Practice refuelling on a PC

For many years now, EDF has been training the teams in charge of loading and unloading fuel at the CETIC, a technical expertise centre in Chalons-sur-Saône, using a replica of the fuel loading machine as installed at its 1300MW nuclear power stations in France. In order to master the key procedures, trainees have a pool with a reactor tank, inert fuel assemblies and a loading machine. A new simulator will add on a virtual spent fuel pool, transfer system and bridge, so operators and supervisors will be able to practice with the full configuration they work with in power plants.

A 1300MW pressurized water reactor core has 193 fuel assemblies, each approximately 4 metres in length and comprising 264 tubes filled with uranium dioxide pellets. It is these assemblies which need to be loaded when the plant is commissioned, and then partially replaced once every 18 months.

The operations take place underwater in two pools. The fuel assemblies stored in the fuel handling building are picked up from above by a grab and placed in a transfer machine: they travel through this to the reactor building. Having previously emptied all cells, the loading machine places these assemblies in the core in certain locations. Spent rod assemblies are returned along the same route. These spent assemblies may be distorted, having been subjected to very high temperatures, so handling them is a delicate operation. In both the fuel and reactor buildings, one person controls fuel assembly handling, under the direction of a supervisor (the loading manager and fuel building manager respectively) to ensure the integrity of the fuel assembly being handled.

It needs to be borne in mind that fuel assembly operations require both excellent mastery of the process and extreme precision. Square fuel assemblies approximately 25cm wide and 4m long have to be transferred between two buildings, via a mechanical tunnel. They must be moved without being bumped in the slightest, and slid into place with millimetre precision once they arrive at their destination. To complicate matters still further, these operations take place 15 metres under water, and spent bars are still very hot – several hundred degrees centigrade!

In real-life situations, the supervisor has an array of cameras which they coordinate using a video control desk. In the simulator, 3D imaging will simulate this system as realistically as possible.

The systems supplied by CORYS, a 66:34 joint venture between Areva and EDF, are not so much virtual models of the machinery as they are of the fuel and environment. The data they provide are sent to the refuelling machine computer: the simulator uses the machine’s Schneider Electric programmable logic controllers (PLCs). “The software model we are providing is allowing it to behave as if it has input from actual device sensors,” says CORYS product manager Pierre-Jean Ajovalasit. To work with PLCs, the simulator vendor has had to define a communication protocol to exchange data through TCP/IP, so that the PLCs can understand it.

There are two main advantages of using real-life PLCs. First, they can replicate all the functionality of the real-life system, for example enabling operators to be able to use all three of the machine’s modes: automatic, semi-automatic and manual. This attribute not only instructs them in those operations, but also reproduces the control issues they might encounter when switching between modes, for example.

Second, the PLCs provide an independent check on every manoeuvre, to make sure that the expected (PLC) and calculated (simulator) positions of the system are consistent. “For any operation, there are procedures to follow, in steps. The operator will not do anything until it has been validated by the supervisor. In standard refuelling operations in France, there are four persons: fuel pool operator, reactor operator, and each has their supervisor. The supervisor is looking after the process, and checking the actual position of the fuel rod, and where it should be according to a pre-set sequence,” says Pascal Gain, vice president, power plant simulation, CORYS.

The new simulator will include screens that virtually represent the spent fuel pool, transfer system and bridge, and will switch between the real world and the virtual one. In practicing removing fuel assemblies, for example, operators will take out a physical fuel assembly model (without any fissile material) from the physical reactor model, and place it on a transfer station. Then, a screen will show a virtual picture of the virtual fuel assembly travelling through the transfer system. They will be able to virtually move the virtual fuel assembly to a virtual rack location in the spent fuel pool, and see a virtual picture of virtual fuel assemblies in the virtual spent fuel pool. However the simulation will not cover re-racking operations.

In addition, a second simulator system will model the entire environment—fuel loading machine, pool and reactor—virtually. Unlike the physical model, this equipment can be moved from one power plant to another for refresher courses just before each refuelling operation, which takes place once every 18 months on average. This is too infrequent for the personnel concerned to remember each procedure when the time comes for them to put it into practice – so much so in fact that the employees in question describe this intervention as an ‘activity’ rather than as part of their core job description.

The arrival of the simulators is taking place as part of the ten-year overhauls of 1300MW power plants alongside on-site upgrades, including computerised supervision that replaces most of the person-to-person information exchanges used previously. Also, the Chalons-sur-Saône simulator is being upgraded with PLCs. The simulators are expected to be completed in?Spring 2011.