Editorial Instructions for the Final Report

THEMATIC NETWORK FOR A PHEBUS FPT-1 INTERNATIONAL STANDARD PROBLEM (THENPHEBISP)

CO-ORDINATOR

B. CLEMENT

IRSN/DPAM, Cadarache

127, Rue des Martyrs ( B. P. 85 )

F - 38054 Grenoble Cedex 9

FRANCE

Tel.:+ 33 7688 3244

Fax:+ 33 7688 5177

LIST OF PARTNERS

1) IRSN, Cadarache (Fr)

2) JRC, Petten (Nl)

3) AEA Technology (UK)

4) AEKI, Budapest (Hu)

5) NRI, Rez (Cz)

6) ENEA, Bologna (It)

7) UPI, Uv. Pisa (It)

8) UPM, Uv. Madrid (Sp)

9) FZK, Karlsruhe (Ge)

10) GRS, Cologne (Ge)

11) PSI, Villigen (CH)

12) SCK, Mol (Be)

13) EDF, Clamart (Fr)

14) JSI, Ljubljana (Sl)

15) ENPROCO (Bu)

CONTRACT N°:FIKS-CT-2001-20151

EU contributionEuro 240899

Starting Date:December 2001

Duration:24 months

CONTENTS

LIST OF ABBREVIATIONS AND SYMBOLS

EXECUTIVE SUMMARY

A.OBJECTIVES AND SCOPE

B.WORK PROGRAMME

B.1Preparation

B.2Intermediate Comparison Workshop

B.3Comparison and Assessment

C.WORK PERFORMED AND RESULTS

C.1Specification Report

C.2Intermediate Comparison Workshop

C.3Comparison and Assessment

C.3.1Representation of the facility

C.3.2Analysis of the results

C.3.3Assessment of Codes and Models

C.3.4Computing Assessment

C.3.5Integral Aspects

C.3.6Implications for Plant Studies

CONCLUSION

REFERENCES

FIGURES

LIST OF ABBREVIATIONS AND SYMBOLS

CSNICommittee for the Safety of Nuclear Insatllations

FPFission Products

FPT-iPhebus FP test n°I

IRSNInstitut de radioprotection et de Sûreté Nucléaire

ISPInternational Standard Problem

OECDOrganisation for Economic Development and Cooperation

PBF-SFDPower Burst facility-Severe Fuel Damage

RCSReactor Coolant System

EXECUTIVE SUMMARY

The thermal behaviour of the fuel bundle and the hydrogen production were generally well captured, and good agreement for the core final state could be obtained with a suitable choice of bulk fuel relocation temperature, however this is unlikely to be representative of all plant studies so sensitivity calculations are needed with the modelling in its current state. Total volatile fission product release was simulated, but its kinetics, and the overall modelling of semi-volatile, low-volatile and structural material release (Ag/In/Cd, Sn) needs improvement. Overall retention in the circuit is well predicted, but calculations underestimate deposits in the upper plenum and overestimate those in the steam generator, also the volatility of some elements could be better predicted. Containment thermal hydraulics and depletion rate of aerosols are well calculated, but with difficulties related to partition amongst the deposition mechanisms. Calculation of iodine chemistry in the containment turned out to be more difficult. Its quality strongly depends of the calculation of release and transport in the integral codes. The major difficulties are related to the existence of gaseous iodine in the primary circuit and to the prediction of the amount of organic iodine in the gas phase.

Beyond the assessment of codes and models, as usually done in International Standard Problems, conclusions were made with respect to plant sequences calculations looking at overall signatures such as the degraded core final state and the fission product source term. A number of recommendations for model development and various implications for plant studies have been identified.

A.OBJECTIVES AND SCOPE

The THENPHEBISP 2-year thematic network started in December 2001, and was concerned with OECD/CSNI International Standard Problem 46, itself based on the Phebus FPT1 core degradation/source term experiment. The aim was to assess the capability of computer codes to model in an integrated way the physical processes taking place during a severe accident in a pressurised water reactor, from the initial stages of core degradation, the fission product transport through the primary circuit and the behaviour of the released fission products in the containment. The ISP-46 provided the first opportunity to assess the capability of integrated severe accident analysis codes in four different areas, corresponding to the phases of the Phebus FP experiments [1], namely: (a) fuel degradation, hydrogen production, release of fission products and of structural materials; (b) fission product and aerosol transport in the circuit; (c) containment thermal hydraulics and aerosol physics; and (d) iodine chemistry in the containment. Participants were encouraged to submit integrated calculations, but detailed calculations for individual phases were also welcome. Choice of noding schemes and model parameters corresponding to plant calculations (base-case) was strongly encouraged, while more detailed optional (best-estimate) studies were also accepted. This formalism made it easier to draw conclusions regarding plant calculations, and to identify ‘user effects’.

B.WORK PROGRAMME

The work programme involved three main phases.

1)Preparation, resulting in the Specification Report for the ISP [2], during this phase a draft was circulated so that feedback from participants on data requirements and modelling needs could be obtained;

2)Calculation, including the organisation of an Intermediate Comparison Workshop, where participants presented their contributions and the coordinators gave their first impressions, at this stage a plan for the detailed analysis was agreed and a schedule arranged for revised contributions as needed;

3)Comparison and Assessment, which included detailed analysis by the coordinators, presentation of the draft Comparison Report at the Final Workshop, and production of the final version of the Comparison Report taking into account all comments received. Feedback from code developers was vigorously encouraged, and valuable information was gained from them.

1.B.1Preparation

The main task of this work package was to issue the specification report for the ISP 46. An initial draft version was distributed to the partners and discussed at the Preliminary Workshop in November 2001.

2.B.2Intermediate Comparison Workshop

During the Intermediate Comparison Workshop, in October 2002, the participants presented their contributions and the coordinators gave their first impressions, at this stage a plan for the detailed analysis was agreed and a schedule arranged for revised contributions as needed.

3.B.3Comparison and Assessment

This phase corresponds to the detailed analysis by the co-ordinator of the results submitted by the participants. This analysis was discussed at the final comparison workshop in March 2003. An application workshop [3] was held in June2003 in order to discuss the implications of the work performed on plant studies.

C.WORK PERFORMED AND RESULTS

4. C.1Specification Report

An initial draft version of the specification report was distributed to the partners and discussed at the Preliminary Workshop in November 2001. The report covers the objectives of the exercise, indicates the timescale, summarises the Phebus facility and the test itself, indicates the boundary conditions and material data required, provides references to the available results of the experiment, details what results are to be provided by the participants, and invites participants to draw their own conclusions with regard to needed model improvements and, importantly, to accident sequence analysis in commercial power plants. Finally, concluding remarks are given. Appendices provide additional data not present in the FPT1 Final Report, recommend source terms at bundle exit (required for calculations considering the circuit only) as well as circuit exit (required for calculations considering the containment only), and a list of participants to the ISP.

The areas covered by the experiment, and therefore by the Standard Problem, are fourfold:

Fuel degradation, hydrogen production, release of fission products, fuel, and structural materials ('bundle' part of the ISP);
Fission product and aerosol transport in the circuit ('circuit' part of the ISP);
Thermal hydraulics and aerosol physics in the containment ('containment' part of the ISP);
Iodine chemistry in the containment ('chemistry' part of the ISP).

Participants were encouraged to perform integral calculations covering all four aspects of the exercise. However, the ISP was so organised that it was also possible for participants to calculate any of the above phases in a stand-alone manner, using detailed-level mechanistic codes that treat for example core degradation or containment thermal hydraulics and aerosol physics on their own. To the latter end, recommendations were made regarding the noding to be used in the analysis, for two cases; a base case with discretisations similar to those that could be used in a reactor study, and for an optional, more detailed, 'best estimate' case more typical of those used in experimental interpretation. A number of participants have been able to perform two sets of calculations for each code they chose to employ, so the effect of fineness of noding could be examined. The submission for each code (not limited to one per organisation) could consist of a base case and a best-estimate case for each of which numerical data would be provided, accompanied by such sensitivity studies to illuminate the results as the participant thinks fit. However the first set of calculations was deemed more important

The sources of information for the ISP were, in descending order of priority:

the Specification Report;

the FPT1 Final Report;

the FPT1 Data Book.

In addition, detailed data were provided in numerical form in supplementary files in electronic form, for example source term data and experimental measurements such as temperatures in the circuit which are not all provided in the Final Report.

5. C.2Intermediate Comparison Workshop

The period for calculations lasted six months, during this period a supplementary Workshop was held to clarify points arising. The Comparison Workshop was held about one year after the start of the project, with participants having had the opportunity to submit revised calculations in the meantime. The ISP was well supported, with participation from 33 institutes, companies etc. in 23 countries and international organisations. The latter comprised EC-JRC, Austria, Belgium, Bulgaria, Canada, Croatia, Czech Republic. France, Germany, Greece, Hungary, Italy, Japan, Korea, Mexico, Russia, Slovenia, Spain, Sweden, Switzerland, Turkey, UK and USA. The participating organisations included utilities, regulators and their technical support organisations, research institutes and private engineering consultancy companies, thus providing a good range of backgrounds to the technical work. Fifteen different codes were used: ASTEC, ATHLET-CD, COCOSYS, CONTAIN, ECART, FEAST, IMPACT/SAMPSON, ICARE/CATHARE, IMPAIR, INSPECT, MAAP4, MELCOR, SCDAP/RELAP5, SCDAPSIM and SOPHAEROS, of these 4 are integral codes (ASTEC, IMPACT/SAMPSON, MAAP4 and MELCOR). For the base case, 47 calculations were received, with 21 for the optional best-estimate version. Of the base case calculations, 14 were integral (at least 3 phases calculated)

During the Comparison Workshop, the participants have presented their results and the co-ordinators their first impressions.

6. C.3Comparison and Assessment

7. C.3.1Representation of the facility

For the base case, a noding scheme was recommended in the specification report The bundle is divided into 11 axial nodes and typically 3-5 radial rings, with normally 1 or 2 thermal hydraulic flow channels. The circuit is divided into 11 nodes, this being the minimum considered necessary for an adequate calculation of deposition. The containment model is simple, with 1 node for the main volume and 1 for the sump, taking advantage of the well-mixed conditions. A typical nodding scheme is given in figure1. For best-estimate calculations, noding density was increased by typically a factor 2 or more, at the choice of the user.

8. C.3.2Analysis of the results

The results were analysed in detail, comparing the results amongst each other and with the FPT1 data. There was considerable scatter amongst the results obtained from each code by different users, the ‘user effect’. To minimise this effect, representative cases were selected where necessary, taking into account the quality of key output variables, completeness and accuracy of the technical reports, and including code developers where possible. This analysis led to an assessment of the main models in each of the four areas considered. These are grouped below, in order of their perceived adequacy. There was on the whole little significant difference between the base and best-estimate cases, with at most a small improvement only in the results of the latter cases, so conclusions could be drawn on the basis of the former.

9. C.3.3Assessment of Codes and Models

The following phenomena/parameters are in general well simulated by the codes:

Bundle – thermal response (figure 2, given adjustment of input nuclear power and shroud thermal properties within experimental uncertainties), hydrogen production (figure 3, including oxidation of relocated melt), bundle final state material distribution (figure 4, given suitable reduction of the bulk fuel relocation temperature from the ceramic value, in the longer term a more mechanistic model is desirable), total release of volatile fission products (figure 5);
Circuit – total retention of fission products and structural materials (figure 6, but after cancellation of errors);
Containment – thermal hydraulic behaviour (as exemplified by average gas temperature, pressure, relative humidity and condensation rate), depletion rates (figure 7);
Chemistry – models of the Ag/I reaction in the liquid phase are adequate for FPT1 (this cannot be extended to other cases where the Ag is not so much in excess with respect to I; due to the large excess of silver, in the experiment, radiolytic production of gaseous iodine and dissociation of silver iodide did not play an important role in the overall iodine behaviour).

The following phenomena/parameters were reasonably well simulated, but some modelling improvement is desirable:

Bundle – outlet coolant temperatures (overprediction), time dependence of volatile FP release (figure 5, generally too fast a release at low temperatures, e.g. for CORSOR-type approaches);
Circuit – distribution of deposition in the circuit (underestimation in the upper plenum where vapour condensation and thermophoresis are the dominant mechanisms, overestimation in the steam generator hot leg where the mechanisms are thermophoresis for all elements + vapour condensation for I and Cd), noting that too coarse a noding leads to underestimation of deposition;
Containment – relative importance of the two main depletion processes (diffusiophoresis and gravitational settling), but it is hard to make firm conclusions owing to the variability in the results;
Chemistry – no items identified.

The following phenomena/parameters were not well simulated and substantial model development is necessary:

Bundle – release of medium and low volatiles (e.g. tendency to calculate low for Mo –figure 8, very high for Ba, reasonable order of magnitude for Ru and U but considerable scatter), and of structural materials (Ag/In/Cd, figure 9, from the control rod where the basic process of evaporation from a molten AIC pool is not captured, tin from the Zircaloy cladding);
Circuit – iodine speciation and physical form;
Containment – no items identified;
Chemistry – gas phase reactions (figure 10), organic iodine reactions (figure 11), including production and destruction through radiolytic processes (definition of optimum parameters for the modelling codes such as adsorption velocity and desorption rate on/from painted surfaces, and the facility to input the gaseous iodine fraction at containment entrance, are recommended).

The good prediction of hydrogen production, generally near the upper bound of the experimental uncertainty range, (+10%), is an important safety-relevant conclusion. The good prediction of the bundle material distribution in the final states requires a suitable reduction of the bulk fuel relocation temperature from the ceramic value. Implications, in the short term and, in the longer term, the need for a more mechanistic model will be discussed later on as integral aspects. Although the structural materials do not themselves have radiological significance, they potentially react with fission products, and their source terms are therefore needed for accurate calculation of chemistry and transport in the circuit. A particular need is to saturate the iodine reaction. The semi-volatile fission products are also of importance, either because of their radio-toxicity and influence on the residual power, or by their propensity to react with other fission products. The structural materials also form the bulk of the aerosol mass, affecting the aerosol concentration and the agglomeration processes.

Concerning the circuit, the overestimation of bundle outlet temperature cannot fully explain the upper plenum results; its main effect is to displace the zone where vapours nucleate. For some elements, part of the discrepancy in the deposition pattern is due to the wrong prediction of the chemical form, and thus of its volatility; Cs is generally calculated as a vapour at 700°C, whereas it was condensed in the experiment. However, this is also not enough to explain the underestimation in the upper plenum and overestimation in the steam generator rising line. Finding explanations is presently part of the work performed in the frame of the interpretation of Phebus-FP tests.

Care is needed in extrapolating the rather good results for the containment directly to the reactor case, as the Phebus containment thermal hydraulics are relatively simple, and the role of gravitational settling is overscaled, with a shorter residence time of aerosols in the atmosphere and probably less effect of agglomeration than for real plant.

Concerning the chemistry, the reaction of iodine with silver, forming non-soluble silver iodide, dominates the phenomenology in the liquid phase. In the FPT1 conditions with a large excess of silver, the models behave sufficiently well, provided enough silver is injected into the sump water. Gas phase chemistry is dominated by early injection of gaseous iodine from the primary circuit, that will be discussed later on as an integral aspect, and by inorganic iodine adsorption on the atmospheric paints followed by organic desorption, together with destruction mechanisms. The results were particularly contrasted, with a large scatter on the total gaseous iodine concentration, and a fraction of organic iodine ranging from less than 10% to nearly 100%. Overall, they range from unreliable to very good (after tuning).