14.092 S  166.654 E   33 N  5.8  6.5  1.1  240  VANUATU ISLANDS. Mo=7.1*10**18 Nm (HRV).
THEN ON APRIL 26 1986(Chernobyl local date) with Mother Earth heavily stressed by damquakes the Chernobyl accident occurred.
Just after the nuclear effect damage in Chernobyl(continuing), Mexico experienced a major 7.0 MM damquake on April 30,1986. This was followed by a 7.7 MM major damquake in Andreanof  Islands,Aleutian Isles on May 7, 7.1 MM major damquake in Papua New Guinea on June 24, a 7.2 MM major damquake on Aug 14 in Molucca passage, on Oct 20 a great 8.1 MM damquake in Kermadec Islands Region, and  a 7.8 MM magnitude damquake in Taiwan on Nov 14, all with significant loss of lives and or damage to infrastructure. Similar to the Narora nuclear effect in 1993, was the event in 2007(July 16) at Kashiwasaki Kariwa of Nuclear effect of the damquakes(continuing) and at Fukushima in  2011(continuing). Note all these were in the Pacific Rim Region.     


The flow exceeded the allowed limit at 01:19. At the same time, the
extra water flow lowered the overall core temperature and reduced the
existing steam voids </wiki/Void_coefficient> in the core.^[30]
<#cite_note-32> Since water also absorbs neutrons (and the higher
density of liquid water makes it a better absorber than steam), turning
on additional pumps decreased the reactor power further still. This
prompted the operators to remove the manual control rods further to
maintain power.^[31] <#cite_note-33>

All these actions led to an extremely unstable reactor configuration.
Nearly all of the control rods were removed, which would limit the value
of the safety rods when initially inserted in a SCRAM condition.
Further, the reactor coolant had reduced boiling, but had limited margin
to boiling, so any power excursion would produce boiling, reducing
neutron absorption </wiki/Neutron_absorption> by the water. The reactor
was in an unstable configuration that was clearly outside the safe
operating envelope established by the designers.

      Experiment and explosion




At 1:23:04 a.m. the experiment began. Four (of eight total) Main
Circulating Pumps (MCP) were active. The steam to the turbines was shut
off, and a run down of the turbine generator began. The diesel generator
started and sequentially picked up loads, which was complete by
01:23:43. During this period, the power for the four MCPs was supplied
by the turbine generator as it coasted down. As the momentum
</wiki/Momentum> of the turbine generator decreased, the water flow rate
decreased, leading to increased formation of steam voids (bubbles) in
the core.

Because of the positive void coefficient of the RBMK reactor at low
reactor power levels, it was now primed to embark on a positive feedback
</wiki/Positive_feedback> loop, in which the formation of steam voids
reduced the ability of the liquid water coolant </wiki/Coolant> to
absorb neutrons, which in turn increased the reactor's power output.
This caused yet more water to flash into steam, giving yet a further
power increase. However, during almost the entire period of the
experiment the automatic control system successfully counteracted this
positive feedback, continuously inserting control rods
</wiki/Control_rod> into the reactor core to limit the power rise.

At 1:23:40, as recorded by the SKALA </wiki/SKALA> centralized control
system, an emergency shutdown of the reactor, which inadvertently
triggered the explosion, was initiated. The SCRAM was started when the
EPS-5 button (also known as the AZ-5 button) of the reactor emergency
protection system was pressed: this fully inserted all control rods,
including the manual control rods that had been incautiously withdrawn
earlier. The reason why the EPS-5 button was pressed is not known,
whether it was done as an emergency measure or simply as a routine
method of shutting down the reactor upon completion of the experiment.

There is a view that the SCRAM may have been ordered as a response to
the unexpected rapid power increase, although there is no recorded data
conclusively proving this. Some have suggested that the button was not
pressed, and instead the signal was automatically produced by the
emergency protection system; however, the SKALA clearly registered a
manual SCRAM signal. In spite of this, the question as to when or even
whether the EPS-5 button was pressed has been the subject of debate.
There are assertions that the pressure was caused by the rapid power
acceleration at the start, and allegations that the button was not
pressed until the reactor began to self-destruct but others assert that
it happened earlier and in calm conditions.^[32] <#cite_note-34> ^:578
^[33] <#cite_note-35>


After the EPS-5 button was pressed, the insertion of control rods into
the reactor core began. The control rod insertion mechanism moved the
rods at 0.4 m/s, so that the rods took 18 to 20 seconds to travel the
full height of the core </wiki/Nuclear_reactor_core>, about 7 meters. A
bigger problem was a flawed graphite-tip control rod design, which
initially displaced coolant before inserting neutron-absorbing material
to slow the reaction. As a result, the SCRAM actually increased the
reaction rate in the lower half of the core.

A few seconds after the start of the SCRAM, a massive power spike
occurred, the core overheated, and seconds later this overheating
resulted in the initial explosion. Some of the fuel rods
</wiki/Fuel_rod> fractured, blocking the control rod columns and causing
the control rods to become stuck at one-third insertion. Within three
seconds the reactor output rose above 530 MW.^[19]
<#cite_note-MedvedevZ-20> ^:31

The subsequent course of events was not registered by instruments: it is
known only as a result of mathematical simulation. Apparently, a great
rise in power first caused an increase in fuel temperature and massive
steam buildup, leading to a rapid increase in steam pressure. This
destroyed fuel elements and ruptured the channels in which these
elements were located.^[34] <#cite_note-36>

Then, according to some estimations, the reactor jumped to around
30,000 MW thermal, ten times the normal operational output. The last
reading on the control panel was 33,000 MW. It was not possible to
reconstruct the precise sequence of the processes that led to the
destruction of the reactor and the power unit building, but a steam
explosion </wiki/Steam_explosion>, like the explosion of a steam boiler
</wiki/Steam_boiler> from excess vapor pressure, appears to have been
the next event. There is a general understanding that it was steam from
the wrecked channels entering the reactor's inner structure that caused
the destruction of the reactor casing, tearing off and lifting the
2,000-ton upper plate, to which the entire reactor assembly is fastened.
Apparently, this was the first explosion that many heard.^[35]
<#cite_note-37> ^:366 This explosion ruptured further fuel channels, and
as a result the remaining coolant flashed to steam and escaped the
reactor core. The total water loss in combination with a high positive
void coefficient further increased the reactor power.

A second, more powerful explosion occurred about two or three seconds
after the first; evidence indicates that the second explosion was from
the core itself undergoing runaway criticality
</wiki/Criticality_accident>.^[36] <#cite_note-Pakhomov2009-38> The
nuclear excursion dispersed the core and effectively terminated the
nuclear chain reaction </wiki/Nuclear_chain_reaction>. However, a
graphite fire was burning by now, greatly contributing to the spread of
radioactive material </wiki/Radioactive_fallout> and the contamination
</wiki/Radioactive_contamination> of outlying areas.^[37] <#cite_note-39>

There were initially several hypotheses about the nature of the second
explosion. One view was, "the second explosion was caused by the
hydrogen </wiki/Hydrogen> which had been produced either by the
overheated steam-zirconium </wiki/Zircaloy> reaction or by the reaction
of red-hot graphite with steam </wiki/Syngas> that produced hydrogen and
carbon monoxide </wiki/Carbon_monoxide>." Another hypothesis was that
the second explosion was a thermal explosion of the reactor as a result
of the uncontrollable escape of fast neutrons </wiki/Fast_neutron>
caused by the complete water loss in the reactor core.^[38]
<#cite_note-40> A third hypothesis was that the explosion was caused by
steam. According to this version, the flow of steam and the steam
pressure caused all the destruction that followed the ejection from the
shaft of a substantial part of the graphite and fuel.

    According to observers outside Unit 4, burning lumps of material and
    sparks shot into the air above the reactor. Some of them fell on to
    the roof of the machine hall and started a fire. About 25 percent of
    the red-hot graphite blocks and overheated material from the fuel
    channels was ejected.... Parts of the graphite blocks and fuel
    channels were out of the reactor building.... As a result of the
    damage to the building an airflow through the core was established
    by the high temperature of the core. The air ignited the hot
    graphite and started a graphite fire.^[19] <#cite_note-MedvedevZ-20>
    ^:32

However, the ratio of xenon radioisotopes </wiki/Isotopes_of_xenon>
released during the event indicates that the second explosion could be a
nuclear power transient. This nuclear transient released 40 billion
joules </wiki/Joule> of energy, the equivalent of about ten tons of TNT
</wiki/TNT_equivalent>. The analysis indicates that the nuclear
excursion was limited to a small portion of the core.^[36]
<#cite_note-Pakhomov2009-38>

Contrary to safety regulations, bitumen </wiki/Bitumen>, a combustible
material, had been used in the construction of the roof of the reactor
building and the turbine hall. Ejected material ignited at least five
fires on the roof of the adjacent reactor 3, which was still operating.
It was imperative to put those fires out and protect the cooling systems
of reactor 3.^[19] <#cite_note-MedvedevZ-20> ^:42 Inside reactor 3, the
chief of the night shift, Yuri Bagdasarov, wanted to shut down the
reactor immediately, but chief engineer Nikolai Fomin would not allow
this. The operators were given respirators </wiki/Respirator> and
potassium iodide </wiki/Potassium_iodide> tablets and told to continue
working. At 05:00, however, Bagdasarov made his own decision to shut
down the reactor, leaving only those operators there who had to work the
emergency cooling systems </wiki/Nuclear_safety_systems>.^[19]
<#cite_note-MedvedevZ-20> ^:44