Fuel fabrication

Hana spends a week at a CANDU fuel plant with a quality assurance auditor from Meridian. The plant presses uranium dioxide powder into pellets the size of a fingertip, loads them into thin zirconium alloy tubes and assembles thirty-seven of those elements into a bundle about half a metre long. "Every bundle that leaves here will sit in a pressure tube at full power for months," the auditor tells her. "If one element leaks, the station sees it within hours. Our job is to make sure that almost never happens."

From powder to pellet

Fuel fabrication turns purified uranium into a precisely engineered product. For enriched fuel, the first step is to convert enriched uranium hexafluoride back to uranium dioxide powder, usually by a dry process that reacts the gas with steam and hydrogen. Natural uranium for CANDU fuel arrives as uranium dioxide powder from the conversion plant. The powder is blended for uniform properties, pressed into cylindrical pellets in high-speed presses, and sintered in hydrogen furnaces at about 1700 °C to form a dense ceramic with about 95 percent of the theoretical density. The pellets are then ground to a precise diameter, inspected and dried.

The ceramic form matters. Uranium dioxide has a very high melting point, around 2800 °C, is chemically stable in hot water, and retains most fission products inside its crystal grains. Its main weakness is low thermal conductivity, which makes the centre of a pellet much hotter than its surface in a reactor, as the thermal hydraulics course explained. Pellet density, grain size, chamfers and dishes on the ends are all controlled to manage swelling and fission gas release, and some fuel uses small additions, such as chromia, to enlarge the grains.

Cladding and assembly

Finished pellets are loaded into tubes made of a zirconium alloy, chosen because zirconium absorbs few neutrons and resists corrosion in hot water. The tubes are filled with helium, sealed by welding end caps, and leak-tested. The sealed tube, called a fuel element or fuel rod, is the second of the barriers in defence in depth, after the pellet itself. Elements or rods are then assembled into the fuel unit the reactor uses, with spacers, end plates or grids holding them in a precise geometry so that coolant can flow between them.

In Canada, CANDU fuel is made by BWXT Nuclear Energy Canada at Toronto and Peterborough and by Cameco Fuel Manufacturing at Port Hope. The bundle design is simple and robust: short elements with thin cladding that collapses onto the pellets under coolant pressure, brazed spacers and bearing pads, and end plates welded to the elements. The simplicity helps keep fuel costs low, and it suits on-power refuelling, where bundles are pushed through the channel by the fuelling machines.

# Energy content of a fuel bundle and an assembly at their discharge burnups.
# Masses and burnups are rounded public values used for illustration.
fuels = [("CANDU 37-element bundle", 19.2, 7500),       # kg U, MWd per t U
         ("PWR 17x17 assembly", 460.0, 50000)]
eff = 0.33
for name, kgU, bu in fuels:
    heat_MWd = kgU / 1000 * bu
    elec_MWh = heat_MWd * 24 * eff
    homes = elec_MWh / 10.0       # an average home uses roughly 10 MWh per year
    print(f"{name:26s} heat {heat_MWd:8.0f} MWd  electricity {elec_MWh:9.0f} MWh  ~{homes:6.0f} home-years")
fissions_per_MWd = 1e6 * 86400 / (200e6 * 1.602e-19)
print(f"one MWd of heat corresponds to about {fissions_per_MWd:.2e} fissions")
print("about 1 g of heavy metal is fissioned for each MWd of heat")

CANDU 37-element bundle    heat      144 MWd  electricity      1140 MWh  ~   114 home-years
PWR 17x17 assembly         heat    23000 MWd  electricity    182160 MWh  ~ 18216 home-years
one MWd of heat corresponds to about 2.70e+21 fissions
about 1 g of heavy metal is fissioned for each MWd of heat

The output shows why the two industries look so different. A CANDU bundle delivers about 1100 megawatt-hours of electricity, enough for about a hundred homes for a year, and a large station uses tens of thousands of bundles a year. A PWR assembly delivers more than 150 times as much, and a reactor loads fewer than a hundred new assemblies per refuelling. Both numbers are large compared with fossil fuels: a single CANDU bundle replaces several hundred tonnes of coal.

Quality assurance

Fuel fabrication is a precision manufacturing business governed by nuclear quality assurance programs. Pellet dimensions, density and enrichment are checked by sampling; cladding tubes are inspected ultrasonically for defects; welds are inspected by radiography or ultrasonic methods; rods are leak-tested with helium; and finished assemblies are checked for dimensions and straightness. Each unit carries a serial number that links it to the records of every batch of powder and every inspection. The aim is a failure rate in the reactor of a few units per million or fewer, and modern plants achieve it.

Fuel defects matter because a leaking element releases fission gases and iodine into the coolant, raising radiation fields and requiring the defective bundle to be located and removed. CANDU stations can find and discharge a defective bundle while operating, which is one advantage of on-power refuelling. Common historical causes of defects include debris fretting, manufacturing flaws, and stress corrosion cracking during power increases, which CANDU fuel designers address with a graphite coating on the inside of the cladding.

Mixed oxide and advanced fuels

Some fuel plants make mixed oxide fuel, called MOX, in which plutonium recovered from used fuel is combined with depleted or natural uranium to replace enriched uranium. MOX is made in France and used in reactors in France and a few other countries. Its fabrication must be done in sealed glove boxes because plutonium is an alpha emitter that is hazardous if inhaled, and it is subject to strict security. MOX is discussed further in lesson 7. Other advanced fuels include the accident-tolerant fuels covered in the materials course, metallic fuels for fast reactors, and the TRISO particle fuel used in high-temperature gas reactors, which needs HALEU.

Safety and regulation in fabrication

Fresh low-enriched fuel is only mildly radioactive, and workers can handle sealed fuel assemblies with gloves. The main hazards in a fabrication plant are inhalation of uranium dioxide powder, chemical hazards from uranium hexafluoride and hydrogen, fire, and criticality. Criticality is controlled by limiting the mass, geometry and moderation of enriched material at every step, for example by using favourable-geometry containers and by keeping water away from powder storage. Natural uranium cannot become critical in light water at all, so criticality is a much smaller concern in CANDU fuel plants than in plants handling enriched uranium.

Fuel performance in the reactor

Once loaded, fuel must perform reliably under intense conditions. The centre of a fuel pellet can reach temperatures above 1500 °C while the outside of the cladding is near the coolant temperature of about 300 °C. As burnup increases, the pellet swells, cracks and releases some of its gaseous fission products, mainly xenon and krypton, into the space inside the cladding, raising the internal pressure. The cladding creeps down onto the pellet under the coolant pressure, and the two may come into contact. Fuel designers use computer codes, validated against irradiation tests, to show that fuel stays within its design limits for temperature, internal pressure, cladding strain and corrosion throughout its life.

Fuel defects are now rare. In the 1970s and 1980s, failures caused by pellet-cladding interaction during power changes, hydriding from moisture left inside the rods, and fretting by debris or vibration were more common. Improvements such as graphite coatings inside CANDU sheaths, barrier liners in boiling water reactor cladding, debris filters at the bottom of light water reactor assemblies and better manufacturing controls have reduced failure rates to a few rods per million in the best fleets. When a defect does occur, fission products leak into the coolant and are detected by monitoring systems.

CANDU stations can remove a defective bundle while the reactor is at power, using the fuelling machines, so a defect has little effect on operation. Delayed neutron and gaseous fission product monitors identify the channel with the failed bundle.

Accident tolerant fuels

After the Fukushima accident, in which the zirconium cladding reacted with steam at high temperature and produced large amounts of hydrogen, the industry and governments began developing accident tolerant fuels. These aim to give operators more time in a severe accident by slowing the cladding's reaction with steam and keeping the fuel intact at higher temperatures. Near-term options include zirconium cladding coated with a thin layer of chromium, and pellets doped with chromium oxide to improve their behaviour. Longer-term options include cladding made of iron-chromium-aluminium alloys or silicon carbide composites.

In Canada, research on enhanced CANDU fuels has looked at similar coatings and at fuels that could reach higher burnups. CANDU fuel bundles are small and cheap, so new designs can be tested in power reactors relatively easily, a few bundles at a time.

Transport and receipt of new fuel

New fuel is only mildly radioactive and can be handled without heavy shielding, so its transport is simpler than that of used fuel. CANDU bundles are packed in boxes and shipped by truck from the fabrication plants to the stations. Light water reactor assemblies are shipped in robust containers designed to protect them from damage, because even small bends or scratches can lead to rejection. At the station, each bundle or assembly is inspected on receipt, its identity checked against the records, and it is stored in a new fuel storage area until it is needed. These records begin the accounting trail that follows the fuel through the reactor and into storage, as lesson 12 describes.

Fuel fabrication is also where the cycle's quality culture is most visible. Every pellet lot, tube and weld is traceable to its raw materials, test records and inspectors, so that if a problem appears in a reactor years later, its cause can be traced back. This traceability is required by regulators in both countries and by the quality assurance standards used across the nuclear industry.