ISO 14152:2001 pdf download

ISO 14152:2001 pdf download.Neutron radiation protection shielding 一Design principles and considerations for the choice of appropriate materials.
4 Basic rules and principles for the definition of neutron shielding
4.1 Introduction
The purpose of neutron shielding is to protect workers and public from the ionizing radiation produced by neutron sources. Neutron emission is always associated with the production of gamma radiation in fission and fusion and some accelerator-produced reactions. Most neutron interactions within the shielding material also produce gamma radiation. Shielding should be designed to protect occupationally exposed workers and the general public from neutron and gamma radiations.
Gamma shielding unit design as defined in ISO 7212 and ISO 9404-1 may be integrated in the neutron shielding design.
4.2 Basic principles
The basic principles of a neutron shielding design for neutrons are the same as those used in the development of gamma radiation shielding. The use of time, distance and shielding apply, albeit differently. The overall approach reduces the personal doses to workers and the collective doses to general public. In review, taking into account the ALARA principle (see 5.5), these principles are applied as follows:
Time of exposure: the time that areas adjacent to the shielding are occupied (time available for exposure) should be carefully determined and minimized.
– Distance: the distance between the exposed person and the neutron source should be optimized (maximized).
Shielding: must be provided to first moderate the neutron, second absorb as far as possible the residual neutron flux and third minimize the gamma radiation originating from the neutron absorption process. Doses due to neutrons depend on the neutron fluence and its spectral distribution.
NOTE In certain special uses, and in addition to the previous requirements, the neutron source strengths can be reduced in order to allow intervention close to the radiation sources. It is the case of experimental reactors where the “neutron source” can be driven and adapted to the required level.
In this International Standard radiological objectives are expressed in terms of ambient dose equivalent, )J*(10), which is considered to give a conservative estimate of effective dose.
When used without modifying quantity, the terms “dose” and “dose rate” will mean ambient dose equivalent and ambient dose equivalent rate respectively.
The quantities used in making the calculation are the particle fluence and the energy distribution of the particle fluence. The results are converted into ambient dose equivalent using a set of energy dependent conversion coefficients.
4.3 Methodology
To determine an operational neutron radiation protection shielding system, an iterative process shall be implemented, taking into account the following steps, considered in the given order:
1) Choice of design criteria — choice of dose objectives and other design criteria.
2) Characterization of the radiation sources.
3) Identification of constraints on placement and construction.
4) Choice of the shielding materials and arrangement within the shield.
5) Choice of the calculation methods.
6) Choice of the final solution.
7) Experimental verification.
The general architecture of this iterative process is given in Figure 1.
5 Choice of radiological objectives and other design criteria
5.1 General
The purpose of this step is to identify all the factors which have to be considered during the dimensioning of the neutron shielding; at first the radiological objectives in terms of ambient dose and ambient dose rate limits and at second all the additional architectural, mechanical, chemical and safety objectives that the shielding has to satisfy, according to the general purpose of the nuclear plant or the operation.
The first criterion, consists of determining the maximal ambient dose equivalent and ambient dose equivalent rate at the different working areas and on the factory boundary, with respect to the radiation protection regulation concerning the exposure of workers and general public to ionizing radiation. In accordance with ICRP recommendations and according to the ALARA principle, these values should be kept as far below the regulatory limits as is reasonably achievable (see 5.4). Additional considerations for the selection of these radiological objectives are given in 5.2 to 5.4.
The second series of criteria concerns all the additional safety requirements with which the shielding shall comply with respect to the general purpose of the plant or the installation (e.g. participation to the containment wall, playing a role of gamma radiation shielding, participation to the fire compartment, need for decontamination, necessity of removable parts). In order to achieve complete implementation, all kinds of hazards or constraints such as mechanical constraints, chemical constraints, fire constraints, thermal constraints, architectural constraints, general safety constraints, shall, in addition, be considered.
Consequently, persons involved in shielding design should review the locally applicable radiation protection regulations and requirements as well as regulations concerning building, fire protection, factory safety or other regulations applicable at the facility or for the operation. Examples of items to be considered are discussed in 5.2.
5.2 Classification of work areas (based upon general occupancy, limited occupancy, etc. and on specific protection requirements).