TOPIC 5 EXPERIMENTS FOR IMPROVING NUCLEAR FUELS MODELS AND PERFORMANCE

Transcription

1 The 2008 Frédéric JOLIOT & Otto HAHN Summer School August 20 August 29, 2008 Aix-en-Provence, France TOPIC 5 EXPERIMENTS FOR IMPROVING NUCLEAR FUELS MODELS AND PERFORMANCE The Importance of In-Pile Measurements Role of MTRs, Integral Validation, High Burn-Up Fuel Margaret McGrath OECD Halden Reactor Project P.O. Box 173, NO-1751 Halden, Norway Phone: , Fax: , margaret.mcgrath@hrp.no

2 Part 1: In-Pile Measurements Why make in-pile measurements on fuel rods? What can be measured? How are measurements made? Part 2: In-Pile Experiments Why do experiments in research reactors? Special demands on capabilities for integral and high burn-up fuel testing Examples of fuel performance experiments and typical data 2

3 Part 1: In-Pile Measurements 3

4 Why make in-pile measurements on fuel rods? economic$ fuel performance code safety develop & improve models validation understanding fuel behaviour in-pile data: operating conditions fuel measurements 4

5 In-Pile Measurements: what can be measured on a fuel rod and to what fuel performance issue does the measurement relate? 1. Fuel Temperature Heat transfer (fuel thermal conductivity, gap conductance) 2. Rod Internal Pressure Change Fission gas release, fuel densification & swelling 3. Fuel Stack Length Change Fuel densification & swelling 4. Fuel Cladding Length Change PCMI, (or when no PCMI, surface heat transfer, growth) 5. Fuel Rod Outer Diameter Change PCMI, (or when no PCMI, creep & growth, corrosion) 5

6 1. Fuel Temperature A good prediction of fuel temperature is an essential requirement for any fuel performance code Fuel centreline temperature at a single point within an annular fuel pellet can be measured with a thermocouple Average fuel centreline temperature can be measured with an expansion thermometer running the length of an annular fuel stack Fuel centreline temperature measurements enable modellers to test their code predictions with respect to: Fuel thermal conductivity and its degradation with burn-up Gap conductance / gas composition (fission gas release) Gap width (fuel densification & swelling, cladding creep) Fuel pellet surface roughness Eccentricity in the fuel stack 6

7 Measuring Fuel Temperature with a Thermocouple High temperature thermocouples suitable for measuring inside a fuel stack are: W / Re Nicrosil / Nisil Strengths of thermocouples Reliable measurement: up to C, thermocouples can last several years and will even manage a few hours at 2000 C Accurate measurement: easy to calculate from measured to solid fuel pellet temperature Weaknesses of thermocouples Tendency to premature failure at high fuel temperatures or in failed rods Need to account for de-calibration due to transmutation at high fluence 7

8 In core connector Thermocouple end plug Instrument base plug Thermocouple Molybdenum tube Pre-irradiated fuel rod segment Measuring Fuel Temperature with a Thermocouple Both fresh and irradiated fuel pellets in a stack can be drilled to allow insertion of a thermocouple (Mo tube) Neutron radiography can show exactly where thermocouple tip (hot junction) is positioned Instrument base plug Pressure transducer end plug 8

9 Test rig structure (grid plate) Fuel rod upper end plug Plenum spring Fuel pellets (annular) Tungsten rod (dia. 1,5mm) Cladding Fuel rod lower end plug Core holder Measuring Fuel Temperature with an Expansion Thermometer A thin tungsten rod is inserted through a centre hole drilled in the entire fuel stack One end of W-rod fixed to fuel rod endplug, other end is free and fitted with a magnetic core, movement of core sensed by an LVDT (Linear Voltage Differential Transformer) Average centreline temperature in fuel stack derived from measured axial expansion of tungsten rod Ferritic core Linear voltage differential transformer (LVDT) Test rig structure (grid plate) 9

10 Linear Voltage Differential Transformer (LVDT) a c d Developed for measuring fuel rod pressure, temperature, fuel stack and cladding elongation. Primary coil with two secondary coils connected in opposition. Movable magnetic core concentrically located inside coil system. Core movement affects the balance of the secondary coils and generates the signal output. b g f e a: b: c: d: e: f: g: Test rod end plug assembly Primary coil Secondary coils Ferritic core Twin-lead signal cables Body Housing 10

11 Measuring Fuel Temperature with an Expansion Thermometer Strengths of expansion thermocouples Suitable alternative to thermocouples for high-temperature measurements No de-calibration with time Expected lifetime is longer than that of a thermocouple Weaknesses of expansion thermocouples Accuracy of the measurement due to the interpretation of the signal that is needed is not as good as the accuracy of a thermocouple There is also a risk that mechanical interaction between the fuel and the tungsten rod will affect the performance of the instrument 11

12 2. Fuel Rod Internal Pressure Change Measuring changes in rod internal pressure provides data on Fission gas release Fuel stack densification and swelling (in absence of FGR) Understanding or being able to adequately model fission gas release mechanisms is important especially for development of new fuel types for better fuel performance Rod pressure measurements often combined with fuel temperature measurements e.g. investigating threshold temperature for FGR onset Fuel rod internal pressure is a key issue for extending the discharge burn-up of fuel for power reactors - most licensing bodies limit allowable fuel rod internal pressure to not exceed system pressure 12

13 Measuring Fuel Rod Internal Pressure Change Gas connection to test rod Bellows support Bellows End plug Support for ferritic core Ferritic core Linear voltage differential transformer (LVDT) Small stainless steel or Inconel sealed bellow unit inserted in a fuel rod end-plug Gas pressure in fuel rod acts on bellows One end of bellows fixed to end-plug, other end is free and fitted with a magnetic core, movement of core sensed by an LVDT In-pile calibration at start of life (know rod pressure), then subsequent signal gives change in rod pressure Different bellows used for different expected measuring ranges: up to 15, 30 or 70 bar ΔP 13

14 Measuring Fuel Rod Internal Pressure Change Bellows experience creep due to the high temperature, stress and radiation environment Use pressurised bellows (to reduce stress) Carry out re-calibrations with data obtained from periods of cool-down and heat-up (no nuclear heating) using gas law Measuring fuel rod pressure in the plenum (end-plug) is subject to accessibility of gas released from the fuel stack No problem with fresh fuel (fuel-clad gap open at power) For fuel rods with a tight gap at power, brief power reductions can be made allowing the fuel-clad gap to re-open and gas to percolate to the plenum 14

15 3. Fuel Stack Length Change Measuring changes in fuel stack length provides data on Fuel densification and swelling (fuel-clad gap open) Fuel densification and swelling are of interest because of the way they affect the development of the fuel-clad gap e.g. as fresh fuel densifies initially, the gap size increases inducing an increase in the fuel temperature Dimensional stability behaviour varies between different fuel types and this is something that fuel models need to capture Fuel density Pellet shape (flat ended versus dished, hollow versus solid) MOX fuel, Gd-doped fuel, other additive fuels 15

16 Measuring Fuel Stack Length Change Linear voltage differential transformer (LVDT) Magnetic core Support for magnetic core End plug Spring Magnetic core holder fitted in fuel rod end-plug and spring loaded against fuel stack end Axial densification / swelling of the fuel stack acts on spring so magnetic core holder position moves Core movement sensed by LVDT In-pile calibration at start of life (zero point), then subsequent signal gives change in fuel stack length Stops being relevant once fuel-clad gap closes Because of connection between fuel dimensional changes and fuel temperature, fuel thermocouple often inserted at other end of same fuel rod Fuel stack 16

17 4. Fuel Cladding Length Change Measuring changes in fuel cladding length can provide data on cladding strain from fuel pellet to cladding mechanical interaction (PCMI) under different conditions which can be used in model development be used for predicting the outcome of situations where clad integrity may be jeopardised When there is no PCMI measurements can provide data on Heat transfer properties of surface layers (oxide, crud) Irradiation growth of cladding Onset of dry-out (in dry-out testing) 17

18 PCMI and Fuel Cladding Length Change Fuel-clad gap closes due to clad creepdown and fuel swelling Power ramps induce fuel swelling and promote gap closure and thus PCMI PCMI has diametral and axial components Plotting cladding elongation (axial) as a function of rod power is typical for interpreting what is occurring Onset of interaction (power at which PCMI first occurs) as function of BU Cladding relaxation during power hold (slippage or fuel creep) Degree of contact (soft or strong) 18

19 Test rig structure (grid plate) Fuel rod upper end plug Plenum spring Fuel pellets Cladding Fuel rod lower end plug Core holder Ferritic core Linear voltage differential transformer (LVDT) Measuring Fuel Cladding Length Change Upper end of fuel rod fixed to test rig structure Magnetic core holder fitted to end-plug at lower (free) end of fuel rod Change in fuel cladding length causes core holder position to move relative to an LVDT In-pile calibration at start of life (zero point), then subsequent signal gives change in fuel cladding length Test rig structure (grid plate) 19

20 5. Fuel Rod Outer Diameter Change Measuring fuel cladding outer diameter changes can provide data on PCMI during power transients (continuous measurement) Cladding creep (measurement made once a week) Oxide / crud build-up on a fuel rod (measurement made once a month) Monitoring diametral in addition to the axial components of PCMI enables better understanding of what occurs during power transients Most fuel performance codes contain models for cladding creep as this affects the development of the fuel-clad gap as well as influencing a fuel rod s PCMI behaviour Discharge burn-ups are often limited by the corrosion behaviour of the fuel rod cladding so knowing how different claddings behave in different water chemistries in-pile is a vital part of alloy development 20

21 b a a b c Measuring Fuel Rod Outer Diameter f a: Primary coil b: Secondary coil c: Ferritic bobbin e d g d: Ferritic armature e: Cross spring suspension f: Feelers g: Fuel rod Instrument based on the LVDT principle Transformer body connected to armature via a pivot point Feelers on opposite sides of the fuel rod trace the fuel rod outer diameter profile Unit is driven along the fuel rod by a hydraulic system Position sensor used to sense axial position of DG along the rod Calibration steps machined into the fuel rod end-plug surface 21

22 In-Pile Measurements: what else can be measured and to what fuel performance issue does the measurement relate? Thermal-hydraulic parameters (temperature, flow, pressure) Nuclear conditions (neutron flux / gamma flux / power) Gas flow through a fuel stack (hydraulic diameter) Gas flushing plus gamma spectroscopy (fission gas inventory) Water Chemistry parameters new developments Electrochemical Corrosion Potential (ECP) High temperature coolant conductivity On-line potential drop corrosion monitor Electrochemical Impedance spectroscopy (EIS) 22

23 Electrochemical Corrosion Potential Electrochemical corrosion potential (ECP) of a corroding metal is the potential difference between it and the standard hydrogen electrode (SHE) A key measurement when the corrosion performance of an in-reactor material is to be assessed or measures taken to optimise its performance Arises from a combination of the surface conditions of the specimen and the concentrations of dissolved oxidants Determined by measuring the potential between a sample and a reference electrode, and adding the (calculated) potential versus SHE of the reference electrode A reference electrode is a half-cell that produces a stable and reproducible potential 23

24 Measuring ECP Shielded signal cable Primary seal Ceramic tube Pt cylinder (supported by secondary seal inside) Mechanical seal in transition between ceramic parts and metal parts - no brazing Platinum reference electrode reliable but does not give SHE values in oxygenated water Palladium reference electrodes show promise for use in BWR conditions Prototype Fe/Fe 3 O 4 reference electrode developed - can be used in hydrogenated and oxygenated water 24

25 Part 2: In-Pile Experiments 25

26 Why do experiments in research reactors? Data needed to develop and validate fuel performance codes Must be generated under controllable and measurable operating conditions Need to consist of in-pile measurements of key parameters for fuel performance Difficult to achieve both in an NPP but research reactors can operate in ways that NPPs can t (safety & economics) Separate effects testing investigate single phenomenon Integral studies as close to real life as possible Accident or safety related studies pushing the boundaries 26

27 Special demands on research reactor capabilities - increasing emphasis on testing fuel under as close to the real situation as possible plus an increasing need for well qualified data from ever higher burn-up fuel Integral testing Specialised and dedicated loop systems built and operated within test reactors to simulate thermal-hydraulic and water chemistry conditions representative of those (current or planned) for commercial NPPs High burn-up fuel To avoid long base irradiation in test reactors an important advance has been the ability to re-fabricate and instrument sections of commercially irradiated fuel rod Also allows in-core performance of fuel in excess of current burn-up limits for power reactors to be studied what would happen next with this exact fuel 27

28 Loop Systems in Test Reactors Test reactors do not necessarily operate with same coolant conditions as commercial NPPs thus not always suitable for corrosion studies Test rigs can be placed in pressure flasks connected to dedicated loop systems - isolated from the reactor coolant Loop systems allow testing of fuel cladding under simulated BWR, PWR or PHWR conditions: Coolant pressure Coolant temperature Water chemistry Representative thermal-hydraulic conditions are also needed for different types of fuel studies e.g. PCMI testing 28

29 Pressure control system Feed water tank Loop Systems in Test Reactors Cooler Water analysis Control valve Purification system For chemistry control Grab samples Filter samples Impurities removed by ion exchange beds / filters Chemicals added to obtain desired water chemistry H 2, O 2 In-core test rig LiOH, B(OH) 3 Zn, Fe, Ni, TiO 2, etc 29

30 High Burn-Up Fuel: Re-fabrication Fuel assembly irradiated in a commercial LWR Fuel rod segment Re-fabricated fuel rod segment instrumented with fuel thermocouple and pressure transducer Re-irradiation in LWR loop Fuel segments taken from commercial NPPs Medium and high burn-up Re-fabricated and instrumented in hot-lab Same instrumentation possibilities as for fresh fuel Use of in-core connectors (with mechanical sealing) for thermocouples and gas lines 30

31 Examples of fuel performance experiments and typical data 1. Thermal behaviour of fuel 2. Fission gas release behaviour of fuel 3. Fuel stack dimensional stability 4. PCMI behaviour of fuel 5. Creep behaviour of fuel cladding 6. Crud formation on fuel cladding 31

32 1. Thermal Behaviour of Fuel Fuel Centre Temperature ( o C) (U,Gd)O Rod Burnup (MWd/kgUO 2 ) UO 2 Comparative irradiation shows conductivity difference of two types of fuel as well as the change of conductivity with burn-up. Data normalised to constant powers in order to identify long-term effects. Early experiments focused on effects related to gap conductance Large number of integral behaviour tests conducted with fuel temperature as one of quantities being monitored Interest in thermal conductivity Different fuels (UO2, MOX, Gd-fuel, IMF) Degradation with burn-up 32

33 Fuel thermal conductivity, W/mK Thermal Behaviour of Fuel Burnup B MWd/kgUO λ = 4040/(464 + a*b + ( *B)*T) *e T W/m/K fresh fuel a = 16 a = 15 (-1 σ) a = 17 (+1 σ) Temperature, o C From in-pile temperature data possible to derive equation describing UO 2 thermal conductivity as a function of burn-up and temperature Includes influence of: Fission products in matrix Micro-cracking Frenkel defects Fission gas bubble formation In fuel modelling codes, the conductivity model is applied locally along the radial burnup profile 33

34 2. Fission Gas Release Behaviour of Fuel Stepwise power increase of a fuel rod instrumented with pressure sensor and thermocouple to establish onset of fission gas release High burn-up fuel - dips necessary for gas communication with plenum Similar testing used to establish 1% FGR release threshold for UO 2 up to 40 MWd/kg oxide 34

35 2. Fission Gas Release Behaviour of Fuel With re-instrumentation technique, onset of FGR at higher burn-up studied Results: Original 1% FGR release criterion overestimates release onset temperature for Bu > 30 MWd/kgUO 2 New FGR code benchmarked against 1% release threshold for UO 2 Empirical modifications Centre Temperature (C) o o Threshold for 1% FGR Code calcs for 1% FGR Experimental 1% data MOX Burn-up (MWd/kgUO 2 ) o 35

36 As the fuel densifies, the gap size increases and the fuel T at a given power increases Data from sibling rods, one with temperature measurement and one with fuel stack extensometer Data have been normalised to constant power in order to more easily identify long term trends Experiments for Improving Nuclear Fuels Models and Performance 3. Fuel Stack Dimensional Stability 36

37 3. Fuel Stack Dimensional Stability Direct measurement of thermal expansion of fuel stack provides data on dimensional stability The amount of densification and swelling are of interest for development of gap closure Aspects that have been covered: Fuel density Pellet shape Resent investigations involve new fuel types like MOX and Gddoped 37

38 4. PCMI Behaviour of Fuel Elongation (mm) grain 22 μm grain 38 μm 0.75 % fuel swelling Burnup (MWd/kgUO 2 ) Cladding elongation response of re-instrumented PWR fuel (61 MWd/kgU) with different grain size during steady state periods Data normalised to constant power Permanent elongation Clad elongation increase reflects fuel swelling Ratcheting Elongation peaks associated with shut-down / start-up (release/onset mismatch) Relaxation Initial relaxation of high power elongation and stress caused by ratcheting is relaxed by fuel creep within a few days 38

39 5. Creep Behaviour of Fuel Cladding In-pile creep data from cladding tested under well defined and controlled conditions of temperature, stress and fast neutron flux used to improve cladding creep models within fuel performance codes Separate effects testing is usually carried out to generate creep data for example if fuelled cladding tubes are used, unrepresentative large fuel-clad gaps are used in order to avoid PCMI during the test Irradiated cladding fabricated into a sealed test rod and connected to a high pressure gas system Rod internal gas pressure changed to achieve nine different levels of hoop stress, each level held for a period of time Changes to the rod diameter measured using a contact scanning diameter gauge 39

40 5. Creep Behaviour of Fuel Cladding Creep curve for each stress period Creep behaviour of high fluence material with stress reversals Primary and secondary creep as a function of hoop stress obtained 40

41 6. Crud Formation on Fuel Cladding Crud formation in PWRs can lead to AOA phenomenon in which boron (absorbs neutrons) is incorporated into crud deposits on upper sections of fuel rods shifting reactor power downwards A full integral test was carried out at Halden to demonstrate that a PWR-type test loop could reproduce the symptoms of AOA A combination of on-line measurement techniques was used with a bundle of 8 fuel rods 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 analysis (lithium return during shutdown) and PIE used in support 41

42 6. Crud Formation on Fuel Cladding Diameter gauge showed crud had deposited on fuel rods Flux depression observed along upper section of fuel Attributed to boron incorporation into the crud Demonstration that the right crud was made 42

43 Summary (1/2) In-pile fuel behaviour and performance testing programmes are aimed at improving understanding and validating fuel performance codes... ultimately with the goal of increasing reactor safety and improving economy For reasons of safety and economy, in-pile testing is best carried out in test reactors Use of reliable and accurate in-core instrumentation to measure all relevant parameters under controlled operating conditions Possibility for separate effects investigations Fuel rods with different sensors enable better understanding and providing consistent data for modelling purposes Re-fabricating commercially irradiated fuel rods extends the relevance of fuel rods that can be studied (high burn-up) 43

44 Summary (2/2) Loop systems allow tests to be conducted under representative thermal-hydraulic conditions, which is important for some forms of investigation e.g. PCMI studies Test reactor loops also allow different water chemistries to be studied, including novel ones It is expected that in the future more emphasis will be placed on having testing facilities and instruments available for water chemistry studies e.g. on-line sensing of cladding corrosion 44