SHIELDING DESIGN AND SAFETY MEASURES AROUND 60Co, 192Ir, AND 252Cf SOURCES IN INDUSTRIAL RADIOGRAPHY FACILITIES BASED ON PHITS MONTE CARLO SIMULATIONS

Abstract

Industrial radiography has been spreading worldwide and its use as a non-destructive testing method for different equipment, goods, and objects of all sorts has proven to be an essential and indispensable technique for objects’ control. Depending on the type of sources used, attention must be paid differently on the protective measures to be taken around a facility built for radiography purposes. In this regard, protection measures around an X-ray generator are different from those necessary to protect people and the environment around a radioisotope, which spontaneously emits radiation, alpha, beta, gamma, or neutron, until it decays to a stable element. As great issues of protection highlighted along with justified activities are the measures to limit the dose to an acceptable limit, the present study was projected to solve the problem of shielding design around industrial radiography facilities using gamma and neutron sources. Gamma sources of Co-60 under 100 Ci and Ir-192 under 50 Ci and a neutron source of Cf-252 with an activity of 100 Ci were considered as typical sources used in fixed industrial radiography. The PHITS Monte Carlo code was used to compute the appropriate concrete wall necessary to shield gamma radiation and the most effective material appropriate to shield neutron source. For the gamma radiation source, the shielding design took into consideration the double and single corner maze, while the neutron radiography testing room design considered only a reinforced shielded door, without maze design. The result of the calculation showed that the minimum concrete wall thicknesses necessary to shield the radiation were 70 cm, 120 cm, and 250 cm for the Ir-192, Co-60, and Cf-252 sources, respectively. All the presented values were obtained with acceptable statistic except the case of Cf-252 where the calculation involving neutron interaction was time-consuming and computing resources demanding. In addition, different materials were subjected to investigation to find out the most appropriate for neutron shielding as the Cf-252 spontaneous fission produce fast neutrons. These fast neutrons undergo moderation in hydrogenous or light material prior to their absorption in borate material where they produce gamma-rays, that are shielded using concrete of dense material (high Z material). It was found that tungsten was efficient in shielding energetic neutron, but is expensive to be used in shielding design. Paraffin, graphite, polyethylene, plastic, and iron could be used for the neutron slowing -down process and the combination of some of these materials is more effective. Water (light and heavy), the most effective neutron moderator or lithium cannot be used in radiographic test rooms as they posed a structure problem (easy leakage, difficult to use in wall form). Also, while using hydrogenous materials, attention should be paid to the safety and security of the design, construction, and operation of the facility as the fire likelihood increases. Gamma-rays shielding design showed that the most appropriate design to shield gamma from sources used in industrial radiography facilities is the double corner maze compared to the single corner maze. The maze length necessary to shield gamma radiation up to 2.5 µSv/h in single corner maze design is almost the triple of the maze length needed in double corner maze design. The length is cost-related. This conclusion highlighted the importance of cost-benefit analysis in the early planning stage prior to its design, construction, commissioning, operation, and decommissioning. Three different positions of the source were evaluated and the outcome showed that the central position of the source in radiography testing facilities should never be allowed unless there are special justifications: this position was found to be the case with the highest exposure rate to the boundary areas enclosing the facility. The choice between the left and right position of the source in the facility merely depends on the length of the maze in the case that the design is a single corner maze. For example, in the case that the source of cobalt-60 is used for neutron imaging, the left position of the source should be preferred when the maze length is less than 7 m and the right position if not. For iridium-192 source in industrial radiography facilities, the single maze design showed that at least 5 m maze length is needed to set the source position on the right side. Neutron shielding is complex in industrial radiography using fast neutrons as those emitted by the spontaneous fission source of Cf-252 investigated in this research. Maze design is not appropriate for neutron shielding and the shielded enclosure, in this case, should be made of reinforced door including materials for neutron slowing down, its absorption, and the gamma-ray shielding. From the computation based Monte Carlo methods, the following materials should be used in combination with iron for slowing the energetic neutron, boron or borate materials for neutron absorption, and concrete or lead at the end of the chain for gamma attenuation. The combination of three to five materials could be enough when well-chosen and the shielded enclosures should be designed and constructed based on a long term planning evaluation. Attention should be paid on materials used to avoid the critical problem of activation as the neutron interaction with an alloy is likely to produce new radioisotope with a high risk of contamination. To keep the effective dose rate as low as reasonably achievable (ALARA) in the areas adjacent to the radiographic facilities, the International Atomic Energy Agency (IAEA) has developed regulatory guidelines, safety reports series, and a various number of technical documents to help governments, institutions, and individuals involved from near or far in the use of radioactive sources in the practice of radiography. These guidelines have been used worldwide to develop a safety culture and network with less risk of radiological accidents from the industrial side. As the National Radiation Protection Agency of Cameroon is the regulatory authority in Cameroon for the practice of industrial radiography, there is a particular need for clear and precise regulations on the use of radioactive sources for non-destructive tests. The regulatory authority has the opportunity to consider the Korean regulations (or USNRC one) and the IAEA case to develop appropriate laws and rules for safe acquisition, possession, planning, siting, designing, construction, commissioning, operation, and decommissioning of a radiography testing facility. The licensees, radiographers, and any individual involved in the use of gamma or neutron sources for activities related to industrial radiography could then have a law as foundation and regulation for their practices without ambiguity