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PHOSPHORUS INFLUENCE ON STEEL AGEING (PISA)

C. English\ F. Sevini2, J-M. Frund3, R. Langer4, A. Donaldsons, F. Gillemot6, M.BrumovsIcy7, P. Nenonen8, A. Ballesteros9, S. SchmauderlO, D. Baconll•

1) AEA Teebnology pIe, Dideot (GB)2) EC-JRC/IAM, Petten3) EDF, Moret-Sur-Loing (FR)4) Framatome ANP, Erlangen (DE)5) BNFL, Gloueestershire (GB)

SUMMARY

6) KFKI, Budapest (H)7) NRI, Rez (CZ)8) VTI', Espoo (Fin)

. 9) Tecnatom, Madrid (ES)10) :MPAIIMWF, Stuttgart (OE)11) University ofLiverpool (GB)

The integrity of the pressure vessel is vital to the safe operation of a nuclear reactor.It is therefore necessary to monitor or predict the changes in the reactor pressure vessel(RPV) material during operation. Exposure to irradiation (or elevated temperatures ) causesthe segregation of phosphorus to grain boundaries iri RPV steels. This, in turn, encouragesbrittle intergrariular failure (IGF) of the material. The object of this programme is toreduce the uncertainties associated with the impact of IGF on the properties of the RPVboth during service and at the end-of-life (BOL) ..

The approach employed involves improving predictability by further developing thephysical understanding of both the segregation process and any resultant changes inmechanical properties. The necessary physical understanding is being developed throughfocussed experimental investigations of irradiated steels and model alloys, and alsothrough associated modelling studies.

The major achievements to date are the completion of two of the three irradiationexperiments at HFR Petten. In addition a review has been completed of existing data, inorder to optimise the detail of the various experimental studies. Many partners havesupplied previously unpublished information. Significant data on phosphorus and carbonsegregation have been obtained from material irradiated at HFR Petten and from previousirradiation experiments .. The PISA programme is starting to establish a database on thelevels of phosphorus and carbon at grain boundaries as a function of steel type, specificheat treatment and irradiation conditions. Modelling activities at MP A, Stuttgart and at theUniversity of Liverpool have gained significant momentum. An advance that has alreadybeen made is the formulation of a physically-based, analytical expression for P segregationto grain boundaries under irradiation.

In this paper we will briefly ouiline the details of the adopted approach, and theachievements of the project to date~ The starting date of the project was 1st December1999 and the duration was originally three years, but detays encountered in the irradiationprogramme have led the partners to request an extension for nirie months.

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A. INTRODUCTION

Tbe aim of this programme is to produce a cIätabaseof phosphorus segregation data,and consequential embrittleinent, relevant to RPV steels. This will be addressed bycollating existing data, including some data held confidentially by the partners, andobtaining fresh data within a carefully designed experiI+lentalprogram. Standardisation oftechniques employed to measure P segregation to grain boundaries will also be covered, inorder to maximise the quality of data recorded in the programme. In parallel with this,informed by the data colleeted on the program, theoretical modelling of P segregationunder irradiation or thermal ageing will be canied out Tbe data and improved models ofPsegregation generated from the programme will help to increase our understanding of themicromechamsms leading 10 P segregation under irradiation, and support quantitativedescription and prediction of this segregation. We hope to be able to answer the questions:

• Can we predict P segregation either qualitatively cr quantitatively, under conditions ofdifferent fluxes, fluences, irradiation temperatures, bulk P levels, material types(different steel compositions or forms)

• What 'will the consequences ofP segregation on embrittlement be? '.• Can we establish whether there is a danger of excessive embrittlement by this P­

related mechanism during the operating life of the RPV of a given reactor?

In addition to predicting P segregation, the PISA programme intends to address themechanical consequences of segregation on mechanical properties.To assess this, thereneeds to be a reliable indicator, from bulk mechanical property measurements, of whethernon-hardening embrittlement is occurring. Tbe ratio of embrittlement (as measured byshift in ductile-to-brittle transition temperature) to hardening (measured increase in yieldstrength) has been used as an indicator of the presence of non-hardening embrittlement(see Ref. [2]). It has proved difficult to define a critical value of this ratio, above whichsignificant inter-granular fracture and'non-hardening embrittlement oeeurs, that applies toall materials, and is supported by relevant microstructural data. Tbe PISA program, by

,combining hardening,data with segregation, embrittlement and fractography data, and bycareful analysis of the doses imposed on the sampies, should enable this critieal ratio to bedefioed more precisely in a number of different RPV steels. In addition, unpublishedsurveillance data to be provided bythe partners should ereate a far larger database ofcorrelated measurements than was previously available; this will help establish where thereis evidenee for non-hardening embrittlement.

Tbe range of the RPV steels to be eonsidered includes the MnMoNi steels employedin European PWRs, the mild steels used in UK Magnox (steel) RPVs, and the steelsemployed in VVER 440s. Intergranular fracture andlor P segregation is considered to beimportant in plant applications involving an three reactor types. Some model alloys willalso be examined, to help clarity proeesses occurring in the more eomplex steels. Tbe steelformsunder consideration are plates, forgings, weId metals and weId heat-affected zones.

B. WORK PROGRAMME

Tbe approach employed to improve predictability was diseussed in ref [I] and isillustrated in Figure I. Tbe work programme to achieve"this' eorisists of experiments,model development and standardisation aetivities. Tbe major elements of these workprogrammes'may be seen in Figure 2. These are briefly discussed below

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n.l Analysis of existing dataDuring the project planning phase ofPISA it was noted that there is a lack of data on

non-hardening embritt1ement. It was considered important that the project partners shouldreview existing published data and that the partners should also contribute unpublisheddata to this review. This review wou1d ensure that the detai1ed PISA programme wascorrect1y focused.

B.2 Examination of representative irradiated materialThe aim of the post-irradiation examinations is to determine both the microstructura1

and the mechanical property changes in materials after irradiation. For the post-irradiationexaminations, the materials will be either irradiated as part of this project or previouslyirradiated as part of past national programs. The partners have supplied the latter sampies.These studies will result in a database of the effect of irradiation and thermal exposure onthe segregation of elements such as P and C to grain boundaries and the concomitantmechanical (impact plus tensile) property changes. This database will be particularlyimportant in obtaining an improved description of the influence of important irradiationand metallurgical variables.

B.3 Examination of thermally-aged materialThe aim of the examination of thermally aged material is primarily to determine the

effect of thermal ageing on P segregation (and the sllbsequent embrittlement). Ofparticular importance is the availability of material that has undergone long term ageing atreactor operating· temperatures, and materials that have received differing heat treatmentsduring fabrication.

B.4 Technique standardisationThis element of the programme focuses on the standardisation of the experimental

measurement of the level of segregants at internal boundaries.

B.5 Theoretical modelling of P and C segregation underirradiation/thermal ageing

Important aspects of the project will be the development of physical models thatdescribe P and C segregation to grain boundaries in irradiated and thennally aged steels,and enable comparison between model predictions and the experimental da18base.

Ca MAIN ACBIEVEMENTS

The majorachievements are describedbelow:

C.! Analysis of existing dataA review of existing da18has been completed, in order to optimise the detail of the

various experimental studies. Many partners have supplied previously unlmblishedinformation. Important technical insight was established on the potential mechanismsunderlying the behaviour in different steels, and it appeared that the threshold level of Psegregated to a boundary, above which non-hardening embrittIement occurs, is simi1arinVVER and MnMoNi steels. No major omissions were found in the planned. PISAprogramme.

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C.2 Examination of representative irradiated ma~erialThe data arising from the examination and testing of material irradiated in tbree

irradiations planned at HFR Petten are critical to the PISA programme. The irradiationconditions are 5xl0lB ncm-2 (E> IMeV) at 200°C (pISA I)t 5x101B ncm-z at 290°C (pISAC)t and 18x101B ncm~2 (pISA B) at 290°C. This irradiation matrix permits the effect ofirradiation temperature and fluence (at 290°C) to be studied. The materials to be irradiatedare aC-Mn platet the IAEA reference PWR plate JRQt a VVER 1000 base metalt and anumber of model alloys. The VVER 1000 base meta! and JRQ were also heat treated toproduce a simulated coarse-grained heat affected· zone microstructure (SCGHAZ), whichwas also subjected to irradiation, as segregation effects are expected to be mostpronounced in this microstructure. Further details may be found in English et al (see Ref.[1]).

The irradiations PISA I and PISA B have been completed and PISA C commencedirradiation in August 2003. This schedule represents a delay on the original timescales dueto unforeseen circumstances, primarily outside the control of the project. 1ms hasnecessitated requesting an extension to the project ofnine months.

Testing of mechanical property specimens has now commenced at AEKI and NRI.Initial results have been reported on JRQ irradiated in PISA 1, and workers at NRI Rezhave commenced testing of VVER 1000 Base Metal - 15Kh2NMFA. Although thisprogramme is in its early stagest interesting observations have been established on theeffect of SCGHAZ on ChaIpy properties.

Data on the segregation (or desegregation of P and C) froin grain boundaries duringirradiation is a key element ofthe PISA programme. Microstructural examination ofbothpreviously irradiated material and material irradiated in PISA I has commenced. It is clearfrom the results that, for th~ fust time, we are establishing a database on P and C on grainboundaries as function of steel type, specific heat treatment and irradiation. Data fromAuger examination of sampies from PISA I are now available from studies at BNFL­Magnox. Most interestingly in every case there is an increase of P and a decrease of C onthe grain boundaries. The magnitude ofthe increase (or decrease) differs in each case.

Previously irradiated sampIes are important as the PISA irradiation programmecannot realise all the conditions of interest to RPV operation or modelling. Data on thelevels of P and C in JRQ plate at SOL and after extensive irradiation outside of PISA to102oncm-zE > IMeV at 270°C have also been obtained. It is apparent there is very little ifany segregation ofP or C during the irradiation. This is a significant 'null' result as loZ°ncm-2 represents an irradiation dose that is beyond the currently planned end-of-life dosefor the majority of civil PWRs.

C.3 Examination of thermally-aged· materialAuger examination has been completed of the VVER 1000 base metal in the initial

condition, and after step cooling. The effect ofthe step cooling is to modify the levels ofPand C on the boundaries. SampIes that have undergone long term thermal ageing havebeen selected and are under preparation for examination.

C.4 Technique standardisationPreliminary data (from model alloys and PISA materials) have been obtained on two

critical aspects ofthe Auger technique.FEGSTEM data on microstructure and segregation

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have been obtained from the VVER-I000 material and a detailed comparison with theequivalent Auger data is underway.

C.5 Theoretical modelling of P and C segregation underirradiation/thermal ageing.

The modelling ofP segregation under both irradiation and thermal ageing are centralto the whole programme. In the first year workers at the University of Liverpoolformulated a physically-based, analytical expression for P segregation to grain boundariesunder irradiation. This was a particularly significant advance. In the current year work hasconcentrated on molecular statics and dynamics simulation ofFe-P mixed dumbbells. Thevalue of these simulations to the PISA project is that the insight and data on importantbinding energies will enable a more 'realistic' fitting of experimental data.

During the current year a Monte Carlo model has been developed at MPA Stuttgart,and the initial detailed parameterization and validation has been undertaken.

D. DISSEMINATION AND EXPLOITATION OF RESUL TS

The main dissemination phase of the PISA project will occur after the results fromthe post examination of the irradiated sampIes has been completed. However, some of theprogress on the project has been disseminated, while workers at the University ofLiverpool have published a letter in Phil Mag. Letters on the physically-based, analyticalexpression for P segregation to grain boundaries under' irradiation. Presentations on theoutcome of the literature review (WP2) were given at an EPSRC meeting on Radiationand Thermal Ageing Effects on Materials Grain Boundaries And Interfaces held in the UKon March 18-19 2002. Finally, an open version of the report onP segregation is to beissued. In its original form this report contains important unpublished data supplied by thepartners. The partners concemed have now given permission for these data to beincorporated into an open publication.

It is anticipated that results of the project will improve the management of plantextension through the prediction of end of life properties of RPVs. Given the importanceofthe RPV this would make a significant contribution to safety of reactors throughoutEurope and the ·stability cif electricit)r supply. In particular, Eastem European VVERreactor designs are inc1udedin the project. The intergranular faUuremechanism is ofverysignificant interest to this reactor system design, and the results of this project will improvenuclear safety in Eastem European plants, which is of essential interest to the EuropeanCommunity. 1t is hoped fuat certain key data emerging from the PISA programme can bereleased for a wider audience. Ifthis occurs it will enable partners in the PISA programmeto discuss relevant results with interested parties in their countries.

In addition, an important element in the programme is the harmonisation of themeasurement of segregation to grain boundaries. The pI:ogramnie will enable individuallaboratories to .assess and improve their ability to characterise relevant microstructura1changes in RPV steels employed in both Westem and Eastem reactor designs.

Finally, there will be information exchange with other relevant 5th Frameworkprojects such as FRAME, and" if,possible, relevant TACIS/PHARE projects. The mostappropriate means of enabling this is through the AMES network.

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E. CONCLUSIONS.

In spite of delays in the irradiation programme the project has made significantadvances in improving the understanding of P segregation in irradiated and thermally agedsteels. The true benefit from the PISA project can only be ässessed after the completion ofthe examination ofthe sampies irradiated at HFR Petten.

REFERENCES

[I] C. English, F. Sevini, JM. Frund, R. Langer, J. Cowan, F. Gillemo't, M. Brumovsky, P.Neonen, A. Ballesteros, S. Schmauder, D. Bacon, "Phosphorus Influence in Steel Ageing(PISA)", Proceedings of PISA 200lEU research in reactor safety, ed G. Van Goethem, A.Zurita, J. Martin Bermejo, P. Manolatos and H. Bischoff, BUR 20281 EN, p. 54-60.

[2] R.B. Jones and CJ. Bolton, "Neutron Radiation Embrittlement Studies In Support OfContinued Operation And Validation By Sampling OfMagnox Reactor Steel Pressure VesselsAnd Components", in Proc. 24th Water Reactor Safety Meeting, USNRC February 1997.NUREG/CP-0157 Vol2 pp25-4.

DEVElDPMENT OFP:B:YSICAL

UND ERSTANDING

INVESTIGATIONS TODETERMINE ROLE

OFP&CINSEGREGATION ANDEMBRITTLEMENT IN

REACTORCOMPONENTS

UNKEDMODEUlNGANDMODEL

AllDYSTUDIFS

STANDARDJSATION OFMICROSTRUCTURAL

TECHNIOUES

Figure 1. Schematic ofthe Aims ofthe PISA Programme

Figure 2.' Elements ofthe PISA Programme.

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2004

EUROPEAN COMMISSIONEURATOM FRAMEWORK PROGRAMME 1998-2002

KEY ACTION 'NUCLEAR FISSION'

FISA 2003EU research in reactor safety

Conclusion symposium onshared-cost and concerted actions

Edited by

G. Van Goethem, A. Zurita, S. Casalta, R Manolatos '

Directorate-General for Research· EUR21026

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