Nuclear fuel cycle facilities

This page refers to "Sustainability Assessment" and presents some background information on NFCFs, discusses their main hazards, and compares NFCFs to nuclear reactors and chemical plants.

Nuclear fuel cycle

A nuclear fuel cycle comprises a number of activities other than reactor operation , the possible combinations of which provide the various fuel cycle options. These activities are:

• Uranium/ thorium mining and milling ;
• Uranium/ thorium refining and conversion;
• Uranium enrichment;
• Fuel fabrication;
• Fuel transportation (including spent fuel transportation);
• Spent nuclear fuel storage;
• Spent nuclear fuel reprocessing including recovered/recycled nuclear material storage;
• Re-fabrication of nuclear fuel using fissile (and fertile) material from reprocessing;
• Radioactive waste management including predisposal waste management and disposal.

Depending upon the requirements and preferences of the individual country, either an open (once through) or closed fuel cycle option can be chosen. In an open fuel cycle, the spent fuel is treated as a waste, i.e. it is (after storage) disposed of directly, without reprocessing. In a closed fuel cycle, spent fuel is reprocessed and the fissile and fissionable elements are used to produce new fuel, and the rest of the fuel elements, e.g. fission products, are disposed of. A comprehensive review of the activities related to nuclear fuel cycles is given in Refs. Developing trends on reactor fuels and their technologies are described in Ref.
The characteristics of nuclear fuel cycles depend mainly upon the type of nuclear reactors. A few examples illustrate this statement: Pressurized heavy water reactors (PHWRs) use mainly natural uranium as fuel, whereas PWRs and BWRs use low enriched uranium (LEU) as fuel. Up till now, fast reactors used several types of fuel, such as high enriched uranium, mixed U/Pu oxide, uranium carbide or uranium /plutonium carbide, and uranium nitride, and metallic fuels. Metallic fuels are also used in thermal (and could be used in fast) gas cooled reactors. The currently predominant (partly) closed nuclear fuel cycle is based on U/Pu, where Pu is obtained by reprocessing of spent U fuel from thermal reactors, to be used either again in thermal reactors or in fast reactors. The ²³²Th/²³³U fuel cycle would require for large-scale use of thorium firstly the conversion of ²³²Th into ²³³U in (thermal) reactors with U fuel. A combination of fast reactors with U/Pu fuel cycle and thermal reactors with ²³²Th/²³³U fuel cycle provides one closed fuel cycle option.
Typical fuel cycle options currently deployed on an industrial scale are the open fuel cycle in heavy water reactors (HWRs) and light water reactors (LWRs), and mono recycling in LWRs. Within the next decades examples of potential industrial developments of fuel cycles include: DUPIC (Direct Use of PWR spent fuel in CANDU reactors), multi recycling of spent fuel in thermal reactors, and application of a partitioning and transmutation concept. Possible long-term industrial developments include: a reactor fleet with a mixture of LWR and fast reactors, a 100 % closed fuel cycle for fast reactors, a thorium fuel cycle, and a fuel cycle for molten salt reactors.

Overview of hazards in NFCFs

Internal and external hazards

In this publication, hazards are considered to be those phenomena or the effects of phenomena that may cause damage to the systems, structures and components relevant to safety of an NFCF, prevent them from performing their functions or alter the processes characteristics beyond the limits defined for normal operation conditions. Generally, hazards are divided into two categories: internal and external. The external hazards can be further split into two sub-categories – natural hazards and human induced hazards.
Internal hazards may involve criticality, fire and explosion, leaks and flooding, radiation and chemical releases, collapse of structures and falling objects, corrosion and erosion, etc., originating from inadequacies in the original design, fabrication or modifications of systems, structures and components or from inadequate processes or procedures, process malfunctions and operational mistakes.
Natural external hazards are associated with meteorological, hydrological, geological and seismic events such as earthquakes, inundation in the flooding, tsunamis, natural fires, and extreme weather conditions, e.g. excessive snowfall, avalanches, tornadoes/ storms/ cyclones, lightning and extreme high or low temperatures. Precaution against these is required at the selection of the site as well as in the design and construction of civil works. The plant and machinery are expected also to be protected per recommended practices and as required by statutory and risk-managing organizations.
Human induced external hazards are associated with the damages/risks caused by man-made factors like potential loss of power supply and consequently loss of control, fire/explosion in the constituent units of the facility or units adjacent to the facility, flying missiles/debris from the neighbourhood/sky/space, accidental or wilful (terrorist) aircraft crash and civil strife including violent strikes, blockages and sabotage.
The risks associated with internal and external hazards are expected to be below acceptable levels. Thus, in the design analyses all potential sources of external hazards need to be identified and the associated event sequences affecting the facility determined. The associated radiological or chemical consequences of any damage caused by an event associated with external hazards need to be evaluated and compared to the acceptance criteria. In the following a few examples of external hazards are presented.
An NFCF is expected to be designed for earthquakes per appropriate standards to ensure that they would not induce a loss of confinement capability (especially of radioactive and/or toxic material such as UF₆ and HF) or a criticality accident by an induced loss of criticality safety functions, such as geometry and moderation, with possible significant consequences for site personnel or members of the public.
Hazards from external fires and explosions need to be covered in the design of an NFCF. They can arise from various sources in the vicinity of the NFCF, such as petrochemical installations, forests, pipelines and road, rail or sea routes used for the transport of flammable material such as gas or oil.
Flooding of an NFCF is expected to be taken into account in its design, if flooding is a credible hazard. For example, the building of the NFCF needs to be robust enough to withstand the impact of a water wave, keeping its integrity as a confinement. In case the material with enrichment higher than 1 % is handled in the facility, the criticality accidents caused by flooding need to be excluded.
An NFCF needs to be protected against extreme weather conditions by including in the design:

• Sufficient strength of structures important to safety to withstand the loads (e.g. mechanical, thermal) caused by these conditions;
• Measures to prevent flooding of the facility, and
• If needed, measures to enable safe shutdown of the facility.

It is necessary to emphasize that the safety requirements adopted for a particular NFCF normally take into account the hazard potential and thus result in a graded approach ensuring that the design and operating philosophies are commensurate with the hazards. The most significant general hazards in NFCFs are briefly discussed in the following sections (see also regulatory guides).

Criticality hazard

Criticality safety is one of the dominant safety issues for NFCFs that handle uranium enriched above 1 % of ²³⁵U, or other fissile material such as ²³³U or plutonium (Pu). As stated before, these facilities employ a great diversity of technologies and processes. Thus, the materials of interest to nuclear safety are distributed throughout the facilities. The nuclear material may be used not only in bulk form (e.g. fuel pellets, fuel elements, fuel rods, fuel assemblies, etc.), but in a distributed and mobile form as well (e.g. in different kinds of solutions, slurries, gases, powders, etc.). As a result, fissile elements may accumulate in some parts of the equipment and may also spill as a result of equipment leakage. The distribution and transfer of potentially critical nuclear materials requires monitoring, alarm systems and operator attention to account for this material throughout the installation and to thus ensure that sub-criticality is maintained and thereby preventing the potentially lethal effects of gamma and neutron radiation doses to workers and the subsequent release of fission products from an inadvertent nuclear criticality.
Nuclear criticality safety is achieved by controlling one or more of the following parameters of the system within sub-critical limits during anticipated operational occurrences (AOO) (e.g. vessel overfilling) and design basis accident conditions:

• Mass and enrichment of fissile material present in a process (e.g. powder in rooms and vessels);
• Geometry of processing equipment (e.g. by safe diameter of storage vessels, separation distances in storage);
• Concentration of fissile material in solutions (e.g. in wet processes for recycling uranium);
• Presence of appropriate neutron absorbers (e.g. in the construction of storage facilities, drums for powder, fuel shipment containers);
• Moderation limitation (e.g. by control of moisture and amount of additives in powder);
• Control of neutron reflectors.

The general procedures to be followed in the criticality analysis are:

• The use of a conservative approach (considering uncertainties on physical parameters, physically possible conditions of optimum moderation, etc.);
• The use of appropriate and qualified (verified and validated) computer codes and cross section libraries within their qualified range.

NFCFs may be split into two groups with regard to criticality:

1. Facilities where a criticality hazard is not credible — mining, milling and conversion of natural uranium facilities, and natural uranium fuel fabrication/storage/transportation.
2. Those where criticality hazards may be credible — enrichment, reprocessing, uranium fuel fabrication, mixed oxide fuel fabrication, fresh fuel storage (and transportation), spent fuel storage (and transportation), waste treatment and waste disposal facilities.

Those facilities in group (2) need to be designed and operated in a manner that provides a high level of assurance that criticality parameters are controlled. Firstly, designs of such facilities need to ensure sub-criticality in all areas utilizing where possible ‘criticality safe’ designed equipment. Secondly, during operation of these facilities, measurement of criticality parameters has to be continuously maintained via monitoring, detection and alarm systems.
A review of some criticality accidents that occurred during operation of NFCFs is provided in Ref. The criticality accident at Tokai Mura, Japan, was the highest level event in the International Nuclear Event Scale reported since 1991. Of the nearly 60 reported criticality accidents that have occurred since 1945, about a third occurred at NFCFs. Two of these occurred in 1997 and 1999. Twenty of these accidents involved processing liquid solutions of fissile materials, while none involved failure of safety equipment or faulty (design) calculations. The main cause of criticality accidents appears to be the failure to identify the range of possible accident scenarios during the design, particularly those involving potential human (operator) errors.

Chemical hazards

NFCFs may also pose hazards to workers (and the public) from releases of chemically toxic and corrosive materials during any of the chemical processing steps in a nuclear fuel cycle. Chemical hazards differ considerably from facility to facility. The production of uranium hexafluoride (UF₆) (in a conversion facility) involves the use of significant amounts of hydrogen fluoride (HF), which is both a powerful reducing agent and chemo-toxic and thus poses a significant hazard to workers. Other examples include the use of strong chemical acids to dissolve nuclear fuel and other materials. These acids are used to chemically dissolve spent nuclear fuel during reprocessing (also to recycle scrap pellets in fuel fabrication facilities), thereby removing the fuel cladding material and enabling separation of the plutonium and uranium from the residual fission products. In addition, residual fission products, which comprise approximately 99 % of total radioactivity and toxicity in spent nuclear fuel, pose a significant radiological hazard in what is typically a complex chemical slurry. During solvent extraction processes, strong acids and organic solvents are used to remove plutonium and uranium from these slurries. These processes can generate toxic chemical by-products that need to be sampled, monitored and controlled. Other chemicals encountered at NFCFs in significant amounts include chemicals such as ammonia, nitric acid, sulphuric acid, phosphoric acid and hydrazine. It is important to recognize that unplanned releases of these toxic chemicals may adversely affect safety controls. For example, a release of hydrogen fluoride could disable an operator who may be relied upon to ensure safe processing. Chemical hazards have caused operational problems and accidents at many NFCFs worldwide. The chemical toxicity hazards associated with UF6 processing were evident in two incidents in 1986 in the USA and Germany and in 2010 in Canada.

Fire and explosions hazards

Many NFCFs use flammable, combustible and explosive materials in their process operations, such as a tri-butyl phosphate-dodecane mixture for solvent extraction, bitumen for conditioning radioactive wastes, hydrogen in calcining furnaces and chemical reactors for oxide reduction. Some flammable and explosive substances may also be generated as bypass products in the production process or as a result of faulty operation when unexpected chemical reactions take place.
Many fire and explosion hazards have been recorded at NFCFs. In 1990, for example, there was an ammonium-nitrate reaction in an off-gas scrubber at a LEU scrap recovery plant in Germany, which injured two workers and destroyed the scrubber. Fire is an especially significant player in accident scenarios because it can be both an initiating event for the accident sequence and can also disable or damage passive and active safety features. It can also provide an energy source to transport radiological and chemical contaminants into uncontrolled areas where they may pose risks to both workers and members of the public. An example of this situation is the fire and explosion at the Tokai Mura reprocessing plant in Japan in March 1997, which contaminated 37 workers with radioactive material.
Therefore, the design of NFCFs is expected to provide for minimum inventories of combustible materials and needs to ensure adequate control of thermal processes and ignition sources to prevent fire and explosions, or at least reduce their potential. For example, extreme care needs to be taken to prevent accumulation of radiolytic hydrogen, which is generated in high activity waste tanks in spent nuclear fuel reprocessing plants.
Fire can also lead to significant releases of radioactive and toxic material. Consequently, fire detection (alarm), suppression, and mitigation controls are usually required. The NFCF design and operation need to consider radiological and other consequences from fires and explosions. Suitable safety controls are supposed to be instituted to protect against potential consequences of fire and explosive hazards. These safety controls need to be designed to provide requisite protection during normal operations, anticipated operational occurrences and credible accidents at a facility.
Summarizing the statements above, it is noted that similar to chemical hazards, fires and explosions that could adversely affect nuclear safety measures need to be given adequate consideration in the design and operation of NFCFs.

Radiation hazards

Radiological safety is an important consideration at NFCFs. Special attention is warranted, when developing and using standards and establishing operational practices to ensure worker safety in operational processes, which may include the open handling and transfer of nuclear materials in routine processing. Although external exposures to radiation fields may be limited, potential intakes of radioactive material require careful control to prevent and minimize internal and external contamination and to adhere to operational dose limits. In addition, releases of radioactive material inside facilities through unmonitored pathways could result in significant exposures to workers, particularly from long lived radiotoxic isotopes. Some facilities, such as MOX fuel fabrication and reprocessing facilities require special shielding design, containment, ventilation and maintenance measures to reduce potential exposures to workers. Fundamental principles whose effective application will ensure appropriate protection and safety in any situation that involves or might involve exposure to radiation are defined in Ref. Based on these principles and objectives, requirements with respect to radiological safety for all types of nuclear installations are established in the International Basic Safety Standards for Radiation Protection and for the Safety of Radiation Sources.

Initiating events

Ref defines an initiating event as “an identified event that leads to anticipated operational occurrences or accident conditions”. The list of internal and external hazards (see Section 4.2.1) is normally used to select initiating events for detailed safety analysis. Postulated initiating events need to be identified on the basis of expert judgement, feedback from operating experience and deterministic assessment, complemented by probabilistic methods where appropriate. The resulting set of identified postulated initiating events has to be confirmed as comprehensive.
Typical initiating events for various NFCFs are discussed in Refs.

Decommissioning of NFCFs

The safety aspects of decommissioning of NFCFs deserve as much attention as the safety aspects of operation of these facilities. The decommissioning of an NFCF has to be factored into the design of the facility and a clear plan for decommissioning needs to be available even at the time of commissioning of the facility. The safety aspects of decommissioning have been dealt with by the IAEA in Refs. The safety aspects related to releasing NFCFs from regulatory control upon termination of practices are discussed in Ref.
In all phases of decommissioning, workers, the public and the environment have to be properly protected from both radiological and non-radiological hazards resulting from the decommissioning activities. Safety issues that are expected to be considered in the decommissioning of NFCFs include:

• “The presence and nature of all types of contamination;
• Hazards associated with the possible in-growth of radionuclides (such as ²⁴¹Am);
• The potential for criticality hazards associated with the possible accumulation of fissile material in the process equipment during operation or during decommissioning actions (such as decontamination);
• The complexity of strategies for waste management owing to the diversity of waste streams;
• For multifacility sites, hazards associated with facilities that are not under decommissioning;
• Inaccessible areas and buried pipes;
• Separation and concentration of material stored in tanks;
• Hazardous chemicals located in SSCs and in buildings, soil, sediment, surface water and groundwater;
• Changes in chemical and physical forms of materials;
• Non-radiological hazards, such as fire or explosion, associated with decommissioning actions.”

The specific characteristics of each type of NFCF will strongly influence the selection of the decommissioning option. A safety assessment is expected to form an integral part of the decommissioning plan. Non-radiological as well as radiological hazards associated with the decommissioning activities need to be identified and evaluated in the safety assessment and factored into the design of the facility. The extent and detail of the safety assessment shall be commensurate with the complexity and the hazards associated with the facility and its operation.

See also

• Safety of NFCFs
• Mining and milling of uranium and thorium
• Uranium refining/conversion and enrichment
• Uranium oxide and MOX fuel fabrication
• Reprocessing of spent nuclear fuel
• Storage of spent nuclear fuel

	[ + ] Assessment Methodology
Areas of INPRO Sustainability Assessment Overview • Economics • Safety (Nuclear Reactors) • Safety (NFCF) • Waste management • Environmental Impact on Stressors • Environmental Impact from Depletion of Resources • Infrastructure Requirements Basic Principle • User requirements • Criteria