Interim storage

Behind the station at Darlington, inside a fenced and monitored building, rows of grey concrete boxes the size of small cars stand on a concrete floor. Each is a dry storage container holding several hundred bundles that spent their first decade in the pool. "These have been the backbone of Canada's used fuel management for decades," Amara tells Hana. "They need no power, no water and no pumps. The question is not whether they work. It is how long we intend to rely on them."

Pool storage

Freshly discharged fuel goes first into a storage pool, a deep, lined concrete basin filled with purified water. The water removes decay heat, which is transferred through heat exchangers to the plant's cooling systems, and it shields workers: a few metres of water reduce the radiation from the fuel to levels at which people can work on the pool deck. Racks hold the fuel in a geometry that keeps it subcritical, using spacing or neutron-absorbing materials such as borated stainless steel, and in light water reactor pools the water itself contains dissolved boron.

Pools are robust but active systems: they depend on cooling, water chemistry control and makeup water. The Fukushima accident drew attention to pool safety because the loss of power stopped pool cooling at several units. Later reviews showed that the pools had kept their fuel covered, but regulators in both countries required improvements, including reliable pool level instruments and means to add water from outside sources, as lesson 11 of the nuclear safety course described. Pools are also limited in capacity. Plants originally expected used fuel to leave for reprocessing or disposal within a few years, and most pools have been re-racked to hold more fuel, then supplemented by dry storage.

Dry storage

After several years in the pool, fuel has cooled enough to be stored dry. In dry storage, fuel is sealed inside a metal canister or cask filled with an inert gas, usually helium, and cooled by natural air circulation around the outside. Thick concrete or steel shields the radiation. The systems are passive: they need no power, pumps or operators to keep the fuel safe, though they are inspected and monitored. Dry storage has been used in North America since the 1980s with an excellent safety record, and it is now the main way used fuel is stored once pools are full.

Canada's CANDU stations use dry storage containers designed for their bundles. At Pickering and Darlington, Ontario Power Generation loads bundles into steel baskets, places the baskets in a dry storage container made of reinforced concrete with an inner and outer steel liner, drains and dries the container, fills it with helium and seals it by welding. Each container holds 384 bundles and weighs about 70 tonnes when loaded. Containers are stored in warehouse-like buildings at the station sites. Bruce Power uses the same design at the Western Waste Management Facility. Hydro-Québec and NB Power use concrete canisters or silos for their bundles, and AECL's Chalk River and Whiteshell sites hold older fuel in similar systems.

Heat limits and cooling time

The main design limit for a dry storage system is heat. The cask must keep the cladding below a temperature limit, typically about 400 °C, to avoid creep and hydride reorientation in the cladding, and must keep the concrete below its own temperature limits. The total heat a cask can accept therefore sets how long fuel must cool in the pool before loading and how many assemblies of what burnup go into each canister. Utilities plan loading campaigns by combining hotter and cooler assemblies so that each canister stays within its licensed heat load.

# Dry storage: how long must fuel cool before a cask is within its heat limit?
# Decay heat per tonne after long cooling is approximated by a fitted power law (illustrative).
def kw_per_tHM(years, burnup_GWd):
    return 1.6 * (burnup_GWd / 45.0) * (years / 10.0) ** -0.55
cases = [("LWR assembly cask, 32 assemblies", 32 * 0.46, 50, 35.0),
         ("CANDU dry storage container, 384 bundles", 384 * 0.0192, 7.5, 3.0)]
for name, tHM, bu, limit_kW in cases:
    for yr in range(1, 41):
        if tHM * kw_per_tHM(yr, bu) <= limit_kW:
            print(f"{name:42s} meets {limit_kW:4.1f} kW after about {yr} years ({tHM*kw_per_tHM(yr,bu):.1f} kW)")
            break
print("high-burnup fuel and larger casks need longer cooling in the pool")

LWR assembly cask, 32 assemblies           meets 35.0 kW after about 6 years (34.7 kW)
CANDU dry storage container, 384 bundles   meets  3.0 kW after about 5 years (2.9 kW)
high-burnup fuel and larger casks need longer cooling in the pool

The output shows that, with these illustrative numbers, both the light water reactor canister and the CANDU container can be loaded after about five or six years in the pool. In practice the time depends on the specific design and licence; high-burnup fuel and large canisters often need longer, and loading patterns put the hottest assemblies in the centre and cooler ones at the edges. The calculation also shows why dry storage is easier for CANDU fuel: each bundle produces little heat, so the limiting factor is often handling time rather than temperature.

Safety and security of storage

Dry storage systems are designed to withstand earthquakes, floods, tornadoes and missiles driven by high winds, and to resist drops during handling. Their safety cases also consider fire and the long-term behaviour of the canister and concrete. Security measures protect the sites against theft and sabotage, under requirements set by the national regulators. Monitoring includes radiation surveys, temperature or air-flow checks on vented systems, and periodic inspections. For welded stainless steel canisters near the sea or in other chloride-rich air, chloride-induced stress corrosion cracking of the canister welds is an ageing mechanism that operators now inspect for.

In Canada, the CNSC licenses waste management facilities at each station, and the licences require monitoring and periodic safety reviews. Used fuel is expected to stay in interim storage at the stations until the deep geological repository described in lessons 10 and 11 is ready, which the NWMO currently plans for the 2040s, followed by decades of transport.

Consolidated interim storage

Canada's plan does not include a separate consolidated interim storage facility. Used fuel will move from station storage directly to the repository when it is ready, with transport packages licensed by the CNSC and Transport Canada. The NWMO has considered an optional shallow underground storage stage at the repository site as part of Adaptive Phased Management.

Transport of used fuel

Used fuel must eventually be moved from reactor sites to a repository or a central storage site, and it is already moved between sites in some cases. Transport packages are designed to the IAEA's Regulations for the Safe Transport of Radioactive Material, which national regulators adopt. Packages for used fuel are Type B packages, which must survive a sequence of tests representing severe accidents: a 9 m drop onto an unyielding surface, a 1 m drop onto a steel bar, a fully engulfing fire at 800 °C for 30 minutes and immersion in water. Full-scale tests in the 1970s and 1980s, in which casks were hit by locomotives and rocket-propelled trucks, showed that they would remain intact in extreme crashes.

In Canada, packages are certified by the CNSC and transport is also regulated by Transport Canada. The NWMO has designed a used fuel transportation package for CANDU bundles, which holds 192 bundles in two modules, and has had it certified by the CNSC. Its transportation planning includes engagement with communities along possible routes, which will mostly be by road, and emergency response arrangements.

Storage after plant shutdown

When a reactor shuts down permanently, its used fuel remains. Operators usually move the remaining fuel from the pool to dry storage within several years so that the pool and the rest of the plant can be decommissioned. The site then contains only the storage facility, its security systems and a small staff. Such sites are sometimes called stranded fuel sites, because the fuel cannot leave until a disposal or central storage facility is available. The cost of operating them, typically several million dollars a year each, continues for as long as the fuel remains.

In Canada, the shutdown of the Gentilly-2 station in Quebec in 2012 and the planned end of operation of some Pickering units show how stations move all their fuel into dry storage. Hydro-Québec has transferred all of Gentilly-2's used fuel to concrete canisters at the site, where it awaits the repository.

Long-term storage research

Interim storage was originally expected to last a few decades. With repositories delayed, storage for a century or more has to be considered. Research programs in both countries are studying how fuel, cladding, canisters, bolted seals and concrete age over such periods. Key questions include whether cladding embrittlement from hydrides could affect later handling, how quickly stress corrosion cracks could grow in canister welds, and how concrete overpacks and storage buildings weather. The results support licence renewals and help design the inspections needed to show that stored fuel remains safe and retrievable when it is finally moved.

Choosing between storage options

Utilities choose among storage options by weighing cost, flexibility and licensing. Re-racking a pool to hold more fuel is cheap but limited, and it increases the heat load and the consequences of a loss of cooling. Dry storage has higher costs at the start, for the pad or building and the handling equipment, but each additional cask or container adds capacity at a predictable cost. Some systems are designed so that the canister can later be placed in a transport overpack and shipped without reopening it, which simplifies the eventual move to a repository or central site. Others, including the CANDU dry storage containers, will need their fuel to be transferred to a different package for transport.

Storage also interacts with the design of the repository. If the repository will use a different container from the storage system, as both the NWMO and most other programs plan, the fuel will have to be handled again at the repository site, in a facility that receives the transport packages, removes the fuel and loads it into disposal containers. The NWMO's design includes such a used fuel packaging plant, where bundles will be placed in the 48-bundle containers and sealed before being taken underground. Planning storage, transport and disposal together reduces the number of times the fuel is handled and the dose to workers.