1. Sandrine BOUTIN – Stéphanie GRAFF – Aude TAISNE – Olivier DUBOIS
REVIEW OF FUEL SAFETY CRITERIA IN FRANCE
Good morning ladies and gentlemen. My name is Sandrine BOUTIN from the Institute for Radiological Protection and Nuclear Safety. I work on fuel topics in the nuclear safety department. Today, I’ll will try to present you the process of French rulemaking regarding the fuel safety criteria which took place in recent years

2. About French rulemaking Review of all acceptance criteria in France
AGENDA
About French rulemaking
Review of all acceptance criteria in France
Summary
So, I have divided my presentation into three main parts. First, I’d like to begin by a short presentation of French rulemaking. Then, I’ll give you an overview of the review of the safety criteria with a focus on some specifics topics and I’ll conclude with a summary

3. ABOUT French rulemaking
US requirements were adopted in France at the start of the French PWR nuclear program
Numerous international research programs have addressed the fuel behavior especially during Loss Of Coolant Accident (LOCA) and Reactivity Initiated Accident (RIA) conditions
 improving the calculation methods and knowledge
The discharge burn-up of the fuel rods has increased notably compared to the situation forty years ago (FA BUlimit = 52 GWd/tU except for EPR : 58 GWd/tU)
 increasing oxide thickness and higher hydrogen pick-up in the cladding material which influence fuel rod behavior under incidental and accidental conditions
Let me first give you the main reasons which are at the origin of the review:
First, US requirements were adopted in France at the start of the French PWR nuclear program
Numerous international research programs have addressed the fuel behavior especially during Loss Of Coolant Accident (LOCA) and Reactivity Initiated Accident (RIA) conditions and lead improving the calculation methods and knowledge
In addition to this, the discharge ….and lead increasing oxide thickness …

4. ABOUT French rulemaking
New cladding materials characterized by enhanced performances (i.e. cladding corrosion in normal operating conditions) have been introduced in French reactors  M5 (AREVA), ZIRLO and Optimized ZIRLO (Westinghouse)
Fresh Zy-4 is no longer loaded in EDF’s reactors
Operating conditions of French plants have changed, notably by stretch-out operating conditions
Clad material
First
Reloads
Generalization M5
2000
2014
ZIRLO 
2006
Optimized
2009
In 2017, EDF has asked for authorization
Some additionnal points :
New cladding materials are …/ it’s the M5 for AREVA and
Let me highlight that fresh Zy-4 is no longer loaded in EDF’s reactors (since the beginning of this year)….
You can see on this table the main steps of the new alloys deployment in EDF NNPs
As a final point, the operating conditions of French ….

5. ABOUT French rulemaking
Because of these evolutions, the French Nuclear Safety Authority (ASN) decided to review fuel safety criteria especially those addressing LOCA and RIA
Technical notices
Technical relationship
Requests
Authorizations
The French regulatory framework is specific: the French utility EDF proposed fuel safety criteria submitted to ASN which were assessed by IRSN
The review of fuel safety criteria took place from 2011 to except for LOCA (review from 2008 to see Eurosafe 2016)
June 2017: meeting of the Advisory Committee for Reactors Safety of ASN about the French rulemaking on fuel safety criteria related to PCC-1, PCC-2, PCC-3 and PCC-4
So, because of these evolutions, the French Nuclear Safety Authority (ASN) decided to review fuel safety criteria especially those addressing LOCA and RIA.
Let me point out that the French regulatory framework is specific: the French operator EDF proposed fuel safety criteria which were submitted to ASN and then assessed by IRSN. This particular organization is depicted on this picture.
In France, the review of fuel safety criteria took place from to 2017 except for those related to LOCA which have bee reviewed from 2008 to 2014 : please refer to Eurosafe two thousand sixteen.
Finally, based on IRSN’s technical assessment, the meeting of the Advisory Committee for Reactors Safety about the French rulemaking was held in June two thousand seventeen

6. Review of all fuel acceptance criteria in france
Impact of Fuel
Assembly distortion on safety analysis
(Neutronic, TH, Mechanic)
Cladding
embrittlement
due to corrosion
Review the adequacy of all the existing fuel acceptance criteria and the pertinence of new criteria (EDF’s proposals)
SCC-PCI clad failure
Impact of in-reactor
leakers on safety
analysis
PCMI clad failure
Fuel melting
Consequences of boiling crisis: Fuel dispersal
Consistency between radiological consequences and safety cases analysis
In this context, the fuel safety criteria were reviewed in order to assess, on the one hand the sufficiency and validity of current requirements and fuel safety criteria relating to all fuel degradation modes in the light of state-of-the-art and operating conditions.
Thus, the review concerned the following limitative physical phenomena:
Cladding embrittlement due to corrosion
Clad failure due to Pellet cladding interaction assisted by Stress corrosion cracking
Clad failure due to pellet cladding mechanical interaction
Consequences of boiling crisis in particular fuel dispersal
And finally, fuel melting
The review concerned also the impact of fuel assembly distorsion and in-reactor leakers on safety analysis since these phenomena aren’t currently taken into account
And last point, the consistency of the fuel rod behavior under the reference PCCs with the assumptions used in radiological consequences studies was also assessed. 6

7. Review of all fuel acceptance criteria in france
Impact of Fuel
Assembly distortion on safety analysis
(Neutronic, TH, Mechanic)
Cladding
embrittlement
due to corrosion
Review the adequacy of all the existing fuel acceptance criteria and the pertinence of new criteria (EDF’s proposals)
SCC-PCI clad failure
Impact of in-reactor
leakers on safety
analysis
See N. Waeckel presentation at FSRM 2017
See N. Waeckel presentation at FSRM 2017
PCMI clad failure
Consequences of boiling crisis: Fuel dispersal
Consistency between radiological consequences and safety cases analysis
As it’s not possible today to present all the cases to you, I’ll focus my presentation on the highlighted subjetcs and specific points. For these two remaining points, please refer to the presentation made at Fuel Safety Research Meeting in Japan last month by Nicolas Weackel from EDF
Fuel melting 7

8. In-reactor corrosion limit
Advanced alloys (M5, Opt ZIRLO) exhibit EOL oxide thickness < 45 µm
REX Zy-4 Europe
REX Zy-4 USA
REX M5 Europe
REX M5 USA
Burn-up (MWd/tU)
Oxide thikness (µm)
M5 : < 25 µm
REX Optimized ZIRLO International
REX Optimized ZIRLO EDF
Opt ZIRLO : < 45 µm
Question: Is the standard 100 µm corrosion limit still relevant ?
Mechanical tests show cladding ductility is not affected if zirconia remains low
IRSN considers it is no longer necessary to verify the oxide thickness criterion for advanced alloys in France  associated hydrogen content is the key parameter regarding PCMI clad behavior (in the future, IRSN will assess EDF’s correlations [H] = f(oxyde thickness) )
Let’s look at the first slide on criteria related to corrosion phenomena and more precisely on this slide to the in-reactor corrosion limit:
- The advanced alloys …
As you can see from the graphs, oxide thickness of M5 remains below than 25 µm and for Optimized Zirlo the limit is arround 45 µm
The question is wether the standard 100 µm corrosion limit still relevant ?
Mechanical tests which were realized by EDF show cladding ductilty is not affected if Zirconia remains low
IRSN …in France but it’s important to note that it’s the associated hydrogen content which is the key parameter regarding PCMI clad behavior and in the future, IRSN will have to assess EDF’s correlation hydrogen content function of oxyde thickness

9. TransienT clad temperature criterion (PCC-3 and PCC-4 – except for LOCA)
Corrosion
Historically: 1482°C (2700°F) comes from LOCA experiments and is appropriate for short transients (less than  30s) - This clad temperature limit ensures core coolability (clad non-failure during rewetting)
Question: Is this criterion appropriate to long time range transients such as uncontrolled control rod assembly withdrawal initiated at power (PCC-3) ?
EDF proposed a new criterion as a transient “Equivalent Cladding Reacted” (ECR) limit depending on maximum clad temperature and based on PBF PCM tests database
1482°C
Non failure area
20% ECR (19 s)
Failed upon quench
Unfailed upon quench
PBF tests (Hesson and Al 1982)
IRSN recommends taking into account experiments which integrate clad ballooning and burst after boiling crisis and secondary hydriding phenomenon
Moving on the next slide which is about the transient clad temperature criterion to verify for PCC-3 and PCC-4
- Historically, the one thousand four hundred heighty two degree celsius comes from … rewetting phase
- The question is wether this criterion appropriate to long time range transients such as Uncontrolled ….?
- EDF proposed a new criterion as a transient « Equivalent …(ECR) limit depending … According to this graph, the new criterion is represented by the red line
IRSN considers that the EDF approach is acceptable but IRSN recommends in particular taking into account experiments which integrate clad ballooning and burst after boiling crisis and secondary …because these phenomena are likely to induce clad embrittlement. EDF will complete its approach and review this new criterion

10. PCMI clad failure: PCC-2 power pulse
Up to now, no criterion for the PCC-2 power pulse: uncontrolled withdrawal of control rod assembly bank(s) at zero power level
Question: Is the historical 1% clad strain acceptance criterion for PCC-2 ramps also applicable to this transient ?
[H] content (ppm)
Transposed total elongation at rupture (%)
Bounding test / H = 805 ppm
Moving on the next slide and criteria related to Pellet cladding mechanical interaction
Up to now, no criterion was established for a PCC-2 power pulse : that is to say uncontrolled withdrawal of control rod assembly bank(s) at zero power level which may induce PCMI clad failure
The question is wether the historical …. ?
Based on appropriate analytical tests carried out in well-adapted conditions, EDF proposed a specific limit of 1% plastic circumferential strain to ensure clad non-failure until the in-reactor hydrogen content below 800 ppm
IRSN concludes that this clad non-failure limit, based on a cautious interpretation of tests results, is relevant
EDF’s proposal: the 1% plastic circumferential strain criterion is applicable to PCC-2 power pulse for M5, ZIRLO and Opt ZIRLO
IRSN considers this clad non-failure criterion relevant

11. PCMI clad failure during REA (at zero power level)
Before the rulemaking
No clad failure due to PCMI
The new French criteria
Requirement
No fuel rod failure due to PCMI and ballooning during boiling crisis
Fuel safety criteria
BUfuel assembly > 47 GWd/tU
Oxide thickness < 108 µm
DH < 57 cal/g
L1/2> 30 ms
Tclad < 700°C
BUfuel assembly > 33 GWd/tU
New criteria expressed by DH and L1/2 whose limits depend on cladding corrosion performances : in reactor hydrogen content
Let’s look at the next slide about related to PCMI clad failure during REA at zero pwer level
In order to prevent from fuel dispersal in the primary coolant, the current requirement is based on the absence of fuel rod failure due to PCMI and ballooning during boiling crisis. In this way, based on the results of full-scale tests carried out in the CABRI facility, EDF established an empirical “safety domain” defined by these four parameters which intends to preclude fuel rod failure for high fuel assembly burn-up, higher than 47 GWd/tU. IRSN considered these criteria as acceptable in 2011.
During French rulemaking, EDF revised these criteria and completed the safety demonstration for transient initiated at zero power level and for fuel assembly burn-up higher than 33 GWd/tU. The new criteria, which are expressed by enthalpy rise and pulse width, aim at precluding PCMI failure.
It is important to note that these limits depend on cladding corrosion performances, more specifically on hydrogen content which is of interest to cope with PCMI loading.

12. PCMI clad failure during REA (at zero power level): BUfA > 33 GWd/tU
IRSN analysis focused on the validity of EDF’s approaches which depend on fuel rod design:
[H] (ppm)
DH (cal/g)
NSRR Unfailure
CABRI Unfailure
NSRR Failure
UO2+ ZIRLO/Opt ZIRLO: No uncertainty concerning experimental data taken into account to define the fuel enthalpy rise limit (EDF’s proposal DH<80 cal/g ) based on the restrictive test (CIP0-1)
 EDF should take into account uncertainties concerning experimental data to define the criterion
The aim of this slide is to present to you the main conclusions of the IRSN analysis about the EDF’s proposals regarding PCMI non failure during REA.
The IRSN analysis focused on the validity of EDF’s approaches which depend on fuel rods types:
- For fuel rods with ZIRLO and Optimized ZIRLO claddings, the EDF’s approach is based on the interpretation of RIA full-scale tests without considering uncertainty about experimental data, especially for the restrictive test used to define the enthalpy rise limit (EDF proposed a limit of 80 cal/g). That is why, IRSN considered that EDF should take into account uncertainties about experimental data to define the criterion. EDF will have to proposed a revised value.
- For fuel rods with M5, alloy wich is characterized by in reactot hydrogen content less than 200ppm, IRSN considered acceptable the criterion based on the fuel enthalpy rise limited to 150 cal/g (one hundred fifteen). However, the definition of the pulse width limit is still in progress.
- For MOX fuel with M5 cladding, EDF will apply the approach based on the interpretation of RIA full-scale tests devoted to MOX fuel in place …..the initial EDF’s proposal is a limit of one hundred thirteen calories per gramm.
Besides, it is important to note that for transient initiated at non-zero power level, EDF developed an approach which aimed at demonstrating that the RIA transient initiated at zero power is the most penalized transient as compared to REAs initiated at higher power levels. For IRSN, this approach is acceptable.
UO2 + M5: DH<150 cal/g acceptable + definition of L1/2 limit in progress
MOX + M5: EDF will apply an approach based on the interpretation of RIA full-scale tests devoted to MOX fuel, in place of transposition calculations with SCANAIR V6.7 code (EDF’s proposal DH<113 cal/g)

13. Consequences of boiling crisis
To calculate radiological doses, the current conservative assumption considers that all fuel rod entering into boiling crisis is failed
EDF suggests to consider only fuel rods susceptible to burst: by applying a fuel rod burn-up threshold (BU burst) calculated with SCANAIR code, some fraction of fuel rods can be excluded from the counting of failed rods
BU burst
Clad burst
No clad failure
Fuel rod BU
BU Boiling crisis
Boiling crisis
I will present you just one point of IRSN assessment about consequences of boiling crisis
Currently, to calculate ….
EDF suggests ……: indeed by applying a fuel rod burnup thresold calculated with SCANAIR code depending on fuel rod design and irradiation, some fraction of fuel rods can be excluded from the counting of failed rods. The population of failed fuel rods is represented on the graph by this red circle.
IRSN considers acceptable this method. However in case of plant operating conditions modifications for the future …finally
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

14. Fuel assembly bowing effects
Fuel Assembly distortion
Phenomenon observed through incomplete/delayed control rods insertion during reactor trip
Can be measured/evaluated with DAMAC measurements during refueling outages for some PWRs (from few mm to 20 mm)
FA distortion potentially leads to the following impacts on safety demonstration:
DNBRmin?
Hot spot
Water gap
Thimble Tube
« Buckling » of grids
My final point is about fuel assemblies bowing effects. Indeed, fuel assemblies may undergo bow in PWRs due to hydraulic loads exerted by the water, mechanical loads applied by the top nozzle, irradiation and temperature.
Phenomenon ….
It can be measured ….(from few mm to 20 mm in case of excessive assembly bow)
This potentially has an impact on the in-core power distribution (at the pin scale) and on the safety analyses supporting the plant operations which rely on the hypothesis of a uniform water gap between fuel assemblies ; IRSN assessment deals with neutronic, thermalhydraulic and mechanical impacts of fuel assembly distortion on safety demonstration but I have just the time to present you the neutronic aspect and please refer to our paper for the two remaining ones
Neutronic Thermal-hydraulic
Mechanical

15. Fuel assembly bowing effects
Fuel Assembly distortion
Neutronic effect: the presence of larger inter-assembly gaps causes power distribution modification which can cause the hot spot value to move to peripheral pins and/or increase
Neutronic
Hot spot
Thimble Tube
EDF developed a new methodology for quantifying and taking into account this effect in the safety demonstration
EDF proposed to consider this impact directly by means of a modification in the power distribution evaluation uncertainty
IRSN assessed neutronic calculations, hypothesis of methodology: IRSN considers this methodology satisfactory

16. SUMMARY
Most (if not all) fuel acceptance criteria or fuel design limits have been reviewed to take into account:
current fuel design
more demanding conditions
current state of the French reactors regarding leakers (not presented here – see paper Eurosafe 2017) and fuel assembly bow
the state-of-the-art concerning physical phenomena: PCMI, cladding embrittlement due to corrosion, clad ballooning and burst during boiling crisis and fuel melting
This review lead to a big commitment in terms of methodologies development, carrying out and interpretation of experiments and technical exchanges between EDF and IRSN
Just to summarize the main points of my talk :
And let me point out that this review lead to a ….

17. SUMMARY
Some of fuel acceptance criteria may be relaxed (not presented here – see paper Eurosafe 2017)
Some of fuel acceptance criteria have evolved (in-reactor hydrogen content instead of oxide thickness) or should be complemented:
clad temperature limit of 1482°C
centerline melting temperature
correlations [H] = function (oxide thickness)
New approaches and fuel acceptance criteria have been defined in order to complete the safety demonstration:
fuel dispersal after clad ballooning-burst during boiling crisis
PCMI during PCC-2 power pulse and REA
SCC-PCI during PCC-3 and PCC-4 (see paper Eurosafe 2017)
After the review, some … they haven’t presented here
Some … (for example, in-reactor hydrogen …) or shouild be complemented : there are listed here
And finally, new approaches and fuel ….have been defined in order to complete the current safety demonstration :
It’s concerned the absence of fuel dispersal …
new criteria aim at precluding PCMI clad failure during PCC2 power pulse and REA
The study of SCC-PCI phenomena during PCC3 and PCC4

18. Thank you for your attention !
Are there any question ?