Bridge cranes
Bridge cranes within the NSC (described in detail on p14) are the workhorses for deconstruction and fuel containing masses (FCM) removal. A tendering process for the custom built cranes is underway. The process is confidential, so we cannot identify the competitors at this time. Two of the cranes are identical classical carriages, one is specialised with a mast for remotely operated tools, and one is specialised for carrying workers. The construction period will last about four to four-and-a-half years.
Environmental conditions for the bridge cranes inside the NSC include an ambient temperature range spanning -22 to +29 degrees C, 100% relative humidity, and gamma radiation doses rate between 0.1 and 1 Gy/hour. Cranes and carriages are suspended from the NSC arch structural members. Four rolling cranes are planned, using three types of carriages capable of being moved from one crane beam (quadrilateral) to another (north-south direction) with their full loading capacity. Travel in the east-west direction is 144m and in the north-south direction 42m. The height above floor level is approximately 80m, and the hook lift height is 75m. Travel speed is variable between 1–10m/min. Positional accuracy is on the order of 10mm. The three types of carriages are:
– Two 50 US ton “classic” lifting carriages
– A 40 US ton secured lifting carriage and “protective box”
– A telescopic mast carriage
The classic lifting carriages were sized to accommodate the largest known loads during deconstruction as well as handling disposal casks for FCM removal. The heaviest loads for deconstruction will be the large beams. All beams are heavier than 40 US tons and measure between 30-55m in length. The lay-down area is non-symmetrical in shape in comparison with the crane tracks, imposing certain constraints on the removal of the longest beams.
Because the bridge cranes will operate above the primary sources of nuclear radiation inside the NSC, the crane operator cannot be located on the cranes, but must operate them from a control room that can be continuously occupied. During conceptual design, several alternatives for communication between the cranes and control room were evaluated, including cable, radio, infrared, laser, and others.
The radio remote control system was chosen because of the absence of a fixed cable connection between the mobile and static equipment. The remote control system allows the control of several pieces of equipment simultaneously, and units are commercially available. The equipment has a record of safe use in other applications and can be upgraded as technologies change over the 100-year life of the facility. The cranes are controlled in normal operating mode from the control room, using video images supplied by a monitoring system consisting of cameras located on the quadrilaterals (two cameras per quadrilateral), on the carriages (three cameras per carriage) and near the tools (three cameras for the telescopic mast).
Sensors and actuators on the cranes are required to enable remote operation and must be environmentally qualified for use in high gamma radiation fields. Positional encoding for two dimensional horizontal position of the carriages and vertical position of the loads carried is further complicated by the mounting of the cranes on the large NSC steel structure. The NSC will move significantly in response to external ambient temperature changes, wind loads, and loads carried by the cranes.
The telescopic mast holds a robotic tool carrier arm and TV cameras. The mast is designed to hold the following tools: concrete drill, demolition jaw crusher or shearer, hydraulic unit to supply the jaw crusher and drill, and a vacuum cleaner with HEPA filtration for removal of loose surface contamination. The drill and jaws are fitted to the end of an articulated, 2.5m-long arm.
The drill and demolition jaw crusher cannot be fitted simultaneously. Tool changes will be carried out in the maintenance garage. The maximum horizontal force considered for the drill is 1,500 daN. The telescopic mast allows for the tools to be used up to level 35m. When the mast leaves its ‘folded’ position, its horizontal movement velocity is limited. The mast must be fitted with a safety device for protection to prevent collision.
The “Protective Box” is an enclosure equipped with biological protection devices (radiation shielding and breathing air) and designed to transport workers to allow them to work safely in radiation or contaminated areas under the NSC.
The carriage is fitted with two lifting devices, each one of 40 US ton capacity. As the workers will be in an extremely hazardous environment, most of the transport, electrical, control, and protective systems must be redundant.
The box and carriage are designed for safe retrieval to the shielded maintenance area under all modes of single-failure.
A shielded maintenance garage will be located at the west end of the NSC. The west end of the building is the furthest from the large radiation sources, but shielding of the garage and stairwells will be required to protect maintenance workers. The garage will be equipped with a hoist in the upper section for disassembly of the main carriage components (lifting gearbox, motor, etc), a rack for the storage of contaminated maintenance tools (drill, jaw crusher, vacuum), and equipment for decontamination, particularly for the telescopic mast. A location in the garage is provided for maintenance/decontamination of the mast. At this location, the mast may be extended to its full length (level 35m) and maintenance workers can access the mast using a suspended platform. Carriages will be stored in the garage when they are not in use.
Conclusion
Due to of the hazardous environment, safety must be considered in all aspects of Chernobyl crane design and operations. This unique application presents numerous challenges to engineers and equipment suppliers. Solving these challenges will be rewarding to all participants in the project and will help remediate one of the worst environmental disasters in the world.
All work described in this paper is made possible by donor nations to the Chernobyl Shelter Fund, which is administered by the European Bank for Reconstruction and Development.
Background: The accident
The Chernobyl Nuclear Power Plant (ChNPP) site is located in the nation of Ukraine on the bank of the Pripyat River, 110km north of the capitol city, Kyiv. All four RBMK (Reactor Bolshoi Moschnosti Kanalynyi) units of the Chernobyl Nuclear Power Plant can be seen in the aerial photograph. Unit 4, which was destroyed by the accident, is the left-most structure and is covered by the Object Shelter. The abandoned city of Pripyat is at the top of the photo.
ChNPP operated four 1000 MWe electric generating units powered by RBMK nuclear reactors. The most catastrophic accident in the history of commercial nuclear power destroyed Unit 4 on April 26 1986. Units 1, 2, and 3 are not operating and are currently being decommissioned. At the time of the accident, two additional units were under construction. Construction of Units 5 and 6 was terminated after the accident.
The partially constructed Unit 5 is surrounded by tower cranes that have remained untouched since the accident in 1986.
Early in the morning of April 26, the operating crew was preparing to shutdown reactor 4 for refueling and to conduct a test of the ability of plant turbine generators to support plant electrical loads in the event of loss of offsite electrical supplies during shutdown.
A combination of operator errors during the test and RBMK design flaws resulted in a steam explosion that lifted the reactor vessel head. In the following hours, nuclear fuel melted and the graphite moderator burned, releasing more than 5% of the core to the atmosphere. Large areas of land in Ukraine, Belarus and Russia were radioactively contaminated.
The roof, north, and south walls of the Unit 4 reactor building were destroyed by the force of the explosion. The extensive damage to the turbine hall, near the bottom of the photo, was due to the fire ignited by burning graphite ejected from the reactor.
The first radiation casualties were nuclear reactor operators and firefighters at Chernobyl, when 19 died within four months of the accident from acute radiation exposure received while extinguishing graphite ignited fires on the roof of the turbine hall. The town of Pripiyat, located only 3km from ChNPP was evacuated and continues to be unpopulated today, inside a 30km “exclusion zone”.
Almost 200,000 people were involved in the immediate liquidation of the accident. In less than nine months, under extremely hazardous conditions, they constructed a steel structure to contain the radioactive remains of the destroyed reactor. This “Object Shelter” has deteriorated in the 20 years since its construction.
Background: New Safe Confinement
The NSC is an enormous steel arch-shaped structure that will be constructed about 450m west of the Object Shelter and then slid into place over the shelter. The NSC is being constructed away from the shelter so that construction workers can work full-time without exceeding their annual allowable radiation exposure.
The approximate dimensions of the arch are 250m wide by 150m long with a centreline height of a little more than 100m. The main structural members are 1m diameter tubular steel. The arch has a chord depth of 12m and will be covered on both the inner and outer surfaces. The eastern and western ends of the NSC will have “skyline” cutouts to accommodate sliding over and sealing to existing structures.
The service life of the NSC, and its equipment, including bridge cranes, is 100 years. The NSC rests on two deep pile foundations running in the east-west direction, 250m apart, each of which is about 450m in length. Piles are 1m diameter, 3m on centre, to various depths ranging from 17-21m. On top of the piles are foundation caps that are each about 18m wide and 4m high.
These concrete foundation caps are the rails on which the completed NSC structure will be slid from its assembly area to its operational location. The sliding force will be provided by hydraulic jacks. Arch sections will be fabricated in the assembly area and hoisted into position by strand jacks. After all sections are connected, the inner and outer surfaces of the NSC will be applied, and internal systems installed. Internal systems include lighting, ventilation, radiation detection, fire detection and fire fighting, closed circuit television, communications and bridge cranes. Preliminary commissioning testing will be conducted in the assembly before the NSC is slid east over the shelter to commence operations.
The first task of the NSC will be to deconstruct unstable structures of the shelter. Deconstruction includes removal of the roof of the shelter, portions of the original concrete structure of Unit 4, accident debris, and many large beams installed immediately after the accident. This material is radioactively contaminated, although most of it was not exposed to neutron radiation and so is not activated. Decontamination will take place inside the NSC and elsewhere at ChNPP before disposal in permitted facilities within the exclusion zone.
After unstable structures in the shelter have been deconstructed, the NSC will be used to remove fuel containing masses (FCM). Before this work can begin, either a deep geological repository must be constructed in Ukraine, or disposal casks designed and a temporary storage facility for them must be identified.
NSC detailed design is currently being performed by an additional consortium, Novarka, composed of Vinci Grands Projets and Bouygues Construction. The final design may differ from this description.
