Evaluation of the resuspension factor for radiological contaminants in conditions of spent nuclear fuel transports from nuclear plants

Transcription

1 Evaluation of the resuspension factor for radiological contaminants in conditions of spent nuclear fuel transports from nuclear plants A. Douce, D. Hameau, C. Letoffe, G. Morel, F. Saintamon, C. Penoty 2 EDF R&D, 6 quai Watier, 840 Chatou. fabrice.saintamon@edf.fr 2 EDF Division Combustibles, Saint Denis. Abstract. Inhalation of resuspended particles is the major hazard linked to non-fixed surface contamination. A wind tunnel called Ermitage has been designed by EDF R&D in order to test resuspension in representative conditions of spent nuclear fuel transports. Under voluntarily penalizing conditions and after statistical analysis, the study, based on 20 tests, has shown that: - the resuspension factor found varies overall between E-6 and 5E-6 per meter, - its average value is 2E-6 per meter, - the resuspension of 0% to 90% of the total activity occurs during the first 2 minutes of air sweeping, - while the total resuspended activity is growing with the speed of wind, the resuspension factor is almost constant. Indeed, the shear stress increases with the speed, but the volume of dilution increases as well. Those two phenomena compensate in the experimentation. - the resuspension is a function of stainless steel surfaces roughness: it is more important on smooth surfaces (Ra 0,02 micrometers), than on rough surfaces (Ra,6 micrometers) because of the increase of a part of the adhesion forces with roughness in the studied range. Generally, this effect depends on the ratio between particles granulometry and the dimension of the surface bumps. These results have been approved by the IAEA and ought to be taken into account in the future official models of risks related to non-fixed surface contamination on spent nuclear fuel transport.. Introduction Health hazard associated to non-fixed radioactive contamination have been assessed by AIEA through the Fairbairn model since 96 []. That means that the official limits of acceptance for non fixed surface contamination on spent fuel transports- 4 Bq/cm 2 for Beta/gamma radionuclides and 0,4 Bq/cm 2 for Alpha radionuclides - come from this model. In 2003, AIEA decided to review this model of assessment by creating a specific working group, the Committee Research Program (CRP), in which EDF R&D was involved. In this context, EDF R&D investigated the whole health hazard related to spent fuel transport: the first step consisted in describing as precisely as possible the type of contamination encountered on nuclear fuel transport. Then, a specific risk analysis was done to identify the more sensitive scenarios for health hazard. Finally, considering that the resuspension factor used in Fairbairn model was not clearly validated, EDF R&D decided to make an experimental assessment of this parameter by developing a windtunnel which was expected to reproduce representative conditions of spent fuel transport. This document presents the main results of this investigation. 2. Description of cases of contamination found on spent nuclear fuel transport from nuclear plants This part of the document relies on a bibliographic study on the existing data related to the surface contamination present on spent fuel casks. This study has enabled to characterize precisely the origin and nature of the radionuclides, to gather the activity levels found on these transports during the past 5 years, and to locate preferential areas of activity concentration on the transports [2].

2 The radionuclides found on the transports are different from the ones that forward in the primary system or in the spent fuel pit, though their origin is the same: radionuclides with short half-life have disappeared and their size is more important probably because of coalescence phenomenon. The size can reach several hundreds µm. The major part of the activity comes from big particles; it is directly proportional to their mass. Particularly, the granulometric distributions in mass are very similar to the granulometric distributions in activity and very different from the ones in number of particles. These contaminants are essentially corrosion products. On an average, Co60 is responsible for 80% of the activity. Mn54, Ag0m and Co58 can be present but in a smaller scale. The contamination levels on the transports have been very variable for these past 5 years: from 4Bq/cm 2 to 3000 Bq/cm 2 with a mean of 50 Bq/cm 2 (values given for the time period between 988 and 998). These levels have been strongly decreasing since the crisis of 998 as well as the number of contaminated spent fuel transports. The most sensible areas in term of contamination are, for the wagons, those which are submitted to liquid flows or liquid concentrations such as retention tanks or recovery pans; for the casks, these are those which are the least protected during the refuelling, i.e. trunnions or the skirt supports (FIG..). Retention tank Trunnions Skirt supports Recovery pans Cask Wagon FIG.. Localization of sensible areas in term of contamination on wagon and cask 3. Analysis of health risks related to spent nuclear fuel transports from nuclear plants 3.. Identification of health hazards related to spent nuclear fuel transports from nuclear plants Health hazards related to non-fixed surface contamination are mainly embodied by the equivalent dose resulting from: - a contamination transfer from the non-fixed surface contamination to the exposed person: inhalation after resuspension, hands contamination, ingestion. - a distant irradiation Quantification of health hazards related to spent nuclear fuel transports from nuclear plants The evaluation of health hazards related to spent nuclear fuel transports has been made using a derived model of the Fairbairn model, applied in representative and realistic (from the EDF R&D point of view) conditions of such transports [3]. Indeed, the hypothesis we ve used to run the model 2

3 are different from the ones used by Fairbairn, what leads to different values of the impact parameters. In this way, we just wanted to point at the specific conditions of these transports. We ll see in chapter the justification of our hypothesis Results The results are shown in table I. Table I. Quantification of health hazards related to spent nuclear fuel transport within realistic conditions Hazard Equivalent dose (msv/year) Permissible equivalent dose for workers (msv/year) Permissible equivalent dose for the public (msv/year) - Inhalation 0, Ingestion 0, Hands contamination - Distant irradiation Whole body Skin It can be noticed that the equivalent doses within representative and realistic conditions of spent nuclear fuel transport are clearly lower than the permissible equivalent doses for workers and for the public whatever the hazard. Nevertheless, inhalation of radioactive particles and hands contamination lead to maximum equivalent doses compared to the other hazards. Hands contamination results from human factors what confers a controllable feature on this hazard. On the other hand, inhalation has a physical origin, what is not linked with a behavioural notion. That is why this last hazard is considered the most important one Impact parameters concerning inhalation hazard As mentioned in the beginning of chapter 3.2., with regards to inhalation hazard, EDF R&D has chosen hypothesis that are different from the Fairbairn s ones; the aim of this chapter is to explain why. 3

4 The equivalent dose linked to inhalation hazard is calculated as: () Di = Li τ r t Ci 0 where Di equivalent dose for radionuclide i (msv/year) Li surface contamination (Bq/cm 2 ) τ resuspension factor (m - ) r respiratory rate (m 3 /hour) t duration of exposition (hours/year) Ci equivalent dose coefficient (Sv/Bq) In table II., all the impact parameters values necessary to run the model are clarified. Table II. Impact parameters used to run the model of risk evaluation related to inhalation of radioactive particles Impact parameters Fairbairn EDF R&D - Resuspension factor (m - ) 4E-5 5E-5 - Duration of exposition (hours/year) Equivalent dose coefficient (Sv/Bq),5E- (Sr90) 4,E-5 (Pu239) 2,E-8 - Respiratory rate (m 3 /hour),2,2 - Surface contamination (Bq/cm²) 0,4 (Pu239) 50 4 (Sr90) The equivalent dose coefficient chosen by EDF R&D is supposed to be representative of the radionuclides that can be found on spent nuclear fuel transport. It is a weighted average between all the radionuclides mentioned in chapter 2. with 0% of Co60. As the spent nuclear fuel transport is not the main activity for nuclear workers, the majorant value of 200 hours/year has been used for the duration of exposition in order to follow our realistic assessment of the risk. We can notice that 2000 hours/year is the result of the product of 40 hours a week per 50 weeks a year what corresponds to a full time job. The value of 50 Bq/cm² for the surface contamination corresponds to a mean value of all the surface contaminations found on spent nuclear fuel transport between 988 and 998. It would be much lower now thanks to the efforts made by EDF during the last years in the field of radiological property of such transports. Finally, one parameter remains uncertain: the resuspension factor. Its variability in the literature is very important -several decades- according to the models and the assessment conditions of this parameter. The wish of the AIEA to re-examines the model of risk related to non-fixed surface contamination underlines again the importance of a realistic assessment of the resuspension factor. That was the aim of the windtunnel called Ermitage built by EDF R&D: an experimental assessment of the resuspension factor in representative conditions of spent fuel nuclear transports. 4

5 4. Description of the windtunnel Ermitage 4.. General description The test bed that has been built, ERMITAGE (for: Essais de ReMIse en suspension pour le Transport des AssemblaGEs irradiés), is a closed windtunnel which enables the resuspension, by air sweeping, of radioactive particles deposited on a plane surface, according to variable parameters: - flow velocity: 3 m/s, 0 m/s, 30 m/s - nature of deposit surfaces: steel of different roughness (stainless steel 0,02 µm or,6 µm) or elastomer - temperature of the deposit plate: ambient or 80 C - test duration: each test lasted 30 minutes, except two special ones which were carried out over 2 first minutes of sweeping, and then 30 minutes after having changed the filter. The objective was to test the effect of the first minutes of sweeping on the total resuspended activity. In order to obtain a conservative evaluation of the resuspension, the aerological technical choices have been made to obtain the most restricting resuspension conditions of particles deposited on the flasks used for nuclear fuel transports. This point was checked during the qualification of the windtunnel Principal components - an experiment cell in which the deposited particles are exposed to an air flow. The cell section is defined to maintain a turbulent flow on the whole range of speeds of air flow (3 to 30 m/s). - two plates, on which the particles are deposited, fit into the higher and lower walls of the cell. - a filter used to collect the resuspended particles carried away by the air flow, - a centrifugal fan with variable air flow, - several tranquillization devices ensuring a stabilized flow at the entry of the experiment cell. - temperature, moisture, pressure and air flow speed sensors. 5. Test procedure and description 5.. Obtaining of a representative contamination 5... Principle of obtaining In order to obtain a representative contamination of the one possibly present on spent nuclear fuel transports, the choice was made to work with the particles at the origin of this contamination, particles present in the spent fuel pool water. The particles were thus taken from the spent fuel pool water of the nuclear power plant of Flamanville by means of a tangential filtration unit enabling: - to concentrate the particles in water - to remove the boric acid from it, in order to avoid the boron precipitation during the contaminants deposit, which would have trapped the particles and prevented their resuspension Particles obtained The characteristics of the particles obtained by this method and concentrated in water were: - radiological spectrum: Co60 (800 kbq/l) and Mn54 (80 kbq/l); - boron content: lower than 2 ppb; - granulometry: > 0,2µm. Note: These particles are scattered around a median or average diameter from 3 to 8 µm. The law of dispersion of the diameters is classically a lognormal law. In the particular case of the experimentation, the particles used had a diameter higher than 0.2 µm, because of the 5

6 threshold diameter of the tangential filtration membrane used. We did not select the particles of diameter lower than 0 µm (threshold of the inhalable particles [4]) what sets the study in penalizing conditions considering its objective Method of particles deposit on the plates The deposit on the plates is prepared from an aqueous suspension that is evaporated on their surface at a temperature of 80 C. A measurement of total activity is then carried out to know the quantity of activity really deposited. After this measurement, the plates are immediately fitted into the higher and lower walls of the experiment cell, before starting the test. There is no cleaning nor sweeping of the plates before the beginning of the test and the resuspension is thus evaluated on this hard deposited contamination without later action Measurements on plates The particles resuspended are recovered on the THE filter, that is changed before each test. A measurement of the activity of each plate is made: - after deposit, - after test, - after cleaning. These measurements have enabled to point out the fact that the cleaning of the plates was not completely efficient. Residual activity after cleaning is then considered to be fixed and is subtracted from the deposited activity in order to evaluate the non-fixed surface contamination really deposited Measurements on filter After the resuspension test, the filter is removed from the windtunnel in a tight vinyl bag. It is then prepared to be measured; the measurement is only carried out on the filtering media and on the packing vinyl bag. The activity present on the filter is measured by two means: - measurement of total activity by a gamma counter, - determination of the spectrum of the radionuclides present on the filter by gamma spectrometry. 6. Results and analysis 6.. Choice of the representative resuspension variable Because of the test procedure of the windtunnel ERMITAGE, the accessible variables for each resuspension test are: - the non-fixed total activity deposited, - the total activity resuspended, - all the recorded physical parameters related to the test: temperatures, moisture, pressure, air flow, duration, type of plate... The objective is to point out a representative variable of resuspension among these ones. In such conditions, the only true variable is the resuspended fraction (ratio between the total activity resuspended and the non-fixed total activity deposited). But the aim of our study is to give elements of reflection about the risk of inhalation of radioactive particles; this risk is assessed thanks to the resuspension factor []. This constraint has conducted us to make a temporal and spatial average of the resuspended activity in order to be able to calculate a mean volumetric activity representative on the height of the vein of experiment. This last variable will enable us to access directly the resuspension factor which will present our results. 6

7 The mean volumetric activity within 0 cm over the surface is: Atot, erm Avol erm= V Serm D, (2) with: Atot,erm total activity resuspended during the test (Bq) V air flow velocity (m/s) Serm section of the experiment cell of Ermitage (m²) D experiment duration (s) Consequently, we propose an evaluation of the resuspension factor and rate valid within the framework of the resuspension conditions in Ermitage: - very close to the inner surface, the vein of experiment being 0 cm high, - for the duration of the experiment (30 minutes); In this case, the resuspension factor is connected to the measured activities by: Avol, erm Fsusp, erm = (3) Asurf, erm with: Fsusp, erm resuspension factor (m - ) Asurf, erm surface activity deposited on the plates (Bq/m²) 6.2. Results and parameters of influence Evaluation of uncertainty on the results of resuspension The resuspension results present an uncertainty related to: - uncertainty due to the measure of activity on the deposit plate; - uncertainty due to the measure of activity on the filter; - uncertainties related to the procedure (loss of contamination cross contamination from one test to another). To limit the risk of cross contamination from one test to the other, the aerodynamics of ERMITAGE was conceived to limit the deposit of particles between the experiment cell and the filter collecting the resuspended particles. During the tests, smears were carried out, and showed the absence of contaminants deposits in this zone. It is thus considered that all the resuspended activity is collected by the filter. The loss of contamination or the cross contamination can occur during the preparation of the filters before measurement. In order to remain restrictive, all the packing vinyls and cutting supports vinyls have been measured and their activity integrated into the total activity used in the evaluation of the resuspension factor. Uncertainties of measurements were evaluated: - for the measurement of activity on the deposit plate 2% - for the measurement of activity on the filter: 0% That gives a total uncertainty on the evaluation of the resuspended activity of 6% Presentation of the results of resuspension In order to test the resuspension factor according to each identified parameter of influence (air flow velocity, nature and roughness of deposit surfaces, temperature of the deposit plate), 20 tests have been carried out, including two no-load tests.

8 A statistical analysis on the dispersion of these results is illustrated in the FIG. 2.: - in X-coordinate, classes of values of resuspension factors, - in ordinate, the number of factors for each of these classes. This analysis shows that all the values are lower than 6E-6 m - except only one, which is definitely higher and is located at 9,6E-6 m -. This value can be explained by the presence of a hot spot i.e. one particle with a high specific activity or located at the extreme of the granulometric distribution, that has been resuspended and collected on the filter. Global statistical analysis occurence Fsusp. (x0e-6) FIG. 2. Global statistical analysis 8

9 Influence of the parameters The influence of the different parameters tested with Ermitage is shown in FIG. 3. Air speed influence Température influence occurence Fsusp. (x0e-6) v30 v0 v3 occurence Fsusp. (x0e-6) t=80 t=20 Roughness influence Effect of the first minutes of sweeping occurence Fsusp. (x0e-6) silic 0,02µ,6µ 00% 80% 60% 40% 20% 0% 2 first minutes 2 first minutes Roughness,6 µm Roughness 0,02 µm FIG. 3. Influence of the parameters Effect of the first minutes of sweeping Two tests enabling the study of resuspension during the first minutes of sweeping showed that over 2 minutes + 30 minutes of sweeping, 0 to 90% of the total activity is resuspended during the first two minutes of sweeping. This is to be brought closer with the fact that the particles deposited on the plates were not treated in any way before resuspension. That point is confirmed in the experiments as well as in the models of resuspension and particularly the model of accumulation of energy of Reeks. Indeed, the phenomenon of particles resuspension by air sweeping is made of two parts [5]: - during the first second of sweeping, the very slightly adherent particles are immediately resuspended; - then, a statistical resuspension of more adherent particles, which is a constant phenomenon in time, occurs Flow velocity There is no notable influence of the speed on the resuspension factor. Indeed, the shear stress exerted on the particles is an increasing function of the friction speed and thus of the flow velocity. The total resuspended fraction during a test is thus an increasing function of the flow velocity. In the case of the tests carried out on Ermitage, we have actually noted an increasing resuspension fraction with the flow velocity (the friction speed has been measured and is indeed an increasing function of the flow velocity). But in a mathematical way, the resuspension factor is inversely proportional to the air flow which causes it, what is explained in a physical way by a greater dilution of the resuspended quantity. 9

10 In the case of Ermitage, these two opposite effects cancelled their reciprocal influence, which results in a resuspension factor almost identical whatever the flow velocity Temperature The variability linked to the temperature appears low in the studied range. Indeed, the temperature has a double influence: - the first one is related to the adhesion forces of Van der Waals which are all the more weak so as the temperature is high. This can lead to a higher resuspension; - but the increase of the temperature leads to a reduction of the air dynamic viscosity, therefore to a reduction of the shear stress; what is an effect contrary to the first. Only a finer modelling of the forces would have enabled to compare our results with the theory. This modelling was not carried out Nature of surface The effect of roughness is clear; it is explained by the increase of a part of the adhesion forces (rough cohesive forces) with roughness in the studied range.. Conclusions on the evaluation of the resuspension factor The experimentation carried out on Ermitage made it possible to evaluate the resuspension factor due to air sweeping under penalizing conditions compared to those of spent nuclear fuel transports. To represent the resuspension, the variable chosen is the resuspension factor, in order to facilitate the comparison with the Fairbairn model. This resuspension factor thus obtained is valid within the framework of the conditions of Ermitage: - very close to the inner surface, the vein of experiment being 0 cm high, - for the duration of the experiment (30 minutes); Its extrapolation in other conditions requires correction factors (in particular a dilution factor to evaluate the concentration in the air at a given distance from the surface). Under these conditions and after statistical analysis, the study has shown that: - the resuspension factor found varies overall between E-6 and 5E -6 m - - its average is of 2E -6 m -. These results have to be compared with the resuspension factor used by Fairbairn in its model, 4E-5 m -, which seems to be overestimated. They have been approved by the IAEA and ought to be taken into account in the future official models of risks related to non-fixed surface contamination on transportation casks [6]. 8. References. International Atomic Energy Agency, Regulations for safe transport of radioactive materials - Notes on certain aspects of the regulation, IAEA, Safety series No, Vienna (96) 2. Letoffe, C., Saintamon, F., Descriptif des cas de contamination des transports de combustibles irradiés, EDF R&D internal document HP-8/0/02/A, Paris (200) 3. Letoffe, C., Saintamon, F., Analyse des risques sanitaires liés à la contamination surfacique des transports de combustible irradié, EDF R&D internal document HP 8/03/022/A, Paris (2003) 4. International Commission on Radiological Protection, Human respiratory tract model for radiological protection, Publication 66. Annals of the ICRP, 24, No -3 (994) 5. Fromentin, A., Particles resuspension from a multy deposit by turbulent flow, Paul Scherrer institut Bericht Nr. 38 (989) 6. International Atomic Energy Agency - Report of the modelling group (version.), IAEA CRP Meeting, Williamsburg VA. (working document publication limited to CRP members - Coordinated Research Project on surface contamination for transport) (2002) 0