RESEARCH ON MANOEUVRING CAPABILITIES OF A NUCLEAR POWER PLANT WHEN SWITCHING IN-USE CONTROL PROGRAMMES

Using a known control programme [1], manoeuvring capacity may be achieved, which brings the consumption schedule of electric power to conformity with the schedule of electric power generation [2]. However, it is impossible to reach full compliance due to an unexpected accident or an inclusion of new customers in an electric network. Therefore, an adaptation of existing power units to new specific working conditions becomes of current importance. It will allow not only nuclear power plants operation at the manoeuvring capacity but also even manoeuvring power control programmes of nuclear power plants (NPP) with the WWER-1000 in operation depending on the selected factors that influence the switching.


Introduction
Using a known control programme [1], manoeuvring capacity may be achieved, which brings the consumption schedule of electric power to conformity with the schedule of electric power generation [2].However, it is impossible to reach full compliance due to an unexpected accident or an inclusion of new customers in an electric network.Therefore, an adaptation of existing power units to new specific working conditions becomes of current importance.It will allow not only nuclear power plants operation at the manoeuvring capacity but also even manoeuvring power control programmes of nuclear power plants (NPP) with the WWER-1000 in operation depending on the selected factors that influence the switching.

Analysis of previous studies and statement of the problem
In [3,4], control programmes were analysed to identify advantages and disadvantages of each control programme.
The main disadvantage was absence of studying control programme modes to explain why power declined with one control programme in use and why it increased while another programme was used.Systematized in [4], the control programmes' defects were attributed to the fact that during their creation special multi-level charts had not been developed.They could be used when working with models in large automation systems.All this would bring clarity to the concepts' presentation at the level of individual objects, states, and processes.
Energy release in the reactor core with manoeuvring power was examined in [5,6].Issues of structural optimization and how the heat flux changed at various control programmes were considered in [7,8].A regulator of boric acid to control the value of nuclear power capacity was offered in [9].A new automated control power system that included the regulator of boric acid was examined in [10].The development of new methods to control the NPP and to increase its safety was relevant and valid, but none of the suggested methods included the switching of power control programmes at the reactor unit being in ЭНЕРГОСБЕРЕГАЮЩИЕ ТЕХНОЛОГИИ И ОБОРУДОВАНИЕ

The purpose and objectives of the study
The purpose of the research is to develop object-oriented analysis (OOA) for the automated control system to enhance the capabilities of a manoeuvring unit with the WWER-1000 by switching the power control programmes of a NPP during its operation.
In accordance with the set purpose, the following research objectives have been identified: -to develop an information model of an automatic power control system; -to construct a state model of the nuclear reactor and the control system; -to design technological algorithms of switching the existing power control programmes at the moment of reduced capacity.

1. The development of an information model of the automatic power control system
An OOA theory has all of the aforementioned positive aspects, and it is advisable to apply it to the analysis of an auto-matic power control system of a NPP with the WWER-1000 in the suggested mode of operation.
According to [5], OOA is developed in three stages: an information model, a state model, and a process model.Abstraction of objects, their properties, and a set of relations in a single chart are located in the first stage.The next one presents a sequence of actions in states and events.This is the second stage of the OOA.Then, a transition diagram of these actions (double pole double throw (DPDT)) is developed at the third stage.
On the theoretical basis [5], a flow chart was built for the information model of an automatic power control system of the reactor plant with the WWER-1000.It is shown in Fig. 1.
From the point of view of analysing the subject area, the following objects were identified: the equipment of the first and second circuits, technical means of the low-level and upper-level control systems.Upon the recommendation [5], to complete the representation of the information model, its description was developed, which is summarized in Table 1.
The attributes marked in the information model were also summarized in Table 2 for description.The mathematical expressions presented below were taken from [2,3].
As shown in Fig. 1, all objects of the information model are linked by three types of connection: for example, a "one-to-one relationship" between the control system and different types of regulators; a "one-to-many relationship" between the nuclear reactor, the steam plant, the pipe conduit of the first circuit, and the turbogenerator with regulators.At the same time, a "super type-to-subtype" connection identifies the base object regulator and copies of this object, as, for example, the regulator of the turbogenerator electric power and the regulator of rotary rotations per minute.The heat capacity is 750 MW; the evaporation is 1,470 t/h; the steam pressure is 64 atm; the steam temperature is 278.5 °С ID of the steam plant (R15); ID of the regulator of the input temperature coolant in the nuclear reactor (R6); ID of the regulator of steam pressure in the SP (R7); ID of the turbogenerator (R16); ID of the timer (R13); steam flow consumption; pressure in the steam plant; the coolant temperature at the outlet of the steam plant n is rotary rotations per minute power frequency = n 50 Hz N el is electric power of the turbogenerator steam flow consumption

2. A state model construction of the nuclear reactor and the control system
In this study, only two states of the model are presented: the nuclear reactor (NR) model and the control system (CS) one.State models of other objects of the information model are not really interesting because they can only be in two states: on/off.Fig. 2 shows the state model of the nuclear reactor (NR).

Fig. 2. A state model of the nuclear reactor (NR)
Table 3 below describes the events of the state model of the nuclear reactor (NR).Nowadays, in-use NPPs are operated under a control programme with a constant pressure in the second circuit, and the power manoeuvring is not carried out.In this regard, it has been proven [6] that still power changing of a NPP, with account for the schedule of daily pressure, can be performed, and it does not affect the value of important technological parameters.
Therefore, it is necessary to consider whether the important technological parameters are supported at a certain level: for example, if the nuclear reactor is unloaded from 100 % to 80 % of power by using one control programme, with 100 % of power reached by using another control programme.
State models of the nuclear reactor and the control system were considered in terms of an infinite cycle.
A state model of the control system is shown in Fig. 3.It was assumed that the initial state of the NPP was "Operation at 100 % of power".After 16 hours, the timer's signal is activated, and the process of power reduction begins at a scheduled rate.If the transient is completed, the system moves to a new steady state of "Operation at 80 % of power".If the process is not completed, calculation of the objective function determines the need to either switch the equipment or to continue the operation in the current configuration.In both cases, the system returns to the standby state of the completing the transition process, and the cycle is repeated.After switching the control programme and after 8 hours, the power increase of the nuclear power unit begins.A new state of "Operation at 100 % of power" begins after the completion of the process.
Below in Table 4, there is a description of the events from the state model of the control system (CS).

3. Technological algorithms design of switching the existing power control programmes at the moment of reduced capacity
The third stage of the OOA implies a development of an action data flow diagram (ADFD), but in [7] the ADFD was replaced by a technological algorithm as a sequence of operations for a leap from one value of the technological parameter to another.In this study, using the proposal of [7], the following technological parameters were selected: the coolant temperature at the inlet and outlet of the reactor, the average temperature of the coolant, the pressure in the second circuit, and the quantitative measure of the reactor stability as the axial offset (AO).One of the problems when applying the disturbance to exploit the reactor is to maintain it in a stable condition [8,9].The change character of technological parameters while changing the power of the unit is determined by control programmes, so it is interesting to consider how the change of technological parameters in different control programmes affects the AO.Fig. 4 shows the characteristics of the NPP with the WWER-1000 in two control programmes. 1 is the point from which the reactor temperatures decline: the outlet, average, inlet and saturated steam temperatures, 1.1 is steam pressure in the second circuit while operating under the control programme at a constant average temperature; 2 is the point from which the reactor temperatures decline: the outlet, average, inlet and saturated steam temperatures, 2.1 is steam pressure in the second circuit while operating under the control programme at a constant temperature at the inlet of the rector Fig. 4 shows the values of such parameters as temperature and pressure at different power levels.The dotted line indicates the transitions from one control programme to another while being switched.These transitions are the key moment of the research that will show if it possible to switch control programmes at 80 % of the reactor power.

Research results of switching control programmes at 80 % of power
An input coolant temperature in the nuclear reactor is calculated according to formula (1): where τ Q( ) is the amount of heat transferred from the first circuit to the second, MW; τ P( ) is the steam pressure in the second circuit, MPa; k is a heat transfer coefficient, W/(m 2 •K); е F is the total effective area of the heating surfaces in the steam plant, m 2 .
The temperature of the coolant at the reactor outlet was calculated by (2): where Ср is the specific heat capacity of the coolant, J/kg•K; 0 ô is the coolant delay time, sec; m is the coolant mass in the reactor core, kg.The average temperature of the coolant calculated by formula (3): The steam pressure in the second circuit was calculated as follows: ( ) where s G is the steam flow rate, kg/s; fw G is the feedwater flow, kg/sec.
The axial offset was calculated by formula (5): where Q t is energy release at the top of the core; Q b is energy release at the bottom of the core [3].
Table 5 shows the solution of the equations (1 through 5) for the two control programmes at 100 % of power of the nuclear unit.Using a mathematical model of the NPP with the WWER-1000 [3], realized on the basis of Matlab Simulink, simulation experiments on switching control programmes were carried out.In total, six switches were made, such as from Т av =const to Т in =const; from Т in =const to Т av =const; from Т av =const to Р 2 =const; from Р 2 =const to Т av =const; from Т in =const to Р 2 =const; from Р 2 =const to Т av =const.In short, the study presents the results of a single switching: from Т av =const to Т in =const.The simulation results confirmed the expectations.The experiment was conducted as follows: the power of the reactor unit was reduced to 80 % when working on the control programme at a constant average coolant temperature.An increase to 100 % of power was carried out under the control programme at a constant temperature of the coolant at the reactor inlet.Below are transient graphics.Below are transient graphics (Fig. 5-9).
For visualization, the simulation results are summarized in Table 6.From the data obtained as a result of simulation, it in obvious that the transient of selected values range from 30 sec to 4.16 min, the deviation is in the range of 0.35 to 3.7 rel.units, and the deviation of the AO values is 0.01 %.These figures are admissible in the procedural options while operating the nuclear power plant.According to Basic rules for operating nuclear power plants, a maximum deviation of the axial offset value is 2.59 %.The value of 2.59 % is the boundary; excess above this regulation is prohibited, and it leads to a forced stop of the reactor unit.This research is a continuation of a number of previous studies.In [9], a regulator of boric acid was suggested for controlling the reactor unit with the WWER-1000.The new power control programme with a constant inlet coolant temperature was invented in [10].The present study is based on the principles of the aforementioned suggestions and innovations in the field of automation of power plants.Hence, the proposition of switching the existing power control programmes was successfully performed.It is for the first time that a minimum deviation of the AO was reached not in a stationary but in a transition mode of the power unit.It indicates a uniform energy release at the top and bottom of the reactor core as well as stability of the reactor unit while switching the existing power control programmes at a reduced output.Thus, the research has been quite beneficial, and its results can be applied at Ukrainian nuclear power plants with the WWER-1000.

Conclusion
1.A complex automatic power control system was developed by using three stages of OOA.This allowed switching power control programmes at Matlab Simulink.The equipment to operate one control programme was disconnected while the equipment responsible for the operation of another control programme was activated at 80 % of power, with minimum deviation values of technological parameters.
2. Moreover, according to the simulation results, control programmes during the operation of the nuclear power plant can be switched.The reactor unit remains in a stable state not only after applying such a disturbance as power reduction or increase but also after switching control programmes, which is evidenced by the values of the axial offset as a quantitative measure of the reactor stability.

Fig. 1 .
Fig. 1.A flow chart of the information model for an automatic power control system

Fig. 4 .
Fig.4.Characteristics of the NPP with the WWER-1000: 1 is the point from which the reactor temperatures decline: the outlet, average, inlet and saturated steam temperatures, 1.1 is steam pressure in the second circuit while operating under the control programme at a constant average temperature; 2 is the point from which the reactor temperatures decline: the outlet, average, inlet and saturated steam temperatures, 2.1 is steam pressure in the second circuit while operating under the control programme at a constant temperature at the inlet of the rector

Fig. 3 .
Fig. 3.A state model of the control system

Fig. 5 .Fig. 6 . 6 .
Fig. 5.A temperature curve at the reactor inlet where the switching point is highlighted by vertical lines and is shown in an enlarged view above

Table 1
Description of the objects of the information modelID of the nuclear reactor; ID of the neutron power regulator (R2); ID of the average temperature regulator -the first circuit coolant (R3); ID of the axial offset (АО) regulator (R4); ID of the pipe conduit of the first circuit (R1); ID of the timer (R11); T in , T out , submerged length of control rods of the 10th group, neutron power, and an axial offset (AO)

Table 2
Description of the information model attributes

Table 3
List of events

Table 4
List of events

Table 5
Solution of the above-mentioned equations for the two control programmes at 100 % power of the nuclear unit

Table 6
Simulation results in numbers