Modelling of core protection and monitoring system for PWR nuclear power plant simulator

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  • Pergamon Ann. Nucl. Energy, Vol. 25, No. 7, pp. 409-420, 1998

    1997 Published by Elsevier Science Ltd. All fights reserved PII: S0306-4549(97)00075-3 Printed in Great Britain

    0306-4549/98 $19.00+0.00




    Department of Electrical Engineering, Chonbuk National University, 664-14 Dukjindong, Chonju, Chonbuk, Korea, 561-756

    (Received 8 July 1997)

    Abstract--A nuclear power plant simulator was developed for Younggwang units 3 and 4 nuclear power plant (YGN Nos 3 and 4) in Korea; it has been in operation on training center since November 1996. The core protection calcu- lator (CPC) and the core operating limit supervisory system (COLSS) for the simulator were also developed. The CPC is a digital computer-based core protection system, which performs on-line calculation of departure from nucleate boiling ratio (DNBR) and local power density (LPD). It initiates reactor trip when the core conditions exceed designated DNBR or LPD lim- itations. The COLSS is designed to assist operators by implementing the lim- iting conditions for operations in the technical specifications. With these systems, it is possible to increase capacity factor and safety of nuclear power plants, because the COLSS data can show accurate operation margin to plant operators and the CPC can protect reactor core. In this study, the function of CPC/COLSS is analyzed in detail, and then simulation model for CPC/COLSS is presented based on the function. Compared with the YGN Nos 3 and 4 plant operation data and CEDIPS/COLSS FORTRAN code test results, the predictions with the model show reasonable results. 1997 Published by Elsevier Science Ltd.


    The safety and reliability of nuclear power plant heavily rely on the plant operator's ability to respond to various emergency situations of a power plant. The importance of operator's ability was well demonstrated in the investigation of the TMI accident in 1979. Investigators for the accident reported that mistaken operations in the emergent situation contributed greatly to the accident. The TMI accident also demonstrated the importance of operator training and the use of simulators for this very purpose. It has become stan- dard industrial practice to use simulators to improve the safety and reliability of nuclear power plant operations. For this purpose, a simulator was developed for Younggwang


  • 410 Jung Kun Lee and Byoung Sung Han

    unit 3 and 4 nuclear power plant, which is the basic model of the Korean standard nuclear power plant. The power plant is the first plant that has several conceptually different fea- tures compared with other nuclear power plants in Korea. Two of them are the core protection calculator and the core operational limit supervisory system. For the simulator to have the same function as the referenced plant, the core protection calculator (CPC) and core operating limit supervisory system (COLSS) simulation model was developed in accordance with the functional design requirement of CPC and COLSS system (CE- NPSD-366, April 1988; CE-NPSD-423, December 1988). The CPC is a digital computer based reactor protection system, which initiates reactor trip when core condition exceeds the departure from nucleate boiling ratio (DNBR) and local power density (LPD) design limit by performing on-line calculation. The COLSS is designed to assist operators in implementing the limiting conditions for operations in technical specifications. The simu- lator hardware systems are designed to provide the same features and appearance as reference plant's main control room, and software systems are in operation under real time UNIX system. For this study, we gathered nuclear and thermohydraulic data during the startup operation to get the parameters of various range of power level from Young- gwang nuclear power plant units 3 and 4, the reference plant for this simulator. These data are used to generate the reference values of parameters by CEDIPS and COLSS FORTRAN provided by plant system vendor. They are also used as inputs for the eva- luation of the simulation model.


    The core protection calculator calculates departure from nucleate boiling ratio and local power density of reactor core. When its values reach to setpoint during certain transients, CPC initiates reactor trip to prevent violation of design limits such as the low DNBR and the high temperature of fuel centerline (Design Spec. 1992).

    The minimum DNBR is calculated as follows:

    q DNn(K) (1) DNBRK - FK " q ~OCAL(K)

    The minimum is selected, and the adjustment terms are applied.


    DNBRsr = EDNm [DNBRMIN + EDNB2],


    where DNBRK = array of DNB ratio in hot channel, q ~NB = critical heat flux of hot channel (BTU/sec ft-2), q ~:OCAL(K)= heat flux distribution of K point hot pin, F/~ = non uniform heating correction factor for hot channel node K, DNBRsr=min imum static DNBR, EDNB1 and EDNB2 = constants for DNBR adjustment terms.

    The local power density is calculated as follows:

  • Modelling of core protection and monitoring system 411

    LPD = BLED" TR. PKMX. PFLeD, (3)

    where BLeD = corrected core average percentage of rated power, PKMX = maximum 3-D peaking factor computed in power distribution program, TR = azimuthal tilt allowance, PFH,o = CEA deviation penalty factor for LPD.

    The control element assembly calculator (CEAC) continuously measures positions of all control element assemblies to detect deviations. It provides signal to the four indepen- dent channels for the DNBR and LPD trip functions. The low DNBR and the high LPD trips assure that the specified acceptable fuel design limits of nuclear fuel is not exceeded during anticipated operational occurrences. They also assist the engineered safety features actuation system by limiting the consequences of certain postulated accidents.

    The core operating limit supervisory system is a part of plant monitoring system. It consists of process measurements and algorithms, which have the role of continuously monitoring the following limiting, conditions for operations: linear heat rate margin, DNBR margin, azimuthal tilt, and axial shape index of power distribution. The COLSS also assists the operator in maintaining core power equal to or below the licensed power. The main parts of COLSS consist of the power level calculation algorithm, the core power distribution algorithm and DNBR and LHR power operating limit calculation algo- rithms. In the core power calculation algorithm, the highest power among the secondary calorimetric power, turbine power and core A T (hot leg temp. - cold leg temp.) power is selected. The DNBR and LHR power operating limit algorithms determine the DNBR and LHR margins. The COLSS should always have some required power margins to meet fuel design criteria for any anticipated operational situations. This margin is called as the required over power margin (ROPM). The COLSS checks every second if the present plant status is within the COLSS limiting conditions for operations using the ROPM and measured state variables. If any of COLSS limiting conditions for operations is exceeded, COLSS alarms and operator action are taken according to the technical specifications.

    Linear heat rate power operating limit, the temperature dependent kW per feet limit, is calculated based on the inlet temperature as follows;


    TLIM1 -- TLIMO (4)

    If TCM1N ~ TLIMO then KLIM = FLIMO, and if TCMIN >~ TLIMI then KLIM = FLIMI, where KLIM = temperature dependent linear heat rate (kWft -~) limit, TCMIN=minimum com- pensated cold leg temperature, TLtMO = minimum temperature in the proportional limit region, TLIM1 =maximum temperature in the proportional limit region, FLIMO =linear heat rate (kWft -1) limit at TLIMO, FLIM! =linear heat rate (kWft -1) limit at TIJM1.

    The following calculations are performed for I = 1 to 40 axial nodes:

    KWFT(1) = T41 x TDpEAK(I) (1 + AZTILT) UNCERT x PP/IO0, (5)

    KWPFPL = (KLIM/KWFT(1)) PP x T42, (6)

  • 412 Jung Kun Lee and Byoung Sung Han

    where KWFT(1)= linear heat rate at node I, T41 = core average linear heat rate at rated power, TD?EAK(1)= 3-D peaking factor, AZriLr=the azimuthal tilt, PP=plant power (%), KLtM =temperature-dependent linear heat rate limit, T42 =adjustment factor for linear heat rate limit, UNCERT= adjustment factor for the calculation of linear heat rate, KWPFPL = the power operating limit.

    The linear heat rate power operating limit, KWPFOPL, is assigned as minimum value of KWPFPL(1) for I= 1 to 40:

    KWPFOPL = MIN[KWPFPL(1)] (7)


    The simulator for the YGN Nos 3 and 4 is a full scope replica type simulator for the training of plant operators. The CPC/COLSS on the simulator was designed to provide the same features as those installed in the reference power plant. To accomplish this, hardware identical to the plant main control room was installed. The hardware includes the same operator modules such as the plasma display unit, Cathode Ray Tube (CRT) and function keyboards. The simulation host computer system consists of one Silicon Graphics Challenge L server and Indy workstations operating under real-time UNIX OS. Table 1 shows the hardware configuration of the actual plant and that of the simulator.


    4.1. Model development process

    The first step in the development of the CPC/COLSS simulation model is the genera- tion of the software requirement specifications (SRS). The SRS is generated by compiling performance requirements and specifications from plant operators and plant data. The SRS defines the scope of simulation for each specific plant system. The next process is the development of detailed design specifications (DDS). In the DDS phase, the algo- rithm of model and system interfaces was developed. The design data for this are the CPC/COLSS functional design requirements (CE-NSPD-335), which are provided by the vendor. After the completion of the DDS, stand-alone test and non-integrated systems

    Table 1. Hardware configuration of the actual plant and the simulator

    Real plant CPC/COLSS Simulator CPC/COLSS

    Process Main control room

    hardware Computer system

    Reactor core, pump, pipe, instrument CRT, plasma display unit

    Computer system for calculation (CONCURRENT 3205/3280MPS)

    Model programs CRT, plasma display unit

    Simulation computer system (Silicon Graphics Challenge L and Indy Workstations)

  • Modelling of core protection and monitoring system 413

    test were implemented for the system model. Then, the system model is integrated with the simulator and put through an integration test. Finally, after the integration test, the software model is integrated with the hardware panel. Figure 1 shows the model develop- ment process of simulator.

    4.2. CPC/COLSS model program structure and process flow

    The model of CPC/COLSS is developed based on commonly used software so that modifications can be easily implemented throughout the simulator. The major tasks of common software are data gathering task, communication manager, scanning task and timer task. The display task receives information such as symbols, value, point ID from current value table, and displays this data on the plasma display unit and CRT in the main control room. The display task also receives operator inputs, and processes the input command. The plant CPC is designed to perform rapid and accurate calculations to maintain certain plant safety parameters within specified limits. To comply with this design requirement, the simulation host computer has a calculation algorithm. The CPC program module is composed of top-down configuration with segment modules placed

    / co,,.o.,on" / ~ . c o--,oo .... I

    Plant sys. SRS [

    I 4 [ Detailed Data [ Analyeis q

    I ' . . . . , . . . . . I DDS 8, Cod ing

    I .oo_,n.o o I" . . . . sys tem tes t

    HW ISW i n tegrat io r |

    es o

    Scope ana lys i s > I

    I Comp. ,,,~,,,. =.,,~ l [ Too,,, req,.,, . . . . . . I I o,3,',-qo, . . . . . . I I I


    Io_o . . . . . ~ . . . . I DDS & Cod ing ~STP E ) ~

    t - -~ .w Oeve 'oo- -n ' I - - - -~ "W O- - io I

    [ Acceptance tes t p rocedure ] [ deve lopment

    [ W,id. ion. Veri,ic.tio,, ]

    Y's ( Readyto Run ~

    I -I ~od" - ~od- ' I

    Y Lyoa ,~

    Fig. 1. Model development process of simulator.

  • 414 Jung Kun Lee and Byoung Sung Han

    in a hierarchical sequence. The structure allows independent compiling of the modules. The control module has the highest place in the program hierarchy. The control module calls segments, component, subroutines for simulation and sets the calculation cycle rate for the various submodules. The segment module is composed of mathematical equa- tions that model the CPC based on the functional design requirements. The segment module calls subroutines. The component modules simulate the control logic of various components in the actual power plant. The subroutines are commonly used for repeated functions. The subroutines are called by control modules, segments, components, and other subroutines. The functions are called by control modules, segments, components, and other functions. The COLSS program consists of five modules with 20 calculation algorithms written in C programming language. Figure 2 shows the configuration of the CPC model program.

    4.3. CPC/COLSS execution flow

    The simulation host computer executes CPC algorithm and the operator display calcu- lates the variables. The simulation systems communicate each other by accessing the data located in the global memory. They send the information through a send server of the communication manager to the receive server on the workstation. The scan tasks read these data and update the shared memory. Finally the display task scans the data in the CVT and displays it on the operator display. The COLSS algorithm calculates the various variables from the data stored in the workstation CVT. The scan task accesses the CVT and sends the information to the operator display terminal. Figure 3 shows the execution and data flow of the CPC/COLSS model program.

















    Fig. 2. The configuration of the CPC model program.

  • Modelling of core protection and monitoring system 415

    Communication Manager

    Global Memory

    Data Measured I I Data Calculated

    f Data ~ Scan Data J Gathering 4 Scan Table Measured ~ Calculated Data / ~ Data

    Send "~ ( Receive Server Server

    Measured Calculated Data Data

    Receive ) ( Send Server Server

    ..... . . / . . : Calculated Data...


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