Cofrentes Nuclear Power Plant is located inland, 64 km from the city of Valencia in eastern Spain. The plant, owned by Iberdrola, is a boiling water reactor (BWR-6, Mark III containment) that was designed and supplied by GE. The reactor was first connected to the grid in 1984 with a thermal power of 2894 MWt. Today, after several upratings, Cofrentes has a licensed power output of 3237 MWt.
Despite weighing 4000kg each, the doors can be opened manually by one person
Recently, new shielding doors were installed at the Cofrentes plant to provide access to the blind flange of the inclined fuel transfer system (IFTS). The IFTS is used to transfer fuel and other irradiated items between the reactor and fuel buildings in the Mark III containment. Fuel bundles are transferred singly or in pairs in a carriage that passes through a water-filled tube. The blind flange of the IFTS is installed to separate the two buildings within this system. Access to the flange is controlled by the International Atomic Energy Agency (IAEA), thanks to a permanent seal which only can be opened with its authorization. The blind flange can only be removed during refueling outages or for maintenance tasks.
Due to the irradiated fuel bundles that are transported by the system, strong radiological protection is required in the area close to the access to the blind flange.
Before the installation of the doors, ten rows of five concrete blocks four columns deep (200 blocks in total) were used to shield this area. Each block was painted with decontaminable paint and weighed around 30 kg. In order to access the blind flange, the blocks had to be removed manually and carried to a storage area outside the reactor building (without any mechanical aids). It took six people four hours to remove or install the blocks, and for each refueling outage four block movements were required. (The blocks had to be removed to access the blind flange at the beginning of the outage and then reinstalled while the irradiated fuel was being transferred from the reactor building to the fuel building. At the end of the outage the blocks were removed again to put the IFTS out of service and were subsequently reinstalled).
Figure 2: The old equipment access to the dry well at Cofrentes was blocked by concrete
Removal and installation of the blocks required an enormous physical effort. Between 1994 and 2005 four industrial accidents were reported, as well as several minor incidents. In addition, due to the high dose rate in the area close to the blind flange, the radiological cost of the operations amounted to approximately 1000 μSv per outage.
The main driver for installation of the new doors was to eliminate industrial accidents and to reduce the collective dose rate associated with these tasks.
The new doors
Work to design the new doors to access the blind flange of the IFTS began in 2008. The doors could have been a similar design to those used to shield other areas in the plant such as the equipment entrance into the dry well, where new doors were installed in 2007 (Figure 2). However, the limited space (10 m2) near the access to the blind flange made the design of the doors more complicated, especially considering the high dose rate in the area during transfer of irradiated fuel bundles. In addition, the aim was to reduce the time necessary to open and close the access to the blind flange and eliminate the industrial risk involved with this task.
To meet these requirements, two new manual doors were designed and made from barite concrete, carbon steel and lead. The design of the doors included these three materials because the space for the door was limited and the total weight of both doors was limited. The doors’ unique design allows them to interlock to prevent a gap where radiation could escape from inside the IFTS (Figure 3).
Plan view of the doors, which meet in a curved, jagged edge to block the passage of radiation
Railings had to be removed to land the doors, hanging on a crane hook, on to a narrow walkway, and then manoeuvred along it around obstacles
Iberdrola Engineering and Construction was responsible for the design of the doors, with two engineers using the Monte Carlo code MCNP for radiological calculations and another two managing the technical design.
The doors were manufactured by MECANOL, a Spanish company that specializes in the manufacture and installation of tools and equipment.
The door’s unusual interlocking surface is visible post-fabrication, as are temporary welded lifting lugs
The doors were moved into the plant, installed and tested over two refuelling outages and one fuel cycle between 2009 and 2011. Iberdrola decided to install the doors during normal operation as not to hinder the tasks that needed to be carried out during an outage.
Approval from the Nuclear Safety Plant Committee was obtained in 2009 prior to the outage. The nuclear safety evaluation of the project included not only the design but also the movement of the doors and the temporary modification of the equipment required. No special evaluation was necessary in order to obtain regulatory approval for the project.
The doors were moved into the containment building during the plant’s 17th refuelling outage in September-October 2009. They entered through a door in the fuel building and were transported to the reactor building using a vehicle. The doors were stored on the refuelling floor inside the reactor building.
After plant start-up the doors were moved to their final location. This was one of the most difficult stages of the project and required the modification of some structures and movement of equipment such as railings, instrument panels, air conditioning equipment and valves. The handling was studied in detail and involved collaboration between the manufacturer, Iberdrola and the engineers responsible for designing the doors. The doors had to be moved above the suppression pool and other safety-related equipment.
Director of MECANOL Ángel Olivera says: “We did not imagine the difficulty of installing the doors when we produced them but the coordination with the maintenance supervisor during the execution phase was essential to resolve all the problems.”
Work to move the doors into the correct location began in March 2010 and it took one week to move each of the doors and an additional four weeks to install them. The installation period was very long because the plant was in normal operation. It was also necessary to modify the hinges and treads of the doors to facilitate maintenance.
Due to the design it is possible for one person to open and close the doors easily. Maintenance supervisor Alberto Vidondo said: “We can open and close the door with no mechanical help. It is incredible because the weight of each door is almost 4000 kg.”
In the 18th refueling outage (September-October 2011), during the movement of irradiated fuel through the IFTS, the shielding capacity of the doors was tested.
Two points were selected to carry out radiological measurements. Prior to the movement of fuel bundles, 10 mR/h was measured at each point. During the movement of the fuel, the measurements were repeated. Readings of 14 mR/h were recorded at the point in contact with the doors and 10 mR/h at a distance of 1 meter from the doors.
ALARA supervisor Baltasar Rodríguez said: “These new doors will avoid almost 1 mSv per outage and all the industrial risks associated with the job. The measurements of dose rate in the area confirm the adequacy of the design.”
Benefits
From the radiological point of view, the new radiological costs for accessing the blind flange of the IFTS during each refueling outage is now estimated to be 4 μSv, 250 times less than the total exposure received before they were installed.
Finally, in relation to outage management, 100 hours of workload have been eliminated per outage. The people that were involved in moving the concrete blocks can be allocated to other more relevant activities. Taking into account that the plant has an outage every two years, the plant is 27 years old and we want to operate the plant at least 60 years, between 1500 and 2000 man-hours will be saved thanks to the installation of the new shielding doors.
Author Info:
Rafael Rubio (rrubio@iberdrola.es), Iberdrola Generation, Tomás Redondo 1, 28003, Madrid, Spain. Javier Fernández, Iberdrola Engineering and Construction, and Baltasar Rodríguez, Iberdrola Generation, Paraje el Plano s/n, 46625, Cofrentes, Valencia, Spain.
This article was first published in the January 2012 issue of Nuclear Engineering International magazine.
