The Advanced Process Simulation Software (APROS), developed by VTT and Fortum, has a track record of modeling and analyzing nuclear power plant designs spanning four decades. Kaj Juslin, the principal designer of APROS, describes how the latest version is being used at the new EPR unit at Olkiluoto.

By: Kaj Juslin, VTT, Finland

At the end of the 1970s there were plans afoot for a fifth nuclear plant in Finland. APROS – Advanced Process Simulation Software – was first employed to support this effort. The key issue for the longevity of APROS and its related success was that sufficient resources were granted for advance planning and specification of the software’s architecture.

Its versatile architecture has allowed the continuous development of its operational range, introduction of additional chemical substances and implementation of new required physical mechanisms. The architecture has also facilitated the easy transportation of both code and application models to new computer and operating system platforms without jeopardizing the large amount of model specifications that the end users create. The initial separation of the simulation engine from the graphics interface was a vital decision. Today, the fourth generation of graphics interfaces is in use.

Key elements of the software architecture

APROS has a real-time relational database used for storage of model type specifications, model instance attribute values, model interconnections, state variables and other calculated variables, as well as specifications related to the scope and control of the simulation experiments. The database supports hierarchical model specification. Subprocess models are made up of process component models that in turn are made up of elementary model building blocks. Also, synthesizing process component models are available that automatically generate the relevant set of elementary models.

End users view of the Apros model hierachy
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Snapshots of the complete model specification database can be written as a binary file and restored to initiate a new simulation run. During the simulation all the state variable values may be stored from time to time to allow for backtrack and replay functions. Also, a specified set of variables may be recorded to an ASCII file with given time intervals.

The model specification language of APROS is used for the specification of the content in the real-time database resident in the APROS simulation engine. There is a line editor for entering specifications. Command queue files can also be edited in advance with any text editor for entering specifications. The whole database content can be written to an ASCII file and read back again. This is a very useful feature for transporting model specifications to new hardware and operating system platforms, as well as to new APROS versions.

Specification is the name of the game

The specification language is also used for communication with the graphical user interface. In fact, most of the process modeling is performed by drawing PI diagrams on the computer screen and filling in query forms to specify the model attributes. It is also foreseen that with the advent of semantic plant and automation design specification databases, the required modeling information can be transformed to the APROS specification language format. Accordingly the required simulation analyses would fit seamlessly into the other design processes.

The large set of versatile elementary models available supports the construction of dedicated process component models. The required physical mechanisms and structures that need to be considered can be specified. For instance, the flow through a pipe segment can be considered as single-phase flow, mixed-phase homogeneous flow or separated-phase, non-equilibrium flow, where the separate phases may have different velocities and temperatures. The dynamic momentum effects on the flow, as well as the momentum flux can be considered.

More complicated heat structures can be combined from plate, cylindrical or spherical elements for calculation of heat conduction. Radiative heat transfer can be considered between heat structures as well. Both thermal and fast reactors can be considered.

APROS is supplied with libraries of process component models for required technical domains. For instance, models of 1-D and 3-D nuclear reactor cores, vertical and horizontal steam generators, tanks, pressurizers, containment structures, steam and gas turbines, condensers, electrical systems and control systems are available.

Subprocess models can be assembled and stored for later reuse. A representative set of generic demonstration plant configurations is also included in relevant APROS delivery.

Fast and accurate data banks of material properties are included. All the required properties, such as density, temperature, viscosity of a homogeneous control volume, as well as the void fraction and relevant properties of the separate phases, are calculated from pressure and specific enthalpy of the mixture and the mass fractions of the included substances.

In addition, Arrhenius-type reaction kinetics can be specified for each control volume. The properties of water and steam have recently been extended beyond the critical point of water. The effect of soluble gases can be considered.

Also very small amounts of substances in the flows can be considered. The introduction of salts is a topical issue in desalination plants.

Fourth-generation nuclear power plants use new coolants, such as liquid sodium, liquid lead and gases such as helium and carbon dioxide. Custom material properties can be included as DLL libraries.

Elementary models are qualified by an extensive verification and validation procedure. Each time the source code of the APROS simulation engine is changed, the procedure has to be renewed. A large set of test models are available that replicate real test equipment. The general assumption is that if the models calculate correctly in each transient recorded from the equipment, then they will also calculate correctly in new configurations and new transients. Also, validation is made with measurements from real plants.

The standard OPC interface makes it easy to connect on-line to DCS software. Several applications of the software have shown the importance of having the possibility of checking the configuration in advance with a simulator instead of the real plant. A specific communication object library is also available for the development of efficient custom interfaces. APROS has a communication library available for efficient connection to native external codes.

Present nuclear applications in Finland

APROS has been used in the analysis required in recent power upgrades and operational lifetime extensions of today’s nuclear plants in Finland. There are two boiling water plants of Swedish design in Olkiluoto having a net capacity of 860 MW each and two pressurized water VVER plants of Russian design in Loviisa having a net capacity of 488 MW each. APROS was also previously used for the advance evaluation of smaller retrofits of these plants

The foundations of the 1600 MW pressurized water EPR under construction at Olkiluoto. APROS was used by VTT for independent analysis of the construction licence application and will be applied in the same way for the operation licence application review. Source: Teollisuuden Voima Oy
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The multi-functional capabilities of APROS are being used in the present control system replacement project in Loviisa. First, it has been used as a simulator for the evaluation of the functionality of the design of the new operators’ displays to replace the present process computers and panel equipment in the control rooms.

The results were used to specify the display design for the new DCS system. In this case the process, the automation systems and the display functionality were simulated. Second, the process model is now in use for factory tests of the functionality of the complete new DCS system before installation at the site. Third, APROS will be used to upgrade the full-scope training simulator at the Loviisa site.

APROS was used by VTT for independent analysis of the construction licence application for AREVA’s 1600 MW pressurized water plant (EPR) under construction in Olkiluoto. APROS will be applied in the same way for the operation licence application review. This is the very first GEN III plant to be delivered.

The APROS software was used in retrofitting the full-scope training simulator for the operators of the Loviisa plants in Finland. A copy of the renewed control rooms will be available at the simulator. Source: Fortum Power and Heat
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Presently there are no measurements from a relevant plant. These analyses exemplify dependable modeling completely based on detailed design data. The modelers at VTT were not familiarized with possible results of analysis calculations with other codes.

Experiences from PLANT modeling

GEN IV nuclear power plants with associated nuclear fuel cycle facilities resemble chemical plants. On-site reprocessing of fuel is planned for GEN IVß nuclear power plants, as well as production of hydrogen and synthetic hydrocarbon fuel. The applicability of APROS tothe simulation of chemical processes has been verified by the extensive simulation of unit operations of Finnish pulp and paper mills, such as digesters and recovery boilers.

Supercritical steam values were already a design issue in conventional power production plants as a way of increasing thermal efficiency and are hence available in APROS. Efficient heat exchange from hot flue gases is already managed in conventional plants.

APROS has also been developed to suite modeling of future completely clean combustion plants including fuel preprocessing and flue gas cleaning processes. APROS is now already in use in 20 countries. The experience derived from applications of the software and feedback from users will benefit its use in GEN4 nuclear plants too.


A new user interface for ubiquitous modeling and simulation access to the APROS simulation engine is already under development. It will support concurrent engineering in virtual workgroups as enabled by the internet.

Version control of the model configuration will be added to the new teamwork features. Cyber security can be promoted by using thin-client software in mobile computers and thus locating confidential data repositories and simulation engines to secure servers.

Intelligent interfaces between APROS and certain CAD tools such as plant design systems have been requested. A semantic, ontology-based approach is applied in order to map the data models of the design systems to the data model in APROS. Standard classifications like ISO 15926 Part 4 can be utilized for mappings. 3-D plant models can be linked to the simulation models using this technique.

Most of the simulation model can be auto-generated and simulation data can be visualized in a 3-D model. Manual redrawing of PI diagrams and retyping of parameter data is perhaps soon to be history, at least with regard to new plant designs. Of course, there are many old plants that will continue to use paper-based documentation.

Porting of the APROS simulation engine to allow it to benefit fully from future multi-core processors has started. In a few years an ordinary PC may be capable of simulations that are more than 100 times faster than real time.

This will enable new powerful engineering solutions, such as improved practices in plant design optimization, efficient implementations of nonlinear model predictive control and novel operator support tools based on predictive simulation.

Kaj Juslin, D.Sc. (Tech.), is the principal designer of APROS. He is chief research scientist at the VTT Technical Research Centre in Finland.