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SYNTHESIS OF THE VERCORS PROGRAMME Main contributions, Pending issues, Prospects B. Clément

SYNTHESIS OF THE VERCORS PROGRAMME Main contributions, Pending issues, Prospects B. Clément Institut de Radioprotection et de Sûreté Nucléaire. Preamble.

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SYNTHESIS OF THE VERCORS PROGRAMME Main contributions, Pending issues, Prospects B. Clément

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  1. SYNTHESIS OF THE VERCORS PROGRAMME Main contributions, Pending issues, Prospects B. Clément Institut de Radioprotection et de Sûreté Nucléaire International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  2. Preamble This presentation is intended to provide highlights on the most important points presented and/or discussed during the seminar. It reflects the thoughts and reactions of the author to what was said during the seminar and is, in that sense, somewhat subjective The presentation is also an introduction to the final panel discussion, during which the panel members and the floor will be asked to intervene International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  3. Experimental database on FP release • A number of experiments have been performed in the past on fission product release from fuel, generally at the scale of one or few pellets, cladded or not, and also on fuel fragments • The integral in-pile experiments (PBF, LOFT, BTF, Phébus…) also provide information, the most precise ones coming from the Phébus-FP programme • The most recent small-scale experiments were performed in the frame of VERCORS (CEA-EDF-IRSN) and VEGA (JAEA) programmes, now completed • The main observations from VERCORS can be summarised as follows: • for volatile fission products (noble gases, Cs, I,...): nearly complete release when reaching fuel collapse temperature – kinetics influenced by oxygen potential International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  4. Experimental database on FP release • The main observations from VERCORS can be summarised as follows: • for medium volatile (Mo, Ba...): overall release can be as high as for volatile but is very sensitive to oxygen potential • for low volatile (Sr, Nb, Ru...): overall release from few to 10% but can be significantly increased for high burn-up fuel or in very oxidising condition • for actinides, uranium and neptunium are more easily released from fuel than plutonium, americium and curium The VEGA experiments provided new information • especially as concerns the effect of pressure volatile FP release – the results are consistent with expectations • the effects of atmosphere (steam vs. He), temperature and MOX fuel were also investigated International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  5. Experimental database on FP release • The number of available experimental data is decreasing with fuel burn-up and is limited for MOX • medium burn-up (45-60 GWd/tU): VI4 (47), VERCORS HT1 (49.4), RT1 (47.3), HT2 (47.7), HT3 (49.3), VEGA 1 to 5 (47), VEGA 6 to 8 (56) • high burn-up (> 60 GWd/tU): VERCORS 6 (60), RT6 (71.8), RT8 (69.6) • MOX: VERCORS RT2 (45.6), RT7 (43), VEGA M1, M2 (43) • Most of the data on release in air come from the Canadian programme on CANDU fuel with moderate burn-up • The VERDON experiments, part of the International Source Term Programme, will extend the database • The ISTC VERONIKA programme on release from VVER high burn-up fuel is likely to be launched International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  6. Understanding and modelling FP behaviour • Although temperature is a key parameter, FP release from fuel is not only temperature driven • it also depends on the location of gases and FP in the fuel, depending on burn-up and irradiation history as well as on fuel structure • chemistry is important (not for gases) – the speciation of fission products within the fuel depends on fuel burn-up and oxygen potential – it evolves during reactor operation and accidental transient International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  7. Understanding and modelling FP behaviour • Two different and complementary approaches are presently used for FP release modelling • system level codes use simplified models in which only dominant phenomena are described, e.g. FP diffusion within fuel grains (Booth model) or evaporation and mass transfer – accounting for oxygen potential influence improves the predictability of release kinetics • mechanistic codes use a detailed description of all involved phenomena – they are used mainly (not only) to interpret the experiments, capture the influence of the most important parameters, allowing thus to improve the simplified models International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  8. Understanding and modelling FP behaviour • The interpretation work performed so far on Vercors experiments allows to derive a consistent and physically sound picture of FP behaviour and release • The hypotheses made for this work suffer however from a certain lack of validation especially concerning the location of FPs in the different phases of the fuel (fuel matrix, precipitates...) and their associations • it is expected to fill this gap by performing detailed micro-analyses on samples of fuel, submitted or not to annealing tests – this is part of the International Source Term Programme A thorough understanding and a detailed description of mechanisms governing fission product release is deemed important for being able to anticipate the behaviour of evolutionary fuel without performing too many new experiments International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  9. Understanding and modelling FP behaviour • The understanding of fuel collapse at rather low temperature observed in VERCORS, and also in Phébus-FP made progress during the recent years • observations: systematic fuel collapse for a temperature range of 2400-2600 K whatever the burn-up from 47 GWd/t to 70 GWd/t – collapse temperature seems to decrease in oxidising conditions • possible role of interactions between materials • melting temperature of hyper-stoichiometric UO2 is decreased as compared with previous data (Latta) according to recent experiments (Manara) International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  10. Fission Product Transport • VERCORS HT experiments provide useful data on FP transport, although limited as compared with integral experiments such as Phebus FP • They are analysed using severe accident codes and used for their validation • The interpretation of the results is made jointly with the one of other experiments (at least Phebus-FP at IRSN) • The main conclusions so far are: • an evidence of possible caesium re-vaporisation from deposits, consistent with what is observed in Phébus-FP and other programmes such as REVAP (European 5th FWP) International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  11. Fission Product Transport • The main conclusions so far are: • in the HT series experiments, caesium is found associated with iodine in the thermal gradient tubes, consistently with the prediction of CsI species by ASTEC - but not consistently with Phébus-FP results • there is no experimental evidence of an influence of Ag-In-Cd or B on FP transport in HT test series – influence of injection mode and/or oxygen potential? • no clear conclusion can be drawn about the possible transport of iodine under a gaseous form in the RCS – not all structural materials present in VERCORS tests – was it possible to track a very small fraction of gaseous iodine in VERCORS experiments? International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  12. Use of acquired Knowledge for Safety Studies Knowledge is acquired through co-operative programmes and is then used by industry and safety authorities or their technical support organisations, according to their own goals The analysis of results from VERCORS and other programmes, including Phébus-FP integral tests, show differences between “best-estimate” evaluations of FP release using the most recent advances in R&D and the “reference” source term evaluations The differences are not such that they dismiss the conclusions of safety evaluations and the decisions made so far by the regulators International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  13. Use of acquired Knowledge for Safety Studies The integration of the most recent R&D results in system-level codes used for safety studies such as ASTEC, MELCOR or MAAP is a rather “continuous” process – code developers (at least for ASTEC and MELCOR) stand close to researchers There is more delay in reflecting those advances in the regulation process, both in the industry and regulatory bodies – there is a generic difficulty in coping with the different timescales of R&D and regulation International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  14. Questions to the panel - 1 The VERCORS programme has contributed to the enhancement of knowledge concerning fission product release from LWR fuel What are, in your opinion, the most significant outcomes of the programme? What is missing or was not sufficiently addressed? International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  15. Questions to the panel - 2 What is your opinion about the following statement concerning LWR fuel: A thorough understanding and a detailed description of mechanisms governing fission product release is deemed important to be able to anticipate the behaviour of evolutionary fuel without performing too many new experiments International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  16. Questions to the panel - 3 • Different kinds of Source Term assessment studies are performed and use different approaches: • general studies (e.g. US NUREG 1465 or French S3) aim at defining a generic Source Term including overall release of various FPs and a rough estimate of the release duration • PSA level 2 studies should be more refined, especially concerning release kinetics and uncertainties Do you think that a full benefit has been drawn so far from FP release programmes or is there still room for improvements, especially for PSA level 2? International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  17. Questions to the panel - 4 An extensive research programme, including VERCORS, has been performed on FP release from LWR fuel For GEN IV fuel, do you think that such an extensive programme will be needed or only few experiments, or none, given the present status of knowledge? International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

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