INTRODUCTION
Physical barriers designed to provide protection from the effects of ionizing radiation also, the technology of providing such protection is termed as radiation shielding. In most instances, protection of human life is the goal of radiation shielding. In other instances, protection may be required for structural materials which would otherwise be exposed to high-intensity radiation or for radiation-sensitive materials such as photographic film and certain electronic components. Ionizing radiation is widely used in industry and medicine, and can present a significant health hazard. Fundamental to radiation protection is the reduction of expected dose and the measurement of human dose uptake.
People and environment could be protected from radiation through three main methods namely time, distance and shielding. Heavy elements like lead or tungsten are ideal materials to be used in radiation shielding, however, those cannot be used in building construction and also Lead metal proved to be toxic. Its lethal effect became eminent. Many developed countries have banned lead usage in various applications. Seeking alternative material to replace lead is a crucial goal.
Over the past years a great deal of concern has been expressed about the toxicity of lead. Human lead toxicity in children as well as adults is well documented. There are also reports on the need for corrective measures due to corrosion of lead sheets when lead is used for structural shielding. These shocking facts steered researchers to look for alternative anti-radiation materials. Other materials such as steel in paraffin/poly-ethylene, hydrogen, silicon or carbon, boron and depleted uranium were proposed for anti-radiation protection. These materials are not easy to be processed, relatively expensive and not abundant; some others are expected to cause cancer like depleted uranium. Based on the above mentioned facts, production of cheap environment friendly non-toxic lead-free radiation shields which provide less weight compared to conventional lead-based shields remains a challenging issue in radiation protection.
Radiation shielding concrete, which can save space and reduce weight of the structure generally used to shield α-rays, β-rays, x-rays, γ-rays and neutron rays, is non-toxic and environmental friendly. Radiation shielding is mostly on γ-rays and neutron rays due to α-rays and β-rays' less penetration and easier absorption. Γ-ray is a kind of hertz wave with high energy, high frequency and huge penetrability, which could only be slowed down by building materials with high density. Neutron rays, mainly produced by nuclear reaction, have no charge but high penetrability. Radiation shielding concrete used for shielding γ-rays and neutrons should have large apparent density and contain a sufficient number of crystal water.
2. SOURCES AND HEALTH HAZARDS OF RADIATION
2.1 SOURCES OF RADIATION
• Medical sources: such as X-ray machines for diagnosing disease.
• Nuclear medicine: Which uses radioactive isotopes to diagnose and treat diseases such as cancer. .
• Industrial sources: Nuclear gauges used to build roads, density gauges that measure the flow of material through pipes in factories and for smoke detectors, some glow-in-the dark exit signs, and to estimate reserves in oil fields.
• Sterilization: This is done by using large, heavily shielded irradiators.
• Nuclear Fuel Cycle: Nuclear power plants (NPPs) use uranium to drive a chain reaction that produces steam, which in turn drives turbines to produce electricity.
• Uranium mines, fuel fabrication plants and radioactive waste facilities
• Exposure through ingestion: Once ingested, these minerals result in internal exposure to natural radiation.
• Exposure through inhalation: Radioactive gases that are produced by radioactive minerals found in soil and bedrock.
• Radon is largest source of natural radiation exposure, which is an odourless and colourless radioactive gas that is produced by the decay of uranium. Health risk not only to uranium miners but also to homeowners if it is left to collect in the home.
• Thoron is a radioactive gas produced by the thorium.
• Exposure from terrestrial radiation: The composition of the earth's crust is a major source of natural radiation. The main contributors are natural deposits of uranium, potassium and thorium which, in the process of natural decay, will release small amounts of ionizing radiation. Uranium and thorium are “ubiquitous”, meaning they are found essentially everywhere. Traces of these minerals are also found in building materials so exposure to natural radiation can occur from indoors as well as outdoors.
• Exposure from cosmic radiation: The earth's outer atmosphere is continually bombarded by cosmic radiation. Regions at higher altitudes receive more cosmic radiation.
• Atmospheric testing of atomic weapon.
2.2 TYPES AND CHARACTERISTICS OF RADIATION
There are two general classes of radiation. They are considered in the design of shields:
2.2.1) Electromagnetic waves
2.2.1.1) GAMMA
2.2.1.2) X RAYS
2.2.2)Nuclear particles
2.2.2.1) ALPHA
2.2.2.2) BETA
2.2.2.3) NEUTRON
2.2.2.4) PROTON
Of the electromagnetic waves, high energy, high frequency waves are known as x-and gamma rays are only types which require shielding for the protection of personnel. They are similar to light rays but of high energy and greater penetrating power. Gamma-rays have high power of penetration but can be adequately absorbed by an appropriate thickness of concrete shield.
A nuclear particle consists of nuclei of atoms or fragments thereof. They include neutrons, protons, alpha and beta particles. Of these all but the neutron possess an electric charge. Neutrons, on the other hand, are uncharged and continue unaffected by electrical fields, until they interact by collision with a nucleus. They have no definite range, and some will penetrate any shield.
Protons, alpha and beta particles carry electrical charges which interact with the electrical field, surrounding the atom of the shielding material and lose their energy considerably. They generally do not constitute a separate shielding problem, although accelerated protons at high energy levels may require heavy shielding comparable to that required for neutron
2.3 HEALTHHAZARDS OF RADIATION
2.3.1 Short-Term Health Effects of Radiation Exposure and Contamination
• Radiation sickness, known as acute radiation syndrome (ARS).
• Coetaneous Radiation Injury (CRI)
It happens when exposure to a large dose of radiation causes injury to the skin.
2.3.2 Long-Term Health Effects of Radiation Exposure and Contamination
• Cancer
People who receive high doses of radiation could have a greater risk of developing cancer later in life, depending on the level of radiation exposure.
• Prenatal Radiation Exposure
It is especially important that pregnant women follow instructions from emergency officials and seek medical attention as soon as emergency officials say it is safe to do so after a radiation emergency.
• Mental Health
Any emergency, including those involving radiation, can cause emotional and psychological distress.
3. RADIATION SHIELDING
Physical barriers designed to provide protection from the effects of ionizing radiation; also, the technology of providing such protectionis termed as radiation shielding. In most instances, protection of human life is the goal of radiation shielding. In other instances, protection may be required for structural materials which would otherwise be exposed to high-intensity radiation or for radiation-sensitive materials such as photographic film and certain electronic components. Ionizing radiation is widely used in industry and medicine, and can present a significant health hazard. Fundamental to radiation protection is the reduction of expected dose and the measurement of human dose uptake.
3.1. DIFFERENT METHOD FOR SHIELDING
3.1.1 LEAD LINING
Lead is a useful and common metal that has been used by humans for thousands of years. It is also a very dangerous poison, particularly for children, when it is accidentally inhaled or ingested.
FORMS OF LEAD USED FOR RADIATION SHIELDING
• Lead Sheet, Slab and Plate- Permanent shield installations
• Lead Shot- Where solid lead is impractical, due to location, shape, and accessibility
• Lead Wool- Filling deep cracks in a radiation barrier
• Lead Epoxy -In-the-field crack filling patching
• Lead Putty- Non-hardening, temporary seal or patch
• Lead Brick- Convenient, easily handled; may be moved and re-used
• Lead Pipe- Shielding of radioactive liquids
• Lead-lined/Lead-clad Pipe- Shielding of radioactive liquids
• Lead Powder- Dispersed in rubber or plastic for flexible shielding; also mixed with concrete and asbestos cement
• Lead Glass- Transparent Shielding
But, Lead metal proved to be toxic.Its lethal effect become eminent. Many countries have banned its use.Issues associated with lead use are long term disposal and the potential characterization as a mixed hazardous waste. Human lead toxicity in children as well as adults is also well documented. There are also reports on the need for corrective measures due to corrosion of lead sheets when lead is used for structural shielding. These shocking facts steered researchers to look for alternative anti-radiation materials.
3.1.2. TUNGSTEN- BRASS COMPOSITE
As density concerns, tungsten-brass composite is a good candidate for lead replacement. The tungsten (W) wt. % in these specimens was ranged from 50 to 80, the balance is brass. To evaluate the radiation shielding performance of these specimens, two gamma ray sources,137Cs and 60Co were utilized. The photon energy levels for these sources were of o.662MeV and 1.25MeV respectively. The measurements were performed using gamma spectrometer contains NaI (Tl) detector. The anti-radiation performance of the tungsten-brass was correlated to that of lead under similar conditions. Vickers micro hardness, relative sintered density, micro structural characterisation and linear attenuation coefficient (μ) were carried out. Samples with the highest Weight percentage of W has the highest hardness value while the one with the lowest Weight percentage of W. The linear attenuation coefficients of the specimens were significantly improved by increasing the W wt. % of the specimen. The linear attenuation coefficients of the tested specimens ranged from 0.85±0.010cm-1 to 1.12±0.049cm-1for 60Co and0.73±0.012 cm-1 to 0.97±0.027 cm-1 for 137Cs. This result indicates that W-brass composites are suitable material for lead replacement as a shielding barrier
Table 1: Tungsten versus lead: Head to Head Properties Comparison
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3.1.3. RUBBER COMPOSITE
The mixture of NR/NBRr (15 wt. % of recycled acrylonitrile-butadiene rubber (NBRr) were added to Natural rubber (NR) to prepare the composite’s matrix part.) was found to be the most appropriate formula for synthesizing light weight and high density rubber shield against radioactive radiations and high speed neutrons. The mixture generated better results than the traditional lead shields which are currently being used in radiotherapy industry. A mixture of rubber blends and pure iron particles can improve the shielding properties of rubber and makes rubber a good absorbent for radioactive radiations. The mixture can be used in medical units, nuclear stations, storage houses to prevent gamma rays and other radioactive particles from escaping into the environment, coating the walls and roofs of storage houses by this mixture can significantly reduce the risk. In order to make the manufacturing procedure cost effective and environment friendly, waste rubber can be used with natural rubber to increase the thickness and density of the shield. The samples prepared were found to be flexible and had high compressive strength. The homogenous composition made the samples impact proof.
3.1.4. RADIATION SHIELDING CONCRETE
The concrete which shields radiation is termed as a radiation shielding concrete. The density of normal concrete is in the order of about 2400 kg per cubic metre. To call the concrete, as high density radiation shielding concrete, it must have unit weight ranging from about 3360 kg per cubic metre to 3840 kg per cubic metre, which is about 50% higher than the unit weight of conventional concrete. They can however be produced with the densities up to about 5280 kg per cubic metre using iron as both fine and coarse aggregate.
The advent of the nuclear energy industry and medical field presents a considerable demand on the concrete technologists. Large scale production of penetrating radiation and radioactive materials, as a result of the use of nuclear reactors, particle accelerator, industrial radiography, and x ray, gamma-ray therapy, require the need of shielding material for the protection of operating personal against biological hazards of such radiation. Concrete which has high density and shielding properties are effective and economic construction material for permanent shielding purposes.
4 SHIELDING ABILITY OF CONCRETE
Concrete is an excellent shielding material that possesses the needed characteristics for both neutron and gamma-ray attenuation, has satisfactory mechanical properties and has a relatively low, initial as well as maintenance cost. Also the ease of construction makes concrete an especially suitable material for radiation shielding. Its only disadvantage is space and weight.
There are many aggregates whose specific gravity is more than 3.5 for making a heavy weight concrete which is capable of radiation shielding. Commercially employed aggregates are, barite, magnetite, ilmenite, limonite, hematite, etc. Steel and iron aggregates in the form of shots are also used. In determining which aggregate to be used, consideration should be given to availability of aggregates locally and their physical properties. In general, heavy-weight aggregates should be clean, strong, inert and relatively free from deleterious material which might impair the strength of concrete.
Since the capacity of various heavy aggregates to absorb gamma-rays is almost directly proportional to their density, also the heavier elements are more effective in absorbing fast neutrons by inelastic collisions than the lighter ones, as heavy aggregate as possible should be used for this purpose. However, density is not only the only factor to be considered in the selection of an aggregate for neutron concrete shield. The desired increase in hydrogen content, required to slow down fast neutrons, can be accomplished by the use of hydrous ores. The materials contain a high percentage of water of hydration. On heating the concrete, some of this fixed water in the aggregate may be lost. Limonite and goethite are available sources of hydrogen as long as shield temperature does not exceed 200 degree Celsius, whereas serpentine is good up to about 400 degree Celsius.
It has already been pointed out the effectiveness of radiation quality of concrete can be increased by increasing the density. Another important requirement of shielding concrete is its structural strength even at high temperature. To produce high density and high strength concrete, it is necessary to control the water cement ratio very strictly. Use of appropriate admixture and vibrators for good compaction are required to be employed. Good quality control be followed.
High density concrete used for shielding differs from normal weight concrete, in that it should contain sufficient material of light atomic weight, which produces hydrogen, serpentine aggregates are used sometimes, because of the ability to retain water of crystallisation at elevated temperature which assures a source of hydrogen, not necessarily available in all heavy weight aggregates.
High modulus of elasticity, low thermal expansion and low elastic and creep deformations are ideal properties for both conventional and high density concrete. High density concrete may contain high cement, in which case, it may exhibit increased creep and shrinkage. Because of the high density of aggregates, there will be a tendency for segregation. To avoid this, pre-placed aggregate method of concreting is adopted. Coarse aggregate may be consisting of only high density mineral aggregate and steel particles or only steel particles. Experiments have indicated that if only cubical pieces of steel or iron are used as coarse aggregate, the compressive strength will not exceed about 21 Mparegardless of the grout mixture or water cement ratio. If sheared reinforcing bars are used as aggregate, with good grout, normal strength will be produced. The grout used in high density preplaced aggregate concrete should be somewhat richer than that used in normal density preplaced concrete. Concreting practice with respect to mixing, transporting, placing as adopted for normal concrete may also be adopted to high density concrete but extra care must be taken with respect to segregation of heavier aggregates from rest of the ingredients. Wear and tear of mixer drum may be high. The form work is required to be made stronger to withstand higher load. Cognisance must also be taken to the strength development of concrete and the dead weight of concrete removing the form work.
4.1 ADVANTAGES OF RST OVER CONVENTIONAL CONCRETE
• High density
• Less space
• Good shielding
• Economical
• High thermal conductivity - minimize the build-up of heat
• Low coefficient of thermal expansion - minimize strains due to temperature gradients
• Low drying shrinkage -minimize differential strains.
4.2. INGREDIENTS FOR HIGH DENSITY SHIELDING CONCRETE
Most of material considerations for HSDC have physical and chemical property requirements which can be challenging to traditional mix design methods. Therefore careful evaluation of these issues is necessary both before and during use of the concretes and grouts. Designers and specifies of HDSC need to be aware that aggregate grading, will frequently fail to comply with more traditional specifications but high quality concrete can still be produced using these materials. It is generally appropriate to design HDSC mixes starting from basics of the mix characteristics in terms of aggregate/cement ratios and fines content, which will often appear to be extreme and unconventional. Water contents need to be minimized to prevent segregation and full use of super plasticisers is normally recommended (in order to achieve workable mixes), though magnetite has been used to produce self-compacting concrete.
The HSDC concrete developed in this research was required to be of sufficient high density to be of a special type needed to fulfil the purpose of neutron and gamma-rays shielding. Normal weight concrete would be too thick if it was considered for this purpose, which would result in an excessive shield size well beyond the space limitations available; it would also be uneconomical. Each identified ingredient used in the mix development had a certain role to play. The following materials were identified for use in this investigation:
• Ordinary Portland cement (OPC), CEM 52.5 N.
• Hematite (natural high density aggregate).
• Iron/steel shots (artificial high density aggregate).
• Portable water.
• Colemanite (boron containing aggregate).
• Galena (natural high density Lead containing aggregate).
• Admixture(water reducing admixture consisting of lignosulfonic acid, carboxylic acid)
The aggregates were divided into two categories consisting of high density aggregates which produce HDSC, attenuate (absorbs) photons (gamma-rays) and scatters neutrons (change the energy from fast to thermal), and the boron containing aggregate that attenuates thermal neutrons.
4.3. EFFECT OF TEMPERATURE ON SHIELDING CONCRETE
Temperature plays an important role in the use of concrete for shielding nuclear reactors.Apart from the general structural requirements, heavy weight radiation shielding concrete should also be capable to maintain its structural integrity and effectiveness as a biological shield over a period of 50 years. Attenuation of radiation results in a rise of temperature of the shielding concrete, as the absorbed energy is converted into heat. Since the energy of absorption, and therefore the heat, varies in an inverse exponential relationship with the distance, the greatest amount of heat is generated in the part of the shield closest to the source of radiation. In addition to the above, the inner face of the concrete shield is often exposed to the direct heat from the reactor core. Concrete has relatively low thermal conductivity, which makes it difficult to remove the heat generated in the shield. As a result, the temperature distribution throughout concrete is non-uniform and the differential thermal stresses arise. To avoid local damage or even, in the extreme case, a structural failure, it is necessary to establish a relationship between the maximum incident energy flux and the allowable, differential compressive and tensile stresses in concrete. For example, (Thomas, D. R., 1965) a 1370 mm thick reinforced concrete shield was capable to resist, without any apparent damage, the incident energy flux of 23g-cal/hr cm2, which resulted in a temperature rise of 520 C. The magnitude of the temperature rise seems to be practically independent of the nature of radiation, be it gamma rays or neutrons. However, without the reinforcement a flux of only 2.8g-cal/hr cm² produced a temperature rise of 8.90 C leading to cracking of the outer concrete surface. A temperature rise of about 650 C produced internal compressive stresses of the order of 7 MPa in this particular shielding concrete. The permissible internal stresses in a concrete shield should always be as low as practically possible, as it is important to insure that no local cracking or deterioration takes place. The desirable properties of radiation shielding concrete are: high thermal conductivity to minimise the build-up of heat, low coefficient of thermal expansion to minimise strains due to temperature gradients, and low drying shrinkage to minimise differential strains. The coefficient of thermal expansion should be as close as possible to that of the reinforcing steel and steel inserts, again to minimise the differential strain
5. ASSESSING AGGREGATES FOR SHIELDING CONCRETE
Aggregate classesASTM C638 provides two classes of aggregates for use in RSC: Class 1 (gamma ray shielding) and Class 2 (neutron shielding)
Figure 1: Different types of aggregates for use in radiation-shielding concrete: (a) Iron ore aggregate (b) Ilmenite aggregate (c) Colemanite aggregate and (d) Metallic slag aggregate.
Table 2: Properties of common naturally occurring high-density aggregates used in RSC
Name | Mohs’s hardness of pure mineral | Properties |
Hematite | 5.5 and 6.5 | Physical properties of rocks may vary considerately. Some are relatively soft and brittle and produce dust in the course of being handled. Some hematite rocks tend to be flaky. |
Limonite | 5.0 to 6.0 | Massive ilmenite deposits can form coarsely crystalline, massive, tough rocks but vary from deposit to deposit. |
Goethite and limonite | 5.0 to 5.5 (goethite) 4.0 to 5.5 (limonite) | Deposit range from hard tough massive rocks to soft crumbling earths. |
Magnetite | 5.5 to 6.5 | Deposit can comprise dense, tough, usually coarse-grained rocks .The crushed aggregate particles may be angular and sharp. |
Barite | 2.5 to 3.5 | The ore contains a large proportion of relatively soft barite particles that may contain open cracks and cleave readily |
*Based on ASTM C638
The aggregates used in RSC should be relatively clean, free of deleterious materials, and chemically inert. Accurate identification and evaluation of these deleterious materials is often the most critical part of petro graphic examination
Table 3: Potentially deleterious materials in aggregates used in RSC
Deleterious materials | Mineral deposits* | Implications** |
Clays | Dry-processed barite and borates frequently contain clays. Goethite and limonite may contain clays and are also likely to be friable so that they may produce considerable amounts of fines. Some sedimentary iron ore aggregates may contains clays. | Clay raises the amount of water need for consistent workability. |
Gypsum, anhydrite, and other sulphate salts | Barite may be associated with anhydrites or gypsum. Gypsum and sulphate salts are found in borate deposits. | Either gypsum or anhydrites in excessive amount can produce false set in freshly mixed concrete. Also, sulphate can react after the concrete has hardened, causing expansion and cracking. |
ASR-reactive constituents | Sedimentary iron ores may contain chert, fine grained/microcrystalline quartz, or a mixture. Some iron-bearing ores of igneous and metamorphic origin may contain a reactive form of silica | ASR-a reaction between unstable silica in aggregates and alkali hydroxide (sodium and potassium from the cement) in the cement paste-can cause expansion and ultimately cracking in hardened concrete. |
Organic impurities | Origin of majority of heavy aggregates makes presence of organic impurities unlikely. However, presence of any organic impurities should be checked. | Organic impurities may interfere with the setting and hardening characteristics of cement. |
*Based on ASTM C638.
**Based on ASTM C33, ASTM C294, and ASTM C295.
Table 4: Common natural Class 1 aggregates for gamma-ray shielding (based on ASTM C638)
Name | Most common sources* | Description |
Hematite | South America, Africa | Most large hematite ore deposits are sourced from altered banded sedimentary formations and rarely from igneous accumulations. Banded formations may contain iron in carbonate or silicates. The impurities associated with hematite include non-ore bedrock and gangue minerals. Sources vary (between and with deposits) in toughness, compactness, amount of impurities, degree of weathering, and suitability for use as a concrete aggregate. |
Ilmenite | Quebec | Ilmenite deposits can comprise coarsely crystalline, massive, tough rocks. Many deposits consist of limonite disseminated in rock rather than concentrated as a major rock-forming mineral. Common impurities include constituents of the associated gabbroic or anorthostic rocks. Sources vary (between and within deposits) in composition, hardness, and suitability for use as concrete aggregate. |
Goethite | Utah, Michigan | Goethite occurs in sedimentary conditions or forms as a primary minerals in hydrothermal deposits. The deposits vary from hard, tough, massive rocks to soft, crumbling earths; these alternations frequently occur within the fractions of an inch. |
Limonite | Utah, Michigan | Limonite is the generic name for hydrous iron oxides of unknown composition; frequently goethite and probably mixture of goethite and hematite. Limonite of high iron content is also called brown iron ores. Frequently, they contain sand ,colloidal silica, clay, and other impurities |
Magnetite | Nevada, Wyoming, Montana | Magnetite ore deposits are associated with metamorphic, igneous, or sedimentary rocks, also in association with hematite and ilmenite. Deposits can form dense, tough, usually coarse-grained rocks. The impurities associated with magnetite may include a wide variety of rock-forming and accessory minerals. |
Barite | Nevada, Tennessee | Barite, also known as barite, occurs in veins transecting many kinds of rocks, concentrated in sedimentary rocks, and as residual nodules in clays formed by the solution of sedimentary rocks. |
*Based on Table 11.1 of ACI 304R. Other sources may be available.
Table 5: Common class 2 aggregates for neutron shielding (based on ASTM C638)
Name | Most common source | Descriptions |
Colemanite | California | Found in evaporate deposits of alkaline lacustrine environments. Common associated minerals include ulexite and other boron minerals, gypsum, calcite, and Celestine.⁴ |
Borocalcite | Turkey | The borocalcite refers to Turkish borate ores, which are probably ulexite or colemanite or mixtures of the two (ASTM C638). Ulexite is found in evaporate deposits in arid regions; it is frequently associated with colemanite and other boron minerals, glauberite, trona, mirabilite, gypsum, and halite.⁴ |
*Gerstley Borate (U.S Borax Inc.) has been historically referenced as a common source of Class 2 aggregates; however, the supply has been recently discontinued
5.1AGGREGATES AND ITS PERFORMANCE (Based on various study results)
Few studies have been carried out earlier on the radiation shielding of concrete utilizing normal and heavyweight aggregates.
Table 6: Aggregates and its performance
SL. NO | SCEINTIST | YEAR | AGGREGATE | RESULT |
1 | Gancel | 2011 | Hematite | • No affection neutrons absorption capacity • Gamma ray attenuation capability and mechanical strength |
2 | Mesbahi | 2011 | Magnetite, Datolite-Galena, Magnetite-Steel, Limonite-Steel, Serpentine. | • Efficiency strongly depends or composition of concrete |
3 | Basyigit& Yilmaz | 2011 | Limonite, Siderite (mineral origin) Compared with mineral and non-mineral aggregates. | • Mineral origin are efficient |
4 | Akkurt | 2010 | Zeolite (at different concentration) | • Concentration increases. No affection shielding perhaps it reduces |
5 | Gencel | (2010 a,b ) | Colemanite | • Up to 30% Colemanite is recommended as best • To achieve a high slump-super plasticiser • To improve the setting time- accelerator(free from chlorides due to high % of steel shots) |
6 | Korkut | 2012 | Colemanite&penclotite rock | • Reduction of wall thickness by 25 cm |
7 | Sharma | 2009 | Fibre reinforced concrete (steel & lead fibres and combination of two (hybrid fibre ) | • Significant enhancement in mechanical and shielding properties |
8 | Kharita | 2009 | Carbon powder on hematite aggregate | • Reduce shielding but 15% increase in strength |
9 | Kharita, Rezali-Ochbelagh | 2009 & 2012 | Lead powder and silica fumes | • 15% silica fumes with lead can be used as gamma shield |
10 | Sayala | Gamma-Guard™ & Neutron-Guard™ | • 74 X neutron shielding & 35 X gamma shielding and hence thickness is also reduced by same amount |
As stated earlier, few studies were conducted to develop radiation shielding concrete; however, there is a need to develop such concrete utilizing cheaper materials, preferably industrial by-products.
6. SHIELDING CONCRETE WITH HEMETITE AS COARSE- AGGREGATE
Heavy weight iron ore is used as the main ingredient of the high density radiation shielding. High density concrete can be made from natural heavy weight aggregates are commonly used having specific gravity ranging from barites (2.5 - 3.5), magnetite (3.5 - 4.0) and hematite (4.0 – 4.5) occasionally. By using iron as a replacement for the portion of either coarse aggregate or fine aggregate, give even greater densities of 5900kg/m³.
Table 7: Physical properties of Hematite
Sl.No | Properties | Results |
1 | Specific gravity | 4.3 |
2 | Bulk density | 2300 kg/m³ |
3 | Particle shape | Angular |
4 | Particle size | 20 mm |
5 | Colour | Reddish |
6 | Water absorption | 3% |
7 | Crushing value | 12.55 |
8 | Impact value | 12.41 |
7. VARIOUS TESTS CONDUCTED
7.1 CHEMICAL ANALYSIS OF AGGREGATES
Small samples were obtained from the identified suppliers and tested for chemical compositions. The purpose of this testing was to ensure that ingredients that could become radioactive due to elements that have long decaying half-lives (i.e. Cobalt, Copper, Nickel, Zinc etc.) were not significantly present in the concrete mix. These tests were also used to confirm the guarantees presented on the suppliers’ product data sheets. The chemical composition analyses of aggregates were conducted using ICP (Inductively Coupled Plasma) and XRF (X-ray Fluorescence methods). It was confirmed that none of the selected aggregates for mix design of HDSC had long half-life elements.
7.2 UNIT WEIGHT
The probability of an incoming photon interacting with a given material per unit path length is usually represented by the linear attenuation (also called linear attenuation coefficient) clearly pertinent for radiation shielding. The attenuation depends on the density of the material. Thus, unit weight of concretes is important. We have determined unit weights and present the results in Fig. 2. Since hematite has higher density than plain concrete, addition of hematite increases the unit weight as expected but also desired result. The higher the density, the smaller the thickness of concrete is required to provide radiation shielding.
7.3 COMPRESSIVE STRENGTH
There is no need to argue that the compressive strength is the most important property of concrete. It was expected that addition of hematite – a material with higher density and higher hardness than cement – will increase the compressive strength. The results are presented in Fig. 2. We see in Fig. 2 that hematite increases the compressive strength of plain concrete for 10 % hematite and only slightly for 20 % hematite. The reason behind it may be the porosity of hematite. The more hematite we have the more pores inside of hematite regions will appear. Using plain concrete again as the reference, changes in the compressive strength are: 4.33 % for H10, 0.48 % for H20, –1.77 % for H30, –2.57 % for H40 and –2.41 % for H50.
Fig.2 Comparison of unit weight compressive and splitting tensile strengths of hardened concretes
7.4 SPLITTING TENSILE STRENGTH
The results are presented in Fig. 2. Effects of hematite addition are not large. Aggregate quality rather than mortar matrix is important for the splitting tensile strength test results. At 10 % hematite, splitting strength is lower with respect to plain concrete because of larger voids between aggregates. Splitting strength is increasing because gap between aggregates is decreasing at 20 % – 30 % replaced of hematite. At 40 % – 50 % hematite, even though gaps between aggregates are smaller, splitting strength values are significantly reduced because more weak points appear – due to oxidation at mortar-aggregate interfaces. Splitting strength with respect to PC decreases as follows: 8.78 % for H10, 4.26 % for H20, 3.52 % for H30, 35.35 % for H40 and 34.97 % for H50.
7.5 ELASTIC MODULUS
The modulus of elasticity, E values were determined after 28 days. A strain-gage with the sensitivity of 0.002 designed for cylindrical specimens was used. E modulus was obtained from σ (ε) curves. The results are shown in Fig. 3. E modulus as a function of concentration of hematite behaves similarly to compressive strength when we use the CEB method. However, the ACI method provides opposite results. This is due to the fact that the ACI method takes into account compressive strength as well as the unit weight. We note differences between the three methods used. Within each method, the effects of hematite addition are not large. In the first method the range of E values is between 43 GPa and 48 GPa.
Fig.3 Variations of concrete E modulus with hematite content
7.6 PULSE VELOCITY
The experimental results of pulse velocity for different types of concrete are presented in Fig. 4. Pulse velocity values as a function of concentration of hematite increase due to porosity of hematite; the effect is smaller than for Cst-I, Cst-II or NSR. As seen in Fig. 4, pulse velocity values range from 4600 m/s to 5100 m/s. The PC had the highest value and addition of hematite decreases the velocity. Long ago Whitehurst classified the concretes as excellent, good, doubtful, poor and very poor for pulse velocity values of 4500 m/s and above, 3500 – 4500, 3000 – 3500, 2000 – 3000, and 2000 m/s, respectively. Thus, all our concretes produced are excellent according to the Whitehurst classification. 4.5. Schmidt hardness The Schmidt hardness test is a popular non-destructive method. A uniform compressive stress of 2.5 MPa is applied to the test specimen along the vertical direction (the same as the casting direction) before striking it with a hammer; this prevents dissipation of the hammer striking energy due to lateral movement of the specimen.
7.7 SCHMIDT HARDNESS
The Schmidt hardness test is a popular non-destructive method. A uniform compressive stress of 2.5 MPa is applied to the test specimen along the vertical direction (the same as the casting direction) before striking it with a hammer; this prevents dissipation of the hammer striking energy due to lateral movement of the specimen.
Striking points were uniformly distributed to reduce the influence of local aggregates distribution and averages of the rebound energy calculated. The results are presented in Fig. 4. Schmidt hardness is a method related to compressive behaviour since it is based on the rebound ratio from surfaces of samples. Therefore, similar behaviour is expected as in Fig. 2. Fig. 4 shows similar behaviour patterns as Figure 2, except for H10 in Figure 2. Thus, the Schmidt hardness values decrease when hematite is added to the PC. The effects are small.
Fig.4. Comparison of Schmidt hardness and Pulse velocity results of concretes
7.8 FREEZE—THAW DURABILITY
Micro-cracks mainly exist at cement paste-aggregate interfaces within concrete even prior to any loading and environmental effects. When the number of freeze-thaw cycles (FTCs) increases, the degree of saturation in pore structures increases by sucking in water near the concrete surface during the thawing process at temperatures above 0 °C. Some of the pore structures are filled fully with water. Below the freezing point of those pores, the volume increase of ice causes tension in the surrounding concrete. If the tensile stress exceeds the tensile strength of concrete, micro-cracks occur. By continuing FTCs, more water can penetrate the existing cracks during thawing, causing higher expansion and more cracks during freezing. The load carrying area will decrease with the initiation and growth of every new crack. Necessarily the compressive strength will decrease with FTCs . The results of Freeze-thaw durability tests are presented in Fig. 5.
We see in Fig. 5 that all concrete types had lost strength in cycling. However, for hematite containing materials the losses are lower. Apparently, specimens containing hematite absorb less water and are thus less affected by FTCs. The strength loss for PC amounts to 21.3 % while for the H10 composite only 7.8 %. Still, the effect in pure PC is acceptable according to the ASTM C 666 code.
Fig.5 Freeze-thaw durability of concretes
7.9 SHRINKAGE
Drying shrinkage is by far the major portion of volume change of concrete. Although several types of volume change due to moisture movement can occur in concrete, volume change due to drying shrinkage is particularly important in radiation-shielding concrete. Stresses resulting from drying shrinkage cause cracking. Although cracking tends to be largely a surface effect, large cracks could affect the effectiveness of the radiation 255 Fig. 6. Shrinkages of concretes shield while extensive micro cracking would reduce the effective density of the shield. We have obtained the appropriate results which are presented in Fig. 6. We see in Fig. 6 that after 15 days or so the drying process is largely completed. And by dependent on hematite content in concrete, shrinkage has decreased as PC (0.43) > H10 (0.10) >> H20 (0.09) > H30 (0.08) > H40 (0.07) > H50 (0.06). Finally, we recall we have not used a plasticizer – which would reduce the volume of voids and thus also enhance the unit weight and other desirable properties.
Fig.6 Shrinkages of concretes
8. CONCRETE MIX DESIGN
Table 8: concrete mix design
TM1 | TM2 | TM3 | TM4 | TM5 | TM6 | TM7 | TM8 | ||
Ingredients mass % | CEM I 52.5N-PPC Water Hematite Stone Hematite Sand Steel shots Colemanite Super plasticisers 1 Super plasticisers 2 Accelerator Silica fume High alumina cement W/C | 8.75 4.38 46.02 21.77 19.08 - - - - - - 0.5 | 8.98 4.03 44.89 21.39 18.62 2.30 - - - - - 0.45 | 8.48 4.24 43.58 20.97 18.37 4.36 - - - - - 0.5 | 8.07 4.86 37.22 19.65 27.92 2.07 0.06 - 0.16 - - 0.5 | 8.07 4.86 33.52 23.38 27.93 2.07 0.05 - 0.12 - - 0.6 | 10.39 4.41 28.96 19.73 35.77 2.31 0.27 - 0.36 - - 0.42 | 7.96 4.41 28.96 19.73 35.77 2.31 0.17 0.14 - 0.69 1.73 0.42 | 7.88 4.36 28.66 19.53 35.40 2.28 0.12 0.05 0.19 0.69 - 0.51 |
Result | Density (kg/m³) Height (mm) Slump Spread (mm) Cohesion 7day strength (Mpa) 28days strength(Mpa) | 4514 50 - Poor 39.35 54 | 4421 NIL - Poor 2.64 41.1 | 4071 NIL - Poor 12.6 33.8 | 4287 10 - Poor - - | 4292 25 - Good - - | 4372 190 - Good 2.6 38.9 | 4220 210 530 Good 20 48 | 4231 230 510 Good 2.51 29.94 |
The final mix design of the high density shielding concrete was workable and cohesive with average 28- day compressive cube strength of 30 MPa, water to cement ratio of 0.51 and density of 4231 kg/m3. The concrete had a high slump with a height and spread of 230 mm and 510 mm respectively. The main special aggregates used in the mix were hematite, steel shots and colemanite. It was observed that colemanite had a strong effect of retarding the setting of concrete. The retardation could be offset by use of high alumina cement; however, consideration should be given to potential conversion of concrete as a result of using high alumina cement. It may be appropriate to avoid using high alumina cement in shielding concrete and instead compensate for set retardation by allowing a long period of setting before theremoval of formwork.
9. PLACING AND CURING
Transporting can be done by dumber or conventional truck mixers on a reduced volume basis and commensurate with density. This affects costs and pours times and small volume may necessitate mixer drums to be pre-grouted. Place using skips, funnels or tremie tubes depending on access and allow for reduced volume in skips. Smaller volumes, for example lead shot, will need to be transported in very small volumes (consider that a standard 10 litre bucket will weigh nearly 90 kg).
Barites and iron oxide mixes can be designed for pumping, even over some considerable horizontal and vertical distances. It is recommended that pumping contractor is made aware of any requirement to pump these materials. Chilcon with natural sand fine aggregate has been pumped with difficulty. Mixes containing iron aggregate should not be pumped as damage to pumps is likely to occur. Placing conventional poker vibrators, generally a large size such as 76.2 mm are used
CONCLUSION
The innovative technologies (enhanced radiation shielding concrete + rubber/tungsten composite) discussed in this seminar have unexpected radiation-shielding capacities for application to various fields such as medical, nuclear and industries (LINAC, X-ray, and PET radiation diagnostic and treatment units and facilities). The enhanced radiation-shielding capacities of these innovative technologies mean that it can be engineered and constructed with thinner walls to meet the same shielding requirements as the conventional technologies. The innovative technologies can offer significant cost savings compared with the costs of conventional concrete and lead-shielding technology products .Furthermore, construction of thinner walls using the innovative technology, can offer considerable space savings. Bearing in mind the ever-increasing cost of real estate, significant cost savings can be realized by constructing the thinner technology-enhanced walls. Considering these two categories of cost savings, the innovative radiation-shielding wall technologies are a viable and cost-effective alternative for the building construction.
REFERENCES
[1] Miller E., High density and radiation shielding concrete and grout, advanced concrete Technology – Processes Chapter 5, Newman & Seng Choo, Elsevier Press., 2003.
[2] Kaplan M.F., Concrete radiation shielding: nuclear physics, concrete properties, design and construction. United Kingdom: Longman group.
[3] Gencel O., Brostow W., Ozel C. and Filiz M. (2010) an investigation on concrete properties containing colemanite-International journal of physical science- Volume 5(3), pp. 216-225.
[4] Kharita MH, Takeyeddin, M, Alnassar M. and Yousef S. (2007), Development of special radiation shielding concrete using natural local materials and evaluation of their shielding characteristics. Progress in nuclear energy –volume 50, pp.33-36, Elsevier.
[5] Mahdy M, Speare, PRS and Abdel-Reheem, AH. (2002) Effect of transient high temperature on heavyweight, high strength concrete. 15th ASCE engineering mechanics conference, June 2-5. New York: University of Columbia.
[6]Dash Sayala, PhDPresident, Science and Technology Applications, Virginia, and Associate, Vulcan Global Manufacturing Solutions, Wisconsin
[7]Applied Mechanics and Materials Vols. 253-255 (2013) pp 303-307 © (2013) Trans Tech Publications, Switzerland
[8] International Journal of Science and Research (IJSR) Volume 4 Issue 3, March 2015
[9] Applied mechanics and materials volume 679(2014) pp 39-44 Trans Tech publications Switzerland
[10]International Nuclear Science and Technology Conference 2014 (INST2014) (IOP Publishing), Journal of Physics: Conference Series 611 (2015) 012019, material plastic, 51, volume No. 3 2014
[11] Concretes Containing Hematite for Use as Shielding Barriers
Osman GENCEL 1, 2 ∗, Witold BROSTOW 2, Cengiz OZEL 3, Mümin FILIZ -3ISSN 1392–1320 MATERIALS SCIENCE (MEDŽIAGOTYRA). Vol. 16, No. 3. 2010
