Uranium oxide and MOX fuel fabrication (Sustainability Assessment)

In this section, firstly, a short description of the main processes in a U oxide and in a U-Pu mixed oxide (MOX) fuel production facility is given and the corresponding specific safety issues are discussed. Secondly, the assessment method is discussed based on the corresponding criteria of the INPRO methodology in the area of safety, which have been, where necessary, adapted to the specific issues of these Nuclear Fuel Cycle Facilities (NFCF).
As there are various types of nuclear fuel cycles, different kinds of fuel are fabricated in different facilities for different reactors. Light water reactors (LWRs) and PHWRs most often use uranium oxide fuel. For LWR, the uranium is enriched in a range from 1.2 % to about 5 % of ²³⁵U. In PHWR, mostly natural uranium is used. In some LWR and fast reactors (FR), MOX fuel is being used. The majority of the research reactors uses metallic/alloy fuel in plate form. Several countries are planning to use metal fuels or nitride fuels in FR to achieve higher breeding ratios. A comprehensive description of fabrication technology of these and several other types of nuclear fuel is documented in Ref [104].
As of end of 2015 the following countries have commercial fuel fabrication facilities (UO₂ and/or MOX) in operation: Argentina (Cordoba, Ezeiza), Belgium (Dessel), Brazil (Resende), Canada (Port Hope, Peterborough), China (Yibin, Baotou), France (Romans, Marcoule), Germany (Lingen), India (Hyderabad, Tarapur, Trombay), Iran (Isfahan), Japan (Yokosuka, Tokai-Mura, Kumatori), Kazakhstan (Ust-Kamenogorsk), Republic of Korea (Taejon), Pakistan (Chashma), Romania (Mioveni-Arges), Russian Federation (Elektrostal, Novosibirsk), Spain (Juzbado), Sweden (Västeras), United Kingdom (Springfields), USA (Richland, Colombia, Wilmington). In this manual the discussion is restricted to the fabrication of fuels most commonly used in power reactors, i.e. natural and enriched UO₂ fuel and UO₂-PuO₂ (MOX) fuel.
Fresh nuclear fuel transportation has not been considered in this manual as an independent stage of nuclear fuel cycle. INPRO methodology implies that safety of fresh fuel transportation is to be considered as part of the INPRO assessment of fuel fabrication facility producing a given type of fuel. Commonly the fuel fabrication facility sets up the requirements to the packaging design, performs fuel package, and also participates in licensing of transportation and organising the transportation.

Natural UO₂ fuel for PHWR

For PHWR starting with uranium concentrate in the form of impure ADU or magnesium di-uranate (MDU) or uranium peroxide or UO₃, high-density UO₂ pellets are prepared by a ‘powder-pellet’ route [42]. The major process steps for fabricating PHWR fuel pellets are as follows:

• Dissolution in nitric acid followed by solvent extraction purification of impure uranium nitrate solution by using tri-butyl phosphate (TBP) in kerosene as solvent;
• Addition of ammonium hydroxide to pure uranium nitrate solution to precipitate pure ADU or addition of NH₃ and CO₂ gases to uranium nitrate solution to precipitate pure ammonium uranyl carbonate (AUC);
• Controlled air-calcination followed by hydrogen reduction and stabilization of ADU or AUC to obtain sinterable grade UO₂ powder; and
• Cold-pelletisation of powder followed by high temperature sintering (1700 to 1725 °C) in hydrogen atmosphere and centreless grinding to achieve the desired diameter.

In most countries, sinterable grade UO₂ powder for PHWR is obtained by adapting the ADU route. This ex-ADU uranium dioxide powder is extremely fine with average particle size < 1.0 μ with specific surface area in the range of 2.5 – 3.5 m²/g and requires a granulation step for making free-flowing press-feed granules. UO₂ granules (1 to 2 mm) are obtained by either ‘roll-compaction-granulation’ or ‘pre-compaction-granulation’.
On the other hand, the ex-AUC uranium dioxide powder is free-flowing, relatively coarse (~10 μ) and porous with specific surface area in the range of 5 m²/g and suitable for direct pelletisation, avoiding the granulation step. In the AUC route, calcination, reduction and stabilization are simultaneously carried out in a vertical fluidized bed reactor.
In the beginning of nuclear power in most countries, cold-pelletisation was carried out by employing conventional hydraulic press with multiple die-punch sets of tungsten carbide or die steel. However, in recent years the high-speed `rotary compaction’ press has been selected. For densification of green pellets, high temperature sintering is carried out at ~1700 °C in continuous sintering furnace.
For example, in India, natural UO<sub>2</sub> powder is produced from MDU. The UO<sub>2</sub> powder is subjected to either `roll compaction-granulation’ or `pre-compaction-granulation’ to obtain free-flowing granules. Lubricant such as zinc stearate is admixed to the granules in a blender. The granules are subjected to final compaction in a double acting hydraulic press with multiple die-punch sets. This is followed by high temperature sintering at ~1700 °C in cracked ammonia in pusher type continuous sintering furnace. The sintered pellets are finally subjected to wet centreless-grinding to obtain UO₂ pellets with designed geometry. The pellets are then loaded into a clad tube filled with a inert gas (e.g. helium) which is subsequently sealed by welding it with an end plug on both sides to become a fuel rod. Several fuel rods are inserted into a fuel element structure (with spacers, upper and lower tie plate, etc.) to produce a fuel assembly (or fuel bundle).

Enriched uranium oxide fuel for LWR

The LWR mostly use Zircaloy cladded LEU oxide fuel assemblies with a ²³⁵U content in the range of about 1.2 to 5 %. The enriched UO₂ pellets are fabricated by ‘powder-pellet’ route involving preparation of enriched UO₂ powder using UF₆ as starting material.
There are dry and wet conversion processes to produce UO₂ from UF₆. In wet processes UF₆ is injected into water to form a UO₂F₂ particulate slurry. Either ammonia (NH₃) is added to this slurry and reacts to produce ADU, or ammonium carbonate ((NH₃)₂CO₃) is added to this slurry and reacts to produce ammonium uranyl carbonate (AUC). In both cases the slurry is filtered, dried and heated in a reducing atmosphere to produce pure UO₂ powder. In most countries the integrated dry route (IDR) is followed for preparation of fine UO₂ powder by reacting UF₆ vapour with a mixture of super-heated dry steam and hydrogen at ~600 – 700 °C. The chemical reaction is as follows:

$UF_{6}+2H_{2}O+H_{2}=UO_{2}+HF;(3)$

The process does not generate any liquid effluent and the only by-product is high purity HF, which could be recovered and reutilized or sold. The specific surface area of the IDR-derived UO₂ powder is low (about 2 m²/g) compared to the powder produced by the wet chemical route. The IDR powder is extremely fine (~0.2μ) and requires granulation. The powder is usually transferred into orbital screw blenders for homogenization. Pore formers such as polyvinyl alcohol, methyl cellulose or U₃O₈ are added at the blending stage. The UO₂ powder is subjected to cold-pelletisation and high temperature sintering in hydrogen atmosphere. Fuel rod production and assembling is similar as for natural uranium fuel.

Mixed oxide fuel

Nuclear fuel containing in addition to U also Pu can be in the form of mixed oxide, carbide or nitride. As plutonium is highly radiotoxic, all operations for fuel fabrication involving Pu have to be carried out in glove boxes or hot cells. Containment and ventilation systems in such a facility need to be very reliable.
Fabrication of Th-Pu mixed oxide fuel can be done in a similar manner. Th-Pu MOX can be sintered in air, which adds to economy and convenience. (Th-²³³U) mixed oxide fuel fabrication calls for development of automated and remote fabrication technology due to the presence of ²³²U.

Safety issues in uranium and MOX fuel fabrication facilities

Safety issues in uranium and MOX fuel fabrication facilities are documented in the IAEA Safety Standards [17, 33, 34] (for supplementary information, see also Refs [99, 105–111]). Criticality accidents and the accidental release of hazardous materials may be the major safety issues in these facilities [112].
The facility needs to be designed to restrict exposure from normal operations to authorised limits. In case of enriched uranium/MOX fuel fabrication, special care is required to minimize contamination. Shielding may be needed for protection of the workers due to higher gamma dose rates compared to natural U fuel production.
Examples of the design principles are prevention of criticality by design (the double contingency principle is the preferred approach, see Annex II of Ref [17]) and confinement of toxic and radioactive chemicals (includes the control of any route into the workplace and the environment).
The main differences regarding hazards between a U and a MOX fuel fabrication facility are:

• The radiotoxicity of plutonium, higher than that of uranium;
• The dry process fabrication method that is preferably used in current industrial-size MOX facilities, which has a higher potential for criticality and for dispersion of radioactive material;
• The thermal power of the plutonium requires that the release of heat be taken into account.

Criticality

Ref [17] requires that criticality safety (see also Section of NFCFs) needs to be achieved by preventive measures (double contingency principle) preferably envisaged in the design but could be also supported by administrative procedures.
In a U and MOX fuel fabrication facility the following parameters [17] need to be kept within subcritical limits during normal operation, AOO and DBA conditions: mass and enrichment of fissile material, geometry characteristics of processing equipment, concentration of fissile material in solutions, characteristics of neutron absorbers, reflectors and moderators. Criticality analysis is supposed to involve potential flooding, mechanical failures, load drops, operator errors, etc. Specific recommendations on how to avoid criticality and how to perform a criticality analysis in a U and MOX fuel fabrication facility are provided in Refs [33, 34].

Internal exposure to radioactive and toxic chemicals

Consideration needs to be given to protecting the workers, public and environment from releases of hazardous material in both normal operational states, anticipated operational occurrences and accident conditions (design basis accidents and design extension conditions) by for example keeping the inventory of liquid UF₆ in the facility to a minimum [33].
Containment is the primary method for protection against the spreading of dust contamination, e.g. from areas where uranium or hazardous substances are held or processed in a powder or gaseous form. In MOX and enriched U fuel fabrication facilities, in addition of a static containment system (physical barriers such as a glove box) also a dynamic containment system (ventilation) needs to be used to cause a flow of air towards parts of equipment or areas that are more contaminated [33].
Ref [33] recommends that in the design of the ventilation and containment systems in the U fuel fabrication facility “account should be taken of criteria such as: (i) the desired pressure difference between different parts of the premises; (ii) the air replacement ratio in the facility; (iii) the types of filters to be used; (iv) the maximum differential pressure across filters; (v) the appropriate flow velocity at the openings in the ventilation and containment systems (e.g. the acceptable range of air speeds at the opening of a hood); and (vi) the dose rate at the filters.”
Guidance on the design of ventilation and containment systems in the MOX fuel fabrication facility is provided in Ref [34]. It involves additional recommendations on the contamination monitoring, gloveboxes ventilation monitoring, buildings compartmentalisation, filters locations etc.
Recommendations on the protection of workers against internal exposure and chemical hazards are provided in Refs [33, 34] and include the following:

• Ventilation systems help to minimize the workers exposure to airborne hazardous material;
• The need for use of respiratory protective equipment can be minimized through careful design of containment and ventilation systems, and the installation as necessary of the monitoring and alarm equipment;
• Location of primary filters as close to the source of contamination as practicable helps to minimize the build-up of uranium oxide and/or MOX powder in the ventilation ducts. Having multiple filters in series is preferable since it avoids reliance on a single barrier;
• Nonporous and easy to clean walls, floors and ceilings in areas of the facility where contamination may occur facilitate the decontamination.

The design of the fuel fabrication facility is expected to provide for the monitoring of the environment of the facility and detection of breaches in the confinement/containment barriers. Uncontrolled dispersion of radioactive substances to the environment from accidents could occur if the containment barrier(s) were impaired. In addition, ventilation of these containment systems, with discharge of exhaust gases through stack via a gas cleaning process such as filtering, can reduce environmental discharges of radioactive materials to very low levels [33, 34]. Efficiency and resistance of these filters to chemicals (e.g. HF), high temperatures in the exhaust gases and fire conditions need to be taken into consideration. There needs to be uninterrupted monitoring and control of the stack exhaust.
Radiation exposure of the public and the environment during normal operation is discussed further in the INPRO methodology manual on environmental impact of stressors [5].

External radiation exposure

External radiation exposure of personnel can be minimized by means of sufficient distance to a radiation source, shielding of it, optimisation of time and quantity of radioactive material storage and processing and restrictions of occupancy near it (see also Section of NFCFs). Personnel radiation monitoring instruments need to be provided for radiation protection.
In U fuel production facilities, a hazard of external radiation exposure exists in areas used for storing UF₆ cylinders, in particular empty ones that have contained reprocessed uranium, and in areas where significant amounts of uranium with a high specific density are present (e.g. in storage areas for pellets and fuels). Shielding provided by vessels and pipework is usually sufficient to control exposure in case of low density UO₂ (used in a conversion or blending unit); however, if reprocessed U is used additional precautions are necessary to limit the exposure of workers to ²³²U decay products such as ²⁰⁸Tl and ²¹²Bi. External exposure in a MOX fuel fabrication facility is possible from ²³⁸Pu and ²⁴⁰Pu isotopes (neutron emission) and ²⁴¹Am (gamma emission) generated by the decay of ²⁴¹Pu during storage. Due to the higher activity of plutonium, shielding provided by vessels and/or glove boxes may not be sufficient to limit exposure adequately. Thus, additional measures can be taken such as limitation of occupancy and proximity, installation of additional shielding, and remote operation of process equipment [34].

Fire and explosion

The facility design needs to account for fire safety (see Section of NFCFs) on the basis of a fire safety analysis and implementation of defence in depth (prevention, detection, control and mitigation). As in all industrial facilities, facilities have to be designed to control fire hazards in order to protect the workers and the public. Fire in these facilities can lead to dispersion of radioactive or toxic materials by destroying the containment barriers or cause a criticality accident by modifying the safe conditions [33].

Ref [33] further recommends:

“Special fire hazard analyses should be carried out for:
(a) Processes involving hydrogen, such as conversion, sintering and reduction of uranium oxide;
(b) Processes involving zirconium in powder form or the mechanical treatment of zirconium metal;
(c) Workshops such as the recycling shop and laboratories where flammable liquids and/or combustible liquids are used in processes such as solvent extraction;
(d) The storage of reactive chemicals (e.g. NH₃, H₂SO₄, HNO₃, H₂O₂, pore formers and lubricants);
(e) Areas with high fire loads, such as waste storage areas;
(f) Waste treatment areas, especially those where incineration is carried out;
(g) Rooms housing safety related equipment, e.g. items such as air filtering systems, whose degradation may lead to radiological consequences that are considered to be unacceptable;
(h) Control rooms.”

Explosions can occur due to gases (H₂ used in conversion process and sintering furnaces, etc.) and chemical compounds (ammonium nitrate in recycling processes) [113]. In some cases, explosion can be prevented by using inert gas atmosphere or dilution systems. Recycling systems need to be regularly monitored to prevent ammonium nitrate deposits. In areas with potentially explosive atmospheres, the electrical network and equipment are expected to be protected according to corresponding industrial safety regulations [33].

Containment of radioactive material and/or hazardous chemicals

Leaks may create several hazards [33]:

• Dispersion of radioactive material (e.g. UO₂, U₃O₈ powder and UF₆) and/or toxic chemicals (e.g. HF) leaking from equipment and components like pumps, valves and pipes;
• Flooding by hydrogenous fluids (water, oil, etc.) which can change the moderation in fissile materials and reduce criticality safety.
• Explosions and/or fire caused by leaks of flammable gases (H₂, natural gas, propane) or liquids.

Ref [33] recommends deploying leak detection systems where leaks could occur and equipping vessels containing significant amounts of nuclear material in liquid form with alarms to prevent overfilling and with secondary containment features to prevent criticality.
As discussed for U conversion and enrichment facilities, UF₆ leakage in a fuel production facility needs to be restricted to less than 0.2 mg/m³ (chemical toxicity limit for natural ‘U’ and up to 2.5 % enrichment) (see Table 6 in 'CR1.6'). The radiation limit is 13 Bq/m³; this implies 80 μg/m³ for U with 5 % enrichment and 32 μg/m³ for U with 10 % enrichment.
To prevent a release of radioactive material and/or toxic chemical to the outside of the plant several barriers (combinations of static and dynamic containments) are necessary: The first barrier is the casing of the equipment (e.g. wall of a vessel or pipe), the second barrier could be a glove box, and the last one is the building of the facility. Additionally, dynamic containments are to be provided by ventilation systems in process equipment and glove boxes but also in the working area of the facility.

Decay heat from MOX fuel material

Isotopes of Pu processed in the MOX fuel fabrication facilities generate heat due to the radioactive decay. Most of the heat is produced from ²³⁸Pu decay and can potentially create essential heat loads on the fuel being produced and on the facility systems, structures and components. Appropriate ventilation of the gloveboxes, storage rooms and other production units containing MOX fuel materials is generally sufficient for the temperature control. Ventilation systems have to be monitored continuously and in case of their unavailability the time interval before damage occurs needs to be adequate for repairing the failure or for taking alternative actions.

External hazards

Fuel production facilities are expected to be designed against all credible external hazards (see Sections 2.1 and 2.6 of NFCFs).