Introduction

The generation of electricity by nuclear power plants involves the operation of a number of facilities, including: nuclear fuel cycle facilities to produce fuel (the ‘front end’ of the fuel cycle), which is then irradiated in a nuclear reactor, after which it is sent either to spent fuel storage or to facilities for the reprocessing of spent fuel (the ‘back end’ of the fuel cycle); facilities required for the management of operational radioactive waste; and facilities for the management of radioactive waste from decommissioning, including facilities for nuclear waste treatment and disposal. The radioactive waste streams are generated either by the operation of these facilities (including the nuclear power plant) or by their decommissioning at the end of their service life.

The front end of the fuel cycle encompasses uranium extraction, conversion, enrichment and fuel fabrication, and the fabrication of plutonium–

uranium mixed oxide (MOX) fuels. The back end covers the storage and/or reprocessing of spent fuel and the management of the resulting operational waste [2, 4–8]. A general overview of the process material streams and routes of the entire fuel cycle is shown in Fig. 1.

The broad spectrum of non-reactor facilities includes some systems and processes similar to those found on reactor sites. These are mainly irradiated fuel storage facilities (wet or dry); radioactive waste handling, treatment and storage facilities; and ancillary facilities such as water purification circuits, ventilation plants, laboratories and maintenance facilities.

The management of radioactive waste produced during the operational period and during the decommissioning of the related facilities involves long timescales and, in many cases, different source terms and pathways. Waste management is to be carried out in such a way that human health and the environment are protected both now and in the future. Effects beyond national borders need to be taken into account, passing undue burdens to future generations is to be avoided, waste is to be minimized, appropriate legal frameworks are to be established and interdependencies among all these steps are to be taken into account [1]. These principles lead to requirements:

— To specify the ultimate safe and satisfactory condition for all types of waste;

— To move waste to the end state as early as is practicable;

— To ensure that intermediate steps do not inhibit or complicate the achievement of the end state, and/or that the design of facilities and waste management practices can be optimized as part of the optimization of the overall system and its life cycle;

— To cover the costs of managing all waste in the life cycle;

— To cover the accumulated liability at all stages of the life cycle [9].

Mining and milling of ore

Yellow cake ← Uranyl nitrate

↓ Enrichment and conversion

(PuO2)

UO2 ↓ Fuel fabrication Fuel assembly ↓

Spent fuel ↓ Spent fuel storage (Once-through option)

↓ ↓

Spent fuel conditioning

↓ Spent fuel ↓ Waste

Spent fuel storage and disposal

Reprocessing waste treatment, storage and disposal Nuclear power

reactor

(Reprocessing option)

Reprocessing

FIG. 1. Simplified nuclear fuel cycle.

Much of the solid radioactive waste arising from the D&D of a nuclear facility is the same as the waste arising during the operational phase [7].

Depending on the nature of the facility, this waste comprises:

— High level waste, and low and intermediate level long lived waste in the form of spent fuel, the products of fuel reprocessing or material contami-nated with long lived radionuclides. These types of waste generally are not associated with the actual dismantling of the facility.

— Low and intermediate level short lived waste in the form of irradiated items and material contaminated with short lived radionuclides. These may include nuclear facility components, equipment and building materials such as steel and concrete containing only small concentrations of radionuclides.

Liquid and gaseous effluents produced during D&D activities are generally similar to those produced during normal operation, except, perhaps, in cases where special chemicals are used during decontamination.

Most of the decommissioning waste is managed using the arrangements in place for dealing with similar waste arising during normal operation. Such arrangements are generally well developed, and their costs are known. Some of the waste, however, is unique to D&D activities, including:

— Very large items within the nuclear power plant, such as heat exchangers.

— Large quantities of graphite containing long lived radionuclides, in some cases constituting a possible fire hazard.

— Mixed waste containing toxic or hazardous material such as sodium, beryllium, lead or asbestos.

— Relatively large quantities of material having radionuclide concentrations close to the clearance levels at which it may be released conditionally or unconditionally upon its further use, depending on local regulations. This may include materials that have been decontaminated, such as steel, concrete or other useful materials.

— Large quantities of waste that is not radioactive but that is subject to regulatory control because it arises on a nuclear licensed site. This is sometimes treated as ‘suspect waste’, because of the possibility of its having become contaminated [7].

The costs of treating, storing and disposing of decommissioning waste may dominate the overall costs of decommissioning. Therefore, it is important to maximize the reuse or recycling of decontaminated and recovered material,

and to minimize the amount of material that will require management as radioactive waste [7].

The principle of clearance has already been used successfully in some countries. Within the European Union (EU), guidance on its practical use is established by the European Commission. Member States of the EU are free to set their own clearance levels. Any inconsistency in the practical application may cause some difficulty for international trade or for transboundary shipment. It is also interesting to note that the maximum radionuclide levels set for clearance of material from sources under nuclear regulation are substan-tially lower than those for the unrestricted use or disposal of materials from conventional industrial sources containing technologically enhanced levels of naturally occurring radionuclides. The rate of production of these materials and their accumulated quantities are orders of magnitude greater than those of the low radionuclide concentration material arising from D&D activities [7].

In most OECD Nuclear Energy Agency (OECD/NEA) Member countries, consideration of D&D and waste management starts at the facility design stage, most often with the selection of appropriate materials and construction techniques. This approach reflects the first basic principle of waste management, namely that “generation of radioactive waste shall be kept to the minimum practicable” [1].

For example, in existing heavy water reactor systems, materials are selected and operating procedures are designed to enhance reactor efficiency and to limit radiological doses to workers and the public. This results in low radionuclide concentrations in the environment and good environmental performance. Many changes have been, or are being, implemented to improve reactors and to further reduce doses and discharges to the environment. Some of the existing heavy water reactors have been retrofitted to include these changes. Many of these improvements not only result in lower doses and reduced discharges of radioactivity, but also achieve reduced discharges of chemicals and products of metal corrosion. Improved reactor efficiency, dose reduction and environmental performance work ‘hand in hand’, and ideally they will be introduced at the planning stage. It is clear that, through design stage selection of appropriate materials and adoption of adequate waste management procedures, the environmental effects due to nuclear facilities can be mitigated to very low levels (to much lower levels) [10].

2.2. GENERATION OF LOW AND INTERMEDIATE LEVEL