1.1.1. General
The term nuclear fuel cycle facilities (FCF) essentially refers to those facilities (other than nuclear reactors) which are meant for fuel fabrication, spent fuel storage, fuel reprocessing and waste management. The Advisory Group meeting (AGM ) of November 1997 identified a wide range of plants, premises and installations that could be considered to be part of the nuclear fuel cycle. These include:
- Mines
- Fuel conversion plants - Enrichment plants
- Fuel fabrication plants - natural U-fabrication plants
- enriched U-fabrication plants
- MOX fabrication plants
- Storage of irradiated fuel - dry storage - wet storage
- Reprocessing plants - decanning - separation
- Waste storage and handling facilities
- for gaseous waste - for liquid waste
- for solid waste : - silos
- cans - compaction plants
- vitrification plants
- incinerators - Other plants - hot cells (sometimes included into FCF) - accelerators
- research facilities related to the nuclear fuel cycle - heavy water plants.
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The majority of existing nuclear fuel cycle plants are in operation, but a significant number of new facilities is under construction particularly in the area of fuel storage and fuel reprocessing.
It was also observed that the existing plants may be subjected to modification, extension or adaptation.
1.1.2. Definition of front-end and back-end facilities
The main activities in the nuclear fuel cycle facilities comprise the following: mining, milling, refining and conversion, enrichment, fuel fabrication, spent fuel storage and reprocessing, waste storage, treatment and disposal and associated research facilities. The scope of this report does not include nuclear power plants.
There are two possible configurations of the fuel cycle:
a) The “once-through” fuel cycle, in which the ore is used to make fuel which is passed through the reactor once and is then stored in a facility waiting for the final disposal.
b) The reprocessing cycle, in which the fuel is passed through the reactor, stored and reprocessed and passed through the reactor again.
The difference between the “once through” cycle and the reprocessing cycle is that the reprocessing cycle makes more efficient use of the fuel through extraction of Pu (Plutonium) and recycling of U235 (Uranium-235).
In this publication, the facilities related to mining, milling, refining and conversion, enrichment and fuel fabrication are regarded as the front-end facilities.
The back-end facilities are related to the spent fuel storage, spent fuel reprocessing, waste storage, treatment and disposal, etc.
1.1.3. Unique features of the nuclear fuel cycle facilities
There are many features which are unique to these facilities, particularly in comparison with nuclear power plants, such as:
(i) the lower pressure and temperature operations,
(ii) the criticality hazard during all the normal and abnormal events,
(iii) the irregular shape of civil structures, which may require an in-depth study of torsional effects,
(iv) the high degree of confinement requested during all the operational states and during abnormal events such as an earthquake,
(v) the provision of multiple containment or confinement barriers to control the spread of radioactivity,
(vi) the shielding considerations giving rise to heavy structures,
(vii) the associated corrosion, erosion and ageing problems under toxic and radiation environments,
(viii) the planning for long-term repair times,
(ix) the associated chemical and fire hazards during their operation (industrial hazard) (x) the special care during component layout considering the repair, maintenance and
replacement strategies in the future,
(xi) the low energies associated with the radioactive inventory,
(xii) the importance of long-term equipment operability and structural integrity, the importance of maintaining long-term passive but secure containment or confinement (e.g., storage tanks, reservoirs),
(xiii) the potential for nuclear waste, spent fuel and material handling accidents (xv) importance of ventilation systems as part of the confinement barriers.
These peculiar features are at the base of the specific safety considerations and the design recommendations in the next chapters.
1.1.4. Other IAEA programmes in related fields
In the field of nuclear safety, design of FCF structures is not covered yet by a dedicated series of IAEA Requirements document or Safety Guides. The basic reference is the Safety Fundamentals. However a series of technical documents is available covering specific aspects, such as seismic design (IAEA-TECDOC-348), accident reporting (INES Data Base), siting (IAEA-TECDOC-403–416).
A general initiative of the IAEA Department of Nuclear Safety has been in progress since 1999, based upon a comprehensive collection of the state-of-the-art regulations in Member States. Its aim is the development of a specific Requirement document for FCF, in addition to the Requirements for NPPs and Research Reactors, and a related series of Safety Guides dealing with:
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Mining, milling and refining•
Conversion and enrichment•
Fuel fabrication — uranium•
Fuel fabrication — MOX•
ReprocessingThis TECDOC represents an authoritative contribution to this development process and, more in detail, to the revision process of IAEA-TECDOC-348, in progress at NS, which will provide detailed procedures for the seismic design of FCF.
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1.2. Objectives
The AGM and the CS collected a state of the art in the experience of Member States in the seismic design of these facilities as a starting point for a discussion on common safety objectives and therefore seismic design principles.
The approaches presented at the AGM as experience in the Member States have some differences among them, but they represent a necessary technical basis for any further steps, both in the derivation of seismic design principles and also in general safety objectives with reference to generic external events.
The main target of this publication is the presentation of these approaches in view of the completion of the general programme requested to support the design of the fuel cycle facilities. In some cases a synthesis has been tried among them as a fall out of the technical discussion and the results are described in the introductory part of this publication.
1.3. Scope
The discussion at the AGM concluded that mining and deep permanent disposal facilities of spent fuel and radio active wastes are beyond the scope of this review for their specific structural problems that could not be treated together with most of the fuel cycle facilities:
therefore they deserve a dedicated discussion elsewhere. Also the nuclear power plants are discussed here only as reference plants where the design principles are in general more evolved and substantiated by application experience.
Moreover it was recognised that the fuel cycle facilities have very low standardisation design among Member States which creates some difficulties to any attempt of a synthesis. Therefore this publication tries to identify common features among such different plants with a final target in the seismic design, disregarding other big differences that mainly affect construction or operation.
1.4. Structure
The publication presents a short introduction on the IAEA approach and on the main outcome of the meeting, with an attempt of a unified approach to the seismic design of the fuel cycle facilities. Many representative contributions from the AGM participants are collected in the appendix in their original form.
2. NUCLEAR SAFETY CONSIDERATIONS