Question 5 of 6: Environmental Impact

Question 5: Environmental Impact (15 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Part (a) — physical disruption to build the plant and supply the fuel. The three technologies disturb the land in quite different ways, and in each case the larger disturbance lies in the fuel supply chain rather than in the power station itself. A coal station occupies perhaps two square kilometres including its ash lagoons and coal yard, but the mines that feed it are the real footprint: at the 1.5 million tonnes a year computed in Question 2, a single 500 MW unit consumes the output of a substantial surface mine, and surface mining removes overburden, destroys soil profiles, alters drainage and leaves highwalls and spoil. Mitigation is well established in Canada under provincial mine-reclamation legislation and the federal Impact Assessment Act: progressive backfilling and contouring behind the working face, salvage and replacement of topsoil, revegetation with native species, treatment of acid rock drainage, and a reclamation bond posted before mining starts. Rail corridors and the station site itself are comparatively minor, and largely reversible.

A nuclear station has the smallest land footprint of the three by a wide margin, because the energy density of the fuel is enormous — a kilogram of fissioned uranium-235 releases some 82 TJ against 27 MJ for a kilogram of coal, a ratio of about three million to one, so the annual fuel requirement of a 1000 MW unit arrives in a few truckloads rather than a train a day. The disruption is concentrated at the uranium mine and mill, where tailings are radioactive and must be isolated from groundwater in engineered impoundments, and at the site itself, which requires an exclusion zone and a large cooling-water intake and outfall. Canadian practice under the Canadian Nuclear Safety Commission places tailings in below-grade pits with engineered covers and long-term monitoring, and requires a funded decommissioning plan for the station from the outset.

Hydroelectric development inverts the picture: the plant itself is the disruption. Impoundment floods valley bottoms, which are precisely the most productive habitat and the land most likely to be occupied, so reservoirs displace communities, drown forest and agricultural land, block fish migration, trap sediment and change the downstream flow, temperature and ice regime for hundreds of kilometres. In Canada these effects fall disproportionately on Indigenous communities, which is why the duty to consult and accommodate, and increasingly impact-benefit agreements and equity participation, are central to any new project. Technical mitigation includes fish ladders and bypasses, clearing the reservoir basin before flooding to limit methane generation and methylmercury formation, minimum-flow and ramping-rate licence conditions to protect downstream habitat, and preference for run-of-river or redevelopment of existing dams over new large storage.

Part (b) — effluents during operation, their mechanisms and their control. Coal combustion is by far the largest source of operational effluent. Oxidising the carbon in the fuel produces carbon dioxide in direct proportion to the carbon burned — 3.66 kg of CO₂ per kilogram of carbon, which for a typical western Canadian sub-bituminous coal is about 98 kg per GJ of fuel, or roughly 925 g per kWh at 38 % efficiency. Sulphur in the coal oxidises to SO₂; nitrogen from both the fuel and the combustion air forms NO_x, thermally at flame temperature and from fuel-bound nitrogen; incomplete capture leaves fine particulate; and trace mercury and other metals are volatilised. Every one of these except CO₂ has a mature control: electrostatic precipitators or fabric filters remove better than 99.5 % of the particulate, wet limestone scrubbers remove 90–98 % of the SO₂, low-NO_x burners with overfire air and selective catalytic reduction cut NO_x by 80 % or more, and activated-carbon injection captures mercury. CO₂ is the exception, because it is a stoichiometric product and not an impurity: the only genuine reductions come from raising efficiency (supercritical and ultra-supercritical steam conditions), switching fuel, or capture and storage, of which SaskPower's Boundary Dam Unit 3 remains the only utility-scale example on a coal unit. Thermal discharge is the other operational effluent, and Question 3 quantified it at 1499 MW for a 1000 MW coal station.

A nuclear station produces essentially no atmospheric effluent in normal operation. Its routine releases are small and monitored: tritium and short-lived noble gases through the stack, tritiated water from the heavy-water systems of a CANDU, and low-level liquid effluent, all held far below CNSC derived release limits. Its dominant operational discharge is heat, and it is larger than the coal plant's — 2147 MW against 1499 MW for the same 1000 MW electrical output, because of the lower cycle efficiency and because the reactor loss is water cooled. That is minimised by cooling towers or by siting on a water body large enough to accept the load, and by mixing-zone limits in the discharge permit. The genuinely serious risk is not routine effluent at all but severe accident release, which is managed by defence in depth: containment, redundant and diverse safety systems, passive heat sinks, and emergency planning zones.

Hydro produces no combustion effluent, but it is not effluent-free. Newly flooded reservoirs decompose submerged organic matter anaerobically, releasing methane and CO₂ for one to three decades and, more insidiously, promoting bacterial methylation of naturally occurring mercury, which bioaccumulates in fish and has closed fisheries at several northern Canadian reservoirs. Supersaturated dissolved gas at spillways causes gas-bubble trauma in fish. Water released from the depth of a reservoir is colder and lower in oxygen than the natural river. All of these are controlled by design and operating practice: clearing vegetation before flooding, multi-level intakes to manage discharge temperature, spillway deflectors to limit gas entrainment, and aeration of releases.

Part (c) — solid wastes and their disposal. Coal produces by far the largest solid waste stream by mass. A coal with 10 % ash burned at 1.5 million tonnes a year yields on the order of 150 000 tonnes of ash annually, split between fly ash carried out with the flue gas and captured in the precipitator, and bottom ash falling into the furnace hopper; scrubbing adds a comparable tonnage of gypsum. These materials are not inert — they concentrate arsenic, selenium, boron and mercury, and wet impoundment has produced serious failures elsewhere. The preferred disposal route is therefore beneficial reuse, and fly ash is a genuinely valuable supplementary cementing material that both consumes the waste and lowers the CO₂ of concrete; scrubber gypsum goes to wallboard. What cannot be reused should go to dry, lined and capped landfill with leachate collection rather than to wet ponds, with progressive capping and revegetation.

Nuclear waste is tiny in volume and enormous in hazard, and is managed by classification. Low- and intermediate-level waste — contaminated clothing, filters, resins, reactor components — is compacted, immobilised and stored in engineered surface or shallow facilities. Spent fuel from a CANDU is discharged at roughly 90 Mg per GW-year, held for six to ten years in water-filled bays where the decay heat falls by two orders of magnitude, then transferred to dry storage in concrete canisters. The permanent solution accepted internationally, and pursued in Canada by the Nuclear Waste Management Organization under the Adaptive Phased Management programme, is a deep geological repository in stable rock several hundred metres down, using multiple engineered barriers — corrosion-resistant containers, bentonite buffer, and the host rock itself — with the waste retrievable for a long period. The mass involved is such that all the used fuel produced in Canada since the 1960s would occupy a volume smaller than a few hockey rinks.

Hydro produces almost no solid waste in operation. What it produces instead is trapped sediment: a reservoir intercepts the bed load and much of the suspended load of its river, which starves the downstream channel and delta of sediment while progressively consuming the storage volume. Management options are sluicing and density-current venting during floods, mechanical dredging, and sediment bypass tunnels on new schemes. The other solid streams are debris and weed removed from the trash racks, which is composted or landfilled, and, at end of life, the concrete and steel of the structure itself.

Part (d) — ranking now and in fifty years. On installed capacity in Canada today the order is unambiguous: hydro first at about 55 % of installed MW, fossil second at roughly a quarter (of which coal is now a minority and falling), and nuclear third at about 9 %, concentrated in Ontario and New Brunswick. Hydro leads because Canada's geography endowed it with exceptional head and flow, because the plant is long-lived and dispatchable, and because it was built when the environmental and consultative constraints described above were not binding.

Fifty years out, the defensible ranking is hydro first, nuclear second and coal last and essentially absent. The reasoning follows directly from parts (a) to (c). Coal is the only one of the three whose principal effluent cannot be controlled by add-on equipment, and Canadian regulation has already legislated the conclusion: conventional coal-fired generation must be phased out by 2030 unless it meets a performance standard that in practice requires carbon capture. Nuclear rises because it is the only firm, high-capacity-factor, near-zero-carbon source that can be sited where the load is; its waste is small, characterised and contained, and small modular reactors address the capital-lumpiness that has held it back. Hydro remains first on installed MW because the existing fleet does not retire and its storage becomes more valuable, not less, as variable wind and solar grow — but its growth will be slow, because the remaining sites are remote and the social and ecological objections are now decisive. The honest qualification to any such ranking is that wind and solar, which this question does not include, are growing faster than all three and will change the composition of the grid around them.