In-core measurements of fuel-clad interactions in the Halden reactor

Transcription

1 In-core measurements of fuel-clad interactions in the Halden reactor Peter Bennett Halden Project IAEA Technical Meeting on Fuel Rod Instrumentation and In-Pile Measurement Techniques Halden, Norway 3 5 September

2 Contents Loop systems for testing under LWR conditions in the Halden reactor In-core, on-line detection of crud induced power shifts (CIPS) Demonstration of the PWR axial offset anomaly (IFA-665) In-core, on-line measurements of fuel clad corrosion using electrochemical impedance spectroscopy Also covered in the accompanying paper Design of test rigs In-core creep monitoring BWR crud thermal conductivity measurements In-core water chemistry monitoring Electrochemical corrosion potential Conductivity 2

3 LWR loop systems The coolant in the Halden reactor is D 2 O, at 235 C and 34 bar Not suitable for corrosion studies Test rigs can be positioned in a pressure flask and connected to a dedicated loop system, and hence isolated from the main coolant. Loop systems allow testing of fuel clad and materials under simulated BWR, PWR or PHWR conditions: Coolant pressure Coolant temperature Water chemistry Two loop types: Large loops (ca 100 litres) for test fuel Small loops (ca 60 litres) for material specimens 3

4 Loop schematic Feed water tank Cooler Vol: l Pressure control system Water analysis Control valve Purification system Flow: 100 l/h - 10 ton/h Pressure: 200 bar Temp: 350 C In-core test rig 4

5 Purification system Mixed Bed Ion Exchanger To main loop Fi Flow: 1 2 loop vol/h Temp: 40 C From main loop Regenerative Heat Exchanger Lithium Removal Bed Boron Removal Bed Cooler Fi 5

6 Sampling / chemical addition system Flow: l/h Temp: 25 C Chemical additions: H 2 /O 2 Li / B Noble metals SO 4 2- / CrO 4 2- Zn etc 6

7 Chemistry control Regular samples taken Grab samples Filter samples Impurities removed by ion exchange beds / filters Chemicals added to obtain desired water chemistry H 2, O 2 LiOH, B(OH) 3 Zn, Fe, Ni, TiO 2, etc 7

8 Water analyses grab samples Atomic absorption spectrometry (Li) Mannitol titration (B) ICP-MS (dissolved transition metals) Capillary Electrophoresis (dissolved anions) ph, total organic carbon (TOC) 8

9 Water analyses filter samples (integrated sampling) Coolant passed through filter packs (particle filters and ion exchange membranes) for approx 3 hours X-ray flourescence spectrometry (soluble and insoluble corrosion products) Gamma spectrometry (soluble and insoluble active nuclides Co-58, Co-60, Fe-59, Mn-54, Cr-51 etc) 9

10 Typical conditions BWR loops Coolant pressure 75 bar Inlet water temperature C Addition of H 2 or O 2 to simulate HWC or NWC (eg) SO 2-4 addition for conductivity control In-core: suppressed boiling or boiling along a specific section of fuel rods 10

11 Typical conditions PWR loops Coolant pressure bar Inlet water temperature C Sub-cooled nucleate boiling control Coolant temperature, flowrate Addition of 2 5 ppm H 2 Addition of LiOH, boric acid (ph control) Other additiions can be made, for example Fe-Ni-EDTA to deposit crud on fuel rods 11

12 Demonstration of the PWR axial offset anomaly (IFA-665) 12

13 Background and objectives AOA is the phenomenon in which boron is incorporated into crud deposits on the upper sections of fuel rods. The boron absorbs neutrons, which results in a shift in reactor power output to the lower sections of the core. The objectives of IFA-665 were to reproduce the symptoms of AOA : Deposit crud on fuel rods Measure flux / power depressions due to boron incorporation In IFA-665, a bundle of 8 test rods was irradiated under PWR water chemistry and thermal-hydraulic conditions. 13

14 Test rods Clad OD (mm) Number of rods 4 Fuel length (mm) 600 Enrichment: lower 200 mm (wt % U-235) (Reference section: no boiling) Enrichment: upper 400 mm (wt % U-235) (Sub-cooled boiling region)

15 Test rig schematic Flow tube Shroud Thermocouples Down comer Hydraulic driven fuel rod Flask Cable flask ND/GT Hydraulic cylinder Position indicator Small diameter rod ND/GT Test rod Shroud Pressure flask Section A-A PWR fuel rod HBWR high flux area ~800mm Active length 600mm A Diameter gauge Outlet coolant thermocouple Down comer Differential thermocouple Neutron detectorsection A-A A γ-thermometer Un-instrumented small diameter fuel rod Instrumented PWR fuel rod Linear voltage differential transformer (LVDT) for cladding elongation detector (EC) Inlet coolant thermocouple Diameter gauge to detect crud on fuel rod surface To detect effect of boron in the crud: Axially and cross-assembly spaced neutron detectors Differential coolant thermocouples to measure differences in heat-up due to power suppression 15

16 On-line measurements indicating AOA Diameter gauge measurements showed that crud had deposited onto the fuel rods. A flux depression was observed along the upper section of the test fuel. There was an accompanying decrease in the coolant heat-up. It was concluded that these effects were due to boron incorporation into the crud. Li return was observed during the shutdown (a common indicator of AOA in PWRs) 16

17 On-line evidence for crud loading 17

18 Neutron detector signals from test rig, pressure flask and neighbouring test rigs 18

19 Test rig and pressure flask neutron detector signals normalised to neighbouring test rig ND signals 19

20 Coolant temp rise and flowrate 20

21 Li return during reactor shutdown 21

22 Summary A combination of on-line measurement techniques was used to demonstrate AOA in the Halden reactor: Diameter gauge to demonstrate crud deposition Coolant flow and temperature measurements to show effect of crud on thermal-hydraulic conditions Neutron detectors to show power depression caused by boron in the crud. Coolant chemistry analyses provided supporting evidence of AOA Lithium return during shutdown PIE showed that the type of crud observed in US plants suffering severe AOA can be reproduced. 22

23 Electrochemical impedance spectroscopy (EIS) A technique that gives information on many corrosion phenomena Apply a variable ac voltage to the sample and measure impedance with frequency of applied voltage Advantages of the technique: Low pertubation signals that do not disturb the system Can be used in low conductivity media Non destructive Calculate oxide thickness from an equivalent electrical circuit (capacitor with plate separation equal to oxide thickness) 23

24 In-core EIS measurements In-core EIS measurements on Zircaloy-4 have been carried out in the Halden reactor.* Water chemistry conditions: 250 ppb LiOH 4 ppm H 2 or 400 ppb O 2 or 5 ppm O 2 Coolant temperature: 260, 270, 290, 310 and 335 C * D M Rishel, K L Eklund and B F Kammenzind, In-situ EIS measurements of irradiated Zircaloy-4 post transition corrosion kinetic behaviour, 15th International Symposium on Zirconium in the Nuclear Industry, ASTM,

25 In-core EIS (cont) 5 samples were tested, under differing gamma and neutron flux conditions. Data acquisition was successful, even with the required long signal cables from the test rig. Measured relative corrosion rates were consistent with PIE. 25

26 Electrode set for EIS measurements 26

27 Effects of water chemistry changes on impedance 27

28 EIS - conclusions It has been demonstrated that EIS measurements can be performed in-core. Relative oxide thicknesses determined from EIS measurements were the same as those measured by PIE. Further developments Measurements in BWR coolant (low conductivity) Measurements on fuelled clad segments 28

29 Summary Loop systems allow testing under LWR thermalhydraulic and water chemistry conditions. A combination of on-line instrumentation allows measurements of complicated phenomena, eg PWR AOA. Techniques are under development to allow on-line measurements of fuel clad corrosion. 29

30 Thank you for your attention! 30