Question 8 of 8: Environmental Impact

Question 8: Environmental Impact (10 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.

The three technologies fail the environment in different currencies, which is why they are so hard to compare. Coal's impact is continuous, diffuse and atmospheric; nuclear's is concentrated, small in volume and very long-lived; hydro's is immediate, local, permanent and entirely physical. A fair comparison has to be made per unit of energy delivered over the plant's life, and the answers below are framed against Canadian practice — the Canadian Environmental Assessment Act 2012 and the Impact Assessment Act 2019, the Canadian Nuclear Safety Commission's licensing regime under the Nuclear Safety and Control Act, the Fisheries Act, and the federal coal-fired electricity performance standards that require conventional units to retire or meet a 420 t CO₂/GWh limit by 2030.

(a) Physical disruption to build the plant and supply the energy

Coal. The station itself occupies a modest site, but the fuel chain is enormous: a 500 MW unit burns roughly 1.5 million tonnes of coal a year, which has to be mined, hauled and stockpiled. Surface mining strips overburden across square kilometres, destroys soil profiles and habitat, lowers water tables and generates acid rock drainage from exposed sulphides; underground mining brings subsidence and methane release. Unit trains or conveyor lines and the associated rail corridors fragment land, and the stockpile itself sheds coal dust and leachate. Cooling, whether once-through or by tower, needs a large water withdrawal and a large intake structure. Alleviation: progressive reclamation and contouring behind the working face with topsoil replaced and native species re-established; capture and treatment of drainage water; enclosed or wetted stockpiles and covered conveyors; methane capture for use rather than venting; siting new units on existing brownfield or mine-mouth sites to shorten the haul; and screened, low-velocity intakes with the fish-passage provisions the Fisheries Act requires.

Nuclear. The station footprint is small for the energy delivered because the fuel chain is tiny — the 83 tonnes of uranium dioxide in the core of Question 4 last several years, against millions of tonnes of coal a year. The disruption is concentrated at the mine and the mill: uranium mining in northern Saskatchewan produces tailings that carry radium-226, thorium-230, arsenic and selenium, and the mill site must be managed in perpetuity. At the station the exclusion zone, the transmission corridor and the cooling-water intake and outfall are the main physical impacts, along with the entrainment and impingement of fish at the intake and the thermal plume at the outfall. Alleviation: subaqueous or engineered tailings management facilities with permanent water cover to prevent oxidation, as at Key Lake and McClean Lake; CNSC-licensed decommissioning funds set aside from first power; low-approach-velocity intakes with fine screens and fish-return systems; diffuser outfalls or cooling towers to disperse the thermal plume; and siting several units on one site to share the corridor and the exclusion zone.

Hydro. This is the most severe and most permanent physical disruption of the three. Impounding a reservoir floods valley bottoms — the most biologically productive land in the watershed — permanently displacing terrestrial habitat, forest and, very often, communities. In Canada this has fallen disproportionately on Indigenous peoples, and the flooding of traditional territory at schemes such as La Grande, Churchill Falls and Site C is the defining environmental issue of the technology. The dam blocks fish migration, traps the sediment that built the downstream delta, changes the flow regime from seasonal to demand-driven, and alters downstream temperature and dissolved gas. Decaying flooded vegetation releases methane and mobilises mercury, which methylates and accumulates in fish for two to three decades after impoundment. Alleviation: genuine consultation and impact-benefit agreements with affected Indigenous communities under the duty to consult; clearing merchantable timber and vegetation before flooding to cut both methane and mercury; fish ladders, lifts or trap-and-haul, and turbine designs with better passage survival; mandated minimum ecological flows and ramping limits to protect downstream habitat and spawning beds; sediment sluicing or bypass; and preferring run-of-river or the redevelopment of existing dams over new impoundments.

(b) Effluents produced during operation, and their minimisation

Coal. Coal is the only one of the three that discharges its principal waste to the atmosphere continuously. Complete combustion of the carbon in the fuel produces carbon dioxide at roughly 850–1000 kg per MWh, which no post-combustion measure short of capture can reduce; sulphur in the fuel oxidises to SO₂, the precursor of acid deposition; nitrogen oxides form both from fuel nitrogen and thermally in the flame; and the incombustible mineral matter leaves as fly ash carrying trace mercury, arsenic, selenium and lead. Thermal discharge from the condenser and blowdown from the boiler and the cooling tower are the liquid effluents. Minimisation: low-NO_x burners with overfire air and staged combustion, followed where necessary by selective catalytic reduction, which together cut NO_x by 80–90 %; wet limestone flue-gas desulphurisation removing 90–98 % of the SO₂ and producing saleable gypsum; electrostatic precipitators or fabric filters removing over 99.5 % of the particulate; activated-carbon injection for mercury; raising cycle efficiency, since every point of efficiency cuts every emission per MWh proportionally — the 43 % of Question 5 against an older unit's 35 % is a 19 % cut in CO₂ per kilowatt-hour; and, ultimately, carbon capture and storage, demonstrated in Canada at Boundary Dam Unit 3.

Nuclear. Operation produces no carbon dioxide, no sulphur or nitrogen oxides and no particulate. The gaseous effluents are small, monitored releases of tritium, carbon-14 and noble gases, which at a CANDU station are dominated by tritium from neutron activation of the heavy-water moderator; liquid effluents are tritiated water and traces of activation and fission products, all released under CNSC-licensed derived release limits that keep public dose to a small fraction of the 1 mSv/year public limit — typically a few microsieverts, well below natural background variation. The largest routine effluent is in fact thermal: because the cycle efficiency is around 33 %, as Question 1 showed, a nuclear station rejects roughly half as much heat again per MWh as a modern coal unit. Minimisation: tritium removal facilities such as Darlington's; leak-tight fuelling and primary-circuit design; ion-exchange and filtration of all liquid streams with monitored batch release and dilution; cooling towers or diffuser outfalls to limit the thermal plume; and defence in depth with containment to keep the accident risk — which is the real environmental concern with this technology, not routine effluent — acceptably low.

Hydro. Operation produces no combustion effluent of any kind. What it does produce is physical and biological: total dissolved gas supersaturation below spillways, which causes gas-bubble trauma in fish; changed temperature and dissolved oxygen in the release, often cold and deoxygenated water drawn from the bottom of a stratified reservoir; methylmercury in the reservoir food web for two to three decades; methane and carbon dioxide from decomposing flooded biomass, which for a shallow boreal reservoir can rival a gas turbine's emissions per MWh in the first years, though it falls to near zero for a deep northern reservoir over its life; and the erosion and stranding caused by rapid flow changes when the plant follows load. Minimisation: spillway aeration and flow deflectors to control dissolved gas; multi-level intakes so that release temperature can be selected; pre-clearing of vegetation to cut both mercury and methane; ramping-rate limits and minimum flows written into the water licence; and fish consumption advisories during the mercury peak.

(c) Solid waste products and their disposal

Coal. By far the largest volume of the three. A 500 MW unit produces roughly 100 000–200 000 tonnes a year of fly ash and bottom ash, plus a comparable tonnage of gypsum if the plant is scrubbed. The ash contains the trace metals concentrated from the fuel and is potentially leachable. Disposal: the preferred route is beneficial use rather than disposal — fly ash is a supplementary cementitious material that replaces 15–30 % of Portland cement in concrete, cutting the cement industry's own emissions; bottom ash serves as structural fill and road base; and scrubber gypsum is used directly in wallboard. What cannot be used goes to engineered landfills with compacted clay or geomembrane liners, leachate collection and treatment, progressive capping and revegetation, and long-term groundwater monitoring. Wet ash ponds, which have failed catastrophically elsewhere, are being converted to dry handling.

Nuclear. The smallest volume and the longest hazard. A CANDU unit discharges its used fuel at roughly 20 m³ a year — a few tens of tonnes — but it remains hazardous for hundreds of thousands of years, and there is also a larger volume of low and intermediate-level waste from filters, resins, protective clothing and eventually the decommissioned reactor components. Disposal: used fuel is first stored in the bay for six to ten years while the short-lived activity and the decay heat fall by orders of magnitude, then transferred to dry storage in concrete canisters or silos on site. The long-term Canadian solution is the Nuclear Waste Management Organization's Adaptive Phased Management — containment and isolation in a deep geological repository in a stable rock formation with multiple engineered barriers (a corrosion-resistant copper-coated container, a bentonite clay buffer, and the host rock itself), monitored and retrievable, with the Wabigoon Lake Ojibway Nation/Ignace area in northwestern Ontario selected as the site in 2024. Low and intermediate-level waste goes to licensed engineered surface or near-surface facilities after volume reduction by compaction and incineration. The programme is funded from the electricity price during operation rather than left to a future generation.

Hydro. Essentially none. Operation generates no solid waste beyond maintenance materials, trash-rack debris and waste oil from the machines, all handled as ordinary industrial waste. The counterpart is the sediment that the reservoir traps — which is not a waste in the regulatory sense but is a genuine long-term problem, since it progressively consumes storage volume and starves the downstream channel and delta of the material that maintains them. Management: sediment sluicing, flushing or bypass tunnels; upstream watershed management to reduce the load; and, at the end of the structure's life, a decommissioning plan that addresses the contaminated sediment already impounded.

Overall comparison. No source is benign, and the choice is between different kinds of harm rather than between harm and none. Coal's damage is the largest in total and the most diffuse, and it is the only one of the three that is fundamentally unfixable without carbon capture, because carbon dioxide is a stoichiometric product of burning the fuel rather than an impurity that can be scrubbed. Nuclear's damage is the smallest in volume and the most completely contained during operation, but it is unique in placing an obligation on societies hundreds of thousands of years away and in carrying a low-probability, high-consequence accident risk. Hydro's damage is the most immediate and the most irreversible — a flooded valley is not recoverable — but it is confined to a known place and a known time, and once built the plant runs for a century with no fuel and no effluent. In Canadian practice the three are complementary rather than competing, and Question 7 is the reason: hydro with storage provides the flexibility, nuclear the low-carbon base load, and the fossil plant the residual peaking duty that is now being displaced by storage and by interties.