Status of R&D Activities related to Axial & Radial Creep of Pressure tubes of Heavy Water Reactors in India

Transcription

1 Status of R&D Activities related to Axial & Radial Creep of Pressure tubes of Heavy Water Reactors in India SK Sinha*, Dr. Dinesh Srivastava*, Dr. Apu Sarkar* B.N. Rath*, G. Ganesha** & Dipti Bhachawat # *Bhabha Atomic Research Centre, ** Nuclear Fuel Complex, Hyderabad, # NuclearPower Corporation

2 Scope of presentation R& D Strength of the organisation Brief introduction about PHWRs in India Operating and Under construction Issues associated with axial elongation and radial expansion due to neutron enhanced creep in pressure tube Status of work done till date Important Points related to present CRP Discussion

3 Pressure tube R & D strength (Out of Pile) Strong multi-disciplinary team supporting R&D activities on pressure tube alloy development, process route development, micro-structure and texture studies, corrosion studies, mechanical and fracture behaviour characterisation New design with emphasis on easy replacement and inspection degradation modelling and simulation inspection, diagnostic and rehabilitation tools development accident analysis and assessment

4 Pressure tube R & D strength (PIE) Post irradiation examination facility includes a large hot cell to accommodate full length active pressure tube CNC machines for preparing specimens for evaluation of mechanical and fracture properties Facilities for guaging the channel for ID, surface examination, visual examination and flaw detection; metallurgical studies Facilities for estimation of hydrogen concentration in zirconium alloy samples Burst test facility for evaluating burst strength and fracture toughness estimation using slit burst test

5 Investigations carried out Texture and microstructure evaluation of offcuts and pressure removed for survellience purpose Fracture toughness and tensile strength for Zr-2 and Zr- 2.5% Nb pressure tubes DHC velocity measurement In past efforts taken up for irradiation creep tests on micro pressure tubes at PFBR

6 Scope of presentation R& D Strength of the organisation Brief introduction about PHWRs in India Operating and Under construction Issues associated with axial elongation and radial expansion due to neutron enhanced creep in pressure tube Status of work done till date Important Points related to present CRP Discussion

7 PHWRS IN INDIA Total 18 Nos. PHWRs Under Operation 16 Nos. are 220 MWe Capacity 2 Nos are of 540 MWe Capacity. 4 Nos of 700 MWe Under Construction.

8 PRESSURE TUBES OF INDIAN PHWRs REACTOR PT MATERIAL GARTER SPRINGS REMARKS RAPS-1 ZR-2 2 NOS LOOSE FIT Not operating RAPS-2 ZR-2.5%NB 4 NOS TIGHT FIT EMCCR DONE RAPS 3&4 ZR-2.5%NB 4 NOS TIGHT FIT RAPS 5&6 ZR-2.5%NB 4 NOS TIGHT FIT MAPS 1&2 ZR-2.5%NB 4 NOS TIGHT FIT EMCCR DONE NAPS 1&2 ZR-2.5%NB 4 NOS TIGHT FIT EMCCR DONE KAPS-1 ZR-2.5%NB 4 NOS TIGHT FIT EMCCR DONE KAPS-2 ZR-2.5%NB 4 NOS TIGHT FIT KAIGA 1&2 ZR-2.5%NB 4 NOS TIGHT FIT KAIGA 3&4 ZR-2.5%NB 4 NOS TIGHT FIT TAPS 3&4 ZR-2.5%NB 4 NOS TIGHT FIT

9 Statistics on Periods of Operation seen by PHWRs with Zr-2.5%Nb Pressure tubes Reactor Units EFPYs REMARKS KAPS LEAD REACTOR OLD SPECIFICATION RAPS-2 RAPS LEAD REACTOR LATEST SPECIFICATION OLD SPECIFICATION RAPS OLD SPECIFICATION KGS-1 & KGS / 9.0 OLD SPECIFICATION Other units have seen even lesser operation and have pressure tubes made with latest specification.

10 Scope of presentation R& D Strength of the organisation Brief introduction about PHWRs in India Operating and Under construction Life limiting Issues associated with axial elongation and radial expansion due to neutron enhanced creep in pressure tube Status of work done till date Important Points related to present CRP Discussion

11 Pressure tube in a coolant channel assembly of PHWRs is a primary barrier against the high pressure and high temperature coolant and the nuclear radiation. Pressure = 10 MPa; Temperature = 250 C 300 C, Neutron flux = 3E13 n/m 2 -sec High flow (~10 kg/sec) (Zr-2/Zr-Nb) (Zr+2.5Nb +0.5Cu) (Zr-2) [350 K] [573 K] (SS 403) 220 MWe PHWR: 306 Channels, typically 5.2 m PT Length & 83 mm PT ID 540 MWe PHWR: 392 Channels, typically 6.2 m PT Length & 104 mm PT ID

12 The pressure tube operates under severe environment and undergoes degradation by Mechanisms PT (Zr-2/ Zr-2.5Nb) Fast neutron Irradiation Enhanced Creep & Growth In-service Corrosion & Hydriding Elongation, Diametral expansion Bending across supports Delayed hydride cracking (DHC), Hydride reorientation, Embrittlement, Hydride blisters Fast neutron Irradiation Enhanced Embrittlement Yes

13 Consequences of unexpectedly large axial elongation and diametral expansion Axial Elongation End fitting coming out of bearing support much early in the design life Feeder to feeder and Grayloc hardware to feeder interactions if differential axial elongation also exists Diametral Expansion Coolant bypassing the fuel fuel failure, financial implication as result of derating Interaction amongst the components of coolant channel like girdle wire, garter spring coil, PT and CT Failure of girdle wire (welded ends) Squeezing of garter spring between PT and CT Loading of CT and its subsequent failure

14 Measured Diameter, mm In a typical channel inside diameter variation profile along the Measured average dia length of a pressure tube, peak occurs at garter spring location. direction S--->N ) eter at FPYs Initial ID GS-4 GS KAPS-2: P-11 Inside Diameter at FPYs (Front end at south, Flow direction S--->N ) Peak location where radial gap between garter spring outer Measured average dia Initial ID GS-4 GS-3 torus and CT ID is the minimum. Courtesy: PIED from Inlet (South to North), mm Diatance from Inlet (South to North), mm

15 Scope of presentation R& D Strength of the organisation Brief introduction about PHWRs in India Operating and Under construction Issues associated with axial elongation and radial expansion due to neutron enhanced creep in pressure tube Status of work done till date Important Points related to present CRP Discussion

16 In-service Inspection (ISI) programme of coolant channel assembly in India is dedicated mainly to the pressure tube. It calls for: Axial elongation measurement every biennial shutdown in (100% channels) Inside diameter measurement every four years for normal trending in selected group of channels ( 16 Nos.) frequency and quantum of inspection can be increased based on feed back from inspection results. (>100 Nos of pressure tubes have been inspected for inside diameter)

17 Statistics of diametral creep rates observed in Indian PHWRs No. of Pressure tubes: 64 In-reactor service life: years

18 Axial elongation measured pressure tubes of KGS-1 at 7.5 Years of operation Effect of Fe contents is clearly visible

19 Texture measurement statistics (1/2) Transverse pole figures No of samples 48 Pole figures obtained from Kearns factors measured using X-ray diffraction Mean Std. dev

20 Texture measurement statistics (1/2) Radial pole figures Mean SD

21 Numerical Models to simulate the in-reactor deformation Phenomenological Model (a) Artificial Neural Network (b) available for axial elongation being developed for diametral expansion E d = E th + E cr + E gr E th = K th C s th Exp(-Q th /T) E cr = K cr (?) s cr f Exp(-Q cr /T) E gr = K gr (?) f Exp(-Q gr /T) Multi parameters function Trained by the a set of measured data Trained network is used for prediction of the remaining sets a) Modelling In-Reactor Diametral Expansion and Axial Elongation in Indian Zr-2.5%Nb Pressure tubes, S.K. Sinha and R.K. Sinha, International conference on Advances in Nuclear Materials , Mumbai, India b) Artificial neural network modeling of in-reactor axial elongation of Zr-2.5%Nb pressure tubes at RAPS 4 PHWR, A. Sarkar, S. K. Sinha, J. K. Chakravartty And R. K. Sinha,Nuclear Technology, VOL. 181 MAR. 2013

22 Inputs required by the Models Pressure tube specific information Axial elongation measured is the gross deformation Length averaged values of stress, temperature, flux, chemical composition, microstructure and texture parameters are required for modelling. chemical composition has been considered as parameters in ANN model but not in Phenomenological model texture parameters, grain size effect yet to be incorporated in both the models Radial expansions measured are location specific effect of all the input parameters identified above. Both the phenomenological and the ANN model have taken location specific values of flux, temperature and coolant pressure. Location specific values of metallurgical parameters are possible to incorporate in the Model. (yet to be implemented) Possible to update the operating hoop stress after each iteration in Phenomenological model but not in ANN Model Reactor operating history Operating period, capacity factor, availability factor

23 Comparison of Model Prediction of inside diameter after creep expansion with the measurements These predictions need to be fine tuned by incorporating precise metallurgical parameters which at present is grossly taken into account by considering UTS as an input variable

24 Comparison of model prediction of maximum creep with the actually observed values Some tuning is required based on metallurgical parameters

25 Comparison between phenomenological model estimated and measured axial elongation of pressure tubes Conservative Un-conservative Predictions are conservative in 95% of the measured data.

26 Comparison of measured axial elongation with ANN predicted values

27 How these in-reactor dimensional changes are taken care of in design? Axial elongation Design provisions are there to accommodate it in the bearing length. Diametral expansion It is not thermal margin but the structural integrity as a result of pinching of garter spring is an issue. Expected service life is around years.

28 Measures being taken to further improve the service life Modifying the garter spring design To accommodate larger creep expansion Optimising manufacturing route To reduce the creep rate

29 Modifying Garter Spring Design to accommodate larger diameter creep rate Reducing coil outside diameter will help accommodating larger diametral creep expansion without pinching of garter spring between CT and PT In case welded girdle wire, it is required to changed its design from wire form to some sort of spring form (this option is being explored)

30 Measured Diameter, mm By reducing coil diameter radial clearance between CT and the Measured average dia Initial ID GS coil OD b is increased. eter at FPYs direction S--->N ) GS-4 GS KAPS-2: P-11 Inside Diameter at FPYs (Front end at south, Flow direction S--->N ) Peak location where radial gap between garter spring outer Measured average dia torus and CT ID is the Initial ID GS-4 GS-3 minimum. Courtesy: PIED from Inlet (South to North), mm Diatance from Inlet (South to North), mm

31 Fabrication route to optimise the texture and other metallurgical parameters New route Current route Reduced number of steps helped better control and consistency in the product

32 Bright field TEM micrographs of pressure tube blanks extruded with different extrusion ratios (a)leading end and (b) trailing end of tube extruded with ER 7.3:1 (c) leading end and (d) trailing end of tube extruded with ER 12.75:1

33 TEM photographs of Final autoclaved microstructure for two stage and single stage pilgered tube Globalised beta Single forging + double pass pilgered tube (a) leading and (b) trailing end Existing extrusion+double pass pilgered tube Doouble forging + single pass pilgered tube (c) leading and (d) trailing end

34 Texture data measured in nearly in specimens from nearly 200 tubes TUBE END Fr Ft Fl <0.1 Average Std. Deviation # O # O

35 MECHANICAL PROPERTIES: Variation along the length of the pressure tubes RT - YS: 85ksi(max) HT - UTS: 68ksi(min), YS: 47ksi(min), %El: 12% (min) Mechanical properties Room temperature High temperature (314C) Leading end Trailing end Leading end Trailing end UTS (ksi) YS (ksi) % Elongation Observations: RT Mech props: trailing end is showing slightly higher strength and lower ductility HT Mech props: Both ends are showing nearly similar strength and ductility

36 Summary on optimisation of production route O is close to upper range of the specification. RT and HT mechanical props are nearly same at both ends. Avg. Texture: Ft =0.69; Ft-Fr = 0.36 nearly same at both ends. Variability in texture values is very low. Grain shape Both alpha and beta are lamaller and beta phase is continuous. Avg. Grain Morphology length =40mm, width = 5mm thickness = ~0.7mm (from SEM measurements) mm ( from TEM) Aspect ratio=1:5:50

37 Normalised creep rate Normalised creep rate variation with radial pole figure Fr (0.29) value in new tube Fr value in Existing tube Fr

38 Normalised creep rate Normalised creep rate variation with difference beteween transverse and radial pole figure Existing tube New tube Ft- Fr

39 Expectation from the optimised optimisation in garter spring spacer product together with design Better creep resistant properties Longer service life

40 Scope of presentation R& D Strength of the organisation Brief introduction about PHWRs in India Operating and Under construction Issues associated with axial elongation and radial expansion due to neutron enhanced creep in pressure tube Status of work done till date Important Points related to present CRP Database scope, limitations, modelling methodology, CRP plan Points of Discussion

41 Database Scope (1/2) Chemical composition identification of elements likely to have effect on creep deformation Pressure tube inspection data manufacturing inspection data initial inside diameter, wall thickness are measured at certain locations along the pressure tube length average value of these data with error band in the measurement should be specified as initial diameter. In-service inspection data Measured ID will have its own error depending upon the tools & technique used Metallurgical Parameters Only offcuts (sampled pieces from the ends) are available. any information generated from the samples from offcuts have to be linearly interpolated for other axial locations

42 Database Scope (2/2) Operating Parameters Inlet & outlet temperatures generic nature in case of CANDU Unit tube specific in case of Indian Units Coolant pressure in each tube Generic value Neutron flux variation along the length of a channel calculated value Reactor operating time Availability factor and capacity factors

43 Limitations of the present exercise Chemical Composition Elements affecting the creep behaviour need to be identified Manufacturing inspection data Average value is available In-service inspection data Presence of error depending upon tool and technique used Material parameters Variation along length is assumed to be linear as only offcuts are available for examination.

44 Modelling Methodology Inspection database Axial Time Average change in length Radial Time Location change in diameter Methodology different approaches are required to be followed incase of axial and radial creep modelling temperature, flux, metallurgical parameters averaged over length can be used as function variables. in case of axial elongation. Location specific flux, temperature, metallurgical parameters can be used as function variables in case of radial elongation. temperature and flux dependence needs to be identified to normalise the inspection data.

45 CRP Tentative Plan (1/2) Identification of a set of inspected pressure tubes from an operating units (done) Compilation of inspection results Compilation of manufacturing inspection data Chemical composition Mechanical properties Dimensional inspection Compilation of operating history Unit operating history Availability factors &capacity factors as a function of years of operation tube specific operating parameters Coolant temperature, Fast flux, Coolant pressure

46 CRP tentative plan (2/2) Preparation of specimens from the off-cuts Generation of texture and microstructure related information basal pole distributions, grain size, grain thickness Building predictive models using operating history, manufacturing inspection data and texture & microstructure data Comparison of model predictions with the results of inspection

47 Issues to be resolved No. of pressure tubes Channel Selection Criteria Chemical Composition Methodology to normalise with respect to temperature Database format Database sharing Material characterisation methodology Material movement Results reporting template

48 Suggestions As the CRP aims to identify material parameters of Zr- 2.5% Nb pressure tube material which can give a service life of 40 years, it would more prudent to include the irradiation of fully characterised specific material samples in a high flux research reactor in the scope of this work.

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