Helping to make refuelling virtually perfect

Norway’s Institute for Energy Technology has worked alongside Russian scientists from the Russian Research Centre Kurchatov Institute and the Leningrad power plant in order to develop a simulator that will provide help to the operators of the refuelling machine at the Leningrad power plant. The simulator was delivered at the end of May, and it uses virtual reality (VR) to reduce human errors during the refuelling process of the plant’s RBMK reactors. The project was financed by grants from Norway and co-ordinated by the Norwegian Radiation Protection Agency.

VR models of the refuelling machine and its local environment are used to extend a traditional display-based simulator. The operator manipulates and controls the simulator system from various “soft panels” on computers that are pictures of the real panels in the real control room. The refuelling process operations are then displayed in real time on several VR screens.

This simulator supports the refuelling machine operation staff by helping them to develop the basic knowledge and skills that they require, and by keeping skilled staff in close touch with the complexities of the machine.

The Leningrad plant is in Sosnovy Bor, which is located some 80km west of St Petersburg. Its RBMK design enables on-line refuelling operations to take place on full power steady state.

Refuelling operations involve withdrawing a spent or damaged fuel assembly and replacing it with a fresh fuel assembly, which is carried out with the help of a complex RM-488 or RM refuelling machine. Annually, over 400 refuelling sessions during full generation are completed on each of the four units at the plant. If any human errors were to take place, they may result in unit shutdown or radioactivity leaks, which would have a significant negative impact on power plant safety, productivity, and operation.

The refuelling machine is a long, upright container that is assembled from various separate elements. It comprises a high pressure housing about 20m high, with an inner diameter of approximately 500mm, and it is equipped with: a 4-seat rotating magazine in which to store long elements (these consisting of fuel bundle, spent fuel, different special products); with locking hermetic gates; a long element grip with a chain drive; and a mating connection with a hermetic wrench.

The housing is placed inside biological protection blocks fastened to each other with special clamps. The whole of the construction is solidly fixed within the body of a mobile trolley, which is itself mounted with special tracks onto a mobile bridge. The bridge is then mounted on special crane tracks inside the reactor hall. Electric drives on the bridge and the trolley move the refuelling machine horizontally.

The lower part of the biological protection section houses a pullout optical device, which is rather like a periscope, for precise sighting of the machine on the channel. The whole machine weighs approximately 350 tons.

The operation of the refuelling machine requires the utmost accuracy. The deviation of the housing axle from the vertical position in any position of the machine above the reactor plateau should not exceed 1°; the manual sighting tolerance of the optical device should not exceed 1mm; the force actions at the hermetic lock should be performed with an accuracy of 5kg relative to the specified value; the up and down axial forces at clamping should be performed with an accuracy of 10kg in relation to the specified value; and any alteration of the clamping axis relative to the casing end mark should be measured with an accuracy of 1mm.

It is possible to control the movements of the refuelling machine from a cabin in the machine. The operator first positions the machine on to the channel port, and the final targeting is done with the assistance of the periscope, at a very low speed. The main control of all the refuelling operation mechanisms is provided from panels located in another special radiation- protected control room.

Simulator structure

The simulator provides a detailed and thorough operation for every stage of the refuelling process, from the initial reactor hall overview and refuelling machine manoeuvring, to the reloading of a spent fuel assembly into the pool intermediate storage facility.

A number of severe accident malfunctions is implemented, with these incidents including fuel assembly leakage, tube damage, containment break and so on. The simulator comprises: •VR models of the reactor hall and of the refuelling machine. These models were developed by the team at the Institute for Energy Technology located in Halden, Norway.

•Soft panels of the refuelling machine cabin control unit as well as the control room control unit, and the mathematical models of the associated hardware and machinery. These items were developed by a team from the Russian Research Centre Kurchatov Institute, which is located in Sosnovy Bor, Russia.

•Jointly developed communication software in order to couple the mathematical models and the VR models.

The Education and Training Centre team at the Leningrad plant has supported the project by providing a lot of information about the refuelling machine’s internals and features.

Refuelling

A huge amount of prepartion is required prior to every refuelling session. A large number of different experts are called in to assist; a large number of different systems, assemblies and constructive devices are engaged; and numerous special preparation procedures and technological processes are applied.

An important part of all of these preparations is the initial check of the reactor hall and the refuelling machine to ensure that all of the equipment is ready and operational, and that the machine’s path is clear of obstacles. The VR part of the simulator plays a major role in this stage of the preparations, by allowing the operator to be able to “walk” around inside the VR model of the reactor hall, checking that it is clear in all respects.

Inside the cabin of the refuelling machine, the operator must move the machine to the wanted location. A soft panel represents the actual control unit located in the cabin. This control unit is used to move the refuelling machine around the reactor hall and to position it exactly over the fuel module that is to be replaced.

The control unit is used in conjunction with VR to see the actual position of the machine. This is helped by using several computer screens with VR pictures, and simulating the views out of the cabin windows. It is implemented by setting “viewpoints” in special places inside the VR model.

Using the periscope, the operator checks that the machine is targeted directly over the port. The periscope view also gives a VR view of the top of the reactor. As in real life, the refuelling machine may be positioned to within 0.1mm. After detailed positioning, the refuelling operation itself is simulated. In real life, this operation is executed from a nearby control room, and this is simulated by the use of soft panels.

The operation of the refuelling machine includes the moving of protection items in order to prevent radiation leakage, connecting to the fuel module, pressurising the machine with water, lowering a gripper, unsealing the fuel channel, moving the spent fuel into the refuelling machine and then exchanging it with a new fuel that is present in the revolver magazine, and disconnecting from the module.

During each stage, the operator controls a variety of hardware. VR views may be used to explore the internals of the refuelling machine at this stage. Transparency modes enable the operators to see the exact positions of every part of the machine and fuel module at any given moment. This will increase their knowledge of the inner functionality of the machine. In real life, however, this information is hidden within the containment of the machine.

After the main operation, the sealing is checked, the spent fuel is moved to the storage pool and the operator performs an integral analysis of the fuelling results and restores the refuelling machine to its initial condition.

Integration

The simulator is implemented on a network consisting of Pentium III-based PCs.

The VR models are in VRML format, and the required views use CosmoPlayer as a rendering platform under NT Workstations. The CosmoPlayer view is displayed as an ActiveX component. RedHat Linux PCs are used for the mathematical calculations and soft panels. Minimal set includes one Linux PC for calculations and soft panel display, and one NT workstation for data communication and VR view. The communication includes COM technology and a communication package called SWBus.

A typical setup includes six workstations. Additional workstations may be added with either the VR or soft panel views in order to improve the usability of the simulator depending on training demands. The simulator may also be used to test upgrades of the refuelling machine design and operation procedures before implementation into the real unit hardware and software.

The simulator development was co-ordinated using e-mail and joint meetings that were held both in Halden and Sosnovy Bor. The project has been financed through grants from the Norwegian authorities, and it is co-ordinated by the Norwegian Radiation Protection Authority. Future developments include 3D core integration into the full scope simulator of the RBMK unit, and extending the accident malfunctions set.