Thread: Chernobyl disaster

A good article on the events that happened prior to, during and after Chernobyl. A little technical but still informative.

Chernobyl disaster - Wikipedia, the free encyclopedia

The Chernobyl disaster was a nuclear accident that occurred on 26 April 1986 at the Chernobyl Nuclear Power Plant in the Ukrainian SSR (now Ukraine). It is considered the worst nuclear power plant accident in history, and it is the only one classified as a level 7 event on the International Nuclear Event Scale.

The disaster began during a systems test on 26 April 1986 at reactor number four of the Chernobyl plant, which is near the town of Pripyat. There was a sudden power output surge, and when an emergency shutdown was attempted, a more extreme spike in power output occurred, which led to a reactor vessel rupture and a series of explosions. This event exposed the graphite moderator components of the reactor to air, causing them to ignite. The resulting fire sent a plume of radioactive fallout into the atmosphere and over an extensive geographical area, including Pripyat. The plume drifted over large parts of the western Soviet Union, Eastern Europe, Western Europe, and Northern Europe. Large areas in Ukraine, Belarus, and Russia were evacuated, and over 336,000 people were resettled. According to official post-Soviet data,[1][2] about 60% of the fallout landed in Belarus.
The accident raised concerns about the safety of the Soviet nuclear power industry, as well as nuclear power in general, slowing its expansion for a number of years and forcing the Soviet government to become less secretive about its procedures.[3][notes 1]

Russia, Ukraine, and Belarus have been burdened with the continuing and substantial decontamination and health care costs of the Chernobyl accident. More than fifty deaths are directly attributed to the accident, all among the reactor staff and emergency workers. Estimates of the total number of deaths attributable to the accident vary enormously, from possibly 4,000 to close to a million.[4][5] Despite the accident, Ukraine continued to operate the remaining reactors at Chernobyl until 2000, when the last reactor at the site was closed down.[6]

Contents [hide]
1 Accident
1.1 The attempted experiment
1.2 Conditions prior to the accident
1.3 Experiment and explosion
1.3.1 Radiation levels
1.3.2 Plant layout
1.3.3 Individual involvement
1.3.4 Deaths and survivors
1.4 Immediate crisis management
1.4.1 Radiation levels
1.4.2 Fire containment
1.4.2.1 Timeline
1.4.3 Evacuation of Pripyat
1.4.4 Steam explosion risk
1.4.5 Debris removal
2 Causes
2.1 Operator error initially faulted
2.2 Operating instructions and design deficiencies found
3 Effects
3.1 International spread of radioactive substances
3.2 Radioactive release
3.3 Health of plant workers and local people
3.4 Residual radioactivity in the environment
3.4.1 Rivers, lakes and reservoirs
3.4.2 Groundwater
3.4.3 Flora and fauna
4 Chernobyl after the disaster
5 Recovery process
5.1 Recovery projects
5.1.1 The Chernobyl Shelter Fund
5.1.2 The United Nations Development Programme
5.1.3 The International Project on the Health Effects of the Chernobyl Accident
6 Assessing the disaster's effects on human health
7 In popular culture
8 Commemoration of the disaster
8.1 Chernobyl 20
9 See also
10 Further reading
10.1 Documents
11 Notes
12 References
13 External links

[edit]Accident
On 26 April 1986, at 01:23 (UTC+3), 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[7]:73 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 contained 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 the normal shutdown procedure.

[edit]The attempted experiment
Even when not actively generating power, nuclear power reactors require cooling, typically provided by coolant flow, to remove decay heat. Pressurized water reactors use water flow at high pressure to remove waste heat. Following an emergency shutdown (scram), the core still generates a significant amount of residual heat, which is initially about seven percent of the total thermal output of the plant. If not removed by coolant systems, the heat could lead to core damage.[8][9]
Following an emergency shutdown, reactor cooling is still required to keep the temperature in the reactor core low enough to avoid fuel damage. The reactor that exploded in Chernobyl consisted of about 1,600 individual fuel channels, and each operational channel required a flow of 28 metric tons (28,000 liters (7,400 USgal)) of water per hour.[7]:7 There had been concerns that in the event of a power grid failure, external power would not have been immediately available to run the plant's cooling water pumps. Chernobyl's reactors had three backup diesel generators. Each generator required 15 seconds to start up but took 60–75 seconds[7]:15 to attain full speed and reach the capacity of 5.5 MW required to run one main cooling water pump.[7]:30

This one-minute power gap was considered unacceptable, and it had been suggested that the mechanical energy (rotational momentum) of the steam turbine could be used to generate electricity to run the main cooling water pumps while the turbine was still spinning down. In theory, analyses indicated that this residual momentum had the potential to provide power for 45 seconds,[7]:16 which would bridge the power gap between the onset of the external power failure and the full availability of electric power from the emergency diesel 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 voltage of the turbine-generator was insufficient; it did not maintain the desired magnetic field during the spin-down. 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.[10]

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 (SCRAM). 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). 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.[11]
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—a missing 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 was not sought from the Soviet nuclear oversight regulator (even though there was a representative at the complex of 4 reactors).[notes 2]:18–20
The experimental procedure was intended to run as follows:
the reactor was to be running at a low power level, between >700 MW & 800 MW
the steam turbine was to be run up to full speed
when these conditions were achieved, the steam supply was to be closed off
the turbines would be allowed to freewheel down
generator performance was to be recorded to determine whether it could provide the bridging power for coolant pumps

[edit]Conditions prior to the accident
The conditions to run the test were established prior to 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.[12] As planned, a gradual reduction in the output of the power unit was begun at 01:06 on 25 April, and the power level had reached 50% of its nominal 3200 MW thermal level by the beginning of the day shift. At this point, another regional power station unexpectedly went off line, 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 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 finalized 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. Further rapid reduction in the power level from 50% was actually executed during the shift change-over. 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.[7]:36–8

The test plan called for the power output of reactor 4 to be gradually reduced to a thermal level of 700–1000 MW.[13] The level established in the test program (700 MW) was achieved at 00:05 on April 26; however, because of the natural production of the neutron absorber xenon-135 in the core, reactor power continued to decrease, even without further operator action. And as the power reached approximately 500 MW, Toptunov committed an error, inserting the control rods too far, bringing the reactor to a near-shutdown state. The exact circumstances are hard to know, because both Akimov and Toptunov died from radiation sickness.

The reactor power dropped to 30 MW thermal (or less)—an almost completely shut down power level, which was approximately 5 percent of the minimum initial power level established as safe for the test.[11]:73 Control-room personnel therefore made the decision to restore the power and extracted the reactor control rods,[14] though several minutes elapsed between their extraction and the point that the power output began to increase and subsequently stabilize at 160–200 MW (thermal). In this case the majority of control rods were withdrawn to their upper limits, but the low value of the operational reactivity margin restricted any further rise of reactor power. 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 of the reactor core by the accumulation of xenon-135. This made it necessary to extract additional control rods from the reactor core in order to counteract the poisoning.

he operation of the reactor at the low power level with a small reactivity margin was accompanied by unstable core temperature and coolant flow, and possibly by instability of neutron flux.[15] The control room received repeated emergency signals of the levels in the steam/water separator drums, of relief valves opened to relieve excess steam into a turbine condenser, of large excursions or variations in the flow rate of feed water, and from the neutron power controller. In the period between 00:35 and 00:45, emergency alarm signals concerning thermal-hydraulic parameters were apparently ignored, apparently to preserve the reactor power level. Emergency signals from the reactor emergency protection system (EPS-5) triggered a trip which turned off both turbine-generators.[16]

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 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 temperature of water, reducing the safety margin. 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 in the core.[17] 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.[18]

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 by the water. The reactor was in an unstable configuration that was clearly outside the safe operating envelope established by the designers.
[edit]Experiment and explosion


Aerial view of the damaged core. Roof of the turbine hall is damaged (image center). Roof of the adjacent reactor 3 (image lower left) shows minor fire damage.


Lumps of graphite moderator ejected from the core. The largest lump shows an intact control rod channel.
At 1:23:04 a.m. the experiment began. The steam to the turbines was shut off, and a run down of the turbine generator began, together with four (of eight total) Main Circulating Pumps (MCP). The diesel generator started and sequentially picked up loads, which was complete by 01:23:43; during this period the power for these four MCPs was supplied by the coasting down turbine generator. As the momentum of the turbine generator that powered the water pumps 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 loop, in which the formation of steam voids reduced the ability of the liquid water 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 into the reactor core to limit the power rise.

At 1:23:40, as recorded by the SKALA centralized control system, an emergency shutdown or SCRAM of the reactor 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 thus fully inserting all control rods, including the manual control rods that had been incautiously withdrawn earlier. The reason 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 convincingly testifying to this. Some have suggested that the button was not pressed but rather that 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 was 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.[19]:578[20] For whatever reason the EPS-5 button was pressed, so the insertion of control rods into the reactor core began. The control rod insertion mechanism operated at a relatively slow speed (0.4 m/s) taking 18–20 seconds for the rods to travel the full approximately 7-meter core length (height). A bigger problem was a flawed graphite-tip control rod design, which initially displaced coolant before neutron-absorbing material was inserted and the reaction slowed. 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 resulted in the initial explosion. Some of the fuel rods fractured, blocking the control rod columns and causing the control rods to become stuck after being inserted only one-third of the way. Within three seconds the reactor output rose above 530 MW.[7]:31 The subsequent course of events was not registered by instruments: it is known only as a result of mathematical simulation. First a great rise in power caused an increase in fuel temperature and massive steam buildup with rapid increase in steam pressure. This destroyed fuel elements and ruptured the channels in which these elements were located.[21] Then, according to some estimations, the reactor jumped to around 30 GW thermal, ten times the normal operational output. The last reading on the control panel was 33 GW. 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. There is a general understanding that it was steam from the wrecked channels entering the reactor inner structure that caused the destruction of the reactor casing, tearing off and lifting by force the 2,000 ton upper plate (to which the entire reactor assembly is fastened). Apparently this was the first explosion that many heard.[22]:366 This was a steam explosion like the explosion of a steam boiler from the excess pressure of vapor. This ruptured further fuel channels—as a result the remaining coolant flashed to steam and escaped the reactor core. The total water loss combined with a high positive void coefficient to increase the reactor power.

A second, more powerful explosion occurred about two or three seconds after the first; evidence indicates that the second explosion resulted from a nuclear excursion.[23] The nuclear excursion dispersed the core and effectively terminated that phase of the event. However, the graphite fire continued, greatly contributing to the spread of radioactive material and the contamination of outlying areas.[24] There were initially several hypotheses about the nature of the second explosion. One view was that "the second explosion was caused by the hydrogen which had been produced either by the overheated steam-zirconium reaction or by the reaction of red-hot graphite with steam that produce hydrogen and carbon monoxide." Another hypothesis posits that the second explosion was a thermal explosion of the reactor as a result of the uncontrollable escape of fast neutrons caused by the complete water loss in the reactor core.[25] A third hypothesis was that the explosion was caused, exceptionally, by steam. According to this version, the flow of steam and the steam pressure caused all the destruction following 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 per cent 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.[7]:32

However, the ratio of xenon radioisotopes released during the event provides compelling evidence that the second explosion was a nuclear power transient. This nuclear transient released ~0.01 kiloton of TNT equivalent (40 GJ) of energy; the analysis indicates that the nuclear excursion was limited to a small portion of the core.[23]
Contrary to safety regulations, a combustible material (bitumen) 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 (still operating) adjacent reactor 3. It was imperative to put those fires out and protect the cooling systems of reactor

3.[7]: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.[why?] The operators were given respirators and 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.[7]:44
[edit]Radiation levels

Link Children of Chornobyl Relief and Development Fund

There always the human side of a disaster. Is the case of Chornobyl here were the children. This organization Children of Chornobyl Relief and Development Fund I was a part of since it's founding until a few years ago.

Please read the link and see what were some of the things we did, In 1991 I was involved with an airflight of medical supplies for children who were being treated for the effects of Chornobyl.

Please read THANK YOU

Children of Chornobyl Relief and Development Fund (CCRDF) is an international non-profit organization working to save the lives of children throughout Ukraine through developed life-saving medical programs. CCRDF’s primary objective is to strengthen the Women’s and Children’s Health Initiative Programs in the fields of neonatology, perinatology, infant cardiac surgery, and pediatric oncology. CCRDF also works to improve the quality of life for orphans with disabilities in Ukraine though nutritional, rehabilitation, and educational programs.




Mission
Established in 1990 following the Chornobyl catastrophe, CCRDF is a humanitarian organization focused on saving and improving the lives of children in Ukraine.

Goal
To provide state-of-the-art medical aid to children in Ukraine with cancer, genetic birth defects, heart diseases, and immune deficiencies as well as to provide aid to orphans.
Track Record
Over the course of 20 years, CCRDF has launched 32 airlifts and 18 sea shipments, delivering over $63,000,000 worth of medical aid to our 31 partner hospitals and 3 orphanages.

Objective
CCRDF’s primary objective is to strengthen the Women’s and Children’s Health Initiative Programs in the fields of neonatology, perinatology, infant cardiac surgery, and pediatric oncology. CCRDF also works to improve the quality of life of children with disabilities year-round at orphanages through nutritional, educational, and physical therapy programs.