Neutron Shielding Properties of Some Vermiculite-Loaded New Samples

Nuclear reactor technology is known as an emerging area of study from past to present. It is an implementation of the nuclear sciences including reactions about atomic nucleus and productions. During the construction of a nuclear reactor, the most important issue is nuclear safety. The term of security can be attributed radiation shielding processes. For nuclear reactors, there are several different materials used to radiation shielding. While determining the most appropriate material to shield, the type and energy of radiation is extremely important.


Introduction
Nuclear reactor technology is known as an emerging area of study from past to present.It is an implementation of the nuclear sciences including reactions about atomic nucleus and productions.During the construction of a nuclear reactor, the most important issue is nuclear safety.The term of security can be attributed radiation shielding processes.For nuclear reactors, there are several different materials used to radiation shielding.While determining the most appropriate material to shield, the type and energy of radiation is extremely important.
There are two types of nuclear reactions reveals very large energies.These are the disintegration of atomic nuclei (fission) and merging small atomic nuclei (fusion) reactions.Therefore, nuclear reactors can be divided into two groups according to the type of reaction occurred during as fission reactors and fusion reactors.Currently a nuclear reactor working with fusion reactions is not available.Today, there are the hundreds of nuclear reactors based on the fission reactions.For the realization of nuclear fission, a large fissile atomic nucleus such as 235 U can absorb a neutron particle.At the end of nuclear fission event, fission products (two or more light nucleus, kinetic energy, gamma radiation and free neutrons) arise.Fission reactions are controls by using neutron attenuators such as heavy water, cadmium, graphite, beryllium and several hydrocarbons.While designing a reactor shield materials against gamma and neutron radiations should be used.
Vermiculite is a monoclinic-prismatic crystal mineral including Al 2 O 3 , H 2 O, MgO, FeO and SiO 2 .It is used in heat applications, as soil conditioner, as loose-fill insulation, as absorber package material and lightweight aggregate for plaster etc.Its chemical formula is known as (MgFe,Al) 3 (Al,Si) 4 O 10 (OH) 2 •4H 2 O and physical density of it about 2.5 g.cm -3 .Melting point of vermiculite is above 1350 0 C.This mineral can be used as additive building material in terms of mineral properties.
Vermiculite is a component of the phyllosilicate or sheet silicate group of minerals.It has high-level exfoliation property.So if vermiculite is heated, it expands to many times its original volume.This feature is a striking ability for a mineral.Because it looks like vermicularis, its name is vermiculite.They molecular structure consists of two tetrahedral layers as silica and alumina and an octahedral layer including O, Mg, Fe and hydroxyl molecules.Water located between layers is an important member in the vermiculite.If mineral heats suddenly, inter-layer water transforms to steam and exfoliation feature occurs.
Vermiculite is clean to handle, odorless and mould durable.It has a wide range of uses as thermal insulation, fire durability, liquid absorption capability, low density and usefulness etc… The main uses of minerals are listed as follows; i. Construction Industry (lightweight concretes, vermiculite-loaded plasters, loosefill insulation) ii.Animal Feedstuff Industry iii.Industrial Insulation for High Temperatures (up to 1100 0 C) iv.Automotive Industry v. Packaging Materials vi.Horticulture In literature, there are several studies about vermiculite and its usage.A comparative study about the effects of grinding and ultrasonic treatment on vermiculite was done.The effect of mechanical treatment and cation type on the clay micro porosity of the Santa Olalla vermiculite untreated and mechanically treated (sonicated and ground) and saturated with different captions was investigated.An experimental study was performed about thermal conductivity of expanded vermiculite based samples.In this paper, measurements were carried out on samples in the temperature range of 300-1100 K.Because of the high heat insulation properties of vermiculite mineral, we frequently encounter studies on thermal properties of it.In another study, researchers produced new materials including vermiculite that can withstand up to 1150 0 C. The cement-vermiculite composition was used to produce new materials to leach 54 Mn and 89 Sr radionuclide.Researchers have received the best results when using 95% Portland cement and 5% vermiculite composition.The effect on ultrasounds on natural macroscopic vermiculite flakes has been studied and effects of ultrasound treatment on the several parameters (particle sizes, crystal structure, surface area, etc...) were investigated.Finally, micron and submicron-sized vermiculites were prepared.High surface area silica was obtained by selectively leaching vermiculite.In this study, also the characteristics of the porous silica obtained from vermiculite are compared with those from other clay minerals.Studies are performed on the material composition and typical characteristics of micaceous minerals of the vermiculite series in the Tebinbulak deposit.Thermal treatments of nano-layered vermiculite samples were studied up to 900 0 C.In another thermal effect study was achieved for 15-800 0 C temperature range vermiculite originated by Tanzania region.Thermal properties of polypropylene-vermiculite composites were investigated using differential scanning calorimetry (DSC) and thermogravimetry (TG) techniques.At the end of this study, new composites with high thermal stability were produced.The effect of sodium ion exchange on the properties of vermiculite was studied by several methodology (scanning electron microscopy, X-ray fluorescence spectroscopy, inductively coupled plasma mass spectroscopy, X-ray diffraction and thermo mechanical analysis) and sodium exchange lowered the exfoliation onset temperature to below 300 °C.A comparative study about oil affinity of expanded and hydrophobized vermiculite was done.According to the results of this study, the expanded vermiculite had a greater affinity for oil than hydrophobized vermiculite.XRD characteristics of Poland vermiculites were studied and crystal structure of it was determined.A general study about typical properties and some parameters of different vermiculites was performed.In this study, especially heat conduction coefficients were commented and an evaluation was done about vermiculite based building materials.In another study, flyash-based fibre-reinforced hybrid phenolic composites filled with vermiculite were fabricated and characterized for their physical, thermal, mechanical and tribological performance.
Neutron shielding studies have a wide range of literature.For example, colemanite and epoxy resin mixtures have been prepared for neutron shielding applications by Okuno, 2005. Agosteo et  In this paper; we investigated usability of vermiculite loaded samples for nuclear reactor shielding processes, because of excellent thermal insulation properties of it.This mineral was doped in cement and new samples including different vermiculite percents were produced.4.5 MeV neutron dose transmission values were determined.Also 4.5 MeV neutron attenuation lengths were calculated for each sample.

Experiments 2.1 Sample preparation
Before the mixing procedure, a part of mixing water at the percentage of water absorption capacity of expanded vermiculite aggregate by weight was added to vermiculite to make it fully saturated with water.Fig. 1 shows the expanded vermiculite particles saturated with water.Then, the rest of the mixing water cement and silica fume or steel fiber were mixed together for 1 minute in a mixer, and finally, expanded vermiculite aggregate saturated with water www.intechopen.comwas added to cement slurry and mixed for 3 minutes again, to get a homogenous structure.Fig. 2 shows the fresh state of the mixture of lightweight mortar.The prepared fresh mortar were cast in standard cube (with an edge of 150 mm) molds, in two layers, each layer being compacted by self-weight on the shaker for 10 s.All the specimens were kept in moulds for 24 h at room temperature of about 20 o C, and then demoulded, and after demoulding all specimens were cured in water at 23 ±2 o C for 27days.After 28 days curing, three plate specimens with a dimension of 150x100x20mm for neutron dose transmission measurements were obtained by cutting the cube specimens using a stone saw.Plate specimens obtained by the way was illustrated in Fig. 3.We obtained 12 different samples.Codes and contents of samples were shown in Table .1.and Radiological Health Handbook, Scintra _Inc., Revised Edition, 1992.).
The NP-100B detector provides us to detect slow and fast neutrons.Tissue equivalent dose rates of the neutron field can be measured by it.The detectors contain a proportional counter which produces pulses resulting from neutron interactions within it.The probes contain components to moderate and attenuate neutrons.So that the net incident flux at the proportional counter is a thermal and low epithermal flux representative of the tissue equivalent dose rate and the neutron field.Because of neutrons have no charge; they can only be detected indirectly through nuclear reactions that create charged particles.The NP100B detector uses 10 B as the conversion target.The charged particle -alpha or proton (respectively) created in the nuclear reaction ionizes the gas.Typical detector properties are shown in Table .3.Equivalent dose rate measurement results have read on RADACS program in system PC.Experimental design is shown in Fig. 4.  We determined dose transmission values of vermiculite loaded samples.Firstly, we counted equivalent dose rate by fast neutrons while there is no sample between source and detector system.And then we measured for each sample neutron equivalent dose rate while there is our sample between 241 Am-Be source box and detector probe.The ratio of two values is called dose transmission.

Results and discussion
Nowadays concrete is often used in radiation shielding process.In several studies, some additive materials were added in concrete to increase its radiation shielding capacity.In this study, as an additive material, we have used vermiculite mineral with a good heat insulation material.Produced samples have three different vermiculite and cement ratio values.4.5 MeV neutron dose transmission values (Fig. 5) and attenuation lengths of samples (Table .4)were obtained.Attenuation length is just equal to the average distance a particle travels before being scattered or absorbed.It is a useful parameter for shielding calculations.Also we calculated experimental 4.5 MeV neutron total macroscopic cross sections (µ) using by dose transmission values.The various types of interactions of neutrons with matter are combined into a total cross-section value: .....
The attenuation relation in the case of neutrons is thus: where I 0 is known as beam intensity value, at a material thickness of x = 0. Equivalent dose rate has been used instead of beam intensity because of our equivalent dose rate measurements.Experimental 4.5 MeV neutron total macroscopic cross sections were shown in Table .5.As can be seen from Fig. 5 and Table .4,dose transmission values and attenuation lengths decrease with increasing fiber steel and silica fume contents.This result indicates that neutron shielding capacity of samples is increased by silica and steel amount.According to the results, there is not a consistent relationship between vermiculite content and neutron shielding capacity of samples except of F15-samples.The sample named 8F15 is the best neutron attenuator in all specimens.The reason of this that, this sample has higher vermiculite and fiber steel content than others.The worst sample is 8S0 which has higher vermiculite but lower silica fume content.As a result, to increase neutron shielding capacity of sample, expanded vermiculite and fiber steel may be added in the mortar.
al. have investigated double differential neutron distribution and neutron attenuation in concrete using 100-250 MeV proton accelerator.In another study, neutron transmission measurements were studied through pyrolytic graphite crystals by Adib et al.Neutron attenuation properties of zirconium borohydrite and zirconium hydride were determined by Hayashi et al., 2009.Sato et al. designed a new material evaluation method by using a pulsed neutron transmission with pixel type detectors.

Fig. 2 .
Fig. 2. Fresh state of lightweight mortar prepared with expanded vermiculite aggregate.

Table 1 .
Codes and Properties of Samples maximum energy alpha particles emitting from 241 Am.Neutron energy value produced by this nuclear reaction is 4.5 MeV.Radiation characteristics of 241 Am/Be neutron source are shown in Table.2 (Dose rate values have been obtained from The Health Physics

Table 2 .
Radiation characteristics of 241 Am-Be neutron source*

Table 3 .
Typical Properties of Detector*

Table 5 .
4.5 MeV neutron total macroscopic cross sections