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
|
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
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
