Разделение изотопов и применение их в ядерном реакторе

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.The nuclear excursion dispersed the core and effectively terminated this phase[clarification needed] of the event. However, a graphite fire was burning by now, greatly contributing to the spread of radioactive material and the contamination of outlying areas 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 produced hydrogen and carbon monoxide." Another hypothesis is 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.A third hypothesis is 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 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.[6]: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 40 GJ of energy, the equivalent of about ten tons of TNT. The analysis indicates that the nuclear excursion was limited to a small portion of the core.

Contrary to safety regulations, 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.[6]: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.[6]:44

Radiation levels

Approximate radiation levels at different locations shortly after the explosion were:[27][not in citation given]Location Radiation (Roentgens per hour) Sieverts per hour (SI Unit)

Vicinity of the reactor core 30,000 300

Fuel fragments 15,000–20,000 150-200

Debris heap at the place of circulation pumps 10,000 100

Debris near the electrolyzers 5,000–15,000 50-150

Water in the Level +25 feedwater room 5,000 50

Level 0 of the turbine hall 500–15,000 5-150

Area of the affected unit 1,000–1,500 10-15

Water in Room 712 1,000 10

Control room, shortly after explosion 3–5 .03-.05

Gidroelektromontazh depot 30 .3

Nearby concrete mixing unit 10–15 .10-.15

Plant layout

Based on the image of the plant[28]Level Objects

Metres Levels are distances above (or below for minus values) ground level at the site.

49.6 Roof of the reactor building, gallery of the refueling mechanism

39.9 Roof of the deaerator gallery

35.5 Floor of the main reactor hall

31.6 Upper side of the upper biological shield, floor of the space for pipes to steam separators

28.3 Lower side of the turbine hall roof

24.0 Deaerator floor, measurement and control instruments room

16.4 Floor of the pipe aisle in the deaerator gallery

12.0 Main floor of the turbine hall, floor of the main circulation pump motor compartments

10.0 Control room, floor under the reactor lower biological shield, main circulation pumps

6.0 Steam distribution corridor

2.2 Upper pressure suppression pool

0.0 Ground level; house switchgear, turbine hall level

-0.5 Lower pressure suppression pool

-5.2, -4.2 Other turbine hall levels

-6.5 Basement floor of the turbine hall

 

Individual involvement

Main article: Individual involvement in the Chernobyl disaster

Deaths and survivors

Main article: Deaths due to the Chernobyl disaster

Immediate crisis management

Radiation levels

Extremely high levels of radioactivity in the lava under the Chernobyl number four reactor in 1986

The radiation levels in the worst-hit areas of the reactor building have been estimated to be 5.6 roentgens per second (R/s) (1.4 milliamperes per kilogram), equivalent to more than 20,000 roentgens per hour. A lethal dose is around 500 roentgens (0.13 coulombs per kilogram) over 5 hours, so in some areas, unprotected workers received fatal doses within minutes. However, a dosimeter capable of measuring up to 1,000 R/s (0.3 A/kg) was inaccessible because of the explosion, and another one failed when turned on. All remaining dosimeters had limits of 0.001 R/s (0.3 µA/kg) and therefore read "off scale." Thus, the reactor crew could ascertain only that the radiation levels were somewhere above 0.001 R/s (3.6 R/h, or 0.3 µA/kg), while the true levels were much, much higher in some areas.

Because of the inaccurate low readings, the reactor crew chief Alexander Akimov assumed that the reactor was intact. The evidence of pieces of graphite and reactor fuel lying around the building was ignored, and the readings of another dosimeter brought in by 04:30 were dismissed under the assumption that the new dosimeter must have been defective.[6]:42–50 Akimov stayed with his crew in the reactor building until morning, trying to pump water into the reactor. None of them wore any protective gear. Most, including Akimov, died from radiation exposure within three weeks.

Fire containment

Firefighter Leonid Telyatnikov, being decorated for bravery

Shortly after the accident, firefighters arrived to try to extinguish the fires. First on the scene was a Chernobyl Power Station firefighter brigade under the command of Lieutenant Volodymyr Pravik, who died on 9 May 1986 of acute radiation sickness. They were not told how dangerously radioactive the smoke and the debris were, and may not even have known that the accident was anything more than a regular electrical fire: "We didn't know it was the reactor. No one had told us."

Grigorii Khmel, the driver of one of the fire engines, later described what happened:

We arrived there at 10 or 15 minutes to two in the morning… We saw graphite scattered about. Misha asked: "What is graphite?" I kicked it away. But one of the fighters on the other truck picked it up. "It's hot," he said. The pieces of graphite were of different sizes, some big, some small enough to pick up…

We didn't know much about radiation. Even those who worked there had no idea. There was no water left in the trucks. Misha filled the cistern and we aimed the water at the top. Then those boys who died went up to the roof—Vashchik Kolya and others, and Volodya Pravik.… They went up the ladder … and I never saw them again.

However, Anatoli Zakharov, a fireman stationed in Chernobyl since 1980, offers a different description:

I remember joking to the others, "There must be an incredible amount of radiation here. We'll be lucky if we're all still alive in the morning."

Twenty years after the disaster, he claimed the firefighters from the Fire Station No. 2 were aware of the risks.

Of course we knew! If we'd followed regulations, we would never have gone near the reactor. But it was a moral obligation—our duty. We were like kamikaze.[32]

The immediate priority was to extinguish fires on the roof of the station and the area around the building containing Reactor No. 4 to protect No. 3 and keep its core cooling systems intact. The fires were extinguished by 5:00, but many firefighters received high doses of radiation. The fire inside reactor 4 continued to burn until 10 May 1986; it is possible that well over half of the graphite burned out.[6]:73 The fire was extinguished by a combined effort of helicopters dropping over 5,000 metric tons of sand, lead, clay, and boron onto the burning reactor and injection of liquid nitrogen. The Ukrainian filmmaker Vladimir Shevchenko captured film footage of an Mi-8 helicopter as it collided with a nearby construction crane, causing the helicopter to fall near the damaged reactor building and killing its four-man crew.[33]

From eyewitness accounts of the firefighters involved before they died (as reported on the CBC television series Witness), one described his experience of the radiation as "tasting like metal," and feeling a sensation similar to that of pins and needles all over his face. (This is similar to the description given by Louis Slotin, a Manhattan Project physicist who died days after a fatal radiation overdose from a criticality accident.)

The explosion and fire threw hot particles of the nuclear fuel and also far more dangerous fission products, radioactive isotopes such as caesium-137, iodine-131, strontium-90 and other radionuclides, into the air: the residents of the surrounding area observed the radioactive cloud on the night of the explosion.

Timeline

1:26:03 – fire alarm activated

1:28 – arrival of local firefighters, Pravik's guard

1:35 – arrival of firefighters from Pripyat, Kibenok's guard

1:40 – arrival of Telyatnikov

2:10 – turbine hall roof fire extinguished

2:30 – main reactor hall roof fires suppressed

3:30 – arrival of Kiev firefighters[35]

4:50 – fires mostly localized

6:35 – all fires extinguished

With the exception of the fire contained inside Reactor 4, which continued to burn for many days.

Evacuation of Pripyat This section may require cleanup to meet Wikipedia's quality standards. Please improve this section if you can. The talk page may contain suggestions. (April 2009)

 

 

View of Chernobyl taken from Pripyat (city)

The nearby city of Pripyat was not immediately evacuated after the incident, for the general population of the Soviet Union was not informed of the disaster until April 29. During that time, all radio broadcasts run by the state were replaced with classical music, which was a common method of preparing the public for an announcement of a tragedy that had taken place. Scientist teams were armed and placed on alert as instructions were awaited.

Only after radiation levels set off alarms at the Forsmark Nuclear Power Plant in Sweden,[37] over one thousand kilometers from the Chernobyl Plant, did the Soviet Union admit that an accident had occurred. Nevertheless, authorities attempted to conceal the scale of the disaster. For example, while evacuating the city of Pripyat, the following warning message was read on local radio: "An accident has occurred at the Chernobyl Nuclear Power Plant. One of the atomic reactors has been damaged. Aid will be given to those affected and a committee of government inquiry has been set up."

The government committee was eventually[when?] formed, and tasked to investigating the accident. It was headed by Valeri Legasov, who arrived at Chernobyl in the evening of 26 April. By the time Legasov arrived, two people had already died and 52 were receiving medical attention in a hospital. By the night of 26–27 April — more than 24 hours after the explosion — Legasov's committee had ample evidence showing extremely high levels of radiation had caused a number of cases of radiation exposure. Based on the evidence at hand, Legasov's committee acknowledged the destruction of the reactor and ordered the evacuation of Pripyat.

The evacuation began at 14:00 on 27 April. In order to expedite the evacuation, the residents were told to bring only what was necessary, as the authorities had said it would only last approximately three days. As a result, most of the residents left their personal belongings, which are still there today. An exclusion zone of 30 km (19 mi) remains in place today, although its shape has changed and its size has been expanded.

Steam explosion risk

Chernobyl Corium lava flows formed by fuel-containing mass in the basement of the plant. Lava flow (1). Concrete (2). Steam pipe (3). Electrical equipment (4).[38]

Two floors of bubbler pools beneath the reactor served as a large water reservoir from the emergency cooling pumps and as a pressure suppression system capable of condensing steam in case of a small broken steam pipe; the third floor above them, below the reactor, served as a steam tunnel. The steam released by a broken pipe was supposed to enter the steam tunnel and be led into the pools to bubble through a layer of water. The pools and the basement were flooded because of ruptured cooling water pipes and accumulated firefighting water. They now constituted a serious steam explosion risk. The smoldering graphite, fuel and other material above, at more than 1200 °C, started to burn through the reactor floor and mixed with molten concrete from the reactor lining, creating corium, a radioactive semi-liquid material comparable to lava. If this mixture had melted through the floor into the pool of water, it would have created a massive steam explosion that would have ejected more radioactive material from the reactor. It became necessary to drain the pool.

The bubbler pool could be drained by opening its sluice gates. Volunteers in diving suits entered the radioactive water and managed to open the gates. These were the engineers Alexei Ananenko (who knew where the valves were) and Valeri Bezpalov, accompanied by a third man, Boris Baranov, who provided them with light from a lamp, though this lamp failed, leaving them to find the valves by feeling their way along a pipe. All of them returned to the surface and according to Ananenko, their colleagues jumped for joy when they heard they had managed to open the valves. Despite their good condition after completion of the task, all of them suffered from radiation sickness, and at least two—Ananenko and Bezpalov—later died.[citation needed] Some sources claim incorrectly that they died in the plant.[42] It is likely that intense alpha radiation hydrolyzed the water, generating a low-pH hydrogen peroxide (H2O2) solution akin to an oxidizing acid.[43] Conversion of bubbler pool water to H2O2 is confirmed by the presence in the Chernobyl lavas of studtite and metastudtite,[44][45] the only minerals that contain peroxide.

Fire brigade pumps were then used to drain the basement. The operation was not completed until 8 May, after 20,000 metric tons of highly radioactive water were pumped out.

With the bubbler pool gone, a meltdown was less likely to produce a powerful steam explosion. To do so, the molten core would now have to reach the water table below the reactor. To reduce the likelihood of this, it was decided to freeze the earth beneath the reactor, which would also stabilize the foundations. Using oil drilling equipment, the injection of liquid nitrogen began on 4 May. It was estimated that 25 metric tons of liquid nitrogen per day would be required to keep the soil frozen at −100 °C.[6]:59 This idea[clarification needed] was soon scrapped and the bottom room where the cooling system would have been installed was filled with concrete.

Debris removal

Chernobyl power plant in 2003 with the sarcophagus containment structure

The worst of the radioactive debris was collected inside what was left of the reactor, much of it shoveled in by liquidators wearing heavy protective gear (dubbed "bio-robots" by the military); these workers could only spend a maximum of 40 seconds at a time working on the rooftops of the surrounding buildings because of the extremely high doses of radiation given off by the blocks of graphite and other debris. The reactor itself was covered with bags of sand, lead, and boric acid dropped from helicopters: some 5,000 metric tons of material were dropped during the week that followed the accident. By December 1986, a large concrete sarcophagus had been erected to seal off the reactor and its contents.

Many of the vehicles used by the "liquidators" remain parked in a field in the Chernobyl area.

Causes

Operator error initially faulted

There were two official explanations of the accident: the first, later acknowledged to be erroneous, was published in August 1986 and effectively placed the blame on the power plant operators. To investigate the causes of the accident the IAEA created a group known as the International Nuclear Safety Advisory Group (INSAG), which in its report of 1986, INSAG-1, on the whole also supported this view, based on the data provided by the Soviets and the oral statements of specialists.[49] In this view, the catastrophic accident was caused by gross violations of operating rules and regulations. "During preparation and testing of the turbine generator under run-down conditions using the auxiliary load, personnel disconnected a series of technical protection systems and breached the most important operational safety provisions for conducting a technical exercise. The operator error was probably due to their lack of knowledge of nuclear reactor physics and engineering, as well as lack of experience and training. According to these allegations, at the time of the accident the reactor was being operated with many key safety systems turned off, most notably the Emergency Core Cooling System (ECCS), LAR (Local Automatic control system), and AZ (emergency power reduction system). Personnel had an insufficiently detailed understanding of technical procedures involved with the nuclear reactor, and knowingly ignored regulations to speed test completion.[50]

The developers of the reactor plant considered this combination of events to be impossible and therefore did not allow for the creation of emergency protection systems capable of preventing the combination of events that led to the crisis, namely the intentional disabling of emergency protection equipment plus the violation of operating procedures. Thus the primary cause of the accident was the extremely improbable combination of rule infringement plus the operational routine allowed by the power station staff.

In this analysis of the causes of the accident, deficiencies in the reactor design and in the operating regulations that made the accident possible were set aside and mentioned only casually. Serious critical observations covered only general questions and did not address the specific reasons for the accident. The following general picture arose from these observations. Several procedural irregularities also helped to make the accident possible. One was insufficient communication between the safety officers and the operators in charge of the experiment being run that night. The reactor operators disabled safety systems down to the generators, which the test was really about. The main process computer, SKALA, was running in such a way that the main control computer could not shut down the reactor or even reduce power. Normally the reactor would have started to insert all of the control rods. The computer would have also started the "Emergency Core Protection System" that introduces 24 control rods into the active zone within 2.5 seconds, which is still slow by 1986 standards. All control was transferred from the process computer to the human operators.

This view is reflected in numerous publications and also artistic works on the theme of the Chernobyl accident that appeared immediately after the accident,[6] and for a long time remained dominant in the public consciousness and in popular publications.

Operating instructions and design deficiencies found

In 1991 a Commission of the USSR State Committee for the Supervision of Safety in Industry and Nuclear Power has reassessed the causes and circumstances of the Chernobyl accident and came to new insights and conclusions. Based on it, in 1992 the IAEA Nuclear Safety Advisory Group (INSAG) published an additional report, INSAG-7,[11] which reviewed "that part of the INSAG-1 report in which primary attention is given to the reasons for the accident." and included the USSR State Comission report as Appendix I. In this INSAG report, most of the earlier accusations against staff for breach of regulations were acknowledged to be either erroneous, based on incorrect information obtained in August 1986, or less relevant. This report reflected another view of the main reasons for the accident, presented in Appendix I. According to this account, the operators' actions in turning off the Emergency Core Cooling System, interfering with the settings on the protection equipment, and blocking the level and pressure in the separator drum did not contribute to the original cause of the accident and its magnitude, although they may have been a breach of regulations. Turning off the emergency system designed to prevent the two turbine generators from stopping was not a violation of regulations.

Human factors contributed to the conditions that led to the disaster. These included operating the reactor at a low power level—less than 700 MW—a level documented in the run-down test program, and operating with a small operational reactivity margin (ORM). The 1986 assertions of Soviet experts notwithstanding, regulations did not prohibit operating the reactor at this low power level.[11]:18 However, regulations did forbid operating the reactor with a small margin of reactivity. Yet "post-accident studies have shown that the way in which the real role of the ORM is reflected in the Operating Procedures and design documentation for the RBMK-1000 is extremely contradictory," and furthermore, "ORM was not treated as an operational safety limit, violation of which could lead to an accident.

Acording to the INSAG-7 Report, the chief reasons for the accident lie in the peculiarities of physics and in the construction of the reactor. There are two such reasons:[11]:18

The reactor had a dangerously large positive void coefficient. The void coefficient is a measurement of how a reactor responds to increased steam formation in the water coolant. Most other reactor designs have a negative coefficient, i.e. the nuclear reaction rate slows when steam bubbles form in the coolant, since as the vapor phase in the reactor increases, fewer neutrons are slowed down. Faster neutrons are less likely to split uranium atoms, so the reactor produces less power (a negative feed-back). Chernobyl's RBMK reactor, however, used solid graphite as a neutron moderator to slow down the neutrons, and the water in it, on the contrary, acts like a harmful neutron absorber. Thus neutrons are slowed down even if steam bubbles form in the water. Furthermore, because steam absorbs neutrons much less readily than water, increasing the intensity of vaporization means that more neutrons are able to split uranium atoms, increasing the reactor's power output. This makes the RBMK design very unstable at low power levels, and prone to suddenly increasing energy production to a dangerous level. This behavior is counter-intuitive, and this property of the reactor was unknown to the crew.

A more significant flaw was in the design of the control rods that are inserted into the reactor to slow down the reaction. In the RBMK reactor design, the lower part of each control rod was made of graphite and was 1.3 meters shorter than necessary, and in the space beneath the rods were hollow channels filled with water. The upper part of the rod—the truly functional part that absorbs the neutrons and thereby halts the reaction—was made of boron carbide. With this design, when the rods are inserted into the reactor from the uppermost position, the graphite parts initially displace some water (which absorbs neutrons, as mentioned above), effectively causing less neutrons to be absorbed initially. Thus for the first few seconds of control rod activation, reactor power output is increased, rather than reduced as desired. This behavior is counter-intuitive and was not known to the reactor operators.

Other deficiencies besides these were noted in the RBMK-1000 reactor design, as were its non-compliance with accepted standards and with the requirements of nuclear reactor safety.