Study of concrete radiation ageing

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To cite this version:

Z. Hlavac, V. Dewynter-Marty, J. Kotatkova, W. Guillot, P. Le Tutour. Study of concrete radiation

ageing. TINCE 2018 – Technological Innovations in Nuclear Civil Engineer-ing, Aug 2018, Palaiseau,

France. �hal-02415501�

N° 000002 Study of concrete radiation ageing

Z. Hlavac 1*; V. Dewynter-Marty 2 ; J. Kotatkova 3 ; W. Guillot 2; P. Le Tutour 2

1

Research Centre Rez, Hlavni 130, Husinec-Rez 250 68, Czech Republic;

2

Commissariat à l'énergie atomique et aux énergies alternatives, Saclay, 91190 Gif Sur Yvette Cedex France;

3

Czech Technical University in Prague, Thakuova 7, Praha 6, Czech Republic *

Corresponding Author, E-mail: Zbynek.Hlavac@cvrez.cz

Concrete is a durable material which is a vital part of engineering barrier for radioactive waste disposal. In many concepts of low and intermediate level waste (e.g. the French concept) and in some cases even of high level waste, concrete is planned to be used for construction of containers carrying the primary packages of the immobi-lized waste. Therefore, it is needed to understand the effect of gamma irradiation on concrete with respect to its durability. A joint experimental program of Czech and French research organizations CVR and CEA respectively was started in order to study the changes in some mechanical properties of cementitious composites after their exposition to gamma irradiation. Samples of cement mortars, manufactured in CVR, were divided into two sets, each of them irradiated and characterized with both CVR and CEA facilities. Samples tested in CVR were ex-posed to cumulative doses of 0.3 to 3.0 106 Gy with dose rates from 0.5 to 6 kGy/h, while samples in CEA, Saclay were exposed to cumulative doses of 0.72 to 3.1106 Gy with dose rates 0.6, 1.6 and 2.6 kGy/h, respec-tively. Some mechanical properties as P-waves with ultrasonic measurements and tensile strengths of irradiated mortars were then defined. A comparison with non-irradiated samples maintained in the same environment was done.

KEYWORDS: concrete strength, gamma radiation, non-destructive testing

Introduction

In order to determine the effect of the gamma irradiation on the concrete properties, 18 concrete prisms were put into two irradiation chambers. Nine prisms were irradiated by the CEA’s laboratory LABRA facilities in Saclay near Paris, 9 were irradiated by the Research Centre Rez (CVR) in the Czech Republic. Nine more prisms were tested without any irradiation to set the reference values. All the 27 samples 40x40x160 mm were mixed between June 12 and 14 2017 in CVR workshop. Wa-ter to cement ratio was 0.5, the cement:aggregate ratio was 1:3. Silica sand particles < 2.5 mm were used as the fine aggregates in order to obtain sufficiently homogenous material, in comparison with the concrete used in the nuclear industry. The average content of cement was 490 kg/m3.

All the 18 samples were exposed to the doses 0.6, 1.0, 1.6, 2.6 or 6 kGy/hour. Moreover, 9 samples were let as the reference ones, being kept in the same conditions, i.e. the relative humidity and the temperature, as the samples being irradiated.

The whole irradiation experiment took 50 days at CEA irradiation chamber and 21 days in CVR’s labo-ratory.

Whereas Pagure irradiation facility at CEA acts by gamma irradiation of 9 cobalt-60 stick sources, each 3 samples obtained constant cumulative doses 3.12 106 Gy, 1.92 106 Gy or 7.20 105 Gy.

CVR’s source of the gamma radiation cobalt-60 has a quasi-dot character that is why the doses and cumulative dose were distributed more widely within the 9 samples.

The number of irradiated samples, their doses and cumulative doses can be seen in Table 1. Details of the fabrication, irradiation and testing of each sample are provided in Tables 2 and 3.

Tab 1 Samples tested by CVR and by CEA and their doses or cumulative doses both CVR CEA Dose [kGy/h] Cumulative dose [MGy] number of tested samples Cumulative dose [MGy] number of tested samples 6.0 3.0 2 2.6 1.3 2 3.2 3 1.6 0.8 2 2,0 3 1.0 0.5 1 0.6 0.3 2 0.7 3 0.0 0.0 9 TOTAL 9 18

Table 2 Samples fabricated in June 14 2017, irradiated and tested by CVR

sample dose [kGy/h] irradiation period [days] cumulative dose [MGy]

age of the samples [days] irradiation testing 2A 6.0 21 3.02 90 190 2B 6.0 21 3.02 90 190 1A 2.6 21 1.31 90 190 3A 2.6 21 1.31 90 190 1B 1.6 21 0.81 90 190 3B 1.6 21 0.81 90 190 2C 1.0 21 0.50 90 190 1C 0.6 21 0.30 90 190 3C 0.6 21 0.30 90 190

Table 3 Samples fabricated in June 12 or 13 2017 by CVR, irradiated and tested by CEA

Samples fabricated in June 12 Samples fabricated in June 12

sample dose [kGy/h] irradiation period [days] cumulative

dose [MGy] sample

dose [kGy/h] irradiation period [days] cumulative dose [MGy] 1A 2.6 50 3.12 A1 2.6 50 3.12 2A 2.6 50 3.12 B2 1.6 50 1.92 2B 1.6 50 1.92 C3 0.6 50 0.72 3B 1.6 50 1.92 A2 0 0 0 1C 0.6 50 0.72 A3 0 0 0 2C 0.6 50 0.72 B1 0 0 0 3A 0 0 0 B3 0 0 0 1B 0 0 0 C1 0 0 0 3C 0 0 0 C2 0 0 0

The samples were irradiated and tested by CEA at the age 48 ÷ 99 days and 188 days, respectively. The dose distribution in the «Czech» samples is in Figure 1. There the red colour represents 1.5 to 3.0 MGy and the dark blue shows less than 0.5 MGy.

The side view of the «Czech» concept is shown in Figure 2. The situation of the «French» prisms in the gamma chamber is in Figure 3.

Figure 1: Distribution of the doses in two perpendicular sections, i.e. in z = 8 cm and in x = 23 cm, in the CVR’s gamma irradiator GOMK

Figure 3: The distribution of the concrete prisms in the CEA irradiator facility

The measurements executed in the CVR’s laboratories

The concrete samples from 2017-06-14 were measured by CVR before and after the irradiation by ultrasonic instrument PUNDIT PL 200 based on the P-waves propagation of frequency 150 kHz. The results can be seen in Figure 4, where the red, orange, yellow and dark green colors represent the levels of irradiation or its doses 6 kGy/h, 2.6 kGy/h, 1.6 kGy/h or 1.0 kGy/h and 0.6 kGy/h, respec-tively. The change of the P-wave speed happened probably due to the water loos during the irradia-tion. It was 10 ± 1 g per sample, i.e. – 1.9 % from the total mass of the sample.

The results of tensile strength bending tests and bulk density measurement are in Figures 5 and 6.

Figure 4 Distribution of ultrasonic P-wave speed of samples 1A to 3C determined in the CVR’s laboratory

4194 4196 4167 4136 4175 4210 4124 4085 4111 3860 3909 3890 3879 3916 3953 3849 ₃₈₂₈ 3870 3600 3700 3800 3900 4000 4100 4200 2A 2B 1A 3A 1B 3B 2C 1C 3C P-wave speed [m/s] 3 samples at 2.6 kGy/h 3 samples at 1.6 kGy/h 3 samples at 0.6 kGy/h

before irradiation on (light green) and after irradiation (various color)

Figure 5 Distribution of tensile bending strength determined in CVR’s laboratory

Figure 6 Distribution of concrete bulk density determined in CVR’s laboratory

On the base of the Figure 5, no extraordinary deviation occurred among the tested samples except the sample 1C. It didn’t sustain a tension 4.2 MPa. Comparing to the other samples, also its P-wave speed was lover, before and also after the irradiation.

A slightly lower bulk density can be mentioned in case of high irradiated samples 2A and 2B.

The measurements executed in the CEA’s laboratory LECBA

Concrete samples from 2017-06-12 and 13 were measured just after the irradiation using ultrasonic instrument PUNDIT Plus using propagation frequency of the transducers 54 kHz.

Results of 3 samples at the dose 2.6 kGy/h, 3 at 1.6 kGy/h, 3 at 0.6 kGy/h and 9 samples at the dose 0.0 kGy/h, i.e. reference samples without irradiation, can be seen in the Figure 7.

Results of tensile strength, record of bending tests and one exemplar photo are in Figures 8, 9 and 11. No irradiation induced decrement of P-wave speed or tensile strength fct can be noticed.

6,20 5,32 6,98 6,12 _5,96 6,06 6,42 4,20 6,21 0,00 2,00 4,00 6,00 2A 2B 1A 3A 1B 3B 2C 1C 3C

Tensile strength fct [MPa]

2,06 2,05 2,10 2,10 2,09 2,07 2,10 _2,09 2,11 2,00 2,02 2,04 2,06 2,08 2,10 2,12 2A 2B 1A 3A 1B 3B 2C 1C 3C Bulk density [g/cm3_]

Figure 7 Distribution of ultrasonic P-wave speed determined by CEA after irradiation (averages values obtained with 3 samples when irradiated and 9 samples if non irradiated, error bars mentioned the

stan-dard deviation)

Figure 8 Distribution of tensile bending strengths determined by CEA

Figure 9 Record of the bending tests executed by CEA’s laboratory LECBA

4 164 4 185 4 177 4 148 3600 3700 3800 3900 4000 4100 4200

2600 Gy/h 1600 Gy/h 600 Gy/h 0 Gy/h P-wave speed [m/s] 7,28 6,48 6,18 5,62 6,41 5,87 6,51 6,30 5,45 4,97 5,86 5,77 5,13 4,89 4,89 5,78 5,41 5,03 0 1 2 3 4 5 6 7 8 9 10 1A 2A A1 2B 3B B2 1C 2C C3 3A A2 A3 1B B1 B3 3C C1 C2 Tensile strength f_ct [MPa] sample

Common results of the two laboratories

The differences of the results of samples tested by 3-point bending test can be seen in Figure 11. In the figure the number of tested specimen is visualized using schematic pictograms.

A big dispersion of the values of samples 1C and 3C from June 14 caused large standard deviation in the region of the cumulative dose 0.3 MGy. No dispersion of the results occurs in the case of a single sample 2C on the level 0.5 MGy.

Figure 10 Comparison of tensile bending strengths fct determined by CVR or CEA

(averages values, error bars mentioned the standard deviation)

Conclusion

This common experiment was prepared in order to approach some mechanical parameters of con-crete samples against the gamma irradiation. Two independent cobalt 60 sources were used for this bilateral experiment.

One, quasi-dot source Co60 was joined in the gamma irradiator The Little Cobalt GOMK in the Re-search Centre Rez near Prague. The source of the activity 155 TBq radiated 9 samples for 21 days. The second source was composed of 9 cobalt-60 sticks, for a total of 740 TBq. This source, used by LECBA laboratory in Saclay near Paris, was radiating 50 day onto 9 concrete samples. Moreover, 9 concrete samples were send to Saclay for the reference tests.

Irradiated samples were tested at first non-destructively, consequently destructively by a classical 3-point bending test.

When looking to the common results of the two laboratories (Figure 10) or to the separate results (Figures 5 and 8), it is clear that the gamma irradiation up to 3106 Gy, i.e. 3108 rad, don’t have any significant effect on the studied concrete properties.

Such conclusions are supported also by Kaplan 1) or Hilsdorf 2) where the limit value was determined as 1108 Gy, i.e. 11010 rad.

These values are used as a worldwide threshold for the civil engineering works such as shielding structures against the x-rays, radioactive waste radiation or gamma radiation of the fuel in the spent fuel pools or other structures of nuclear power plants.

The opposite result was published by Vodak 3) where the cumulative dose 300 kGy caused 10 % decrement of the tensile strength of the concrete.

[HODNOTA] _{5,76 [HODNOTA]} 6,55 _{6,01 [HODNOTA] 6,42}

5,20 [HODNOTA] 0 2 4 6 8 10

3.2 MGy 3.0 MGy 2,0 MGy 1.3 MGy 0.8 MGy 0.7 MGy 0.5 MGy 0.3 MGy 0.0 MGy

Acknowledgment

Irradiation of concrete was financially supported by the Ministry of Education, Youth and Sport Czech Republic - project LQ1603 Research for SUSEN. Gamma irradiator was completed thanks to

the project SUSEN CZ.1.05/2.1.00/03.0108, CZ.02.1.01/0.0/0.0/15_008/0000293 and Infrastructure SUSEN LM2015093.

Concrete samples were fabricated within the project VI20152018016 from Security Research Pro-gramme 2015 - 2020 of the Ministry of the Interior of the Czech Republic.

Special thanks belong to Vit Rosnecky, an operator of the GOMK irradiation facility in Rez, who took care about the samples all the irradiation period.

References

1) M.F. Kaplan, Concrete Radiation Shielding, Harlow: Longman Scientific, 1989.

2) H.K. Hilsdorf, J. Kropp and H.J. Koch, “The Effects of Nuclear Radiation on the Mechanicals Properties of Concrete,” American Concrete Institute Special Publication, SP- 55, 223-251 (1978).

3) F. Vodák et alt, “Effect of γ-irradiation on strength of concrete for nuclear-safety structures,” Cement and Con-crete Research, 35, 1447-1451 (2005).