18-Env-B5 Industrial & Hazardous Waste Management · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Reference texts: Nemerow & Dasgupta, Industrial and Hazardous Waste Treatment, 2nd ed.; Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery, 5th ed.; Davis & Cornwell, Introduction to Environmental Engineering, 6th ed.; LaGrega, Buckingham & Evans, Hazardous Waste Management, 2nd ed.; CCME, Guidelines for the Management of Biomedical Waste in Canada (1992); Canadian Nuclear Safety Commission (CNSC) regulatory framework and NWMO Adaptive Phased Management; Canadian Environmental Protection Act (CEPA), 1999.
All nineteen questions are compulsory on this paper and are answered in full below.
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 heavy water reactor — the Canadian CANDU design being the principal example — uses natural, unenriched uranium as fuel, because heavy water (D2O) is a far more efficient neutron moderator than ordinary water; this is the single feature that most distinguishes its fuel cycle from a light-water reactor's, which requires uranium enriched to roughly 3–5% U-235. The once-through CANDU fuel cycle therefore has no enrichment stage at all.
The cycle proceeds as follows. Mining extracts uranium ore (Canada's major deposits are in northern Saskatchewan). Milling, at a mill near the mine site, crushes and leaches the ore and precipitates a concentrate known as yellowcake (predominantly U3O8). Refining and conversion purifies the yellowcake and converts it to uranium dioxide (UO2) powder — with no enrichment step in between, since the natural U-235 fraction (≈0.7%) is retained. Fuel fabrication presses and sinters the UO2 powder into ceramic pellets, which are loaded into zirconium-alloy tubes bundled into the fuel bundles a CANDU reactor uses. In the reactor, the bundles are fissioned to generate heat while heavy water simultaneously moderates the neutrons and, in a separate loop, removes heat to raise steam. Used fuel discharged from the reactor is first placed in a shielded, water-filled bay at the reactor site for roughly a decade to allow short-lived activity and decay heat to fall, then transferred to passively-cooled dry storage in concrete canisters, and is ultimately destined for a Deep Geological Repository for permanent isolation — the final stage of the once-through cycle, since CANDU used fuel in Canada is not currently reprocessed.
