atw 2018-1

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atw Vol. 63 (2018) | Issue 1 ı January FUEL 40 8) The CFHR criterion adopted in the French safety demonstration results from the interpretation of critical flux tests performed for a given fuel assembly. For this reason, the CHFR criterion is likely to undergo changes in case of modification to the fuel materials and design. 9) A RIA is caused by a control REA, which is defined as the mechanical failure of a Rod Cluster Control Assembly (RCCA) drive mechanism casing, located on top of the reactor pressure vessel which is ejected vertically from the reactor core due to the high coolant pressure. Such a RIA is characterized by a very rapid increase of reactivity and power in some rods of the reactor. 10) EDF’s safety domain for REA: Oxide thickness, enthalpy variation, pulse width, clad temperature. 11) ECR : Equivalent Cladding Reacted. 12) Expansion due to compression using various PWR cladding alloys and performed at 350°C and 10 -4 s -1 . 13) Uni-axial tensile tests using transverse samples and carried out from 280°C to 400°C at 10 -2 s -1 . fuel melting. In the 1990s, the absence of clad failure due to PCI-SCC was added. Criteria were thus defined: • in order to avoid DNB, the Critical Heat Flux Ratio (CHFR) must remain above a critical value d epending on the fuel assembly 8 ; • in order to avoid fuel melting, the maximum Linear Power Density (LPD) must remain below 590 W/cm; • in order to avoid clad failure due to PCI-SCC, some thermo- mechanical limits must be verified. In addition, fuel rod design criteria were used to check that fuel rods behave correctly during transients as regards to: • cladding corrosion. In PCC-1, oxide thickness shall not exceed 100 µm. In PCC-2, clad temperature at the interface between the metal and the oxide shall not exceed 425 °C; • PCMI. In PCC-1 and PCC-2, the circumferential clad strain shall not exceed 1 %. 2.2 In PCC-3 and PCC-4 In France, at the start of the industrial exploitation of NPPs, specific requirements and empirical criteria were defined to demonstrate core coolability, especially for Rod Ejection Accident (REA) 9 : • to ensure that there is no hot or molten fuel dispersal in the primary coolant during REA, the maximum fuel enthalpy is limited to 200 cal/g, the limit coming from Westinghouse’s extrapolation of fuel behavior established on the basis of RIA full-scale SPERT-CDC tests carried out at zero-power on fresh and very low irradiated UO 2 fuel. This criterion is applicable for mean fuel assembly burn-up up to 33 GWd/tU; • regarding PCMI, the progressive increase of fuel assembly discharge burn-up led ASN to ask EDF to demonstrate that the previous criteria were still applicable for REA. Thus, some full-scale tests carried out in the French CABRI test reactor and in the Japanese NSRR test reactor using high burn-up fuel rods led to fuel dispersal in the primary coolant for fuel enthalpy far below 200 cal/g. These tests clearly showed that this criterion was no longer relevant. Based on the results of full-scale tests, EDF established an empirical safety domain defined by four parameters 10 which intends precluding PCMI clad failure and burst during boiling crisis for high mean fuel assembly burn-up (> 47 GWd/tU); • the maximum peak clad temperature must remain below 1,482 °C (2,700 °F). This limit was taken from fuel failure boundary for LOCA conditions. The rational for retaining a higher temperature limit for non-LOCA transients, such as REA, is that film boiling occurs briefly during those transients, so that fuel rods could withstand this brief dry-out without suffering serious damage. In addition, the number of fuel rod failures must be calculated so that the radiological doses to the public can be estimated. A requirement is defined to limit the number of fuel rods affected by DNB. The conservative assumption is that all fuel rods entering into boiling crisis are assumed to fail. Thus, the percentage of fuel rods likely to suffer DNB is limited to 5 % in PCC-3 and to 10 % in PCC-4. Besides, all fuel rods that experience fuel melting, especially for REA, are assumed to be failed for radiological doses calculations. Nevertheless, only a limited amount of fuel melting is accepted, less than 10 % of pellet volume. 3 Evolution of fuel safety criteria 3.1 Clad embrittlement due to corrosion During operating conditions, it is no longer necessary, for cladding alloys loaded in EDF’s reactors (M5, and Optimized ZIRLO), to verify the oxide thickness criterion limited to 100 µm because of their improved corrosion resistance. However, as in-reactor hydrogen content has a major impact on clad behavior under PCMI during incidental and accidental conditions, the validity of the various criteria ensuring clad non-failure under PCMI conditions relies on compliance with limits of hydrogen content (see § 4.2.1). During incidental conditions, the absence of corrosion acceleration is not likely to occur for cladding alloys loaded in EDF’s reactors because of their corrosion resistance and the temperatures likely to be reached during PCC-2. Verification that the clad temperature at the interface between the metal and the oxide remains below 425°C is therefore no longer necessary. In accidental conditions, the current clad temperature criterion limited to 1482°C does not take into account the time spent at high temperature during boiling crisis, even though cladding oxidation rate is dependent on this. By analysing experimental results available in the literature, EDF plans to complete this criterion by defining a new oxidation rate (ECR 11 ) limit, which is expressed as a function of maximum clad temperature and based on DNB tests carried out in PBF reactor [7]. IRSN considered that, although this approach is acceptable, EDF hasn’t taken into account all physical phenomena that are likely to induce clad embrittlement nor measurement uncertainties to define the ECR limit. EDF will complete its approach and review this new criterion. 3.2 Clad failure due to PCMI and PCI-SCC 3.2.1 PCMI clad failure For PCC-2 power ramps likely to induce PCMI clad failure, the clad strain limit of 2 % is raised instead of 1 % until the in-reactor hydrogen content is below 250 ppm, based on representative analytical tests 12 . In addition, the uncontrolled with drawal of control rod assembly bank(s) at zero power is a particular PCC-2 transient leading to a rapid power excursion, which may also induce PCMI clad failure. Up to now, no criterion was established for this transient. That is why, a specific limit of 1 % of plastic clad strain has been defined to ensure clad non-failure until the in-reactor hydrogen content is below 805 ppm. This criterion is based on appropriate analytical tests 13 . IRSN concludes that these evolutions, based on a cautious interpretation of tests results, are acceptable. No requirement and fuel safety criterion ensuring core coolability were defined for mean fuel assembly burn-up between 33 and 47 GWd/tU in REA transients. Moreover, SPERT, CABRI and NSRR tests were carried out at zero-power while French safety demonstration requires REA studies for all initial power levels. That is why, EDF has revised existing criteria and completed the safety demonstration for fuel assembly burn-up higher than 33 GWd/tU. The new acceptance criteria, expressed by enthalpy rise and pulse width, aim at precluding PCMI clad failure. Their limits depend on cladding corrosion performances, more specifically on in-reactor hydrogen content which is of interest to cope with PCMI behavior. More precisely, EDF’s approach to define the Fuel Review of Fuel Safety Criteria in France ı Sandrine Boutin, Stephanie Graff, Aude Foucher-Taisne and Olivier Dubois
atw Vol. 63 (2018) | Issue 1 ı January new REA criteria depends on the fuel rods types: • for UO 2 fuel rods with ZIRLO, Optimized ZIRLO and M5 claddings, the approach has been based on the interpretation with SCANAIR code [17] of some full-scale RIA tests carried out in CABRI and NSRR reactors and associated with PCMI issue. But, the threshold values of enthalpy rise and pulse width are different for M5 than for ZIRLO and Optimized ZIRLO due to specific cladding corrosion performances. Regarding M5, IRSN considers acceptable the 150 cal/g of enthalpy rise criterion (the pulse width limit definition being in progress and the hydrogen content limit is 160 ppm). However, concerning ZIRLO and Optimized ZIRLO, IRSN identifies that no uncertainty about experimental data has been taken into account by EDF to calculate the enthalpy rise limit from the restrictive test, CABRI CIP0-1 14 , which will lead EDF to review the definition of the associated criterion; • for MOX fuel rods with M5 cladding, EDF has used SCANAIR code to reproduce PCMI behavior for MOX fuel based on a specific RIA test carried out on UO 2 fuel and related to ballooning. IRSN considers that the approach is complicated and unsupported. Eventually, EDF plans to define fuel safety criteria for MOX fuel rods with M5 on the basis of the analysis of specific integral RIA tests devoted to MOX, as it has been done for UO 2 fuel rods. For REA initiated at non-zero power levels, EDF has developed an approach which aims at demonstrating that the REA initiated at zero-power is the most limiting compared to transients initiated at higher power levels. IRSN estimates that EDF’s approach, based on the comparison of thermo- mechanical parameters calculated with SCANAIR code for the PCMI behavior, is acceptable. EDF will apply this approach for each NPPs series. As in-reactor hydrogen content plays an important role in the definition of criteria related to PCMI, IRSN will assess EDF’s correlations giving hydrogen content as a function of oxide thickness. 3.2.2 PCI-SCC clad failure The risk of PCI-SCC clad failure is currently taken into account in PCC-2 studies for which fuel rods integrity must be demonstrated. However, some PCC-3 or PCC-4 transients lead to PCI-SCC. If the corresponding clad failure mode is not likely to lead to a loss of core coolability, the risk still needs to be assessed for PCC-3 and PCC-4 transients in order to ensure that the radiological consequences of the concerned accidents are conservatively assessed. Thus, EDF has developed an approach to verify the absence of any risk of clad failure in case of Uncontrolled Control Rod Withdrawal accident at non-zero power level (PCC-3). IRSN considers this approach to be acceptable. Another transient, the Steam Line Break accident initiated at non-zero power level (PCC-4) is also likely to lead to PCI-SCC clad failure. EDF has provided justification concerning some reactors concluding that the PCI-SCC clad failure risk is no greater than for PCC-2 transients. For IRSN, the justification still needs to be confirmed and extended to all reactors. 3.3 Consequences of DNB In order to demonstrate the absence of fuel dispersal in the primary coolant after clads ballooning and burst during boiling crisis, EDF has proposed two approaches depending on transients: • for REA, the approach is based on the comparison between the restrictive PCMI criterion and results of various full-scale tests associated with ballooning and burst (IGR, BIGR, NSRR, PBF – [18, 19, 20]). In the available experimental database, no fuel dispersal is observed up to EDF fuel rods burn-up discharge limit (57 GWd/ tU) and up to the enthalpy rise limit of 150 cal/g (see § 4.2.1); • for Uncontrolled Control Rod With drawal at non-zero power level (PCC-3) and Locked Rotor (PCC-4) accidents, EDF has compared the maximum fuel rod burn-up calculated beyond which boiling crisis is avoided and the current non-dispersal threshold 15 . However, as the absence of fuel dispersal has been demonstrated with a very small margin, IRSN considers that EDF will have to update its safety demonstration for each ten-yearly outage review or in case of modifications deemed to impact this conclusion. Besides, questionning the conservative assumption is that all fuel rods entering into boiling crisis are assumed to fail, EDF foresees to limit (up to 5 % for PCC-3 or 10 % for PCC-4) the number of broken rods due to ballooning during boiling crisis. From EDF’s point of view, the current criterion related to radiological doses calculations is based on a very conservative assumption considering that all fuel rod entering into boiling crisis is supposed to be failed [21]. By applying a fuel rod burn-up threshold calculated with SCANAIR code [17] depending on fuel rod design and irradiation, some fraction of fuel rods can be excluded from the counting of failed rods. IRSN considers acceptable this method. However, in case of plant operating conditions modifications (for the future), EDF’s evolution could lead to increase radiological consequences, which is not acceptable for IRSN. Finally, regarding on-going RIA investigations and research programs, IRSN considers namely that Cabri International Project (CIP 16 ) tests planned in the CABRI-water loop facility may be used to analyse clad behavior during boiling crisis notably for high fuel burn-up and will improve knowledge on the MOX fuel behavior. 3.4 Consequences of fuel melting In the current safety demonstration, no requirement associated with fuel safety criterion was defined concerning fuel melting risk during PCC-3. In order to adress this gap, EDF plans to verify the limit of 10 % molten fuel at the pellet centre for the Uncontrolled Control Rod Withdrawal accident initiated at non-zero power level. For IRSN, this evolution is acceptable, but the radiological doses calculations related to this transient will have to be assessed consistently with the new criterion. Moreover, like the NRC’s requirement, a limited amount of fuel melting is acceptable provided it is restricted to the fuel centerline region and is less than 10% of pellet volume [22]. Indeed, during REA (PCC-4), due to the effects of edge peaked power and lower solidus temperature, fuel rods may undergo fuel melting in the pellet periphery. Thus, fuel melting outside the centerline region is precluded to avoid molten fuel coolant interaction. Therefore, EDF will demonstrate that this requirement is satisfied based on appropriate analysis rules. Besides, with regard to the 200 cal/g of maximum fuel enthalpy criterion for REA (applied to fuel assemblies with burn-ups up to 33 GWd/tU), EDF confirmed its validity for MOX fuel on the basis of the CABRI REP-Na9 test 17 . However, IRSN 14) For CIP0-1, the measured hydrogen content is 1000 ppm. 15) Established at 55,2 GWd/tU in mean fuel rod burn-up, based on Halden and Studsvik LOCA tests. 16) CABRI CIP: Tests with water coolant loop plan to start in 2018. 17) CABRI REP-Na9 was carried out on MOX fuel with a low clad corrosion and a fuel rod burn-up of 28 GWd/tU. The tested fuel rod was not failed for a maximum fuel enthalpy of 200 cal/g. FUEL 41 Fuel Review of Fuel Safety Criteria in France ı Sandrine Boutin, Stephanie Graff, Aude Foucher-Taisne and Olivier Dubois
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