1

Instrumentation systemsof BWR

400～800 fuel assemblies are loaded in a reactor core.

Core

Fuel rod

Fuel assembly

Fuel clad

Fuel pellet

Pressure vessel

Steam separator

Steam dryer

Main steam tube

Feedwatersupply tube

Control rod driving mechanism

Jet pump

Reactor core

Water from recirculation pump

Control rod

Recirculation pump

Reactor vessel :15～22cm thickness of steel, height of 21m, diameter of 7m

Reactor core and pressure vessel of BWR

2

Lining: 3cm thickness of steel Height: 32～34m

Diameter of spherical part: 20m

Boiling Water reactor (BWR) Systems (USNRC)より

Spent fuel storage pool

Pressure vessel

Containment vessel

Reactor building

Pressure suppression pool

3

Containment vessel of BWR

Unlike NIS of PWR, ex-core neutron detectors are NOT used in BWRs.

- Pressure vessels of BWRs are much larger than those of PWRs.

- As neutron detectors are short sight, they can only observe neutron flux in their neighboring regions.

- Neutron couplings between core regions are weak, and therefore power distribution can be regionally independent.

Examples of RPV diameterOh-i unit-1 (PWR, 1175MWe): 4.4 mKashiwazaki-kariwa unit-1 (BWR, 1100MWe): 6.4 m

4

NIS of BWR

Fuel assemblies of PWR and BWR

BWR PWR

– SRM: Source range monitor – IRM: Intermediate range monitor – PRM: Power range monitor – TIP: Traversing in-core probe

(for calibration)

In-core neutron detectors are used in BWRs as NIS.

Measurement regions covered by these monitors are almost the same as those of PWR NIS.

SRNR: Start-up range neutron monitor

(wide range monitor)

6

NIS of BWR

7

◎ Local power range monitor (LPRM): 124 (31 x 4 axial positions)

■ Intermediate range monitor (IRM): 8

Source range monitor (SRM): 4

▽ Neutron source (NS): 5＋ Control rod (CD): 137

Maki BWR power plant (2,436[MWt])

×○

Positions of neutron detectors

• LPRMs are combined into some groups, eg., 6 groups.• Averaged value of LPRM signals belonging to the same

group is given as a signal of APRM of that group. • LPRMs in the respective groups are selected to measure

power level correctly over the entire reactor region.

Example of the same APRM group

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Average power range monitor (APRM)

MeasurementRange Detector type Signal mode

Source range Movable fission chamber Pulse mode

Intermediate power range

Movable fissionchamber

Campbell (*MSV) mode

Power range Miniature fission chamber Current mode

Detector types of neutron monitors

*MSV (the Mean Square Voltage) mode :The mean square voltage of signal is proportional to neutron flux level.

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NIS of BWR

Detector driving mechanism

Detector (SRM or IRM)

Upper support plate

Core support plate

Reactor vessel

Detector driving mechanism controller

Record & surveillance system

Containment vessel penetration

- SRM and IRM are located at prescribed positions in the guide tubes by the detector driving mechanism when they are used. - When out of service, they are moved in a position under the core support plate.

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SRM and IRM (movable detector)

Reactor vessel

Signal cable

Core support plate

Upper support plate

LPRM position A

LPRM position B

LPRM position C

LPRM position DGuide tube for movable detector

*TIP can be inserted with a driving mechanism.

1111

Four LPRM positions in a guide tube

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Active materials >90%235U, U3O8 (or 90%234U-10%235U)Active length 25.4 mmDetector diameter 6.3 mmIonization gas Ar gasApplied voltage 75～175 V

AnodePlate coated with 235U

Insulation(eg. Al2O3)Casing (cathode)

Coaxial cable

Power & signal cable

In-core miniature fission chamber for PRM

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235U

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0 2 4 6 8 10

Total thermal neutron irradiation (1021n cm-2)

Rel

ativ

e se

nsiti

vity

Sensitivity of 235U fission chamber (FC) becomes small during thermal neutron irradiation.

(Example)One year irradiation with the thermal neutron flux of 1013 [n cm-2s-1]

13 2010 3600 24 356 3.2 10

234 235U n U

The sensitivity reduces by about 20%.

Measurements with 235U FCs must be properly corrected with the change of sensitivity.

In-core miniature fission chamber for PRMQ3. Answer how to solve this problem?

14

90%234U-10%235U

235U

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0 2 4 6 8 10

Total thermal neutron irradiation (1021n cm-2)

Rel

ativ

e se

nsiti

vity

(Example)One year irradiation with the thermal neutron flux of 1013 [n cm-2s-1]

13 2010 3600 24 356 3.2 10

regenerative fission chamber

234 235U n U

The sensitivity reduces by about 20%.

Measurements with 235U FCs must be properly corrected with the change of sensitivity.

In-core miniature fission chamber for PRM

Sensitivity of 235U fission chamber (FC) becomes small during thermal neutron irradiation.

Pressure transmitter

Pressure vessel

core

Condensing chamber

Reference Leg

Containment vessel

15

Reactor pressure meter

Pressure transmitter

Pressure vessel

core

Steam

Steam condensation

Condensing chamber

Reference Leg

Containment vessel

Steam entering from steam drum in pressure vessel into condensing chamber is condensed.

16

Reactor pressure meter

Pressure transmitter

Pressure vessel

core

Condensed water

Steam condensation

Condensing chamber

Reference Leg

Containment vessel

Condensed water above reference water level goes back to steam drum in pressure vessel.

17

Reactor pressure meter

Pressure transmitter

Pressure vessel

core

Condensing chamber

Reference Leg

Containment vessel

Reference water level

Reference water level in condensing chamber is always keeping constant.

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Reactor pressure meter

Pressure transmitter

Pressure vessel

core

Condensing chamber

Reference Leg

Containment vessel

Reference water level

Water head from the pressure transmitter

Reactor pressure:19

Reactor pressure meter

Differential pressure transducer

Active Leg

CoreTAF

BAF

Condensing chamber

Reference Leg

rahWater level

Reference water level

rH

ah

2 1p P P

Pressure vessel

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Water level meter

Differential pressure transducer

Active Leg

CoreTAF

BAF

Condensing chamber

Reference Leg

rahWater level

Reference water level

rH

ah

Pressure vessel

1 r rP P H 2 r aP P h

2 1

a r

ra

p P Ph Hh

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Water level meter

PDifferential pressure transducer

Pressure vessel

Active Leg

Core

Condensing chamber

Reference Leg

water level

Reference water level

TAF+5000mm

TAF+0m

TAF-3700mm

TAF

Water level ∆P=P2-P1

TAF+5000 mm -11.51[kPa]

TAF-3700mm -95.35[kPa]

Ex. Fukushima Daiich Unit-2

1P

2P

TAF+5000mm

TAF-3700mm

-11.51kPa -95.35kPa

Water level 2 1p P P

TAF : Top of Active Fuel

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Water level meter

TAF

+1000mm

+2000mm

-1000mm

-2000mm

Water level meter indication

3/120:00

3/126:00

3/12 12:00

3/12 18:00

3/11 21:30

Water level meter indicated water level above bottom of reactor core. (TAF-3700mm)

Level meter ALevel meter B

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Water level meter in the 1F1 accident

TAF

+1000mm

+2000mm

-1000mm

-2000mm

Water level meter indication

3/120:00

3/126:00

3/12 12:00

3/12 18:00

3/11 21:30

Water level meter indicated water level above bottom of reactor core. (TAF-3700mm)

Level meter ALevel meter B

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Water level meter in the 1F1 accident

From MAAP analysis, during this period, water level went down below bottom of reactor core.

TAF

+1000mm

+2000mm

-1000mm

-2000mm

Water level meter indication

3/120:00

3/126:00

3/12 12:00

3/12 18:00

3/11 21:30

Water level meter indicated water level above bottom of reactor core. (TAF-3700mm)

Level meter ALevel meter B

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Water level meter in the 1F1 accident

From MAAP analysis, during this period, water level went down below bottom of reactor core.

?

TAF

+1000mm

+2000mm

-1000mm

-2000mm

Water level meter indication

3/120:00

3/126:00

3/12 12:00

3/12 18:00

3/11 21:30

Water level meter indicated water level above bottom of reactor core. (TAF-3700mm)

Level meter ALevel meter B

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Water level meter in the 1F1 accident

From MAAP analysis, during this period, water level went down below bottom of reactor core.

?

Q4. Answer the reason.

Differential pressure transducer

Pressure vessel

Containment vessel

Active leg

Condensing chamber

Reference leg

1P2P

Water level

In the Fukushima Daiichi Unit-1 accident, water in condensing chamber and a part of reference leg was evaporated because of depressurization or superheated condition in pressure vessel.

Depressurization

Superheat

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Water level meter in the 1F1 accident

In the Fukushima Daiichi Unit-1 accident, water in condensing chamber and a part of reference leg was evaporated because of depressurization or superheated condition in pressure vessel.

Differential pressure transducer

Pressure vessel

Containment vessel

Active leg

Condensing chamber

Reference leg

1P2P

Water level

Depressurization

Superheat

The precondition for water level meter that reference water level in condensing chamber is kept constant is violated.

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Water level meter in the 1F1 accident

differential pressure transducer

Pressure Vessel

Active Leg

core

Condensing Chamber

Reference Leg

reference water level

1P

2 1p P P differential pressure transducer

Active Leg

core

Condensing Chamber

Reference Leg 1P

2P 2P

Pressure Vessel

rah

rah

Accident state Normal operation state

Water level

Misjudgment

2 1

ra

p P Ph

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Water level meter in the 1F1 accident

differential pressure transducer

Containment Vessel

Active Leg

coreReference Leg

When water in reference and active legs in containment vessel are completely evaporated, pressure difference is unchanged even if water level went down to the bottom of pressure vessel.In this situation, it looks like water level is unchanged.

Water level 1P2P

rah2 1p P P

Unchanging

Pressure vesselCondensing chamber

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Water level meter in the 1F1 accident

Controllers and process instrumentationof BWRs

31

Main steam flow rate

Recirculation pump

Pressure control

Turbine control

Main controller

Neutron Flux controller

Flow controller

Averaged Power (neutron flux)

Turbine

Condenser

Bypass valve

Feedwatercontroller

Water level

Feedwaterflow rate

Generator

Steam governor valve

Setting of water level

Manual setting

L water level F flow rateR rotation speedP pressure