Q1. In situ leaching (ISL) of uranium requires a specific geological setting. Which of the following is an essential requirement?
(a) The ore body must be located above the water table in fractured granite (b) The deposit must be in a permeable confined aquifer bounded by impermeable strata (c) The ore must be located in an ignite seam at depths exceeding 500 m (d) The deposit must be in volcanic rock with natural fissures for solution flow
Answer: (b) — ISL requires a permeable formation (typically sandstone) confined above and below by impermeable layers, and the deposit must be below the water table. This ensures the leaching solution contacts the ore and can be recovered without contaminating surrounding aquifers.
Q2. Yellowcake, the tradeable product of uranium milling, is commonly described as containing:
(a) Pure metallic uranium powder (b) Uranium hexafluoride (UF₆) (c) Ammonium diuranate / triuranium octoxide (U₃O₈) (d) Uranium tetrafluoride (UF₄, “green salt”)
Answer: (c) — Yellowcake is a mixture of uranium oxides, predominantly U₃O₈ after calcination. The bright yellow colour comes from ammonium diuranate (ADU), the intermediate precipitate. It is traded and priced in units of $/lb U₃O₈.
Q3. At what approximate temperature does uranium hexafluoride (UF₆) sublime at atmospheric pressure?
(a) 28.5 degrees C (b) 56.5 degrees C (c) 114 degrees C (d) 210 degrees C
Answer: (b) — UF₆ sublimes (transitions directly from solid to gas) at 56.5 degrees C at atmospheric pressure. This conveniently low temperature makes it practical to handle UF₆ in the gas phase for enrichment processes.
Q4. Why is uranium hexafluoride (UF₆) the compound of choice for isotopic enrichment of uranium?
(a) It is the most chemically stable uranium compound and therefore safest to handle (b) It is the only readily available gaseous uranium compound, and fluorine has only one stable isotope (F-19), so mass differences between molecules are due solely to the uranium isotope (c) It has the highest uranium density of any compound, maximising throughput (d) It is non-reactive with water, making it easy to store and transport
Answer: (b) — Both gaseous diffusion and gas centrifuge methods require the uranium to be in gaseous form. UF₆ is gaseous above 56.5 degrees C. Crucially, fluorine is mono-isotopic (only F-19), so any mass difference between UF₆ molecules arises entirely from the uranium isotope (U-235 vs U-238), enabling effective mass-based separation.
Q5. In the context of uranium enrichment, which of the following changes would increase the number of Separative Work Units (SWU) required?
(a) Increasing the feed enrichment above natural uranium (b) Increasing the tails assay (c) Decreasing the product enrichment (d) Increasing the product enrichment and/or decreasing the tails assay
Answer: (d) — SWU increases when the enrichment plant must work harder: either producing a higher-enriched product or squeezing more U-235 out of the feed (lower tails assay). Increasing feed enrichment or tails assay, or decreasing product enrichment, all reduce the required SWU.
Q6. Compared to gaseous diffusion, the energy consumption of a modern gas centrifuge enrichment plant per SWU is approximately:
(a) The same — both methods require similar energy input (b) 5 times less (centrifuge uses ~20% of diffusion energy) (c) 50 times less (centrifuge uses ~2% of diffusion energy) (d) 500 times less (centrifuge uses ~0.2% of diffusion energy)
Answer: (c) — Gas centrifuge enrichment requires approximately 50 kWh/SWU compared to approximately 2,400 kWh/SWU for gaseous diffusion — roughly 50 times less energy. This enormous energy saving is the principal reason centrifuge technology has completely replaced gaseous diffusion commercially.
Q7. An enrichment plant operator is deciding between a tails assay of 0.30% and 0.20%. Compared to 0.30% tails, choosing a tails assay of 0.20% will:
(a) Require more natural uranium feed but fewer SWU (b) Require less natural uranium feed but more SWU (c) Require both less feed and fewer SWU (d) Require both more feed and more SWU
Answer: (b) — Lowering the tails assay extracts more U-235 from each kilogram of feed, reducing the feed requirement. However, the enrichment plant must do more separative work to achieve this, increasing the SWU cost. The economically optimal tails assay balances the cost of natural uranium against the cost of enrichment.
Q8. If a container of UF₆ is breached and the contents contact atmospheric moisture, the primary immediate hazard to workers is:
(a) A criticality excursion from moderation by the water (b) High gamma dose rates from the exposed uranium (c) Chemical toxicity from hydrofluoric acid (HF) produced by the reaction (d) Neutron radiation from spontaneous fission of U-238
Answer: (c) — UF₆ reacts violently with water to produce uranyl fluoride (UO₂F₂) and hydrofluoric acid (HF). HF is extremely corrosive and toxic, and represents the dominant immediate hazard. At natural or low enrichment, the radiological hazard is far less significant than the chemical toxicity of HF.
Q9. The principal radiological hazard to workers in underground uranium mines is:
(a) External gamma radiation from U-238 in the ore (b) Neutron radiation from spontaneous fission in the ore body (c) Inhalation of radon-222 (Rn-222) gas and its short-lived daughter products (d) Ingestion of uranium-contaminated groundwater
Answer: (c) — Radon-222 is a radioactive noble gas produced by the decay of radium-226 in the uranium decay chain. It seeps out of the rock into mine air. Inhalation of Rn-222 and its short-lived alpha-emitting daughters (which deposit on lung tissue) is the dominant radiological exposure pathway and historically caused elevated lung cancer rates among uranium miners.
Q10. The mass fraction of the fissile isotope U-235 in natural uranium is approximately:
(a) 0.0055% (55 ppm) (b) 0.711% (c) 3.5% (d) 7.11%
Answer: (b) — Natural uranium contains 0.711% U-235 by mass (with 99.274% U-238 and 0.006% U-234). This is the standard feed assay used in enrichment calculations. Light water reactors require enrichment to 3-5% U-235.