Predictability of CNEA PHWR MOX Experiments by Mean of TRANSURANUS Code, From the IFPE Database. Rozzia D, M. Adorni, A. Del Nevo, F.

Transcription

1 Predictability of CNEA PHWR MO Experiments by Mean of TRANSURANUS Code, From the IFPE Database Rozzia D, M. Adorni, A. Del Nevo, F. D Auria University of Pisa Gruppo di Ricerca Nucleare di San Piero a Grado (GRNSPG) Via Diotisalvi 2, Pisa daviderozzia@libero.it, m.adorni@ing.unipi.it, a.delnevo@ing.unipi.it, f.dauria@ing.unipi.it ABSTRACT Investigations of fuel behavior are carried out in close connection with experimental research, operation feedback and computational analyses. The comprehensive understanding of fuel rod behavior and accurate prediction of the lifetime in normal operation and in accident conditions are part of the defense in depth concept. In this connection, OECD NEA sets up the public domain database on nuclear fuel performance experiments International Fuel Performance Experiments (IFPE) database, with the aim of providing a comprehensive and well-qualified database on UO 2 fuel with Zr cladding for models development and codes validation. The CNEA s PHWR MO Experiment belong to this database. This experiment was carried out in the High Flux Reactor (HFR) of Petten, Holland. It involves six MO rods prepared and controlled in the CNEA s Alpha Facility (Argentina). The objective of the experiment was to verify the fabrication processes and the study of the fuel behavior with respect to cladding failure due to Stress Corrosion Cracking (SCC) under Pellet Cladding Interaction (PCI) conditions. These rods were irradiated from until 15. The code TRANSURANUS version v1m1j9 is assessed against the database CNEA PHWR MO in order to verify the capability of the code in predicting the cladding failures due to SCC. Comparisons with the experimental data and the results obtained with BACO code developed by CNEA of Bariloche are presented. Sensitivity calculations are also performed for supporting the analyses of the results, improving the level of understanding of the code capabilities. The main conclusion is that the clad failure propensity of the rods belonging to the PHWR CNEA MO experiment is conservatively assessed. 1 INTRODUCTION The present activity is focused on the behavior of the fuel component. The aim is to study the PCI phenomenon during power ramp in heavy water nuclear reactor, i.e. PHWR. The relevance of PCI in nuclear technology is connected with the prevention of fuel failures due to stress corrosion cracking (SCC), involving the lost of integrity of the first and second barriers (defence in depth concepts), during normal, off normal and accident conditions. The objective is the assessment of TRANSURANUS [1][2][3] code performance in predicting fuel and cladding behavior under pellet cladding interaction conditions using one experimental database based on PHWR rods at ranging from to 15: the 4.1

2 4.2 CNEA MO experiments [4]. The datasets of the CNEA-MO experiments, are part of the International Fuel Performance Experiments (IFPE) [5][6]. TRANSURANUS is a computer program for the thermal and mechanical analysis of fuel rods in nuclear reactors. The TRANSURANUS code consists of a clearly defined mechanical mathematical framework into which physical models can easily be incorporated. The mechanical mathematical concept consists of a superposition of a one-dimensional radial and axial description (the so-called quasi two-dimensional or 1½-D model). The code was specifically designed for the analysis of a whole rod. TRANSURANUS code incorporates physical models for simulating the thermal and radiation densification of the fuel, the fuel swelling, the fuel cracking and relocation, the generation of fission gases, the redistribution of oxygen and plutonium, etc. Mainly research institutions, industries and license bodies exploit the code. Besides its flexibility for fuel rod design, the TRANSURANUS code can deal with a wide range of different situations, as given in experiments, under normal, off-normal and accident conditions. The time scale of the problems to be treated may range from milliseconds to years. The code has a comprehensive material data bank for oxide, mixed oxide, carbide and nitride fuels, Zircaloy and steel claddings and several different coolants. It can be employed in two different versions: as a deterministic and as a statistical code. 2 CNEA MO EPERIMENTS 2.1 Description The irradiation of the first prototypes of MO fuels fabricated in Argentina began in 1986 [4]. The six rods were fabricated in the α Facility (GAID-CNEA-Argentina). The original design was made for irradiating the fuel rods in MZFR reactor (Karlsruhe, Germany). This reactor was decommissioning before the test execution, therefore the experiments were finally carried out in the HFR-Petten reactor, Holland. The first rod has been used for destructive pre-irradiation analysis. The second one was a Pathfinder (A1-4). Pathfinder rod [7] was irradiated with the purpose to calibrate the instruments of the HFR reactor. That experience verified the response of the HFR reactor detection systems with the MO rod. The duration of the irradiation was about hours. It included a final ramp test. Two additional rods (A-3, A-4) [8] included iodine doped pellets (one of them Cs-I and the second one elemental iodine). The concentration of iodine was calculated to simulate a of 15. The power histories were defined with the BACO code [9]. An irradiation period of 15 days including two power cycling and a final ramp was designed, no SCC failures were observed. The experiment named BU15 [][11] was performed with the last two rods (A1-2, A1-3). The goal of this experiment was to verify the fabrication processes and to study the fuel behavior with respect to PCI-SCC. Both rods were irradiated together for a long period and then one of them underwent a final power ramp after a short preconditioning irradiation. The final was 15. The power level during irradiation was low and without important demands, only the normal shutdowns of the HFR. The final ramp was applied to A.1.3 rod only; it was similar to that applied to the iodine test rods. In this case A.1.3 experiences a PCI/SCC failure. The rods subjected to these experiments have same pellets and cladding properties. Must be mentioned that the gap size (6 µm) is out of the TU correlations tested range therefore the activity is aimed also to verify the TU performance in this condition.

3 Modeling The input deck has been prepared respecting the information available in the code manual [3]. The models selected are generally the ones standard for the transient to be simulated. Only the active part of the fuel is accounted for the simulation. The active part has been divided into 1 or 5 axial slices, according to the experimental data available. For the reference calculations, the nominal geometrical values were used when available. The input deck of each rod differs from the others in: Boundary conditions:, linear heat rate, ramp terminal level; Geometry: rod length (A.3 and A.4 are different from A.1 type); Physical proprieties: A.3 and A.4 are Iodine an Cs-I, Mo doped. The boundary conditions implemented for the analysis are: 1) linear heat rate at the axial position according to the ASCII files; 2) cladding temperature histories at same position of linear heat rate; 3) fast neutron flux and 4) pressure. Figure 1 reports the linear heat rate versus time for the rod A.1.3, as sample. lhr1 lhr3 lhr5 lhr2 lhr4 Kw/m-h Ramp Preconditioning LHR [KW/m] LHR [KW/m] lhr1 lhr2 lhr3 lhr4 lhr5 6 Kw/m-h a) Base irradiation. b) Ramp. Figure 1: CNEA MO exp., A.1.3 rod LHR history. A.3 and A.4 rods have been irradiated 15 days. They are doped with Iodine for simulating an equivalent average burn up of 15 MW/kgU. TU code does not have the capability to simulate such experimental procedure. Moreover, the code does not treat the Iodine production and migration to the gap. The code applies the rough approximation that reaches the concentration capable to produce SCC (after 5). Then a specific subroutine is in charge of evaluating the cladding failure on the base of an empirical failure function (see Ref. [12]). In order to perform the analyses of the rods A3 and A4, it was decided to modify the source of the code, allowing the rupture due to PCI/SCC since values of burn up equal to zero. Details on the input decks are reported in Ref. [13]. 3 VALIDATION OF TREANSURANUS CODE AGAINST CNEA MO EPERIMENTS The present section provides a summary of the main results achieved (see also Ref. [13] for more details). Table 1 summarizes the conclusions of the activity, which are hereafter discussed. The first consideration is related with the fuel gap size in the database, which is 6 µm. This value is larger than the range of validity of TU correlations and also than in the typical PHWR fuel. This parameter, together with the neutron fast flux, delta pressure between coolant and gap have a relevant influence on the clad creep and swelling: this is connected with the not predictable parameter in Table 1.

4 4.4 Table 1: CNEA MO exp., TU validation related to PCI/SCC experiments. CNEA-MO EPERIMENTS: VALIDATION OF TU Gap initial size Neutron fast flux Delta pressure gap-coolant 6 [µm] 15 [n/(cm 2 s)] 144 bar Parameter Predictable Partially predictable Not predictable Not suitable for code validation or further analyses needed Cladding creep down Fuel relocation Gap size evolution Fuel grain size Fuel central temperature FGR Burn-up Cladding failure More in detail, the fuel behavior may be affected by the initial gap size mainly because the following correlations: Cladding creep and swelling; Gap conductance; Fuel relocation. The main effect is expected to perturb the early gap conductance and, therefore, the fuel central temperature at the beginning of irradiation. The simulations of the gap width at the end of the irradiation (cold conditions) are reported in Table 2. Experimental measures (when available) and the BACO code predictions [4] are also given. TU simulations are in good agreement with BACO results and experimental values only in the case of the rods irradiated until 15. On the contrary, over predictions of the gap final dimensions is observed in the case of rods irradiated at low values of burn-up, corresponding at values of Table 3 highlights the fuel outer at the end of the experiments. The comparison is between BACO and TU results. TU simulations show a pellet increase, which is proportional with burn-up: it is due to relocation and fuel swelling phenomena. On the contrary, the BACO simulations reveal a different behavior: the pellet at the end of irradiation decreases in the case of the rods irradiated at 15. No deduction about this results can be provided. In addition, no experimental data are available. Table 4 summarizes the cladding outer results. The comparison involves TU, BACO and experimental (when available) results. Experimental as well as BACO results are lower than TU predictions. Therefore, the cladding creepdown is under-predicted by TU (i.e. the early creep behavior is correctly not reproduced). Table 5 reports the maximum fuel central temperature. Due to the gap dimension in the early irradiation phase, a very low gap conductance is predicted, therefore, the fuel centreline temperature appears strongly over-predicted with respect to BACO results. This is reflected in a low heat exchange from the pellet to the clad. Figure 2 depicts the rod geometry evolution for two sample rods (the first irradiated at 15, the second at,29 ).

5 4.5 Table 2: CNEA MO exp., gap size at end of the irradiation. EP gap CC 1 BACO Calc Gap ZP 2 TU Calc. Gap CC 1.15 Path µm A A.1.2 µm A.1.3 µm A.3.52 A.3 µm A.4.52 A.4 µm CC: cold conditions ( C) 2 ZP: zero power (LHR=, Tclad=337 C Table 3: CNEA MO experiments, fuel outer at end of the irradiation. Initial nominal value BACO fuel outer TU Calc. fuel outer.15 Path mm A A.1.2 mm A.1.3 mm A.3.52 A.3 mm A.4.52 A.4 mm Table 4: CNEA MO exp., clad outer at end of the irradiation. Initial nominal value EP clad outer BACO clad outer TU Calc. Clad outer.15 Path mm A A.1.2 mm A.1.3 mm A.3.52 A.3 mm A.4.52 A.4 mm Table 5: CNEA MO exp., fuel central temperature max values. BACO Calc. TU Calc. Ref.15 Path C $ (27)$$ A A1.2 C A.1.3 C (18) A3.52 A3 C (28) A4.52 A4 C (265) $ Pre-Ramp $$ During Ramp Radius [mm] / Gap hundreds of [µm] TU calc. clad outer TU calc. fuel outer TU calc. gap EP gap 1 BACO calc. clad outer BACO calc. fuel outer BACO calc. gap size LHR in peak axial position (a) A.1.3 rod LHR [KW/m] Radius [mm] / Gap hundreds of [µm] TU calc. clad outer slice1 TU calc. fuel outer slice1 TU calc. gap slice1 EP avg.clad outer BACO calc. clad outer BACO calc. fuel outer BACO calc. gap size LHR in peak axial position (a) A.3 rod Figure 2: CNEA MO., rod geometry time trend. LHR [KW/m]

6 4.6 The fuel centreline temperature is mainly influenced by the relocation model as function of, LHR and gap initial size (when the reference/standard model is applied). Another option is a simplified model option (RELOC 5), which considers only the as fabricated gap dimension. The other models available in the code are similar with the reference model, with RELOC 5 or it considers the actual value of the gap dimension during the transient. No major difference of the results among the six models is evidenced from the point of view of the maximum centreline fuel temperature, exception for the rod A1.3, which is irradiated at 15 (Figure 3). Relocation model Reloc 3 reproduces the best simulation but in any case at least irradiating hours are needed to reach the gap dimension at which the correlations are tested. The gap conductance is investigated with the following set of sensitivities: 1) two gap conductivity models, 2) minimum gap dimension obtained from geometrical tolerances 3) constant gap conductance (1 W/mmK). The results are reported in Table 6: and Figure 4; assuming a constant gap conductance, the fuel central temperatures remain always acceptable. This is reflected in many parameters as the simulation of grain diameter and FGR at the end of the experiment (Figure 5, Figure 6). Table 6: CNEA MO exp., effect of gap conductance on max fuel central temperature. BACO Calc. TU Calc. TU Calc. TU Calc. TU Calc. TU Calc. Ref Ihgap4 Ihgap5 Min. gap Gap cond. (1) (2) (3) (4).15 Path C $ 2744)$$ 2 (2721) 93 (2721) 89 (2718) 52 (1645) A1.2 C A A.1.3 C (18) 2532 (1821) 2531 (1814) 24 (1821) 1427 (1773) A3.52 A3 C (28) 219 (289) 219 (289) 2224 (28) 96 (1724) A4.52 A4 C (265) 194 (2544) 194 (2544) 1912 (2572) 933 (19) $ Max temperature Pre-Ramp $$ Max temperature During Ramp Fuel central temperature [ C] (a) A.1.3 rod (sample rod at 15) TU calc Ref. Slice5 TU calc. reloc2 Slice5 TU calc. reloc3 Slice5 TU calc. reloc4 Slice5 TU calc. reloc5 Slice5 BACO calc. Max central T. Fuel central temperature [ C] TU calc. Ref. TU calc. reloc2 TU calc. reloc3 TU calc. reloc4 TU calc. reloc5 BACO calc. Max central T (a) A.3 rod (sample rod at.5 Figure 3: CNEA MO experiments: influence of relocation on fuel central temperature. Fuel central temperature [ C] (a) A.1.3 rod (sample rod at 15) TU calc Ref. Slice5 TU calc. constant gap cond. Slice5 TU calc. min-gap Slice5 TU calc. ihgap4 Slice5 TU calc. ihgap5 Slice5 BACO calc. Max central T. Fuel central temperature [ C] TU calc. Ref. TU calc. constant gap cond. TU calc. min-gap TU calc. ihgap4 TU calc. ihgap5 BACO calc. Max central T (a) A.3 rod (sample rod at.5 Figure 4: CNEA MO exp., influence of gap conductance on fuel max temperature.

7 4.7 Calculated grain diameter [µm] TU calc. constant gap cond. BACO calc. TU calc. Ref. A grain diameter [µm] (a) TU Reference against TU sensitivity, BACO and experiment at the end of the exp. A.1.2 A.3 A.4 Calculated grain diameter [µm] TU calc. constant gap cond. BACO calc. +% A.1.3 -% A.1.2 A.3 A.4 grain diameter [µm] (a) Zoom on TU sensitivity against BACO and experiment at the end of the exp Figure 5: CNEA MO exp., centre pellet grain diameter, influence of gap conductance. TU FGR Ref. TU FGR constant gap cond. BACO FGR LHR TU FGR Ref. TU FGR constant gap cond. avg LHR FGR [%] LHR [kw/m] FGR [%] 25 LHR [kw/m] (a) A.1.3 rod (sample rod at 15) (a) A.3 rod (sample rod at.5 Figure 6: CNEA MO exp., FGR, influence of gap conductance. Finally, in Table 7, the cladding failure predictions by PCI/SCC are analyzed. Considering the reference results, they are conservatively predicted: TU simulates, not correctly, the cladding failure of rod A.1.2. This rod was not ramped and the failure occurs during a change of power in the base irradiation. The failures of the rods A.1.2 and A:1.3 are influenced by the high fuel centreline temperature (see above) and the closure of the gap, as predicted by TU. In the cases at burn-up approximately equal to, the following considerations are applied: the Pathfinder does not fail because Iodine production is not enough to trigger the SCC mechanism, the experimental results of the rods A.3 and A.4 (doped) demonstrate that the effect of the Iodine is not effective in causing the failure. Doped rods are more resistant to SCC comparatively with irradiated rods (at fixed ). Table 7 CNEA MO exp., cladding failure assessment. Initial gap Width [µm] Initial pellet grain [µm] Avg. LHR [kw/m] Avg. Neutron Fast flux 14 [n/cm 2 s] Meas. Burnup [] RTL [kw/m] Hold Time [hrs] EP and BACO F/NF 1 TU Ref. F/NF TU gap cond Path & NF NF NF A.1 A $ NF F 2 NF A $ F F F A.3 A & NF NF NF A4 A & NF NF NF & Average value calculated from ASCII data 1 F/NF: failure / not failure Qualitative values 2 Blue color indicates wrong prediction in a conservative way Calculated from ASCII data In peak axial position F/NF

8 4.8 The sensitivity analysis on gap conductance (constant gap conductance at 1 W/mmK) allows the best prediction of failures, because the fuel centreline temperature. Other sensitivity analyses were conducted and discussed in Ref. [13]. 4 CONCLUSIONS The TU predictability of these parameters/phenomena are considered satisfactory: Burnup; Cladding failure by SCC mechanism, which is the objective of the activity. The predictability of this parameters/phenomena are judged partially satisfactory: Gap dimension at the end of the experiment (15 only); Pellet grain size at the end of the experiment (all the rods); Fission Gas Release. The TU predictability of these parameters/phenomena are evaluated not satisfactory: Cladding creep down/expansion time trend (all rods); Gap size time trend (all rods). The term partially predictable means that the difference from the experimental results is well understood and improvements of the simulations are achieved in sensitivity analysis. In conclusion, the PCI failure criterion implemented in TRANSURANUS code is conservative in the case of PHWR CNEA MO experiments. ACKNOWLEDGMENTS The authors wish to express their thanks to Enrico Sartori, for his effort in creating, maintaining and making available the OECD/NEA/NSC International Fuel Performance Experiments database. Gratitude is also expressed to John Killeen that organized the IAEA FUME III meeting in Pisa and allowed an excellent and useful exchange of expertise among the participants. The authors gratefully acknowledge the helpful technical support of the ITU fuel performance team. REFERENCES [1] K. Lassmann, TRANSURANUS: a fuel rod analysis code ready for use, Journal of Nuclear Material 188, 1992, pp [2] P. Van Uffelen, Modeling of Nuclear Fuel Behavior, Publications Office, JRC Publications, Report EUR EN, European Commission, 6. [3] K. Lassmann, A. Schubert, P. Van Uffelen, Cs. Gyory, J. van de Laar, Transuranus Handbook Version v1m1j6, EC, JRC, ITU, July 6. [4] A. C. Marino, E. Perez, P. Adelfang, Irradiation of Argentine MO fuels. Post irradiation results and analysis with the BACO code, Journal of Nuclear Materials, 229, 1996, pp [5] OECD/NEA, The Public Domain Database on Nuclear Fuel Performance Experiments for the Purpose of Code Development and Validation, International Fuel Performance Experiments (IFPE),

9 4.9 [6] J. C. Killeen, E. Sartori, J.A. Turnbull, Experimental Data on PCI and PCMI within the IFPE Database, Proceedings of Seminar on Pellet-clad Interaction in Water Reactor Fuels, Aix-en-Provence, France, 9-11 March 4. [7] J. F. W. Markgraf, E. J. de Haan, H. Rother, B. G. Fischer Technical memorandum, JRC, Petten establishment, HFR Pathfinder power history, Petten, [8] J. F. W. Markgraf, E. J. de Haan, H. Rother, B. G. Fischer, Technical memorandum, JRC, Petten establishment, HFR A.3/A.4 power history, Petten, [9] S. Harriague, G. Coroli, E. Savino, BACO (BArra Combustible), a computer code for simulating a reactor fuel rod performance, Nuclear Engineering and Design 56(198)91. [] S. McAllister, J. Markgraf, B. Seaber, Technical memorandum power history of the pre-irradiation phase in the HFR core, JRC, Petten establishment, HFR test BU- 15 power history, Petten, [11] S. McAllister, J. Markgraf, B. Seaber, Technical memorandum power history of the pre-irradiation phase in the PSF, JRC, Petten establishment, HFR test BU-15 power history, Petten, [12] IAEA, Status and advances in MO fuel technology, Technical Report series n 415, Vienna 3. [13] D. Rozzia, A. Del Nevo, M. Adorni, TRANSURANUS Predictability of CNA-MO Experiments (from IFPE database), FUEL/FUME-III/3(), GRNSPG, UNIPI DIMNP, July.