Invisible rays such as X-rays, gamma rays, neutron, beta rays, etc., can penetrate surfaces and pose radiation hazards. These ionizing radiations are used/emitted from hospitals, nuclear power plants, particle accelerators, industrial radiography, etc. It is important to protect the public, patients, and the environment from harmful radiation.
Radiation shielding materials are used to prevent the hazardous spread of radiation. The thickness of the radiation shield varies with respect to the energy, type of radiation and attenuation characteristics of the shield. For example, 1 to 2.3mm thick lead sheets are used on the wall of X-ray rooms, CT scanner rooms, etc., to shield X-rays. But 450 mm thick lead or 2300 mm wide concrete is used to build 10 MeV radiotherapy bunkers.
The lightweight (2.3g/cc) and heavyweight (>2.6 g/cc) concretes are commonly used to build radiation shielding structure in radiotherapy bunkers, nuclear power plants, sterilization plants, etc. But these have their inherent disadvantages like using up space, being expensive, non-replenishable and thermally unstable.
Red mud is a hazardous alumina industry waste that is very alkaline and has heavy element leaching. Globally, 170 million tonnes of red mud are generated annually and is left unused in the disposal plants due to inadequate technologies for its large-scale utilization. Currently, only 3- 4 % of red mud is used to fabricate useful products. The rest is getting accumulated in the disposal plants and possess a serious threat to the public, environment, and wildlife (Figure 1).
CSIR-AMPRI has developed the technology to convert the iron-rich red mud into X-ray, gamma-ray and neutron shielding materials. It is a green process and is economically viable. A certain weight percentage of BaSO4 and kaolin clay binder is added to the red mud to increase its X-ray attenuation characteristics and mechanical strength, respectively.
The X-ray shielding tiles were fabricated by mixing appropriate proportion of red mud, BaSO4 and kaolin clay powder in a ball mill. The red mud:BaSO4:kaolin compound mixtures were wet ground with water, and then green tiles were made by applying ≈530 kg/cm2 pressure. The developed tiles were dried in a hot air oven at 90°C and then sintered between 900 - 1150°C in ambient conditions (Figure 2a).
The developed tiles possess the breaking strength of 1880 N and the modulus of rupture of >25 N/mm2. Nearly 12 - 15mm thick tile possess the attenuation equivalent to 2 mm lead at 100 kVp. Moreover, no heavy metals were found to leach from the tiles above the permissible limit.
Joint free X-ray shielding tiles (2500 ft2) were fabricated and demonstrated in two X-ray diagnoses, one CT scanner and one Cath Lab of M/s Saideep Healthcare and Research Pvt Ltd., Ahmednagar, Maharashtra. The X-ray installations are currently in operation (Figure 2b).
The technology for the fabrication of “Lead-Free X-ray Shielding Tiles” was transferred to M/s Prism Johnson Ltd., on 10/06/2019 at CSIR, New Delhi. CSIR-AMPRI and M/s Prism Johnson Ltd have worked together and solved various problems like white efflorescence, cracking, melt-sticking and attenuation, which evolved during upscaling of this technology from the lab to the industrial scale. All the problems were solved, and X-ray shielding tiles were made on a pilot scale on September 2021 (Figure 3). The product will be launched soon. The 12 -15 mm thick tiles possess the attenuation of 2 mm lead at 100 kVp and have sufficient mechanical strength. The developed tiles are cheaper than lead and barite boards.
The developed tiles are suitable for shielding diagnostic X-rays. However, it requires a thick shield to attenuate very high energy X-rays, gamma rays and neutrons due to their porosity and less density.
CSIR-AMPRI has also recently developed gamma-ray shielding blocks by hot compacting red mud. The density of the shield is found to be double than that of the radiation shielding blocks fabricated through the conventional ceramic route. In many places, both the neutron and gamma-ray shielding materials are required as neutrons are generated as secondary radiations. These shields are capable of shielding both the neutron as well as high energy X- and gamma rays.
This technology can promote the large-scale utilization of red mud and reduce the usage of toxic lead, primary mining, deforestation, and environmental pollution.
Dr. Shabi Thankaraj Salammal
Dr. Deepti Mishra
Dr. Mohd. Akram Khan
Dr. S K S Rathore
Dr. Avanish Kumar Srivastava