Question 6 of 6: Environmental Impact — comparing coal, nuclear and hydro generation

Question 6: Environmental Impact — comparing coal, nuclear and hydro generation (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.

(a) Physical disruption to build the plant and supply the energy (3 marks)

Coal generation disturbs far more land off site than on. The station itself occupies a few square kilometres including the ash lagoons and the cooling towers, but the fuel supply chain is the dominant footprint: the 9.9 million tonnes a year computed in Question 1 for a six-unit station implies a large surface mine or a continuously worked underground field, with overburden stripping, altered drainage, acid rock drainage from exposed sulphides, and either a dedicated rail corridor or, as at Matla, a mine-mouth conveyor. Alleviation is mostly a matter of planning and progressive rehabilitation: mine-mouth siting to eliminate the transport corridor, contemporaneous backfilling and revegetation rather than deferred reclamation, sealing of spoil to prevent acid drainage, and financial-assurance instruments so that closure is funded before it is needed. In Canada this is regulated through provincial mine permitting and the federal Impact Assessment Act for designated projects.

Nuclear generation has the smallest physical footprint per megawatt of the three, because the energy density of the fuel is some two million times that of coal: the 83 tonnes of uranium dioxide in the Question 4 core replaces millions of tonnes of coal per year. Disruption is concentrated at the uranium mine and mill — in Canada, the Athabasca Basin operations in northern Saskatchewan — where the concerns are tailings management, radon release and radiologically contaminated water, and at the station itself, where a large once-through or tower-cooled circulating water system creates thermal and entrainment effects. Alleviation includes underground mining with raise-bore or jet-boring methods and freeze walls to isolate ore from groundwater, engineered tailings facilities, and cooling towers or offshore diffusers in place of once-through discharge. Sites are also fenced for security, so land use is exclusive but small.

Hydro generation reverses the pattern: it has no fuel chain at all, and almost all of its impact is the one-off physical rearrangement of a river system. Reservoir creation floods valley bottoms, which are precisely the most biologically productive and most often Indigenous-occupied land; the dam blocks fish migration and sediment transport, alters the downstream thermal and flow regime, and induces bank erosion and sometimes reservoir-triggered seismicity. Construction itself moves enormous volumes of rock and concrete. Alleviation is design-led: run-of-river and low-head schemes in place of large storage where the hydrology permits, fish ladders and downstream bypass or trap-and-haul, environmental flow releases and ramping-rate limits, selective withdrawal to manage downstream temperature, clearing of vegetation before flooding to limit methylmercury and methane generation, and negotiated impact-benefit agreements with affected Indigenous nations, which in the Canadian setting is a legal duty flowing from section 35 of the Constitution Act, 1982 as much as an environmental measure.

(b) Effluents during operation, their mechanisms and their minimisation (5 marks)

Coal is by far the largest effluent producer of the three, and the mechanisms are well understood. Complete oxidation of the fuel carbon yields the 3300 Mg/h of carbon dioxide computed in Question 1, some 21.7 million tonnes a year for the station, and there is no end-of-pipe treatment for it short of capture and geological storage, which imposes a 20 to 30 % energy penalty; the only large lever is efficiency, since carbon dioxide per kilowatt hour is inversely proportional to the plant efficiency, which is why supercritical and ultra-supercritical steam conditions matter environmentally and not merely economically. Sulphur in the coal oxidises to sulphur dioxide, controlled to above 90 % removal by wet limestone flue gas desulphurisation. Nitrogen oxides arise both from fuel-bound nitrogen and from thermal fixation of atmospheric nitrogen in the flame, and are controlled first by combustion measures — low-NO_x staged burners and overfire air, the air-staging discussed in Question 5 — and then by selective catalytic reduction. Particulate matter, which for this station means most of the 375 Mg/h of ash, is captured to better than 99.5 % by electrostatic precipitators or fabric filters. Trace mercury and other heavy metals are volatilised from the coal and are captured incidentally on fly ash and in the scrubber, or deliberately by activated carbon injection; Canada is party to the Minamata Convention on mercury. Finally, the cooling tower rejects 650 MW per unit to the atmosphere and consumes the 5405 m³/h of make-up water calculated in Question 1, with a blowdown stream carrying concentrated dissolved solids and treatment chemicals that requires its own management.

Nuclear operation produces essentially no atmospheric effluent: no carbon dioxide, no sulphur or nitrogen oxides, no particulate. Its routine releases are small and radiological — tritium and short-lived noble gases and iodines in gaseous and liquid effluent, controlled by delay tanks that let short half-lives decay, by filtration and ion exchange, and by dilution, and licensed by the Canadian Nuclear Safety Commission to derived release limits that in practice keep public dose to a small fraction of the roughly 1.8 mSv/y natural background. The environmentally significant continuous effluent is heat: the same first-law arithmetic as Question 1 applies, and because a light water reactor cycle operates at a lower peak temperature than a modern coal boiler its efficiency is nearer 33 than 43 %, so it rejects proportionally more heat per kilowatt hour. Minimisation is by cooling tower rather than once-through cooling, by diffuser design, and by thermal-plume limits in the operating licence.

Hydro generation produces no combustion effluent at all, which is its central advantage, but it is not effluent-free. Newly flooded reservoirs decompose submerged organic carbon anaerobically, releasing methane — a greenhouse gas some 28 times more potent than carbon dioxide over a century — and generating methylmercury, which bioaccumulates in fish and has led to consumption advisories at Canadian reservoirs including Muskrat Falls and the La Grande complex. Water passing the turbines may be supersaturated with dissolved nitrogen at spillways, causing gas-bubble trauma in fish, and hypolimnetic releases are cold and low in oxygen. Minimisation is again design-led: pre-flooding clearance of biomass, spillway aeration and flow deflectors to control total dissolved gas, selective-withdrawal intakes, and minimum-flow and ramping-rate licence conditions to protect the downstream reach.

(c) Solid waste products and their disposal (5 marks)

Coal produces solid waste in industrial quantities: the 375 Mg/h of ash in Question 1 amounts to about 2.46 million tonnes a year for the station, of which roughly four fifths is fly ash and the remainder bottom ash, plus a further large tonnage of calcium sulphate from the desulphurisation plant. The material is not inert — it concentrates arsenic, selenium, boron and mercury, and it is alkaline — so wet impoundment in ash lagoons carries a real risk of groundwater leaching and of catastrophic dyke failure. The environmentally preferable route is beneficial reuse: fly ash is a well-established supplementary cementitious material that displaces Portland cement, in itself a large carbon saving, and is used in structural fill, road base and lightweight aggregate, while scrubber gypsum is sold to the wallboard industry. Ash that cannot be sold should go to dry, lined and capped landfill rather than a wet lagoon, with leachate collection and groundwater monitoring.

Nuclear waste is tiny in volume and extreme in specific hazard, which inverts the whole disposal problem. The Question 4 core discharges of the order of twenty tonnes of spent fuel a year, a volume measured in cubic metres rather than millions of tonnes, but it contains fission products requiring isolation for centuries and actinides for hundreds of thousands of years. Canadian practice is a graded system: spent fuel cools for seven to ten years in at-reactor pools where water provides both cooling and shielding, is then transferred to dry storage in concrete canisters on site, and is destined for a deep geological repository under the Nuclear Fuel Waste Act, implemented by the Nuclear Waste Management Organization as adaptive phased management, with a willing host community identified in the Ignace and Wabigoon Lake area of northwestern Ontario. Low and intermediate level waste — ion exchange resins, filters, contaminated clothing and eventually decommissioning rubble — is far larger in volume and is compacted, incinerated or grouted and disposed of in engineered near-surface or intermediate-depth facilities. The defence is multi-barrier: ceramic fuel matrix, corrosion-resistant container, engineered bentonite buffer and stable host rock.

Hydro generation produces almost no operational solid waste. What it does accumulate is trapped sediment, which is a waste stream in the sense that it must be managed: reservoirs intercept the bedload and suspended load that would otherwise nourish downstream channels and deltas, so storage capacity is progressively lost while the downstream reach is starved and erodes. Management options are sediment sluicing and flushing through low-level outlets, bypass tunnels around the reservoir, and mechanical dredging with beneficial use of the dredgate. Trash-rack screenings, waste lubricating oil and grease from the turbine and governor systems, and eventually the concrete and steel of decommissioning are the only other solid streams, all of them ordinary industrial wastes handled by recycling or licensed landfill. Because reservoir sediment may carry historical contaminants and methylmercury, dredged material should be characterised before beneficial use.

(d) Ranking by installed capacity, now and in fifty years (2 marks)

Present ranking, on installed megawatts in the Canadian system, is hydro first, nuclear second and coal third. That order reflects geography and history rather than the environmental analysis above: Canada happens to possess exceptional hydraulic resources in British Columbia, Manitoba, Quebec, Labrador and Ontario, and developed them because they were the cheapest firm capacity available. Nuclear is concentrated in Ontario and at Point Lepreau in New Brunswick and supplies the majority of Ontario’s energy from a modest share of its capacity, because it runs base load at high capacity factor. Coal, once dominant in Alberta, Saskatchewan and Nova Scotia, has been displaced by gas and renewables and is being closed under the federal carbon dioxide performance standards, SOR/2018-263, which require conventional coal units to cease operating or meet a gas-equivalent emission intensity by 2030.

In fifty years the ranking is likely to be unchanged in order but very different in character. Hydro remains first because the installed base is long-lived, refurbishment extends it almost indefinitely, and the storage it provides becomes more valuable, not less, as intermittent wind and solar capacity grows and needs balancing; new large-storage development, however, will be limited by the very impacts described in part (a) and by Indigenous consent requirements, so growth will come mainly from uprating and from pumped storage. Nuclear remains second and probably grows, because it is the only dispatchable source that is both carbon-free and independent of site hydrology; small modular reactors at Darlington and in Saskatchewan are the vehicle, and the constraint is public confidence in the waste solution rather than physics. Coal falls to essentially zero for conventional units, surviving if at all only where carbon capture and storage is retrofitted to a unit with a nearby geological sink, because its carbon dioxide, its 2.5 million tonnes a year of ash and its regulated status make it the most environmentally burdened of the three on every criterion in parts (a) to (c). The overall pattern is worth stating plainly: the environmental cost of coal is continuous and diffuse, that of nuclear is concentrated and manageable but requires multi-generational institutional commitment, and that of hydro is a large one-off ecological rearrangement followed by decades of clean operation.