Chernobyl nuclear power plant and sarcophagus
The true story
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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. After 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. 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 US gallons) of water per hour.: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 :15 to attain full speed and reach the capacity of 5.5 MW required to run one main cooling water pump. :30 This one-minute power gap was considered unacceptable, and it had been suggested that the rotational energy (or angular momentum) of the steam turbine and residual steam pressure (with turbine valves closed) could be used to generate electricity to run the main cooling water pumps while the emergency diesel generators were reaching the correct rotational speed (RPM) and voltage. In theory, analyses indicated that this residual momentum and steam pressure had the potential to provide power for 45 seconds, :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 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 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. Before and after |
The nuclear reactor after the disaster. Reactor 4 (center). Turbine building (lower left). Reactor 3 (center right)
On the left a view of Chernobyl from Google-Maps
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 :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 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 the normal shutdown procedure. The nuclear reactor after the disaster. Reactor 4 (center). Turbine building (lower left). Reactor 3 (center right)
Aerial view of the damaged core on May 3, 1986. Roof of the turbine hall is damaged (image center)
The abandoned city of Pripyat with Chernobyl plant in the distance
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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). 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.
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). :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 and 800 MW.
- The steam-turbine generator was to be run up to full speed.
- When these conditions were achieved, the steam supply for the turbine generator was to be closed off.
- 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.
- After the emergency generators reached normal operating speed and voltage, the turbine generator would be allowed to freewheel down.
Design and construction
International competition In 1992, Ukraine's Government held an international competition for proposals to replace the hastily constructed sarcophagus.
Of the 394 entries, only the British submission proposed a sliding arch approach. The outcome of the competition was no overall winner, but the French submission came 2nd with the UK and German proposals coming joint 3rd.
Subsequently, a pan-European study (the TACIS programme) re-examined the proposals of the top three finalists of the competition. The study selected the sliding arch proposal as the best solution for their further investigations and recommendations, primarily to reduce the chance of the construction workers receiving a harmful dose of radiation.
On 17 September 2007 Vinci Construction Grands Projets and Bouygues Travaux Publics announced that they won the contract to build the New Safe Confinement as 50/50 partners of a French consortium named Novarka. The original 432 million euros contract comprises the design and construction of the NSC and planned to employ 900 people at its peak.
Design goals The New Safe Confinement (NSC) was designed with several design goals in mind:
The arches are constructed of tubular steel members, and are externally clad with three layer sandwich panels. These external panels will also be used on the end walls of the structure. Internally, each arch will be covered in polycarbonate (Lexan) to prevent the accumulation of radioactive particles on the frame members themselves.
Large parts of the arches will be shop fabricated and transported to the assembly site, 180 metres (590 ft) west of reactor unit 4. Each of the steel tubes will be high-strength steel in order to reduce cost and assembly weight. The steel used in construction of the tubular members will have a yield strength of no less than 2,500 kg/cm2 (250 MPa; 36,000 psi).
More extensive detail of the structural composition and design of the arches can be found in Section II.B., "Structural Design Process" of Conceptual Design of the Chornobyl New Safe Confinement — an Overview.
An air-conditioning system is used to prevent corrosion. Condensation will be avoided by maintaining a temperature difference.
International competition In 1992, Ukraine's Government held an international competition for proposals to replace the hastily constructed sarcophagus.
Of the 394 entries, only the British submission proposed a sliding arch approach. The outcome of the competition was no overall winner, but the French submission came 2nd with the UK and German proposals coming joint 3rd.
Subsequently, a pan-European study (the TACIS programme) re-examined the proposals of the top three finalists of the competition. The study selected the sliding arch proposal as the best solution for their further investigations and recommendations, primarily to reduce the chance of the construction workers receiving a harmful dose of radiation.
On 17 September 2007 Vinci Construction Grands Projets and Bouygues Travaux Publics announced that they won the contract to build the New Safe Confinement as 50/50 partners of a French consortium named Novarka. The original 432 million euros contract comprises the design and construction of the NSC and planned to employ 900 people at its peak.
Design goals The New Safe Confinement (NSC) was designed with several design goals in mind:
- Convert the destroyed ChNPP Unit 4 into an environmentally safe system (i.e. contain the radioactive materials at the site to prevent further environmental contamination)
- Reduce corrosion and weathering of the existing shelter and the Unit 4 reactor building
- Mitigate the consequences of a potential collapse of either the existing shelter or the Unit 4 reactor building, particularly in terms of containing the radioactive dust that would be produced by such a collapse.
- Enable safe deconstruction of unstable structures (such as the roof of the existing shelter) by providing remotely operated equipment for their deconstruction.
The arches are constructed of tubular steel members, and are externally clad with three layer sandwich panels. These external panels will also be used on the end walls of the structure. Internally, each arch will be covered in polycarbonate (Lexan) to prevent the accumulation of radioactive particles on the frame members themselves.
Large parts of the arches will be shop fabricated and transported to the assembly site, 180 metres (590 ft) west of reactor unit 4. Each of the steel tubes will be high-strength steel in order to reduce cost and assembly weight. The steel used in construction of the tubular members will have a yield strength of no less than 2,500 kg/cm2 (250 MPa; 36,000 psi).
More extensive detail of the structural composition and design of the arches can be found in Section II.B., "Structural Design Process" of Conceptual Design of the Chornobyl New Safe Confinement — an Overview.
An air-conditioning system is used to prevent corrosion. Condensation will be avoided by maintaining a temperature difference.
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