Corresponding author: Dmitry B. Statsura ( statsuradb@nvnpp1.rosenergoatom.ru ) Academic editor: Yury Korovin
© 2021 Dmitry B. Statsura, Maksim Yu. Tuchkov, Pyotr V. Povarov, Aleksandr I. Tikhonov, Sergey P. Padun, Aleksandr P. Vorobyov, Margarita M. Mayorova.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Statsura DB, Tuchkov MYu, Povarov PV, Tikhonov AI, Padun SP, Vorobyov AP, Mayorova MM (2021) Using the unit software model to improve design solutions and optimize process management. Nuclear Energy and Technology 7(1): 3339. https://doi.org/10.3897/nucet.7.64982

Advanced design power units are distinguished by a high degree of digital transformation. Therefore, of particular interest are operator information (intelligent) support systems, which can reduce the workload on operating personnel as well as predict possible deviations long before they evolve into severe emergencies.
The article analyzes the current standard process documentation that requires solutions to support the operator and determines the list of system functions that should be provided to improve the safety level of nuclear power plants. A brief overview of the world experience in implementing such solutions is also provided.
As an example of the further development of operator support systems, the authors consider the operator information support system (OISS), which is being developed at the NvNPP pilot unit with the VVER1200 reactor. The OISS functions will make it possible to fulfill the requirements of standard process documentation that are currently not implemented in the power unit design.
The key features of the OISS under development are stepbystep interactive procedures and the unit software model. The authors provide a brief description of the power unit software model and consider several examples of its practical application as part of the OISS to improve design solutions and optimize automatic process control. In the years ahead, it is proposed to implement the OISS at power units under construction in order to reduce the information overload of operators and create conditions for a stepbystep increase in the automation level of the power unit control.
OISS, technological process, interactive procedure, power unit software model, decision making, monitoring, design solutions, optimization, APCS, algorithms
In accordance with the Nuclear Safety Regulations for NPP Reactors (
The comments to NP00115 (
The requirements for the operator information support are more fully set out in GOST R IEC 609642012. Nuclear power plants. Control Rooms. Design. (
Using the power unit software model, it is possible to diagnose the NPP state when developing guidelines for managing beyonddesignbasis events (including severe accidents).
As far as possible, the functions should be integrated into the overall control room design.
At the Biblis NPP in Germany in the 1980s, the STAR abnormal situation analysis system was introduced (
As part of the R&D implementation plan of Rosenergoatom Concern JSC, the requirements and basic solutions for the creation of an operator’s intellectual (information) support system (OISS) are being developed.
The interested organizations have agreed on the following list of functions of the OISS:
The functions of the OISS ensure the fulfillment of the operator support requirements set in (
Innovative Company SNIIPATOM JSC has developed a model of the operator information support system (MOISS), which includes a power unit software model and interactive applications that can be divided into logic units. The first version of the model has been functioning at NvNPPII1 from the date of startup.
The unit of interactive procedures contains applications designed to control the safe performance of work and support the operator in the stepbystep execution of unit startup/shutdown programs and standard switch cards. This tool cuts the time for processing information and reduces the likelihood of possible human errors and failures.
An example of interactive procedures is the unit startup procedure, which contains active links to operational documentation and has the ability to display the values of process protections and interlocks of equipment, control time delays for various operating modes of the power unit. Validation of interactive procedures in 2019 at NVNPPII1 confirmed that the procedures meet the requirements for them and are convenient for use by the MCR operating personnel.
The unit of interactive action charts (IACs) includes applications for analyzing the process behavior that reduce the load on the operator in situations with limited decision time. The IACs are designed for personnel to perform a number of sequential actions, when a process alarm is triggered on the MCRs, aimed at bringing the parameters and equipment to a state in which they do not exceed the operational limits and/or conditions of safe operation. The charts contain descriptions of possible causes of alarms and the operator’s actions to eliminate them. This unit was implemented in MOISS in accordance with (
The division into intelligent and information operator support (
Work on the creation of simulation models of complex thermal power facilities was carried out at the Institute for Problems in MachineBuilding (IPMash) of the National Academy of Sciences of Ukraine, where simulation models of condensing steam turbine plants for thermal power plants and nuclear power plants were developed (
The model of an NPP power unit with VVER1000 includes mathematical and software models as well as simulation modeling (
In 2010, the US Department of Energy established the Consortium for Advanced Simulation of Light Water Reactors (CASL). Its goal is to develop advanced computational models of light water reactors (LWRs) that can be used by utilities, fuel vendors, universities, and national laboratories to help improve the performance of existing and future nuclear reactors (
Similar work is being carried out for Generation IV reactors under the Nuclear Energy Advanced Modeling and Simulation (NEAMS) Program (
The Russian hardware/software package, “Virtualdigital NPP with VVER”, (
It is obvious that the operator’s decision support system (“Recommendations for optimizing technological process control”) is an intellectual component of the OISS, which is based on a power unit model that makes it possible to forecast various operating modes.
The main distinctive feature of the power unit software model used in the MOISS is that it was developed specifically for use in the APCS. Therefore,
At the same time, the MOISS uses not simplified models typical for training simulators but a neutronic code of increased accuracy. Calculations of the core power distribution are performed using the SVC program certified by Rostekhnadzor, which is based on a direct solution of the neutron transport equation and does not use, in contrast to training programs, the homogenization of computational cells and the diffusion approximation.
A detailed thermohydraulic model of the core (163×16 design elements) is characterized by the following features:
High discretization of the calculation model of the core makes it possible to obtain a detailed threedimensional field of thermohydraulic parameters throughout the core, i.e., to estimate more accurately the critical power ratio indicators and the thermomechanical operating conditions of the fuel elements in each fuel assembly.
The transverse leakages in the core should be taken into account in the case of using jacketless fuel assemblies, in order to assess the redistribution of flow rates across the core section, which occurs due to uneven power density, as well as in the case of using mixed fuel loads consisting of fuel assemblies with different hydraulic characteristics.
The thermohydraulic model is based on the solution of a system of fundamental conservation equations and closing relations. The system of main equations describing the processes of heat and mass transfer in the primary and secondary circuits of the power unit includes the continuity equation, momentum conservation equation (NavierStokes equation), energy conservation equation and state equations written for each phase. In addition to the basic equations, an insoluble impuritytransport equation is written to simulate the boron absorber transfer in the primary reactor coolant circuit.
Due to the fact that the equations to be solved are nonstationary, the resulting solution is a set of states of the system under consideration for different time slices to an arbitrary forecasting depth from a given initial state.
The system of basic equations after discretization is solved using numerical methods. Discretization implies splitting the elements of the calculation scheme into separate nodes and links.
The power unit thermohydraulic model is created from a set of computational schemes, which are sets of interconnected equipment elements. Calculation schemes may include
The software model is linked to the UULS signals, which makes it possible to initiate the simulation process from any current state of the power unit. The performance of the model is provided at the level of real time (or significantly higher). To make a forecast, it is possible to use the state of the power unit at an arbitrary moment in the past as an initial one with an accuracy of one second, while the entire operation history of the unit is archived. Using the over relaxation method, one can find a numerical solution to the system of equations describing thermohydraulic processes in an iterative manner, while the rate of convergence of the numerical solution makes it possible to achieve the required accuracy in a smaller number of iterations. Calculating the power unit parameters at a speed exceeding real time, even when models with high detail are used, is a prerequisite for the function of dynamic monitoring of the state of the power unit based on the comparison of the calculated data with the current sensor readings.
The APCS model is implemented on the basis of a separate software module that calculates the operating logic of model monitoring and control schemes. A set of calculation schemes is formed for it using the scheme editor, which is an integral part of the KRUIZ software environment. Sets of jointly compiled schemes reproduce the operating algorithms of the power unit main controllers as well as process protections and interlocks. Calculation schemes are formed on the basis of GET diagrams of typical software and hardware as well as the power unit APCS technical design and the RP general designer’ documentation. Therefore, the APCS model as a whole repeats the logic laid down in the APCS of the power unit. It is important to note that the operating algorithms for the unit controllers and process protections and interlocks are verified in the unit software model according to the results of the commissioning stage. In the same way, various technological systems and equipment of the secondary circuit, as well as auxiliary systems of the power unit, are simulated in detail.
As part of the OISS, the unit software model should be used to solve such problems as
The description of the sequence of operator’s control actions during forecasting at request (forecast scenario) is formed by the operator step by step as the calculation is performed. For standard sequences of actions, a prearranged scenario can be used. During forecasting, normal operation processes are simulated, including the unit startup from a cold state to operation at nominal parameters, unloading and shutdown, maneuvering power, as well as processes in the case of normal operation failure, including those accompanied by the fire protection activation and the unit accelerated unloading.
In addition to information support, the unit software model can be used to solve the following tasks:
Example 1. Increasing the power unit dynamic stability.
An example of using the unit software model for adjusting the process protections and interlocks and time delays is the numerical simulation of transient processes during experiments at NvNPPII1 when one feed electric pump (FEP) is switched off and the standby pump is not switched on. The level controllers in SG14 (SG LC) switch to “standby” mode when the flow rate at the head of any FEP increases over 2000 m^{3}/h. At the same time, they cover themselves, keeping the maximum feed water flow through the FEP in the range of 2000 m^{3}/h. Standby mode is removed when the level in the SG is restored to the nominal value. The unit software model was used to simulate the change in the main parameters (Fig.
Before FEP1 was switched off, the total flow rate at the feed water head was 7422 m^{3}/h. The flow rate on the switched off FEP decreased to zero in seven seconds. The flow rates of FEP2, 3, 4 increased up to 1953–2050 m^{3}/h at each FEP, and the steadystate flow time of at least 2050 m^{3}/h at FEP3 exceeded the setting of 90 s. As the power of the reactor plant decreased, the flow rates at the head of each FEP stabilized at the level of 1700 m^{3}/h. At the 220^{th} second, after the level in SG3 had increased to the nominal value, the SG level controllers switched to level maintenance mode. The FEP head pressure did not drop below 8.18 MPa. Based on the operation of the unit software model, it was concluded that the delay time to switch off should be increased to 300 s the operating FEPs according to the flow rate and pressure at their heads. The results obtained on the power unit model coincided with the results obtained by JSC VNIIAES, due to which it became possible to change the algorithms and successfully carry out dynamic tests.
One of the effective ways of improving the stability and ensuring the “survivability” of the NPP power unit is to develop methods for unloading the reactor when the main equipment of the primary and secondary circuits is shut down.
To analyze the unit dynamic stability, the conditions associated with switching off the FEP and the condensate electric pump (CEP) were simulated.
If one of the operating FEP is switched off and the standby one is not switched on, at 100% power, it is possible to fundamentally change the reactor unloading, replacing the operation of the power limiting controller (PLC) with the activation of the accelerated preventive protection (APP). When the APP is activated, the imbalance between the reactor power and the feed water consumption is eliminated almost immediately, and the power unit main controllers, including the feed unit of steam generators (SG), modify the secondary disturbances from switching off the FEP and triggering the APP.
An almost similar process occurs when the CEP is switched off, while a longterm imbalance remains between the reactor power and the secondary circuit power, which is determined by the condensate flow into the deaerator.
By simulating transient processes during shutdown of various types of equipment, it will be possible to make optimal decisions on changing the ways of unloading the unit as well as adjusting the settings for the process protections and interlocks and time delays for shutting down the equipment.
Example 2. Independent verification of algorithms for functional group control.
As part of the commissioning of the systems and equipment of the startup complex of NvNPPII1, 2, autonomous adjustment of the functional group control (FGC) was carried out. For a number of technological and logical reasons, the complex adjustment was partial.
In “forecast” mode, the power unit model can be used for performing the initial check and adjustment of the design FGC algorithms or developing new ones as a result of their approbation. To put the FGC into operation, it is necessary to check the operability of the design control and signaling algorithms in the interaction of technological equipment. It is also required to update the settings of thresholds of technological parameters, time characteristics of processes, FGC algorithms and corresponding changes in GET projects. These possibilities are provided by the unit software model.
In the future, the software tools tested as part of the OISS and during the FGC validation will allow the transition from functional group control of individual subsystems (lower level FGC) in the sequence described below to functional group control of the power unit in complex normal operation modes, such as the power unit startup/shutdown (upper level FGC).
Stage 1. Before each startup of the unit, based on the algorithms and implemented interactive stepbystep procedures, a description of the expected sequence of operator’s control actions (draft control scenario) is automatically generated. The operator is provided with the results of the forecast of the process advancement calculated by the power unit model with the possibility of correction. After the operator’s confirmation, the draft scenario becomes a scenario according to which
Stage 2. At each step of the interactive procedure, after automatic determination of the readiness for a control action, the operator allows this action.
Stage 3. For individual steps and/or groups of steps, the right to control is delegated to the program with a delay before issuing a timeout control command, during which the operator can block the command. The operator still has the opportunity to completely stop the FGC program. With the accumulated experience and trust in automation, the number of automatically performed steps will increase, and timeouts may decrease.
Example 3. During the operation of the turbine building nonessential services cooling water system at NvNPPII1, the problem of increased temperature of the cooling water (more than 31 °C) was revealed annually from May to September, which leads to accelerated contamination of heat exchange surfaces and outlet pipes of small diameter (DN32 and less) with carbonate deposits. To ensure the required .temperature regime of technological systems and fulfill the plan for generating electrical power, it is necessary to include standby equipment in parallel operation, which increases the risk of deviations in the unit operation or leads to the need for an unplanned reduction in the electrical load when it is shut down. The temperature of the cooling tower makeup water during the hottest period of the year does not exceed 25 °C. It is proposed to install an additional pipeline to supply makeup water with a flow rate of up to 1000 m^{3}/h directly to the suction of two service water pumps (SWP), which will reduce the cooling water temperature due to mixing. Using the software model, it became possible to calculate the change in the parameters of the cooling water after an additional pipeline was installed.
The following parameters were set for the calculation:
The results of calculating the maximum throughput of pipelines of various diameters and the corresponding decrease in water temperature are given in Table
Maximum throughput of pipelines of various diametersand the corresponding decrease in water temperature
Pipe diameter  Maximum flow rate, m^{3}/h  Water temperature at the pump head,°C 

DN200  286  31.4 
DN300  783  30.3 
DN400  1578  28.4 
DN500  2701  25.8 
Installing an additional external 500 mm diameter makeup water pipeline to the SWP suction will provide an acceptable temperature of the cooling water for the turbine hall nonessential services during the period of peak temperature loads.
Example 4. As a result of testing the SG blowdown operating modes, a low efficiency of the regenerative heat exchanger (RHE) of the SG was revealed (
The operation of the SG RHE is simulated using the unit software model. The most probable reasons for the RHE inefficiency were identified as follows:
The article presents the authors’ view of possible further stages in developing operator support systems based on the OISS, the implementation of the functions of which is currently being tested at NvNPPII1. The backbone components of the MOISS are the power unit software model and interactive procedures. The introduction of the OISS at the new generation power units, whether operating or under construction, will eliminate the information overload of operators and create conditions for a stepbystep increase in the automation level of the power unit control.
It is shown that the unit software model as part of the OISS can be used for