Radiation Protection Today - Spring 2022 - Page 22

Specialist Spotlight –

The Shielding Design Process

Peter Evans is a Principal Radiation Shielding Consultant with 12 years ' experience . He is the Shielding Team Leader at Cerberus Nuclear .
Radiation Shielding is crucial for ensuring that dose rates in a facility or process including radioactive sources are as Low As Reasonably Practicable ( ALARP ). Since shielding is generally a passive safety feature ( once it is installed , nothing has to be done for it to start working ), it is often the primary means of keeping operators safe from radiation . It can also have major impacts on the design of a plant , including on cost , space management and even structural stability . It is , therefore , necessary that the design of such shielding be optimised as far as possible . This is typically achieved by following the Shielding Design Process .
The Shielding Design Process is a framework used to structure the process of designing or assessing radiation shielding for a project involving radioactive sources . It can be used for a project of any size or complexity , but generally , the larger and more complex , the more essential the Shielding Design Process becomes . Following the Shielding Design Process is considered to be relevant good practice , and its use is promoted by The Shielding Forum .
The first step of the process is to prepare a Basis of Design document which specifies the purpose of the shielding and what it is required to shield against . Perhaps the most crucial part of this is defining a radiation source term to be used throughout the rest of the shielding assessment , describing the radiation sources that might be present , which the shielding must protect against .
The Basis of Design document also sets out the dose criteria that the shielding will allow the project to meet . The dose limits set by nuclear site operators are often stricter than the statutory dose limits in the Ionising Radiations Regulations ( IRR17 ).
It is often the job of a shielding assessor to convince a project or client of the need for the Basis of Design document , as its output is not immediately useful to the client ( it does not , for example , tell them how thick any of their walls need to be ), but it is essential if the rest of the shielding assessment process is to go smoothly .
Following this , the second step is the Designation of Areas , sometimes known as the Radiological Classification of Areas . IRR17 requires that areas of an active facility be designated as either monitored or controlled , based on the dose uptake that a person working in such an area might receive over the course of a year . In practice , this is often achieved by setting dose rate targets for each area of a facility , along with access restrictions that limit operator access to higher-risk areas . These dose rate targets will form the target criteria for the actual shielding to achieve .
Once a source term is defined in the Basis of Design , and a dose rate target set in the Designation of Areas , the shielding requirements can be defined . Shielding assessments are completed to determine how much shielding ( and of what material ) is needed to reduce dose rates to below the target . Shielding Assessment also takes into account more complex issues such as cable penetrations through walls , gaps around the edges of shield doors , and even scatter in the air above a source . It can cover many different types of radiation , such as gamma
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