Design Provisions for Core Instrumentation of PFBR

BHAVINIBHAVINIKALPAKKAMKALPAKKAM

Design Provisions for Design Provisions for

Core Instrumentation of PFBRCore Instrumentation of PFBR

C. Paramasivan Pillai

Indo-French Technical Seminar, France, 3rd – 8th April 2006.

The core instrumentation system consists of

Neutron Flux Monitoring System

Core Temperature Monitoring System

Flow Monitoring System

Failed Fuel Detection System

Design provision for core instrumentation of PFBR

The functions of the system are

To monitor the core status in all states of the reactor –

shutdown, fuel handling, start-up, intermediate & power

ranges and during design basis events.To detect errors during fuel handling

Type and Location of Neutron Detectors

The neutron flux around the reactor is very low as neutron

shields are provided to protect the components housed inside

the main vessel and to reduce the secondary sodium activity.

NEUTRON FLUX MONITORING

The neutron flux (total) at core centre varies from

1.7x107 nv (n/cm2/s) at shutdown to 8x1015 nv at

nominal power.

The neutron flux at the control plug location varies

from 6nv (with auxiliary neutron source) to 2.7x109 nv

(U235 equivalent thermal flux) from shutdown to nominal

power.

The neutron flux at the under vessel location varies

from 1.1x10-5 nv to 2x105 nv (U235 equivalent thermal

flux).



Detectors under vessel for power operation

Detectors in central subassembly

Six high temperature fission chambers during startup and intermediate operation (3 safety, 2 control, 1 insitu spare)

Detectors for Normal Operation

It is proposed to provide 6 (3 for safety, 2 for control and 1 insitu spare) high temperature fission chambers at 3 locations 120o apart in the control plug

These detectors facilitate monitoring the measurement range from shutdown to intermediate power range with wide range channels having pulse, Campbell modes from 1 nv to 3.1 x 107 nv.



Above 1% power, the response time and accuracy of the control plug detectors in campbell and dc modes of operation are not meeting the requirements due to leakage and gamma currents.

Hence 6 fission chambers of sensitivity 0.2 cps/nv working in pulse mode (3 for safety, 2 for control and one in-situ spare) are proposed to be placed under the safety vessel with concrete over the detectors removed for better flux availability during power range.




1.7x107

8x101515 16x1015

25 166 2160 92.7x10 5.4x109

3.6 Wt 15 Wt 100 Wt 1 kWt 10 kWt 100 kWt 1 MWt 10 MWt 100 MWt 1000 1250 1500 2500MWt MWt MWt MWt

NOMINAL POWER (Pn) SHUTDOWN

POWER

CORE CENTRE FLUX

FLUX AT CP DETECTOR LOCATION

CRITICALITYat 144 cps

1 100 1000 10 10 104 5 6

2x10 cps6

PULSE MODE

25 kWt 250 kWt 2.5 MWt 25 MWt 250 MWt 2500 MWt

CAMPBELL MODE

PULSE MODE(LINEAR)

ABSOLUTE POWER TRIPLin N at 110% Pn

(1375 MWt)

TRIP ON CAMPBELL CHANNELS,Log P at 20% Pn (250 MWt)

INTERLOCK ON TAKE OVER OF Lin PCHANNELS FROM CAMPBELLCHANNELS, Lin N1-IL at 5% Pn

(62.5 MWt)

TRIP ON PULSE CHANNELS,Log N at 800 kWt (10 cps)6

INTERLOCK ON TAKE OVER OF CAMPBELLCHANNELS FROM PULSE CHANNELS,

Log P1-IL at 200 kWt

NON-AVAILIBILITY OF ADEQUATENEUTRONS, Log No at 3 cps

FUEL LOADING INTERLOCK,Log N1-IL at 25 cps

HTFC(0.2 cps/nv)WITH WIDE

150 MWt 1500 MWt(120% Pn)

RANGE CHANNEL

6 2.16x104 5 6 7 8 9

7 8 9 10 11 12 13 14 15

U THERMAL EQUIVALENT FLUX235

9.6x10

NEUTRON FLUX MONITORING - SAFETY CHANNELS -NORMAL OPERATION

*

7.1x10 4.7x10 6.4x10 6.4x10 6.4x10 6.4x10 6.4x10 6.4x10 6.4x10

2.16x10 2.16x10 2.16x10 2.16x10 2.16x10(nv)

(nv)

3.24x109

(k = 0.93)eff

15 MWt

(BOEC CORE, WITH SOURCE)

(0.2 cps/nv)UNDER VESSEL

FISSION CHAMBER

101.2

FLUX AT UV DETECTOR LOCATION* (nv) 2x102x103 4 1.66x1052x105

3.3x105

Requirement of source

During the first 4 fuel cycles, before the BOEC (Beginning of Equilibrium Cycle) is achieved, the shutdown count rates on control plug location are very less. It is planned to load source subassemblies containing fresh Antimony Berilium pins during first startup.

These source assemblies get activated during reactor operation during first fuel cycle and provide a min. shutdown countrate of 6 cps on control plug detectors after 4 months of operation.


Till the source gets activated a special instrumentation namely Instrumented Central Sub Assembly (ICSA) monitors the core during shutdown and startup.

The ICSA contains 3 high temperature fission chambers of 0.1 cps/nv connected to pulse counting channels for monitoring flux and provides trips in the range from 10 nv to 107 nv (Core centre, U 235 thermal equivalent flux) (~2 kWt).



3 fission chambers in ICSA for flux monitoring during core loading and low power

The detectors in control plug takes over safety and control action from the ICSA detectors arond ~103 nv at control plug location well before the ICSA detectors saturate. As the power goes up the ICSA is withdrawn and kept at a higher elevation at ~ 1 kWt power.

In case of long shutdown extending more than 4 months during reactor operation , the shutdown count rate goes < 3 cps as the source decays with a half-life of 60 days, The subsequent reactor startup is done again with ICSA, till the source gets reactivated.



1.7x1078x10

151516x10

15

25 166 21609

2.7x105.4x109

3.6 Wt 15 Wt 100 Wt 1 kWt 10 kWt 100 kWt 1 MWt 10 MWt 100 MWt 1000 1250 1500 2500MWt MWt MWt MWt

NOMINAL POWER (Pn) SHUTDOWN

POWER

CORE CENTRE FLUX

FLUX AT CP DETECTOR LOCATION

CRITICALITYat 144 cps

1 100 1000 10 10 104 5 62x10 cps

6

PULSE MODE

NON-AVAILIBILITY OF ADEQUATE NEUTRONS

FUEL LOADING INTERLOCK,Log N1-IL at 25 cps

HTFC(0.2 cps/nv)WITH WIDE

RANGE CHANNEL

6 2.16x104 5 6 7 8 9

7 8 9 10 11 12 13 14 15

U THERMAL EQUIVALENT FLUX235

9.6x10

NEUTRON FLUX MONITORING - SAFETY CHANNELS

*

7.1x10 4.7x10 6.4x10 6.4x10 6.4x10 6.4x10 6.4x10 6.4x10 6.4x10

2.16x10 2.16x10 2.16x10 2.16x10 2.16x10(nv)

(nv)

3.24x109

(k = 0.93)eff

10

2x10 cps6

610105104100010010

PULSE MODE

PULSE WITH

(0.1cps/nv)HTFC

CHANNEL

NON-AVAILABILITY OF

ADEQUATE NEUTRONS TRIP ON PULSE CHANNELS,

Log C at 10 cps (~ 2 kWt)Log Co at 3 cps

IL ON TAKE OVER OF CP DETECTORSFROM ICSA CHANNELS

Log N2-IL at 200 cps

6

IN ICSA

1.2

(ICSA AND SOURCE RANGE)

Log No at 3 cps

Design Basis Events (DBE)

For the following DBE, Neutron Flux monitoring system initiates automatic safety action.

Transient over power at low powerTransient over power at start-up, Transient over power at power operationShort period during power raiseAbnormal reactivity changes


Functions Monitor the temperatures at the outlets of FSA and core

inlets Derive parameters such as the

Mean core outlet temperature (M), Mean temperature rise in the core (M) Deviation in individual subassembly sodium outlet

temperature over the expected value (I). To provide SCRAM signals on

Central sub assembly temperature (CSA) Mean temperature rise in the core (M) Deviation in individual subassembly sodium outlet

temperature over the expected value (I). Core inlet temperature (RI)

CORE TEMPERATURE MONITORING SYSTEM

Specification Thermocouple type: K type (Chromel-Alumel) Mineral

insulated, SS sheathed, ungrounded T/C Size : 1 mm. Sensitivity : 41 V / K Range : 400 to 1100 K Overall accuracy : 3 K Time constant : 300 ms (for the TC for CSA)

: 62 s (for the TC for other FSA)

Scan interval : 1 Sec.


Signal Processing Central Sub-assembly outlet and core inlet sodium

temperature signals are triplicated and processed by hardwired electronics

For all other fuel sub-assemblies sodium outlet temperature is monitored with 2 T/C (in thermowell) and each T/C signal is triplicated after signal conditioning and processed by 3 different Real Time Computers (RTC)


Central SA Outlet Sodium Temperature Measurement

TC are located on the central canal plug - directly

in contact with sodium

Signals are processed by hardwired electronics

SCRAM signal is generated through 2/3 logic.

Connected to the RTC for calculating

Mean core outlet temperature (M)

Deviation in individual subassembly sodium outlet

temperature over the expected value (I)



Thermocouple in central canal plug

2 /

3 L

OG

IC

SCU

SCU

SCU

Central Sub-assembly Outlet Sodium Temperature Monitoring System

TCs in Central canal plug

CR


FSA Outlet Sodium Temperature Measurement

Two TC in thermowells at the outlets of each SA (except central sub assembly).

RTC calculates M, M, I

RTC initiates SCRAM on M

and I For calculating M and I,

Minimum of the Two RI is taken as RI for calculation

SCRAM on I is not effective when (csa - RI )below a setpoint



TC – A210 Nos.

RTC-A

RTC-B

RTC-CRCB Side

-RI-1B, -RI-2B, -CSA-B

-RI-1A, -RI-2A, -CSA-A

-RI-1C, -RI-2C, -CSA-C

2 /

3 L

OG

IC

FSA Outlet sodium Temperature Monitoring System


ISO AMP

ISO AMP

ISO AMP

ISO AMP

TC – B210 Nos.

ISO AMP

ISO AMP

ISO AMP

ISO AMP

Fig. 1 : LOCATION OF EDDY CURRENT FLOW METERS IN PUMPS

Thermocouple Probes


A A

Detail-A

Thermocouple Probe Location


EL. 22740

THERMOCOUPLEPROBE

PUMP SUCTION

Thermocouple Probe

EL. 22740

PSP suction

Reactor Inlet Temperature Monitoring System

Thermocouple Probe


GRIPPER

GUIDE RING

SEAL PLUG

CONNECTOR

2-THERMOCOUPLES

EXTENSION

INTERMEDIATE

TUBE Ø4/5

Ø1mm

PIECE



2 /

3 L

OG

IC

SCU

SCU

SCU

2 /

3 L

OG

IC

SCU

SCU

SCU

TCs in PSP-1

TCs in PSP-2

CR BCR


Eddy current Flowmeter

Functions Monitor sodium flow

through the core Protect the reactor in

case of events like one primary sodium pump seizure/ trip, transient over power, pipe rupture and off-site power failure

Signal processing by hardwired electronics

SCRAM on P/Q > 1.1 - 2/3 voting logic

These probes can be easily withdrawn for maintenance/replacement

CORE FLOW MONITORING

P/Q>1.1FLOW METER-1A

FLOW METER-2A

2 /

3 L

OG

IC

FLOW METER-1B

FLOW METER-2B

FLOW METER-1C

FLOW METER-2C

P/Q

P/Q

P/Q

P/Q>1.1

P/Q>1.1

SCUQ=Q1+Q2

CR

PRIMARY SODIUM FLOW MEASUREMENT (P/Q)

SCUQ=Q1+Q2

Q1

Q2

Q1

Q2

SCUQ=Q1+Q2

Q1

Q2

CRCRT


Fig. 1 : LOCATION OF EDDY CURRENT FLOW METERS IN PUMPS

Eddy current Flowmeters


Ø1850

1330

1445

INTERMEDIATE SUPPORT SKIRT

SHAFT

HYDROSTATICBEARING BUSH

HYDROSTATICBEARING JOURNAL

SUCTION CASING

IMPELLER

DIFFUSER

EDDY CURRENTFLOW METER

Fig.5 : LOCATION & GENERAL ARRANGEMENT OF EDDY CURRENT FLOW METER IN PUMP

Eddy current Flowmeters


5050

50

VIEW-A

A

GENERAL ARRANGEMENT OF

EDDY CURRENT FLOW METER IN PRIMARY SODIUM PUMP

EL. 21865

EL.21740

EL. 22915

EL. 21254

SUCTIONCASING

WEB INSUCTIONCASING

SUCTIONCASING


8386

20

Ø14

Ø12

102

94

100

94

50

EL. 21274

SE

NS

OR

-1S

EN

SO

R-2

0.1 mm SS FOIL COVER

SECONDARY COIL

PRIMARY COIL

SECONDARY COIL

IRON CORE

11.8 mm OD

EDDY CURRENT PROBE ASSEMBLYEddy current Flowmeter Probe




For the following DBE, Core temperature monitoring

system initiates automatic safety action.

1. Transient over power

2. Sub-assembly faults

3. One primary sodium pump (PSP) trip

4. One PSP seizure

5. Primary pipe rupture

6. Off-site power failure

7. One secondary sodium pump (SSP) trip

8. One SSP seizure

9. One BFP trip with the standby not starting

10. Loss of feed water to SG

This system provides immediate indication and safety action (SCRAM) on wet rupture of the failed fuel.

The system consists of high temperature fission chambers of sensitivity 0.2 cps/nv placed at the inlets of each of the four IHX,

It monitors the delayed neutrons emitted by the solid fission products that get into sodium due to the fuel clad failure (Bulk DND)

FAILED FUEL DETECTION SYSTEM

* The detector assemblies, each consisting of three high temperature fission chambers surrounded by graphite to thermalize the delayed neutrons and B4C for shielding the streaming core neutrons background, are installed in pockets at 8 locations near IHX inlet windows.

* The studies carried out shows that a rupture of 10 cm2 clad surface area can be detected with an accuracy of 20 % with a total response time 16 to 59 s

BULK DND SENSITIVITYBULK DND SENSITIVITY


CP

IHX

LRP

1

IHX

IHX

IHX

SRP

BULK DND PITS

TOP VIEW OF DND LOCATIONS


BUILK DND THIMBLE



INNER VESSEL

MAIN VESSEL

CP

HOT POOL

COLD POOL

Na LEVEL

DND PIT

IHX

EL 25910

2

IHX

LRP SRPCP

Na

Argon

ACTIVECORE

BULK DND SYSTEM FOR PFBR

Bulk DND location

DND Thimble


For the following DBE, Failed Fuel

Detection System initiates automatic

safety action.

Sub-assembly faults Blockage


• Failure of the redundant signals

• The channels are designed with fail safe criteria. Failure of any component or power supply of the redundant channel results in trip condition of respective parameter and “system unhealthy” trip corresponding to faulty channel is initiated.

• In addition to the above, discordance system constantly monitors all the trip parameters and thresholds. Any malfunction or failure of any of the above results in alarm in control room.


• Failure of the redundant signals

• Instrumented channels are checked by simulated signals daily.

• Healthiness of the TC are checked online by RTC.

• Healthiness of the RTC is checked by online diagnostics.


Design Basis Events and SCRAM parameter (in the order of their appearance)


CSARIOne BFP trip and standby not starting

CSARIOne SSP seizure

CSARIOne SSP trip

CSA and MP/Q and Off-site power failure

CSAP/Q, and LinPOne PSP seizure

DND and I and DNDSA faults

CSA and M, p and P/QTOP at low power / start-up

CSARILoss of feedwater to SG

CSAP/Q, and LinPPrimary pipe rupture

CSA and MP/QOne PSP trip

CSA and MLinP, P/Q, TOP power operation

SDS-2SDS-1DBE

Instrumented Central Sub Assembly to

monitor the core from shutdown to 1 kWt till

the source gets activated during reactor

operation.

High temperature fission chambers of 0.2

cps/nv sensitivity in control plug for startup

and intermediate power range operation.

Under vessel fission chambers of 0.2 cps/nv

sensitivity in pulse mode for power operation

SUMMARY

K type thermocouples in control plug for

monitoring sodium outlet temperatures of all

the fuel sub assemblies.

Thermocouples in the primary Na pump to

measure the primary sodium inlet temperature

Eddy current flow meters in the by pass flow on

discharge side of primary sodium pumps

In vessel high temperature fission chambers on

either side of IHX for monitoring failed fuel

SUMMARY

Design Provisions for Core Instrumentation of PFBR

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