Real Time Computer for Plugging Indicator Control of Prototype Fast Breeder Reactor

M.Manimaran, P.Manoj, A.Shanmugam, N.Murali, S.A.V.Satya Murty

Abstract—Prototype Fast Breeder Reactor (PFBR) is in the advanced stage of construction at Kalpakkam, India. Liquid sodium is used as coolant to transfer the heat produced in the reactor core to steam water circuit. Impurities present in the sodium are removed using purification circuit. Plugging indicator is a device used to measure the purity of the sodium. Versa Module Europa bus based Real Time Computer (RTC) system is used for plugging indicator control. Hot standby architecture consisting of dual redundant RTC system with switch over logic system is the configuration adopted to achieve fault tolerance. Plugging indicator can be controlled in two modes namely continuous and discontinuous mode. Software based Proportional-Integral-Derivative (PID) algorithms are developed for plugging indicator control wherein the set point changes dynamically for every scan interval of the RTC system. Set points and PID constants are kept as configurable in runtime in order to control the process in very efficient manner, which calls for reliable communication between RTC system and control station, hence TCP/IP protocol is adopted. Performance of the RTC system for plugging indicator control was thoroughly studied in the laboratory by simulating the inputs and monitored the control outputs. The control outputs were also monitored for different PID constants. Continuous and discontinuous mode plots were generated.

Keywords— flow control, hot standby architecture, plugging indicator, real time computer, soft PID, sodium purification, temperature control

I. INTRODUCTION ROTOTYPE Fast Breeder Reactor (PFBR) is a 500 MWe liquid sodium cooled, mixed oxide fuelled, pool type

reactor which is in the advanced stage of construction at Kalpakkam, India. Primary sodium circuit is used to transfer the heat generated from the core to secondary sodium circuit which in turn transfers the heat to steam water circuit. The entire primary sodium circuit is contained in a single vessel called main vessel which consists of core, two primary sodium pumps and four intermediate heat exchangers. Cold and hot primary sodium has been separated by inner vessel which is inside the main vessel. Cold sodium enters through the core using two primary sodium pumps which removes the heat from the core and becomes hot sodium. The hot primary sodium is radioactive and is not directly used to produce steam. Hence two independent secondary sodium circuits,

All authors are with Real Time Systems Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, India. (maran@igcar.gov.in, manojp@igcar.gov.in, shanmugam@igcar.gov.in, murali@igcar.gov.in, satya@igcar.gov.in)

each having a sodium pump, two intermediate heat exchangers and four steam generators are used for heat transfer [1].

The presence of impurities in liquid sodium can cause blockage in fuel subassemblies, hindrance in fuel handling mechanism and solidification of sodium at temperature higher than melting temperature, leads to several undesirable consequences in reactor systems. Sodium purification circuit is provided in order to maintain the required level of sodium purity. In PFBR, separate purification circuits are provided for purifying the sodium in primary circuit and secondary circuit. Cold trap and plugging indicator are part of purification circuits. Cold trap is used to purify the sodium and plugging indicator is used to measure the purity of sodium. Plugging indicator needs closed loop control system to measure the sodium purity.

Instrumentation & Control systems of PFBR are classified into three classes namely safety class-1, safety class-2 (SC2) and non-nuclear safety systems, based on safety considerations [2]. The signals of primary sodium purification circuit are classified as SC2. Versa Module Europa (VME) bus based Real Time Computer (RTC) systems are deployed for I&C of sodium purification circuit. Fault tolerant architecture is used for sodium purification in order to meet the single failure criterion as stipulated by Atomic Energy Regulatory Board, India [3],[4].

Proportional-Integral-Derivative (PID) controllers are widely used in process control industry. According to the literature, PID controllers are the most dominating form of controllers and more than 90% of control loops are based on PID [5],[6]. Most loops are in fact PI because derivative action is used only when there is sudden change in process. The strength of PID control is that it deals with actuator saturation, integral windup and tuning of PID parameters [5]. Software based PID control algorithm is used for plugging indicator control.

The organization of this paper is as follows: Section 2 describes about the operation of purification circuit and the need for plugging indicator & its control. Section 3 discusses the hardware configuration and the fault tolerant architecture adopted for plugging indicator control. Section 4 details about software development and implementation of PID algorithm in software. Section 5 explains about implementation of hot standby architecture in laboratory and performance study of control algorithm and section 6 provides the conclusion.

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II. OPERATION OF PURIFICATION CIRCUIT The purification circuit consists of main vessel,

electromagnetic pump, exchanger economiser, cold trap and plugging indicator. Sodium is drawn from the main vessel for purification and then it is sent back to main vessel after purification. Electromagnetic pump is used to draw the sodium from main vessel and is transferred to cold trap through exchanger economiser. Sodium is purified in cold trap and the purified sodium is sent back to main vessel through exchanger economiser. Sodium is sampled from the return line of main vessel and the purity of sodium is measured using plugging indicator [7].

A. Principle of Sodium Purification Impurities such as sodium oxide and sodium hydride get dissolved in sodium at higher temperature. When the sodium temperature is lowered then the dissolved impurities get precipitated. This principle is used for sodium purification. Cold Trap is a tank in which the sodium temperature is lowered and passed through a stainless steel wire mesh placed in it. As the temperature of sodium is reduced the dissolved impurities get precipitated and trapped in the wire mesh which results in purified sodium [8].

B. Working principle of Plugging Indicator Plugging indicator is a device used to measure the purity of the sodium and it works on the same principle as that of sodium purification. By decreasing the temperature of sodium flowing through the plugging indicator, plugging of the orifices by sodium impurity takes place. When the sodium flow through the orifice plate drops to 80% of the unplugged orifice flow, the temperature measured at the cold point of the plugging indicator is called as plugging temperature which gives the indication of the impurities in sodium [8],[9].

C. Plugging Indicator and its Controls Fig.1 shows typical schematic of plugging indicator. It

consists of economiser, cooler and an orifice. The economiser is made up of two concentric tubes one inside the other. Sampled sodium is passed to cooler through inner tube of economiser and the outlet of cooler is connected to main line through the outer tube of economiser. The economiser helps to reduce the heat load on the cooler and also to increase the temperature of outlet sodium of the cooler in order to reduce the thermal shock on the connection point where the sampled sodium is mixed with main line. At the end of economiser cooler is provided. Nitrogen is passed through the cooler in order to reduce the temperature of sodium. Orifice plate is provided at the end of the cooler in the annulus between inner and outer tube. Electromagnetic flow meter is provided near to orifice to monitor the flow through the plugging indicator. K-type thermocouple is provided at the outlet of cooler to monitor the cold point temperature of plugging indicator [7].

Separate nitrogen cooling circuit consists of nitrogen blower and control valve is used for cooling the sodium of plugging indicator. Blower operates at constant speed and the control

valve positioner is adjusted to achieve different rate of cooling. Fig.2 shows the closed loop control for plugging indicator which is achieved using VME bus based RTC system.

Plugging indicator can be operated in ‘manual’ mode or ‘auto’ mode. RTC system does the following operation for plugging indicator control [10].

• In ‘manual’ mode, signal for the control valve is given by the RTC system based on the value obtained from Graphical User Interface (GUI).

• In ‘auto’ mode, the plugging indicator can be operated in continuous/ discontinuous mode.

• Auto/manual selection, continuous/ discontinuous mode selections are accomplished through GUI

1) Continuous Mode of Operation: In the continuous mode of operation, partial plugging of orifice corresponding to 80% of the unplugged orifice flow through plugging indicator is maintained. The nitrogen flow is adjusted so that the orifice is always kept partially plugged. If the impurity level increases/decreases, sodium flow tends to reduce/increase and hence the corresponding control signal is given to control valve positioner in order to maintain 80% of nominal flow. 2) Discontinuous Mode of Operation: In the discontinuous mode of operation, the sodium temperature is gradually lowered to the plugging temperature by varying the nitrogen flow to the plugging indicator. To prevent sudden plugging of the orifice plate, the rate of cooling is maintained at 3 to 10°C/min upto 150°C or selected value. Below 150°C or selected value, the sodium is cooled at a slower rate of 0.5 to 3°C/min till the plugging temperature is reached. After reaching the plugging temperature nitrogen blower is stopped. The value for the slow cooling rate, fast cooling

Fig. 2. Block diagram of plugging indicator control system

Fig. 1. Plugging Indicator

rate, temperature at which change over from fast to slow cooling rate occurs and interval between two cycles are fed from graphical user interface.

III. HARDWARE CONFIGURATION AND FAULT TOLERANT ARCHITECTURE

Hot standby architecture comprising of dual redundant RTC system (RTC1, RTC2) with switch over logic system (SOLS) is adopted for plugging indicator control in order to achieve fault tolerance (Fig. 3). RTC system consists of VME bus based 68020 CPU card [11], analog input card, analog output (AO) card and relay output (RO) card mounted on a 19”, 6U card frame. SOLS contains switch over logic card (SOLC) and ORing logic card which are implemented using relay based logic [12].

CPU card contains dual redundant hardware TCP/IP stack (commercially off the shelf product) for network connectivity. Each RTC system is connected to process computer and display stations (for data storage and display purposes respectively) using TCP/IP protocol. Each RTC system also receives various commands like auto/manual mode,

continuous/ discontinuous mode etc. from process computer Both the RTC systems contain same control software fused into the EPROM of respective CPU card. Control software runs directly on hardware without any operating system. Physical sensors such as thermocouple and flow sensor inputs of plugging indicator are duplicated and connected to analog input card of both the RTC systems. RTC systems scan the physical (field) inputs, receive PID constants, set points and mode selections from process computer, do the processing as per processing logic, generate physical outputs (for control & window alarm purpose) and send the plant parameters to process computer & display stations (for storage and display purposes) using TCP/IP protocol. RTC system does all these functions in real time with a scan interval of 1 second. Physical outputs from RTC systems are sent to the plant through ORing logic card of SOLS. Both the systems send its healthiness information to

SOLC of SOLS. When both the RTC systems are healthy, SOLS routes RTC1 outputs to plant. If RTC1 fails, RTC2 output is routed to plant. If both the systems fail, then the control output will be in stay put condition. The RTC system whose outputs are routed to the plant is called as online system.

A. Fault Detection and Switchover Fault detection is a crucial step in any type of fault tolerant

architecture. Systems shall have the capability to detect all types of faults that may occur during operation. CPU card, analog input/output cards and relay output card have the provision for diagnostics in hardware.

CPU card has watchdog timer, analog input card has onboard reference voltages and grounds, analog output and relay output cards have readback feature for the outputs generated from them. As part of RTC self test, each RTC system checks the healthiness of each card present in the system by executing the diagnostic routines. Analog input card healthiness is ensured by checking the onboard reference voltages and ground values are within the tolerable limit. Analog output and relay output cards healthiness are ensured by reading back the latched values. CPU card healthiness is ensured by using watchdog timer. Control software and

Fig. 3. Hot standby architecture

Fig. 4. Flow chart for fault detection & switch over

diagnostic software are embedded in the same EPROM of the CPU card. During each scan cycle both the software get executed.

Fig.4 shows the flow chart for fault detection and switch over. When there is fault in any of the cards then it is considered as system failure. When both the systems are healthy, SOLS sends RTC1 outputs to plant. When the SOLS receives system unhealthiness status from RTC1 then stops RTC1 outputs reaching the plant and switches the RTC2 outputs to the plant. If RTC2 is also unhealthy then it sends stay put output (AO) and fail safe output (RO) of RTC2 to the plant.

IV. SOFTWARE DEVELOPMENT AND SOFT PID ALGORITHM Two types of software namely control software and GUI

software was developed for plugging indicator control. Control software was developed in C language using Tasking C cross compiler and embedded in EPROM of 68020 CPU card. Flow chart for the control software is shown in Fig.5.

This software scans the temperature and flow inputs, does engineering unit conversion, executes the soft PID based control algorithm on the plugging indicator modes, generates analog output to adjust the control valve positioner and sends the information to GUI station using TCP/IP protocol for storage and display purposes. Cycle time of control software is 1 second. This software receives the information like plugging indicator modes, PID constants, set point for flow, set point for temperature, fast cooling rate, slow cooling rate, change over temperature, cycle repeat time etc. from the GUI station.

A. PID Algorithm PID based control algorithm is used in various process

control applications for temperature, pressure, flow control etc. Implementation of PID algorithm using software based

system is in practice now-a-days [13]. For plugging indicator control soft PID based control algorithm is developed in order to generate 4-20 mA control output for controlling the control valve positioner from 0 to 100%.

The proportional (P), integral (I) and derivative (D) component of control output is calculated as per the following algorithm.

Control output = ((P+I+D) * 0.16) + 4.0

P = Kp * Proportional error

I = Ki * Integral error

D = Kd * Derivative error

Where Kp, Ki, Kd are proportional, integral and derivative constants.

B. Soft PID Algorithm for Continuous Mode Flow chart for continuous mode of operation is shown in

Fig.6. In this mode controlling parameter is sodium flow passing

through the orifice. Control output is generated based on the soft PID algorithm. When the measured flow is equal to the set point (80% of nominal flow) the temperature monitored at the outlet of cooler is recorded as plugging temperature. The control output is adjusted in order to maintain the set point.

Fig. 5. Flow chart for control software

Start

RAM check with 0x5555,0xAAAA pattern

STOP theprocessor

Stop

No

Receive softcommands

and set points

Initialization ofperipherals andnetwork ports

if RAM checkpassed Generate control

output

Generate softoutputs

Load timer with 1 sec

scan temperatureand flow inputs

Yes

Yesdiagnostics logic

IFTimer expired

No

Soft PID algorithmfor continuous mode

Soft PID algorithmfor discontinuous mode

Fig.6. Flow chart for continuous mode

START

control output = 4.0intgeral error=

integral_error - error

Yes

VoidReturn

Yes

proportional_error = flow input - set pointintegral_error =integral_error + errorderivative_error = error - prev_errorprev_error = errorP = PID_constants[Kp] * proportional_errorI = PID_constants[Ki] * integral_errorD = PID_constants[Kd] * derivative_errorcontrol output = ((P + I + D) * 0.16) + 4.0

No

plugging temperature reached

IF flow = set point

IFcontrol output < 4.0

No

Generate control output

IFcontrol output > 20.0

control output = 20.0intgeral error=

integral_error - errorNo

Yes

C. Soft PID Algorithm for Discontinuous Mode Flow chart for discontinuous mode of operation is shown in

Fig.7. In this mode the controlling parameter is temperature which

is gradually reduced in order to exactly find out the plugging temperature. Sodium temperature is decreased at 10° C/min (fast cooling rate) till the change over temperature (default value 150° C) is reached. Sodium temperature is decreased at 3° C/min (slow cooling rate) below the change over temperature. Slow cooling rate, fast cooling rate and change over temperature are configurable parameters through GUI. The temperature set point gets changed at each scan cycle and the control output is generated based on the difference between the set point and the process value (temperature). During this process, flow is also monitored continuously in each scan cycle. When the flow reaches 80% of nominal value then the temperature measured at the cold point is recorded as plugging temperature and 4 mA is generated as control output in order to close the control valve so that further cooling of sodium is avoided. Hence sodium flow increases to nominal flow and the discontinuous cycle is repeated based on cycle repeat time. Default value for cycle repeat time is 4 hours.

V. PERFORMANCE STUDY OF CONTROL ALGORITHM Hardware was configured as per hot standby architecture

and the performance of RTC system for plugging indicator control was thoroughly studied in the laboratory by simulating the temperature and flow inputs using hardware simulators. GUI used for configuring the various parameters is given in

Fig. 8. GUI has the provision for manual/auto mode selection, continuous/discontinuous mode selection and display for temperature and flow values. GUI also has provision to change the cooling rate, temperature set point, flow set point, change over temperature, cycle repeat time, PID constants etc.

Various testing carried out on RTC system is detailed below.

Continuous mode was selected from GUI and the flow set point was given as 160 l/hour (nominal flow = 200 l/hour). P, I, D constants were set as 2.0, 0.01, 0 respectively. Since sudden change in flow is not expected in this process, the derivative constant was set as 0. The flow input was gradually reduced from 200 l/hour and the control output was observed. The control output was also observed by changing the P and I constants to 2.5 and 0.05 respectively. Fig.9 shows the control outputs delivered for different P, I constants.

Proper selection of P and I constants is highly recommended for good operation of plugging indicator. When P value is 2.5 then the control output saturates at 20 mA. (ie, control valve is fully opened allowing for maximum cooling). Since the flow reduction would not happen until temperature comes down to certain value (based on impurities in sodium), the integral error increases which leads to 20 mA output for longer duration. When the temperature reduces below certain value, impurities start precipitating at the orifice which results in flow reduction to less than 160 l/hour. Hence it is preferred to maintain the control output around 15 mA at the beginning so that there would not be much of flow reduction less than 160 l/hour.

When there are no impurities in sodium then there would not be flow reduction. But the plugging temperature decreases

Fig. 7. Flow chart for discontinuous mode

START

Set point = Setpoint - Fastcooling rate

Yes

Yes

IFcontrol output > 20.0

IFcontrol output < 4.0

control output = 20.0intgeral error =

integral_error - error

Yes

control output = 4.0intgeral error =

integral_error - error

Yes

VoidReturn

error = temperature - set pointintegral error =integral_error + errorderivative _error = error - prev_errorprev_error = errorP = PID_constants[Kp] * errorI = PID_constants[Ki] * integral_errorD = PID_constants[Kd] * derivative_errorcontrol output = ((P + I + D) * 0.16) +4.0

No

IFflow = flow set point

Set point = Setpoint -slow cooling rate

No

No

No

Wait for cyclerepeat time to expire

Generate control output

control output = 4.0

IFcycle repeat time expired

IFtemperature > Change

over setpointRecord the

plugging temperature

Yes

No

Fig.8. GUI snapshot

to below 110 °C and the RTC system generates 4 mA as control output to close the control valve in order to stop

subsequent cooling. Fig.10a shows the control output and flow rate with respect

to time and Fig.10b shows proportional and integral component with respect to time. P and I constants are kept as 2.0 and 0.01 respectively while observing these outputs. Flow rate was maintained at 200 l/hour for 25 seconds to simulate the near field condition. During this flow rate, increase in integral error and control output were observed and there was no change in proportional error. After 25 seconds the flow rate was gradually reduced and observed that the proportional error and the control output were reducing while the integral error was increasing. When the flow rate reaches 160 l/hour, 4 mA was driven as control output which indicates the control valve is fully closed when 80% of rated flow is reached.

Discontinuous mode was selected from GUI and observed the control output. For this mode of operation the controlling parameter is temperature. Unlike continuous mode the set point for control parameter keeps on changing in each scan

interval based on the fast and slow cooling rate. Discontinuous mode was tested by simulating the following values.

Fast cooling rate - 10°C/min, slow cooling rate - 3°C/min,

change over temperature - 150 °C/min. P, I, D values were set as 2, 0.5, 0 respectively. In this mode, the contribution of

Fig.9. Continuous mode plot

Fig. 10a. Continuous mode plot (flow rate and control output)

Fig. 10b. Continuous mode plot (proportional and integral

component)

Fig. 11a. Discontinuous mode plot (integral component and

control output)

Fig. 11b. Discontinuous mode plot (temperature)

proportional error for control output is very minimal. The integral error plays very important role for the control output. At the beginning only 4mA is driven as control output and over the period of time the output gradually increases as shown in Fig. 11a. When the flow rate comes to 160 l/hour or when the temperature comes to 110° C then the control valve is fully closed. Fig.11b shows the cold point temperature of plugging indicator. The slope of the profile changes from 10 °C/min to 3 °C/min at 150 °C. Control output was also monitored by changing the P and I constants as 3.0 and 0.25 respectively. There was no appreciable difference in temperature and control output profile. Hence the profiles were not depicted separately.

VI. CONCLUSION Soft PID based control algorithm for both continuous and

discontinuous modes of plugging indicator were tested by using VME bus based Real time computer system. Performance of the system was verified by varying the set points, PID constants, slow and fast cooling rate etc. Since the parameters are kept as configurable, tuning of the PID parameters for proper operation of the plugging indicator control system is easier which will lead to less commissioning time at PFBR site.

ACKNOWLEDGMENT The authors thank Mr.G.K.Mishra, Mrs.Varuna,

Mr.Saktivel, Mr.S.L.N.Swamy and Mr.K.Madhusoodanan of Power plant group, IGCAR for their valuable inputs and their involvement for testing the plugging indicator control system at laboratory. The authors also thank the independent Verification & Validation team for ‘Computer based systems of PFBR’ for their excellent suggestions at various stages which helped to develop a good control system.

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