In-pile fuel rod performance characterisation in the Halden Reactor

Transcription

1 In-pile fuel rod performance characterisation in the Halden Reactor T. Tverberg Institutt for energiteknikk OECD Halden Reactor Project TECHNICAL MEETING ON FUEL ROD INSTRUMENTATION AND IN-PILE MEASUREMENT TECHNIQUES 3-5 September 2007 Park Hotel, Halden, Norway 1

2 Contents Examples of in-pile measurements from fuel rod instrumentation: a. Fuel temperature Thermocouple / Expansion thermometer b. Fuel stack elongation detector a. densification b. fuel swelling c. relation to fuel temperature c. Cladding extensometer a. PCMI d. Fuel rod pressure transducer a. Fission gas release e. Re-instrumentation of commercially irradiated LWR fuel a. Investigations on high burnup fuel 2

3 Introduction For modelling it is invaluable to get access to direct measurements of aspects related to in-pile performance of LWR fuel Fuel temperature Dimensional stability Amount of fuel densification and rate of solid fission product swelling Fission gas release behaviour Temperature onset as function of burnup Cladding strain Pellet cladding mechanical interaction (PCMI) With the instrument capabilities the Halden Reactor Project has developed over the years, direct (and simultaneous) measurements are possible on-line Different operating modes can be studied Short- and long-term effects can be studied Vast experience 3

4 Fuel temperature A good prediction of fuel temperatures is an essential requirement for any code used in design, fuel performance and safety evaluations With fuel thermocouples, fuel centreline temperatures can be measured accurately and reliably for temperatures up to ~ C. For short-term measurements temperatures as high as ~2000 C have been measured Early experiments were focused on effects related to gap conductance gas composition and gap size pellet eccentricity surface roughness In recent years, focus has been more on high burnup effects and new fuel types Fuel conductivity degradation MOX fuel, Gd fuel,... A large number of integral behaviour tests have been conducted, with fuel temperature as one of the quantities being monitored Fuel centre temperature, o C Xenon, concentric Xenon, eccentric Helium data local power, kw/m The figure shows data from an experiment related to the effect of eccentric pellets on temperature. The nonlinear relation of temperature vs. power is typical of Xe or fission gas filled rods. 4

5 Fuel Stack Elongation Detector Direct measurement of thermal expansion of the fuel stack provides data on dimensional stability of the fuel In particular the amount of densification and swelling are of interest Development of gap closure Aspects that have been covered: Fuel density Pellet shape Flat ended vs. dished Hollow vs. solid Resent investigation involve investigation of new fuel types MOX Gd 5

6 Fuel dimensional changes and temperature 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 Densification phase Temperature increases as gap opens Swelling phase Temperature given by gap closure and fuel conductivity degradation 6

7 % ΔV/V o Fuel elongation, mm Fuel Stack Elongation Detector UO 2 fuel (EF1) Gd 2 O 3 fuel (EF2) Gd 2 O 3 (EF2 at HSB) UO 2 (EF1 at HSB) 0.5% dv/v / 10 MWd/kgUO Rod burnup, MWd/kgUO 2 Swelling of UO 2 and Gd-UO 2 fuel derived from fuel stack length change at hot stand-by Densification and swelling Irradiation of production line UO 2 and Gd fuel (8 w /o absorbing Gd) No densification is observed for Gd fuel Probably related to the low power and temperature at BOL Both fuel types show a swelling rate of about 0.5% ΔV/V 7

8 Fuel temperature and fission gas release The combined measurements of fuel temperature and rod pressure allows for direct investigation of onset of fission gas release Example shows MOX rod instrumented with fuel thermocouple and rod pressure sensor Power increase at ~10 MWd/kg to cross release threshold Onset of fission gas release 8

9 Fuel temperature and fission gas release Data like those shown on the previous slide, have assisted in the establishing the Halden (Vitanza) 1% FGR threshold established for UO 2 up to ~40 MWd/kgUO 2 Onset of fission gas release Newer data have shown that the release onset of MOX and Gd fuel is very similar to that of UO 2 Example of investigation of temperature threshold for onset of fission gas release The data point from the MOX test on the rpevious slide is indicated and compared against the Vitanza threshold 9

10 Thermal Conductivity Derived from in-pile Fuel Temperature Data Fuel Centre Temperature ( o C) (U,Gd)O Rod Burnup (MWd/kgUO 2 ) UO 2 This comparative irradiation shows conductivity difference of two types of fuel as well as the change of conductivity with burn-up. Data have been normalised to constant powers in order to identify long-term effects. For small gap rods, the influence on gap thermal conductance change with gap size change (densification / swelling) and FGR is small The fuel themal conductivity of different fuels can be investigated Amount of change with burnup can be quantified Halden has a large database from numerous fuel rods that have been equipped with thermocouples or expansion thermometers Database includes different fuel types (UO 2, MOX, Gd-fuel, IMF) Supplements laboratory measurements (laser flash) on irradiated fuel samples 10

11 UO 2 Thermal Conductivity Derived from In-pile Fuel Temperature Data Large data base from low to >65 MWd/kg burnup Direct link to quantity of interest (fuel 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 Different fuel types have also been investigated MOX Gd Fuel thermal conductivity, W/mK 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 11

12 Cladding extensometer and pellet cladding mechanical interaction (PCMI) Cladding elongation sensor On-line measurements, reliable instrument Rigs with in-pile diameter gauge moving along the length of the rod also available Provide data on cladding strain in different conditions model development and prediction of situations where clad integrity may be prejudiced The figure shows early power ramps for a rod with cladding elongation sensor Features of early-in-life PCMI are seen: First power ramp: very early onset of interaction Following power ramps: shift of PCMI onset to higher power Relaxation of axial strain during power holds Immediate continuation of elongation when power increases (strong contact) 12

13 Development of Onset of interaction Fuel-clad accommodation NOTE: In the figure, subsequent ramp curves have been shifted for clarity The onset of interaction moves to lower power with increasing burnup and decreasing power The accommodation of fuel and cladding to each other result in small interaction tails as long as power does not exceed previously reached levels. 13

14 Investigations on high burnup fuel: Re-instrumentation of commercially irradiated LWR fuel With the move to higher discharge burnup there is a need for well qualified data for bunrups in excess of 60 MWd By taking irradiated LWR rods and re-instrumenting them, high burnup fuel behaviour can be investigated in-core Fuel temperature measurements Conductivity change with burnup Impact of fission gas release on temperature Rod pressure Fission gas release behaviour at elevated burnup Gas communication through fuel column Cladding strain PCMI at higher burnup 14

15 Fuel centre temperature ( o C) Re-instrumentation of irradiated fuel rods fresh fuel, 30μm gap TUBRNP power distrib. 1 + cond. par. a= rim porosity 3 + cond. par. a=16 measured data Local heat rate (kw/m) Thermal behaviour of high burnup fuel High BU (59 MWd/kgUO 2 ) UO 2 rod re-instrumented with TF and PF Appreciable difference to temperatures of fresh fuel Important factors: - conductivity degradation - power distribution - rim porosity (High Burnup Structure) The model for UO 2 conductivity degradation derived from other inpile temperature data is suitable for explaining the differences 15

16 Re-instrumented PWR fuel: Gas flow through the fuel column ( High BU ) Internal rod pressure (bar) steady state 45% FG / 55% He He Average heat rate (kw/m) The fuel column becomes permeable after a limited power reduction. The release of the inner overpressure to the fuel rod plenum is detected then. The released fission gas results in a gas mixture of 45% FG / 55% He The impact on measured fuel temperature is small, due to the tight bonding between fuel and cladding 16

17 LWR fuel re-instrumented with fuel thermocouple and pressure sensor: Fission gas release kinetics Temperature and FGR history LWR MOX rod Re-instrumented with fuel thermocouple and pressure sensor Stepwise power / temperature increase to establish onset of fission gas release Again it is seen that for high burnup fuel, power dips are necessary in order to obtain communication with the plenum and PF instrument (tight fuel column) Envelope of release curve indicates diffusion controlled release 17

18 LWR fuel re-instrumented with fuel thermocouple and pressure sensor: Onset of Fission Gas Release at High Burnup With the re-instrumentation technique, the onset of FGR at even higher burnups can be studied Results: Original 1% FGR release criterion overestimates release onset temperature for Bu > 30 MWd/kgUO 2 The curve to the right shows an FGR code benchmarked against 1% release threshold (Halden data) for UO 2 Empirical modifications: resolution parameter burnup dependent diffusion coefficient Centre Temperature (C) o o Threshold for 1% FGR Code calcs for 1% FGR Experimental 1% data MOX Burn-up (MWd/kgUO 2 ) Comparison of original and revised criterion for fission gas release onset o 18

19 LWR fuel re-instrumented with cladding elongation sensor: PCMI: Axial racheting Burnup MWd/kgUO In the figure, subsequent ramp curves have been shifted for clarity The slight mismatch between release and onset of interaction causes axial ratcheting and elongation peaks 19

20 LWR fuel re-instrumented with cladding elongation sensor: PCMI behaviour at high burnup (UO 2 ) 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. Normalised data. Permanent elongation Clad elongation increase reflects fuel swelling Ratcheting Elongation peaks associated with shut-down / start-up (release/onset mismatch) Relaxation Inital relaxation of high power elongation. Stress caused by ratcheting is relaxed by fuel creep within a few days 20

21 Summary With the instrument capabilities the Halden Reactor Project has developed over the years, direct measurements of important parameters related to fuel in-pile behaviour are possible on-line: Fuel temperature Feedback from fission gas release and dimensional changes Dimensional stability Quantification of fuel densification and rate of fuel swelling Fission gas release behaviour Temperature onset as function of burnup Cladding strain Pellet cladding mechanical interaction (PCMI) The data provide for detailed modelling of LWR fuel 21

22 Other measuremetns and techniques Other information than what is shown in the previous slides can be obtained from the measurement techniques discussed. These include e.g: Noise analysis (at steady state) Fuel thermocouple provide information on fuel thermal conductivity and gap conductance (fission gas release) Cladding elongation detector state of contact between fuel and cladding (PCMI) SCRAM data Fuel thermocouple Fuel time constant, fuel thermal conductivity, gap conductance Cladding elongation detector Stored heat 22

23 Other measurements and techniques Examples of additional instrumentation techniques at the Halden Reactor Project related to fuel and cladding behaviour: Gas lines connected to fuel rods (also pre-irradiated fuel): Measurements of radioactive fission gas release on-line Axial communication in fuel rod (hydraulic diameter) Can change gas composition on-line to change fuel temperature Information on onset of clad lift-off (by applying large overpressure) In-pile diameter gauges Cladding creep under variable loading conditions CRUD build-up Cladding thermocouples (re-instrumented LWR fuel) LOCA transient studies Dry-out studies And many more... 23