Review Questions
Q1. Which is NOT an advantage of ceramic uranium fuel over metallic fuel?
(a) Higher thermal conductivity (b) Higher melting point (c) Lower fabrication costs (d) Resistance to oxidation
Answer: (a) — Ceramic UO fuel actually has poor thermal conductivity compared to metallic fuel. This is one of its main disadvantages. Its advantages include a very high melting point, chemical stability, resistance to oxidation, and low fabrication costs.
Q2. Which statement is INCORRECT?
(a) New nuclear fuel poses a very small radiological hazard compared to irradiated fuel (b) MOX fuels may pose a more significant inhalation hazard if involved in a fire than uranium oxide fuels (c) The activity of U-234 in enriched nuclear fuel is negligible (d) Reprocessed uranium fuel normally presents greater radiological hazards than ‘fresh’ fuel
Answer: (c) — This statement is incorrect. As shown in the worked example, U-234 actually contributes the majority (~81%) of the total activity in enriched UO fuel, despite being present in very small mass fractions. Its short half-life makes its activity far from negligible.
Q3. Which type of fuel produces the greatest radiological hazard in terms of external dose rate?
(a) Fresh metallic fuel (b) Fresh oxide fuel (c) Recycled oxide fuel (d) MOX fuel using recycled uranium
Answer: (d) — MOX fuel produces the greatest external dose rate due to the neutron hazard from spontaneous fission of Pu-240, the gamma hazard from Am-241, and the contributions from recycled uranium isotopes. Contact dose rates from MOX can be ~30 times higher than from fresh UO.
Q4. Recycled fuel has specific activity:
(a) Lower than fresh fuel (b) About the same as fresh fuel (c) A little higher than fresh fuel (d) A lot higher than fresh fuel
Answer: (d) — Recycled uranium fuel has a significantly higher specific activity than fresh fuel, primarily due to the presence of U-232 (with its very short 68.9-year half-life and strongly gamma-emitting daughter products) and higher concentrations of U-234. Surface dose rates are typically 3—4 times higher than for fresh fuel.
Q5. Which of the following is an important consideration in a MOX fabrication plant?
(a) High dose rates from Pu (b) Low critical mass of Pu (c) High radiotoxicity of Pu (d) High fire risk from Pu
Answer: (c) — The most important consideration is the high radiotoxicity of plutonium, particularly the inhalation hazard from airborne PuO particulates during fabrication. Plutonium is one of the most radiotoxic substances when inhaled, due to its alpha-emitting isotopes being deposited in the lungs. While (a) and (b) are also concerns, the inhalation/radiotoxicity hazard is the primary driver for the extensive containment and automation required in MOX plants.
Q6. The dose rate from Pu fuel increases with time mainly owing to:
(a) Spontaneous fission of Pu-240 (b) Spontaneous fission of Pu-242 (c) Production of Am-240 (d) Production of Am-241
Answer: (d) — The gamma dose rate from MOX fuel increases with time due to the build-up of Am-241 from the beta decay of Pu-241 ( = 14.3 years). Am-241 emits a 59.5 keV gamma ray with high abundance. This is why MOX fuel has a limited shelf-life before an americium removal step is needed. Note: option (c) says “Am-240” which does not exist as a significant product — the correct nuclide is Am-241.
Q7. Which of the following is INCREASED by using MOX fuel?
(a) The negative temperature coefficient of reactivity (b) The rod worth (c) The xenon worth (d) The delayed neutron fraction
Answer: (a) — Using MOX fuel increases the negative temperature coefficient of reactivity. This is because the Pu-239 fission cross-section has a greater sensitivity to neutron energy changes with temperature compared to U-235. The delayed neutron fraction is actually decreased with MOX fuel (because Pu-239 produces fewer delayed neutrons than U-235), and rod worth is typically reduced.
Q8. Weapons-grade plutonium is normally that with:
(a) High Pu-241 content (b) High Pu-240 content (c) Low Pu-241 content (d) Low Pu-240 content
Answer: (d) — Weapons-grade plutonium has a low Pu-240 content (typically <7%). This is because Pu-240 undergoes spontaneous fission at a high rate, which would cause pre-detonation in a nuclear weapon. Weapons-grade Pu is therefore produced with short irradiation times to minimise Pu-240 build-up, resulting in high Pu-239 content. Reactor-grade plutonium (from typical power reactor operations with longer irradiation) has much higher Pu-240 content (~25%) and is unsuitable for weapons.
Q9. The thorium cycle would use which fissile isotope as the main fuel source?
(a) Th-233 (b) U-233 (c) U-234 (d) U-235
Answer: (b) — The thorium fuel cycle uses U-233 as its primary fissile material. Th-232 (which is fertile, not fissile) absorbs a neutron to form Th-233, which beta-decays to Pa-233, which in turn beta-decays to U-233. U-233 is an excellent fissile material that produces more thermal neutrons per fission than U-235.
Q10. Order the following in terms of increasing proliferation risk:
MOX Fuel, Thorium-oxide based fuel, Uranium-oxide based fuel, Depleted uranium
Answer: From lowest to highest proliferation risk: Depleted uranium < Thorium-oxide fuel < Uranium-oxide fuel < MOX fuel — Depleted uranium has the lowest risk — it is almost entirely U-238 and cannot sustain a chain reaction. Thorium-oxide fuel has low risk because Th-232 is not directly fissile, and the U-233 produced in the thorium cycle is contaminated with U-232 (making it difficult to handle and process for weapons use). Uranium-oxide fuel carries moderate risk because it contains U-235 which, if further enriched, could be used in a weapon. MOX fuel carries the highest risk because it contains separated plutonium, which is directly usable in nuclear weapons.