Instrumentation systems of BWR

Instrumentation systems of BWR 1

Reactor core and pressure vessel of BWR Fuel rod Fuel assembly Reactor vessel :15~22cm thickness of steel, height of 21m, diameter of 7m Steam dryer Pressure vessel Main steam tube Core Steam separator Feedwater supply tube Fuel clad Fuel pellet Reactor core Control rod Water from recirculation pump Jet pump Recirculation pump 400~800 fuel assemblies are loaded in a reactor core. Control rod driving mechanism 2

Containment vessel of BWR Reactor building Lining: 3cm thickness of steel Height: 32~34m Spent fuel storage pool Diameter of spherical part: 20m Pressure vessel Containment vessel Pressure suppression pool Boiling Water reactor (BWR) Systems (USNRC) より 3

NIS 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 diameter Oh-i unit-1 (PWR, 1175MWe): 4.4 m Kashiwazaki-kariwa unit-1 (BWR, 1100MWe): 6.4 m 4

Fuel assemblies of PWR and BWR BWR PWR

NIS of BWR In-core neutron detectors are used in BWRs as NIS. SRM: Source range monitor IRM: Intermediate range monitor PRM: Power range monitor TIP: Traversing in-core probe (for calibration) SRNR: Start-up range neutron monitor (wide range monitor) Measurement regions covered by these monitors are almost the same as those of PWR NIS. 6

Positions of neutron detectors Maki BWR power plant (2,436[MWt]) 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 7

Average power range monitor (APRM) 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 8

NIS of BWR Detector types of neutron monitors Measurement Range Source range Intermediate power range Power range Detector type Movable fission chamber Movable fission chamber Miniature fission chamber Signal mode Pulse mode Campbell (*MSV) mode Current mode *MSV (the Mean Square Voltage) mode : The mean square voltage of signal is proportional to neutron flux level. 9

Reactor vessel SRM and IRM (movable detector) Upper support plate Detector (SRM or IRM) Core support plate - 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. Detector driving mechanism controller Detector driving mechanism Record & surveillance system Containment vessel penetration 10

Four LPRM positions in a guide tube Upper support plate LPRM position D LPRM position C LPRM position B Guide tube for movable detector *TIP can be inserted with a driving mechanism. LPRM position A Core support plate Signal cable Reactor vessel 111

In-core miniature fission chamber for PRM Active materials >90% 235 U, U 3 O 8 (or 90% 234 U-10% 235 U) Active length 25.4 mm Detector diameter 6.3 mm Ionization gas Ar gas Applied voltage 75~175 V Coaxial cable Power & signal cable Casing (cathode) Plate coated with 235 U Anode Insulation(eg. Al 2 O 3 ) 12

In-core miniature fission chamber for PRM Sensitivity of 235 U fission chamber (FC) becomes small during thermal neutron irradiation. (Example) One year irradiation with the thermal neutron flux of 10 13 [n cm -2 s -1 ] 13 20 10 360024356 3.210 The sensitivity reduces by about 20%. Relative sensitivity 1.4 1.2 1.0 0.8 0.6 0.4 0.2 Q3. Answer how to solve this problem? U n U 234 235 235 U Measurements with 235 U FCs must be properly corrected with the change of sensitivity. 0 2 4 6 8 10 Total thermal neutron irradiation (10 21 n cm -2 ) 13

In-core miniature fission chamber for PRM Sensitivity of 235 U fission chamber (FC) becomes small during thermal neutron irradiation. 1.4 1.2 regenerative fission chamber 90% 234 U-10% 235 U (Example) One year irradiation with the thermal neutron flux of 10 13 [n cm -2 s -1 ] 13 20 10 360024356 3.210 The sensitivity reduces by about 20%. Relative sensitivity 1.0 0.8 0.6 0.4 0.2 U n U 234 235 235 U Measurements with 235 U FCs must be properly corrected with the change of sensitivity. 0 2 4 6 8 10 Total thermal neutron irradiation (10 21 n cm -2 ) 14

Reactor pressure meter Pressure vessel Condensing chamber core Reference Leg Containment vessel Pressure transmitter 15

Reactor pressure meter Pressure vessel Steam Condensing chamber Steam condensation Steam entering from steam drum in pressure vessel into condensing chamber is condensed. core Reference Leg Containment vessel Pressure transmitter 16

Reactor pressure meter Pressure vessel Condensed water Condensing chamber Steam condensation Condensed water above reference water level goes back to steam drum in pressure vessel. core Reference Leg Containment vessel Pressure transmitter 17

Reactor pressure meter Pressure vessel core Condensing chamber Reference Leg Reference water level Reference water level in condensing chamber is always keeping constant. Containment vessel Pressure transmitter 18

Reactor pressure meter Pressure vessel Condensing chamber Reference water level core Reference Leg Water head from the pressure transmitter Containment vessel Pressure transmitter Reactor pressure: 19

Water level meter Pressure vessel Condensing chamber Reference water level Water level h ra TAF Core Reference Leg H r BAF Active Leg h a Differential pressure transducer p P2 P1 20

Water level meter Pressure vessel Condensing chamber Reference water level Water level h ra TAF BAF Core Reference Leg Active Leg h a H r p P P 2 1 h a h ra H r P2 Pr ha Differential pressure transducer P1 Pr Hr 21

Water level meter Ex. Fukushima Daiich Unit-2 Water level P=P 2 -P 1 TAF+5000 mm TAF-3700 mm -11.51[kPa] -95.35[kPa] TAF : Top of Active Fuel Pressure vessel TAF+5000mm water level TAF+0m TAF-3700mm Core Condensing chamber Reference water level TAF P Reference Leg 1 P Active Leg 2-11.51kPa P -95.35kPa TAF+5000mm Differential pressure transducer Water level p P2 P1 TAF-3700mm 22

Water level meter in the 1F1 accident Water level meter indication Level meter A Level meter B +2000mm +1000mm TAF From MAAP analysis, during this period, water level went down below bottom of reactor core. -1000mm -2000mm Water level meter indicated water level above bottom of reactor core. (TAF-3700mm) 3/11 21:30 0:00 6:00 12:00 18:00 24

Water level meter in the 1F1 accident Water level meter indication Level meter A Level meter B +2000mm +1000mm TAF -1000mm -2000mm Water level meter indicated water level above bottom of reactor core. (TAF-3700mm) 3/11 21:30 0:00 6:00 From MAAP analysis, during this period, water level went down below bottom of reactor core. 12:00 18:00? 25

Water level meter in the 1F1 accident Pressure vessel Depressurization Superheat Condensing chamber 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. Water level Reference leg Active leg Containment vessel P 2 P 1 Differential pressure transducer 27

Water level meter in the 1F1 accident Pressure vessel Depressurization Superheat Condensing chamber Water level Reference leg Active leg 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. The precondition for water level meter that reference water level in condensing chamber is kept constant is violated. Containment vessel P 2 P 1 Differential pressure transducer 28

Water level meter in the 1F1 accident Accident state Normal operation state Pressure Vessel Condensing Chamber Pressure Vessel Condensing Chamber reference water level Misjudgment h ra P 1 core h ra Reference Leg Water level Active Leg Reference Leg P 1 core P2 P 2 Active Leg p PP 2 1 h ra differential pressure transducer p P P 2 1 differential pressure transducer 29

Water level meter in the 1F1 accident Pressure vessel Condensing chamber 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. core Reference Leg Water level Active Leg Containment Vessel h ra P 2 P 1 differential pressure transducer p P P 2 1 Unchanging 30

Controllers and process instrumentation of BWRs Main steam flow rate Pressure control Turbine control Water level Steam governor valve Recirculation pump Bypass valve Turbine Condenser Generator Feedwater flow rate Flow controller Main controller Manual setting Feedwater controller Neutron Flux controller Averaged Power (neutron flux) Setting of water level L water level P pressure F flow rate R rotation speed 31