Movements of radioactive material on nuclear licensed sites

Abstract

The regulations governing the transport of radioactive materials are prepared by the International Atomic Energy Agency (IAEA) and then introduced into modal regulations and national legislation. These regulations are based on a graded approach to contents limits for packages and conveyances and to performance standards applied to package designs depending upon the hazard of the radioactive contents.

They apply to the transport of radioactive material in the public domain in which the packages can be conveyed by road, rail, sea, inland waterways or air transport modes and may share transport routes with movements of people and cargoes in close proximity.

In contrast, the movement of radioactive materials on nuclear sites is a much more controlled operation. Normally, only road or rail transport is involved, there are much lower volumes of other traffic and any hazards during the movement are generally less severe than the test conditions in the IAEA transport regulations representing accident conditions of transport.

Furthermore, there is no internationally accepted set of design standards applicable to packages intended purely for onsite movements.

In the UK, suitable safety cases need to be prepared to demonstrate the acceptability of the onsite movement of radioactive material to the regulator, the Nuclear Installations Inspectorate (NII). The safety case includes engineering substantiation against appropriate design standards. However, the criteria in the design standards do not need to be as demanding as those in the IAEA transport regulations because of the controlled environment within which onsite movements take place.

The principles of the graded approach in the IAEA transport regulations can be applied to onsite movements of radioactive material. However, the high level of safety resulting from compliance with these regulations can be achieved for movements of radioactive material packages on a nuclear licensed site by amending limits and test criteria to take account of the stringent onsite controls and environment. Examples of this are increasing the package contents limits for a particular package type, reducing the package test requirements or a combination of the two.

There are also general requirements in the IAEA transport regulations for all packagings and packages, and aspects of these can be applicable for packages used for onsite movements of radioactive material. However, there are aspects of these where the detailed implementation can be relaxed for onsite movements, such as the acceleration values experienced at the typically low speeds of onsite movements and the limited ambient temperature and pressure ranges for a specific site.

The present paper discusses various differences between transport of radioactive material in the public domain and on nuclear licensed sites.     

Composition of self-assembled quantum dots

Abstract

Semiconductor quantum dot (QD) nanostructures have attracted increased interest in recent years because of their electronic and optical properties. A common way to make QDs is to grow a thin layer of material on a substrate with a different lattice constant. The strain between the layers induces the formation of three-dimensional islands. The electronic properties of the islands are mainly determined by their size, shape and composition. While the size and shape of QDs have been the focus of many studies, only recently has their composition been investigated. Experimental studies of the composition of QDs are reviewed and compared with the available theoretical models of QD growth. It is found that no model in the literature can satisfactorily predict QD size, shape and composition. Experimental results from studies of QDs grown under similar conditions vary substantially. Most authors, however, agree that the average composition of the QDs is different from the nominal composition of the deposited material. The composition is also found to vary from top to bottom of the QDs, which is found to have a significant influence on the electronic properties.     

Exposure Addition Effects and Their Terminology

Studies of the increases in overall sensitivity, in red light sensitivity, and in silver sulphide during sulphur sensitization of a photographic emulsion have been carried out. They suggest that there are two quite distinct mechanisms by which the silver sulphide can give rise to increase of light sensitivity.     

没有规矩不成方圆

尾货，不知不觉中已经成为2007～2008年热点词语之一，所谓尾货，简单地说，就是外贸订单剩余和库存积压的商品，具体的经营模式和利润收支，本刊去年曾经详细介绍过，5块钱一件衬衫，7块钱一条休闲裤……服装尾货市场以出人意料的超低商品价格出尽风头，自然也吸引了无数人的目光。