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Lecture 18

The Nuclear Cycle

– Concerns with nuclear power

– Characteristics of U

– Radioactivity

– Formation of Uranium deposits

– Mill tailings

– Nuclear Waste

– Accidents

– Proliferation

Options for the nuclear cycle (Fig. 1)

Radioactivity

• Major reactions;

– Alpha decay ^MA à ^M-4B + ⁴He + Q

• ⁴He is called an alpha particle

– Beta decay ^MA_N à ^MB_N-1 + e^- + n + Q

• e^- is called a beta (-) particle

– Gamma decay A* à A + g

• g rays are at the high-energy end of the radiation spectrum

Penetration of radioactive particles (Fig. 2)

Units of Radioactivity (Fig. 3)

• rem = QF x rad

– QF = ‘Quality Factor’

• = 1 for X-ray; g, b

• = 3 for thermal neutrons

• = 10 for a, fast neutrons, protons

• 1 Sievert (Sv) = 100 rem

• 1 gray (Gy) = 100 rad

• Lethal doses ~ 100 rem

• LD³⁰₅₀ = 350 rem à whole-body does of 350 rem delivered suddenly causes death of 50% within 30 days.

Radioactivity – health considerations

• High energy particles can interact with biologic tissue and break up molecular bonds à mutation

– Displacement of atoms and electrons

– Large energy release in small volume

– Production of impurities (conversion of atoms)

Radiation Exposure

• Natural background radiation:

– Cosmic rays

– Radiation related to U and Th decay

• Anthropogenic additions:

– X-rays and similar medical exposure

Typical exposure levels (Fig. 4)

Potential relations between radiation dose and biological damage (Fig. 5)

Decay of U and Th isotopes (Fig. 6)

Chemical characteristics of U

• Chemical behavior: occurs in two main valence states: U⁺⁴ and U⁺⁶

• The two types indicate different ways to react with oxygen

• High O₂ content: U⁺⁶ soluble in water

• Low O₂ content : U⁺⁴ insoluble in water

à Formation of Roll Type U deposits (Fig. 7)

Mining and milling of uranium leaves ‘Mill Tailings’ containing radioactive products of the U decay chains.

Nuclear waste

• Spent Fuel: fuel rods extracted from reactors after use, without further treatment

• Fission products: fragments resulting from fission of ²³⁵U à all are initially b active

• Depleted U à ²³⁸U

• Transuranium elements, ²³⁹Pu etc.

• n-activated: tritium etc.

• Intermediate level waste: fluids which have been in contact with fuel rods

• Low-level waste: Clothing etc. exposed to radiation

• USA:

– Currently, all nuclear waste produced by civilian facilities is kept in water pools at reactor sites

– Yucca Mountain, the only site under investigation is in the planning state, potential opening in 2017

– Intermediate storage facilities are available for low and intermediate level waste

• World:

– No permanent storage place has been constructed in any country

– Several countries (e.g. France, Japan) have active reprocessing plants which separate U and Pu from fission products

The Yucca Mountain site (Fig. 8)

Nuclear accidents –Chernobyl 1986 (Fig. 9)