Software for Shielding Calculation Based on NCRP #147 Methodology
Mustafa Majali, Ph.D.
Radiation Safety Specialist
Federal Authority for Nuclear Regulation Abu Dhabi, United Arab Emirates
[email protected]
Introduction
Structural radiation protection for diagnostic X ray facilities is most commonly performed following the recommendations of National Council on Radiation Protection and Measurements report No. 49 which has effectively remained the primary guide for diagnostic x-ray structural shielding design in the U.S. in particular and worldwide in general for more than a quarter of a century.
Reasons for turn over to Report 147
• NCRP Report 116 mandated substantially lower design dose limits than those given in NCRP 49. • NCRP 49 exposure limits selected are 100 mR/Wk. (5 rem /year) for occupationally exposed persons and 10 mR /Wk. (0.5 rem/year) for public. • NCRP Report 116 recommended that for the design of new facilities or the introduction of new practices should be designed to limit annual exposures 10 mSv/year (currently 5 mSv/year and1 mSv/year for public.
Reasons for turn over to Report 147 • Rapidly changes in workload for diagnostic radiology due to use high speed film/ screen system. • inappropriateness of the suggested use factors and occupancy factors provided in NCRP 49 for today imaging environment. • Report 49 doesn't address the new modalities such as CT, mammography and digital imaging. • Report 49 doesn't address the shielding for multiple sources is not addressed. • Intensive use of shielding materials other than lead and concrete.
concepts of shielding calculations based on NCRP No. 147 • The concepts of shielding calculation depend on: • • • • • •
Shielding design goals (P), Distance (d) to occupied areas, The occupancy factor (T) Workload and its distribution (W), Use factor (U). Shielding materials
Shielding design goal • The weekly shielding design goals for controlled areas is 0.1 mGy/Wk. and 0.02 mGy/Wk. for uncontrolled areas. • It is also most likely to use the concept of dose constraint in as shielding design goals for purpose of optimization of safety and protection of workers and public.
• the proposed design limits is reduced NCRP 49 levels by a factor of ten for controlled areas and by a factor of five for uncontrolled areas.
Occupancy factor & Use factor • The occupancy factor (T) for an area is defined as the average fraction of time that the maximally exposed individual is present. • Flexibility to determine the occupancy factor is also available.
• The use factor (U) is the fraction of the primary beam workload that is directed toward a given primary barrier. • The value of U will depend on the type of radiation installation and the barrier of concern.
Workload • The weekly workload (W) is the time integral of the x-ray tube current over a specified period (mAs). • The normalized workload (Wnorm) is also defined as the average workload per patient. • It is important to distinguish between the number of patients examined in a week (N) and the number of “examinations” performed in a given x-ray room. • For shielding design, the distribution of workload as a function of kVp is much more important than the magnitude of the workload.
Shielding Materials • The typical shielding materials are lead and concrete. • Today, other materials have been used for shielding purposes such as Gypsum, Steel, and wood and little guidance or attenuation data is provided in NCRP 49. • Evidence shows that these barriers have proven to be sufficient to reduce doses to required levels. • Also, it is avoid costly and wasteful over shielding, • it is prudent to use more realistic and accurate estimates of the required shielding.
The Software design. • Development of Software for performing shielding calculations is based on NCRP report No.147 algebraic computation Model by using a given tabulated data and fitting factors. 𝛾
𝑥𝑏𝑎𝑟𝑟𝑖𝑒𝑟 =
𝑥𝑏𝑎𝑟𝑟𝑖𝑒𝑟
1 𝑙𝑛 𝛼𝛾
1 = 𝑙𝑛 𝛼𝛾
𝑁𝑇𝑈𝐾 −1 𝛽 + 𝛼 𝑃𝑑2 𝛽 1+𝛼 𝑁𝑇𝐾 −1 𝑃𝑑2
𝛾
𝛽 1+𝛼
+
𝛽 𝛼
𝑓𝑜𝑟 𝑝𝑟𝑖𝑚𝑎𝑟𝑦 𝑏𝑎𝑟𝑟𝑖𝑒𝑟𝑠
𝑓𝑜𝑟 𝑠𝑒𝑐𝑜𝑛𝑑𝑎𝑟𝑦 𝑏𝑎𝑟𝑟𝑖𝑒𝑟𝑠
Software for shielding calculation. • Since the effective and efficient use of shielding materials and development of optimal design requires qualified expert either for performing or for evaluation and reviewing. • The software enables user to enter the related parameters via simple user interface and performs the shielding calculation and provides the user with result for the appropriate shielding thickness required to achieve safety goals and provide adequate protection to workers and public form radiation.
Software for shielding calculation.
Software for shielding calculation. • • • • •
Selected type of barriers and the targeted modality is available. Tableted and fitting data are loaded into software. Chose or entering the appropriate occupancy factor, use factor and design goal is available. Distance and no. of patients entering by user. Shielding material is selected.
Software for shielding calculation.
the result of calculation is also generated in form of Assessment report to be used as part of submission to regulatory authority.
conclusion.
The software is able to perform shielding calculations effectively, reliably and being a costeffective and time-saving tool. Using of the NCRP report # 147 shielding methodology achieve the goal of radiation safety for workers and public with insignificant increase of shielding cost.
