Toxics, Vol. 10, Pages 697: Radiation Attenuation Assessment of Serpentinite Rocks from a Geological Perspective

Recently, all countries are trying to gather the three equation sides: cost, power supply, and preservation of the environment, to fill the economic gap in energy technologies. Nuclear power can achieve this complex equation. In addition to the high energy density supplied by nuclear reactors, these reactors are not costly in the long term. In addition, the trivial emissions of CO2 or other greenhouse gases from these reactors compared to other energy sources enforce our planet protection for future generations [1]. Despite these potential outcomes of nuclear energy, it poses an inevitable threat to humanity. These reactors are vulnerable to radiation leakage or explosion anytime, resulting in more disastrous radiation emissions. On the other hand, the management of these reactor-induced nuclear wastes is another challenge faced by any country that seeks nuclear technology [2]. The threat of these radiations, especially neutrons and γ-rays, lies in their high energy and capability to penetrate the human body [3]. This can lead to more diseases related to cancers and tumors [4]. All previous threats explain the reluctance to use nuclear techniques in practice. Therefore, it was imperative to find a radiation shield for protection from these radiations. As a result, scientists of nuclear engineering, physics, and geology have always sought to find and improve new materials that can effectively shield these radiations in recent years.Three factors govern the shielding performance of any material. The first is the material density, which is responsible for the attenuation effectiveness against γ-rays as can be illustrated in heavy-weight minerals [5], rocks [6], and heavy-weight concrete [7,8]. The second is the high content of structural or crystalline H2O, which is responsible for the attenuation effectiveness of fast neutrons, as demonstrated in hydrous minerals (i.e., high-crystalline H2O content) [9] and concrete [10]. The third is suitable additives such as boron or carbon for thermal and fast neutron attenuation, respectively, and this can be observed in minerals of colemanite or boron-containing concrete mixes [11,12,13,14]. Recently, geologists have started to shed light on the effect of the mineralogical composition of minerals and rocks on their radiation shielding performance [10,15]. This trend stems from the principles of applied mineralogy. Applied mineralogy is a branch of geology concerned with the natural barriers comprised of minerals or rocks, which are used to confine radioactive elements [16]. Although concrete is widely applied in natural or artificial crises [13,17], the utilization of raw materials (i.e., minerals and rocks in radiation shielding) has become a method attracting the attention of many studies as an alternative to concrete and cement pastes [18]. This can be credited to many advantages, as follows: (1) lesser cement consumption in concrete and subsequent lesser energy, cost, and CO2 emissions, (2) utilization of unexploited mineral resources, (3) space conservation when using thinner shielding walls, (4) less maintenance and longer life compared to concrete. Besides concrete, the lead element is one of the conventional choices to reduce radiation exposure from X-rays and γ-rays. However, it is not encouraged for use because of its toxicity [19]. More specifically, during this period of global economic crisis, local natural rocks are preferred to be used in research to identify the best quality products that can compete in profitably with those currently imported from other countries [20,21]. Moreover, some rocks exhibited superior shielding efficiency over concrete [22,23]. Generally, there are three types of rocks: igneous, metamorphic, and sedimentary. At first, many studies have considered the attenuation properties of igneous rocks, which are categorized into plutonic (subsurface) and volcanic (on the surface) rocks based on the position of formation. Radiation shielding properties of plutonic igneous rocks have attracted the attention of many researchers. Such rocks include dunite, carbonatite [24], peridotite, pyroxenite, gabbro [25,26], syenite [24], granodiorite [18,27], and granite [18,22,27,28,29,30,31,32]. On the other hand, further studies discussed the radiation shielding properties of volcanic igneous rocks such as basalt [6,22,30,33], andesite [18], dolerite [22], rhyolite [6,33], and volcanic tuff [34,35]. Moreover, other volcanic igneous rocks were evaluated by the Monte Carlo method via the Geant4 simulation toolkit for neutrons and the SRIM program for charged particles [36]. The findings proved that one of these rocks was superior to its counterparts in thermal and fast neutrons shielding, as well as charged particles (e.g., electron, alpha, proton, and carbon ion) [37]. As for sedimentary rocks, both clastics (e.g., sandstone) [22] and non-clastics (e.g., limestone) [6,30,33,38] were evaluated as a radiation shield. As for metamorphic rocks such as marble [5,29,30,39,40], metagabbro [26], gneiss, and charnokite [38] were found to be more effective shields against γ-rays compared to concrete [30].Serpentinite is a metamorphic rock mainly composed of serpentine minerals and associated with minor occurrences of magnetite and carbonate (e.g., dolomite) minerals with a density and crystalline H2O ranging from 2.5–2.7 g/cm3 and 11–16%, respectively [41]. The differences in magnetite content are one of the main reasons for the variation of serpentinite colour (e.g., grey, greyish black, and green) [42]. Commonly, the most prevalent application of serpentinite rocks is ornamental stones due to their aesthetic characteristics, which renders them commercially named “green marble” [20,43]. A massive series of serpentinite mountains is located in Egypt as a part of the ophiolitic assemblages [44], which are not exploited as ornamental stones due to problems related to their structural and durability properties [45]. Other studies investigated serpentinite rocks as aggregates in normal concrete [46]. Moreover, the serpentine mineral (i.e., serpentinite rock-forming mineral) was investigated as ore, in its native state, for only γ-ray shielding [9] and others considered it as aggregates in the radiation-shielding concrete, RSC, for fast neutron and γ-ray attenuation [47,48,49]. Therefore, all varieties of serpentinite rocks display a common feature that enables them to be eligible for use in radiation shielding. However, to the best of our knowledge, no studies deeply investigated the employment of serpentinite rocks (i.e., in their native status) with their different variations in mineralogy and geochemistry in radiation shielding. This can be understandably attributed to the difficulty of obtaining these rock types separately. Additionally, such rocks are not known by many physicists and engineers. Hence, this study aims to utilize some varieties of Egyptian serpentinite rocks instead of concrete as a geologic repository for nuclear waste disposal or in tile production for lining the walls of radiotherapy centers or nuclear facilities. In addition, the impact of their geochemical, morphological, and mineralogical compositions upon their attenuation ability against γ-rays and fast neutrons using a collimated PuBe source and a stilbene detector will be correlated.

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