Fire Events Effect on Concrete Shielding of 60Co ... · PDF fileConcrete is the most important structural which is used in radiation ... the measuring instrument for qualifying the

Arab Journal of Nuclear Science and Applications, 45(4), 109-121 (2012)

109

Fire Events Effect on Concrete Shielding of 60Co Industrial Irradiation Facilities

Keshk, A. B.1 and Aly, R. A.2

1 - Radiation Engineering Department. National Center for Radiation Research and Technology, e-mail:[email protected]

2- Radiation Safety Department . National Center for Nuclear Safety and Radiation control, Atomicemail:[email protected]

Energy Authority, Egypt Received: 4/3/2010 Accepted: 2/8/2010

ABSTRACT

Concrete is the most important structural which is used in radiation shielding for owing to its low price and good shielding performance. In concrete structures, of industrial irradiation facilities cracks occur due to thermal stress, hydration he at, weather, dry storage radiation of sources and fire events nuclear reaction. Shielding performance is effected according to the crack size geometry (width, depth and bending). However, there are no design criteria providing the allowable crack size limits for concrete shielding. Three different methods are applied in this work wbere the measuring instrument for qualifying the effect of cracks on shielding performance in concrete mass. The correlation between crack size and concrete shielding performance is deduced. The surface dose rate increased logarithmically according to the increase in crack width. The results are compared by the effects of fire events inside industrial irradiation facilities "cold sterilization" which led to change the effected mechanical parts and electrical components. Radiation concrete shielding is repaired. Radiation standard measurements outside irradiation concrete facility recorded normal radiation values. Radiation safety is maintained.

Key Words: Concrete Shielding / Industrial Irradiation Facilities/ Concrete Events/

Thermal Cracks Size/ Instrument/ Shielding Performance and Exposure Doses.

INTRODUCTION

Concrete and shot Crete are used to construct the radiation shielding of reactors, nuclear power plants, dry storage and industrial irradiation concrete facilities (wet storages). In cobalt-60 industrial irradiation unit-radia tion sources (60Co pencils) arranged in a source rack which can be lifted into a radiation concrete design (radiation room) for processing and after wards lowered back into a safe position at the source is stationary and product boxes are tranced on a conveyor around the radiation source [6, 23].

Table (1) shows different thickness of radiation concrete shielding walls 60Co irradiation units

[14]. An irradiation room is surrounded by calculated concrete shielding to prevent radiation exposure of the operation and public. The concrete shielding thickness is determined on the basis of the maximum foreseen activity and the criterion that the radiation level outside the radiation room remain lower than the maximum permissible level. Very often it is significantly lower than radiation level and back ground natural radiation level [6,13]. Product boxes enter the radiation area through a specially designed maze and the personnel entrance is strictly controlled. Fig (1 (a+b)) [6,13 , 22].

Arab Journal Of Nuclear Science And Applications, 45(4), (109-121) 2012

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Table (1): Show different thickness of radiation concrete shielding walls for various 60Co irradiation units [20,21, 24].

Maximum dose rate (0.25 MR/Hour (2.5µSv/h)

Maximum radiation leakage on the basis that no person shall receive more than 10 mrcm in A 40 hour work week

Wall thickness

Concrete thickness-147 LB/FT3 (2350 KG/M3) occupancy Dose rate mR/Hour 300.000 Ci

12 terra Bq3 1.000.00 Ci

37.2 terra Bq3 2.000.000 Ci

47.4 terra Bq3 HRS/Week maximum average inches Meters inches inches meters inches

A 68 1.73 72 1.83 75 1.91 10 4.0 1.0 B 64 1.63 66 1.86 69 1.75 20 2.0 0.5 BB 62 1.57 66 1.86 69 1.75 10 4.0 1.0 C 30 0.76 33 0.84 36 0.91 6.7 6.0 1.5 D 27 0.69 30 0.76 33 0.84 6.7 6.0 1.5 E 64 1.63 38 1.73 71 1.80 0.1 300 100 F 27 0.69 30 0.76 32 0.81 2.5 16.0 4.0 G 21 0.53 22 0.56 23 0.58 2.5 16.0 4.0 H 64 1.63 68 1.73 71 1.80 0.1 300 100 Control 40 1.0 0.25

Fig. (1-a) Show plan of industrial irradiation facility and concrete shielding thicknesses

Fig.(1-b) Show 60Co industrial irradiation facility and concrete shielding for walls and roof.

The proportional of the final mix design to obtained 1 m3 of concrete is given in Table (2).


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Table (2): Concrete proportional mix design [1,16].

Concrete sample

Material Cement content

(Kg/m3 of concrete)

Fine aggregate (< 5 mm) (Kg/m3 of concrete)

Coarse aggregate (5-20 mm) (Kg/m3 of

concrete)

Mixing water (w/c = 40%)

(lit m3 of concrete)

Super plasticizer (sikament-163)

(lit/ m3 of concrete)

Ordinary 400 755 1125 1160 -- Barite 400 1230 1516 160 1.5 Ilmenite 400 1040 1930 160 2.1

The mechanical and physical characteristic of concrete shielding are shown in Tables (3) and (4) [1,16].

Table (3): Show mechanical characteristic of concrete shielding.

Type of concrete

Mechanical characteristic Compressive strength (ft)

MPa

Tensile strength (ft)

MPa

Flexural (bending)

strength (ft) MPa

Bond strength (ft)

MPa

Modulus of elasticity GPa

Ordinary 44 2.55 2.43 5.4 20.79 Barite 47 2.85 4.42 6.89 29.03 Limonite 51 3.53 5.4 6.87 34.43

Table (4): Show the physical characteristic of concrete shielding.

Type of concrete

Physical Characteristic Slump (according to

astm C-149 – 90a) mm. Density after 28 days

(Kg/mm3) Absorption % after 28 days (astm C 462-90)

Ordinary 100-120 2.35x10-6 3.75 Barite 50-70 3.25x10-6 2.8 Ilmenite 20-30 3.45x10-6 2.5

Cracks form when the stress on the concrete structure exceeds the characteristic strength of the structure [16]. According to a study conducted by CREIPI in Japan and Idaho National Laboratory [7], long-term exposure to high temperature can cause deterioration of concrete ,and as temperature rises, compression strength and tensile strength decrease [25].

According to the studies conducted in Japan [17], the USA [3], and the U.K. [5], it is estimated that

cracking width and the number of bends or jags in the concrete cracking pattern influence shielding performance, and especially surface dose rate.

Work Technique

The experimental work

The experiment work shows, a design for measuring instrument for quantifying the effect of cracks on performance of radiation concrete shielding (structure) which is presented in radiation engineering department N.C.R.R.T. The correlation between crack width and concrete shielding performance is deduced. Crack shapes are irregular in radiation safety aspect. The most conservative crack shape among various patterns is assumed. A comparison is done between the experiment results, the real effects of fire events which were recorded inside gamma irradiation facilities and the finial results after repearing.

MATERIAL AND METHODS


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Work Technique Build up factor-shielding calculation:

When gamma-ray photons strike a detector after passing through a medium, the intensity of the gamma ray is increased or decreased, according to the thickness or density of the medium. In the case of a concrete structure with cracking, because the density of air is different from that of the concrete, there is a difference in radiation intensity when the ray passes through concrete with air-filled cracks and concrete with no cracking [8, 11].

Gamma rays may reach the detector directly or after having scattered in the media as secondary

photon production. This result is an exponential attenuation for gamma rays [Eq. (1)]. [1,2 , 3]. t)(µ/ )e

yE(t, B

0II −= (1)

where I this intensity of radiation; I0 is the intensity of radiation before attenuation; B is the buildup factor; µ/ ? is the mass attenuation coefficient of medium ; ? is the density of medium; t is the thickness of medium; and Ey is the energy of gamma ray.The buildup factor is due to buildup of scattered photons from absorption of the original gamma rays. The buildup factor depends on the type of gamma-ray detector used as well as the geometry and type of medium. As such, it is difficult to experimentally derive B. The measuring system for removing or minimizing the effect of build up .The buildup effect from scattering and secondary photon emission can be reduced by proper beam collimation [18].

Measuring Design Radiation source and detector

60Co is employed as a gamma radiation source in these experiments, because 60Co is widely for nondestructive detection, strong activity through industrial irradiation facility. It is relatively easy to secure [19]. In addition, because it has remarkable penetration ability, it is estimated to be suitable for the present experiment, where thick concrete is considered as a sample. Moreover, as only two peaks, 1.17 and 1.33 MeV, are detected in 60Co data handling is relatively straightforward [11].A Nal scintillation detector is used. The detector offers relatively high output and smaller decay time [10]. Collimator

The collimator used in gamma ray and X-ray imaging in filters a stream of photons such the only those traveling, parallel to a specified direction are allowed through work system [18]. The collimator is composed of a hole for a narrow beam and an empty space for the source. Geometrical simulations by MCNI show that a hole distance over 10 cm is required to obtain high-quality narrow beams [9]. The collimator was fabricated from tungsten in the center and is surrounded by lead. In order to assess whether the narrow beam is transmitted to the detector through the crack in the concrete medium, a simulation was carried out with geometry tube (T) type and plane (?) type cracks in concrete, as shown in Fig.(2-a). Because tube type cracks in concrete play a role similar to that of a collimator, they can be employed as a standard of judgment regarding maintenance of the narrow beam to the detector. The geometry of the plane type crack is mode led as a real collinear crack as shown in Fig.(2-b).The results of the simulation for cases of a collimator hole diameter smaller or larger than the crack width are shown in Fig (3a) and Fig (3b), respectively. The crack width corresponds with the tube diameter or plane width, in the case of tube and plane type cracks, respectively. [1, 2].


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Fig. (2-a): Schematic diagram of experimental

principle.

Fig. (2-b): Tube type and plane type crack.

Fig. (3-a): Crack width (w) is < hole diameter (h).

Fig. (3-b): Crack width (w) is < hole

diameter (h).

Fig. (4): Simulation results are maintained by using two collimators to improve performance.

The simulation results for plane type cracks were similar to those of tube type cracks for a hole

diameter that is smaller than crack width. However, for a hole diameter larger than the crack width, it was difficult to differentiate between the plane type crack and tube type crack (Fig 3 b).As a result, to improve the accuracy of the system, the hole diameter in the collimator must be under 0.4 mm, However, fabrication of a collimator with hole under 1 mm in diameter with a transmission distance of


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10 cm distance is difficult. In this study, a second collimator was used in order to improve the performance of the system with holes over 1 mm diameter [4].

The hole distance (thickness) of the collimator for the detector was taken as 3 cm half value

layer. The result of a simulation with two collimators is shown in Fig (4).It show the hole thickness of the collimator for the detector which maintain simulation results by using two collimators for improving performance.

Positioning system

The positioning system is evaluated in this study stage. In order to improve the accuracy of the positioning system, step motors that move 1 mm with 1 µm error were employed (Fig.5). A remote control system was used for the positioning system.

Fig.(6):-Schematic diagram of the experimental

concrete sample

Experiments 2.2.1. Concrete sample :

The concrete sample is shown in Fig (6) as a medium for the collinear crack in the experiment. Experimental process

The crack width was increased from 0 to 2 mm. In order to accomplish this, the set-up was used. Detection was carried out by shifting one side to another by 1 mm increments, since the hole diameter is 1 mm. Detection was carried out vertically for five lines, and the average value was taken. Using a NaI scintillation detector, counting was carried out for 1 min of live time at 1.33 MeV. The experiment was carried out with concrete samples of 10, 15 and 20 cm thickness.

A collimator is usually a straight cylindrical duct and its is also a limiting case for shield-

penetration design. With an extended (plane) source cobalt60. There are three components of the flux density and dose at the detector: 1) line of sight component for radiation passing entirely through the duct (usually air with attenuation neglected). 2) a component from radiation penetrating part of the roof shield before entering the duct, and 3) primary gamma rays scattered from the duct wall or secondary gamma rays incident on the wall. The line of sight component can be calculated.


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RESULTS AND DISCUSSION The count rates of 10 cm thick sample are shown in Fig. (7).

Fig. (7): Variation of count rate for 10 cm thick concrete.

While the width of the sample crack increases to right, the samples right part is moved to right to increase the width and the peak of the graph shifts to right too. Different results for various samples thicknesses are shown in Figs (8,9). Figs (8) show total count rates for different thickness of concrete samples.

It is estimated that the variance of in the detection values from this experiment is due to the

difference in density of the concrete samples as a results of thermal effect. The determination of the correlation between the width of thermal crack and intensity revision of the density of each sample. The results after reviving the detection value according to the density are shown in Fig (9) This work stage proposes the following expression for the prediction of intensity according to the width of thermal crack in concrete shield as shown by dotted lines in Fig (9).

Fig. (8): Total count rate variation and intensity.

Fig. (9): Correlation between crack width for thick concerete.

This is the attenuation low for gamma rays in concrete shielding with a collinear crack.


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+−= 10

txb

log x µteoTI (3)

Where I (is) the intensity of radiation, Io is the intensity of radiation before attenuation, µ is the liner

attenuation coefficient of medium, t is the thickness of medium, x is the crack width (0-2.5 mm) and b is a constant that suggests a value of 140-150.

Results during normal operation (Industrial gamma units)

Cobalt-60 industrial irradiation facility consists of calculated concrete shielding for primary and secondary barriers (walls and ceiling for radiation protection and entries are through mazes for operators and products). It is constructed above ground from standard density concrete (2.3 g/c3) and it is ventilated by exhaust fan. The source rack holds cobalt-60 gamma source elements, in the form of source pencils. The radiation source is shielded when not in use by lowering it into a pool of water; it comprises that area delineated on Fig 1(a+b). Source activity = 1.000.000 curies= 37.2 pbq

Shielding calculation ( cobalt-60 industrial irradiation Unit): The concrete shielding is designed to reduce gamma radiation leakage level to outside from 12.3

pbq (0.3 mega curies) to 47.7 pbq (2.0 mega curies) to an average exposure rate of less than 2.5 µSv/h (0.25 m. rem/h). This allows a person to work near the shield 40 hours/week and under normal working conditions no person in the vicinity of the irradiator receive more than an average of 10.0 m.rem/week or a maximum of 500.0 m.rem/5 years (Safety series), No-115 Dose limits (20 m Sv/y)[3,14]. The exposure rate from 1 curie point source of cobalt-60 is 1.3x103 mrh and various inversely with the square of the distance. [13, 14].

Calculating the transmission of cobalt-60 gamma radiation in concrete after fire event depend

on calculating the maximum exposure rate outside the concrete shielding wall for a point source and correcting for source geometry, and absorption within the source plaque. [14].

2

D2D1

x x A E

shield ET

= (2)

Where

E shield = maximum exposure rate outside the primary shield E: The exposure rate from 1 curie point source of cobalt60 is 1.3x103 m.rem and varies inversely

with the square the distance. A source activity (60Co point source), D2 distance from source plane to exterior surface of primary shielding; D2 distance from source plane to interior surface of the primary shielding; T transmission of 60Co gamma rays in concrete; F self absorption and (F2) geometry factor for transforming point source calculation Fig.1(a+b).

Length x width of the source rack. = 0.65

Maximum exposure rate = mrh that is compared by the design level of 2.0 mrh maximum (wall shielding).Self absorption and geometry factor for vertical source plaque = 0.2 Maximum exposure rate that is compared by the design level of 15.0 mrh maximum (roof shielding).


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3.3.1. Measurement of maximum radiation field outside external concrete shield parallel to source plaque = 1.18 mrh.

Self absorption factor F1 and geometric factor F2 for transforming point source calculation for a

36.0 in (91.4 cm) x 60.0 in (152.4 cm) plaque source = 0.65 Measurement of maximum exposure rate outside shield = H H ,calculated exposure x (F1+F2) 0.77 = 1.18 x 0.65 Concrete shield thickness = 67.0 in (170.1 cm). Distance from source plane to exterior concrete shield surface = 139.0 in (3353.1 / cm). The need to determine the transmission of cobalt-60 gamma rays in concrete shield = T.

2

139

39.37 x 610 x 310 x 1.3

1.182

2D

1DA x x E

shield ET

==∴

T = 9x10-8

Calculation of Transmission of cobalt-60 gamma ray in (roof concrete shield).

Roof concrete thickness = 57.0 in (144.8 cm)

Assume point source to roof = 119.0 in (302.3 cm). Source activity = 1.000.000 curies = 37.2 pbq

Measurement of exposure rate due to point source of 1.000.000 Curies of cobalt60 = 27.4 mrh

Self absorption and geometry factor for vertical source plaque = 0.2 Maximum exposure rate = 5.5 mrh This is better than the roof shield design level of 15.0 mrh maximum

72.1x102

11939.27

x610 x 310 x 1.3

27.42T −==∴

Transmission of cobalt-60 gamma radiation in concrete are shown in Table (1) and Fig (10) [13,14].

Table (5): Show transmission of cobalt-60 gamma radiation in concrete thickness (before fire).

Concrete thick

Transmission Concrete thickness


Transmission

70 65 57 55 50

6x10-8 1.8x10-7

1.9x10-7 2.7x10-6 0.7x10-5

45 40 35 30 25

6x 10-5 0.8x10-4 1 x10-4 4.1x10-3 1.5x10-2

20 15 10 5

7x10-2

3.5x10-1

0.8x10 7x10

Fire Events: (inside industrial irradiation units).


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Fig.(10):- Transmission of 60Co industrial irradiation unit in concrete before and after fire

event.

In the fire event inside 60Co radiation unit [J.S. 6000] in Indonesia. The sound from audible alarm was ring, but it did not cut . It reflected that the radiation source rack did not trance to the safe position inside the storage pool because the source rack was stuck by jammed product boxes.The surfaces of concrete shielding exposed to heat are significantly affected. At fire surface the heat flow is caused by convection and radiation. In both the two fire events inside irradiation facilities the results cleared that great damages and bad effects were recorded by fire events especially on the concrete shielding roof where the high temperatures ranging of long duration which recorded bad effects on the properties of roof concrete shielding. [6, 13, 14]. Fire event inside 60Co radiation unit [J.S. 6500] in Egypt. The sound from audible alarm was ring and then stopped. It reflected that the radiation source rack trance to the safe position on the bottom of the storage pool of water at 5.5m.depth . When the main door was opened “ from outside under specific conditions.“. The operators noticed heavy smoke inside the first leg of the main maze and they could not resist fire inside irradiation room and they needed help from firemen. Table (6): Show effected parts which are changed after fire event in Js 6500, [6].

Part Job No Carton Box Electrical cable (meter) Rubber ring Stainless steel 200x1x0.1 Micro switch Almonium roal Fire detector Survey meter

Product boxes Electrical system cables Air pressure system Source shroud Electrical system Conveyor system Protection Radiation protection

60 200 30 2 50 25 1 1


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Results showed that concrete layers which cover the steel of reinforced concrete roof were felt

down inside irradiation room of each irradiation facility.

3.4.1. Effect of thermal crack on performance of concrete shielding.

The goal of this work was to estimate the effect of thermal crack width after any fire event on performance of concrete shielding of industrial irradiation facilities. It based on deduction of a correlation between intensity and thermal crack width. The surface dose rate increased logarithmically according to the increase in crack width and depth based on (the experiment work and the results of recorded fire events) The work suggests the attenuation equation for gamma ray in a concrete structure with collinear cracking. If the thickness of concrete shielding exceeds 20 cm and the thermal crack width is 0.4 mm, it is cleared that the effect on concrete shielding performance will be less than 10%. A crack width of 0.4 mm is the cererition for structural safety destined by [12].

3.4.2.Effect of thermal crack on concrete layers.

Because thermal crack for motion is irregular, and the falling of concrete layer which covered the

steel of reinforced roof concrete shielding by fire events, the work results can be applied to assess the real surface dose rate outside the concrete shield of 60Co irradiation facilities [2-20]. Table (7): The measurement of radiations levels outside and around the two radiation units during the two fire events.

Loc

atio

n

Sour

ce is

sto

ried

in

side

sto

rage

poo

l

Irradiation position During Fire event

J.S . 6000 J.S .6500 J.S . 6000 J.S . 6500

Source dose Source Dose Main door • 90-140 100-150

Stuc

k ja

mm

ing

and

"On"

90-14

"Dow

n"

10 Trench • 600 700 600 10 Box entry • 40 45 40 12 Box exit • 50 50 50 10 PI4 • 90 100 490 13 Under track • 900 1000 900 16 Conv.opening • 180 200 180 10

All radiation dose are micro roentgen/h

• Back ground

Table (8): Radiation doses outside J.S. 6500 radiation unit during fire event

Table (9): Radiation doses inside J.S.6500 radiation unit Sources was down

Location Radiation dose Location Radiation dose Main Door 10 Micro roentgen /h First maze leg 12 Micro roentgen /h Trench 15 Micro roentgen /h Middle maze leg 10 Micro roentgen /h Box entry 10 Micro roentgen /h End maze leg 12 Micro roentgen /h Box exit 10 Micro roentgen /h First of second leg 12 Micro roentgen /h PI3 15 Micro roentgen /h Middle second leg 12 Micro roentgen /h Under track 15 Micro roentgen /h End second leg 15 Micro roentgen /h Conv / opening 10 Micro roentgen /h


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Radiation doses which were recorded outside J.S. 6500 unit cleared that radiation source rack was storied in radiation safe position Table (7) and (9). The radiation levels (standards) in the three legs of the maze (main entrance) were recorded from 10 to 15 micro roentgen/h. because radiation source rack was storied in the safe position inside the storage pool Table (9).

They used to establish for radiation safety in concrete shielding especially to determine the

transmission of cobalt-60 gamma radiation in concrete shielding after fire events.Transmissions of cobalt-60 gamma rays in concrete after fire event are shown in Fig (10) and [2-20]. Table (10): Show transmission of cobalt-60 gamma radiation in concrete thickness inside

irradiation room after fire event.

Concrete thickness



Transmission

70 65 60 57 55

1.2x10-8 1.8x10-8

1.35x10-7

8x10-6 1.7x10-6

50 45 40 35 30

1.6x10-5

1.25x10-5

1.9x10-5

1.4x10-4

1.14x10-3

25 20 15 10 5

1.7x10-3

1.28x10-2

1.1x10-1

4x10-1 1.9x10-1

3.5 Work Results Lead to 1-If a calculated concrete room of an industrial irradiation unit by 60Co irradiator (cold sterilization)

was fired, the roof concrete shielding recorded the biggest bad effect depending on correlation between intensity and cracks sizes (width, depth and bending) and the surface dose rate increased logarithmically according to the increase in crack size. The concrete layer (shell ) that cover the steel of reinforced concrete roof (which located over the fired product boxes) were fell down inside irradiation room .

2-Performance of concrete shielding will be less than 10%. 3-The work results may be used to establish a new standards transmission of 60Co gamma rays in

concrete after repairing to maintain safety of concrete shielding.

CONCLUSION

The work estimate the effect of thermal crack size on concrete shielding performance of industrial cobalt-60 irradiation facilities depending on correlation between intensity and crack size (width, depth and bending).The surface dose rate increased logarithmically according to the increase in crack size. The work suggests the attenuation equation for gamma radiation in a concrete structure and the transmission of cobalt-60 gamma rays in concrete shells after fire events in such facilities. If the irradiation room of industrial irradiation facility was fired the roof concrete shielding recorded the biggest bad effect and the shielding performance will be less than 10%. The results may be used to establish a standard transmission to maintain radiation safety in concrete shielding of industrial 60cobalt irradiation facilities after repairing.


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Fire Events Effect on Concrete Shielding of 60Co ... · PDF fileConcrete is the most important structural which is used in radiation ... the measuring instrument for qualifying the