THE GEOCHEMISTRY OF NATURAL WATERS

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THE GEOCHEMISTRY OF NATURAL WATERS

• ENVIRONMENTAL GEOCHEMISTRY AND RADIOACTIVE WASTE
  DISPOSAL
• Characteristics of nuclear waste

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LEARNING OBJECTIVES

• Define types of nuclear waste and their
  characteristics.
• Consider the pros and cons of geological burial
  of nuclear waste.
• Understand the geochemistry of uranium.
• Understand the geochemistry of the fission
  products (Tc) and neutron capture products (Am,
  Pu and Np).
• Learn about analog studies, in particular the
  Oklo, Gabon reactor.

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NUCLEAR REACTORS

• Generate heat via a controlled chain reaction
  based on fission of 235U by thermal neutrons.
• Fission-product radionuclides and transuranium
  elements (Pu, Np, Am, Cm - formed by neutron
  capture reactions with 235U and 238U) accumulate
  in the fuels rods.
• Fuel rods become highly radioactive and do not
  decay back to level of U ore until after
  10,000-100,000 years.
• Wastes are radiotoxic and generate heat.

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IMPORTANT UNITS - I

• Becquerel (Bq) - One disintegration per second. A
  measure of the rate of decay of a radionuclide.
• Curie (Ci) - 3.70?1012 Bq.
• The decay rates encountered in nuclear waste
  range from 1 TBq (1012 Bq) to 1 PBq (1015 Bq).
• Note T Tera P Peta

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IMPORTANT UNITS - II

• Exposure to ionizing radiation (?-, ?-, ?- and
  X-rays) is measured in terms of the energy
  released when the radiation reacts with a unit
  weight of the absorber (e.g., biological tissue).
• Gray (Gy) - Basic unit of dose equal to 1 J kg-1.
• Radiation absorbed dose (rad) - 100 rad 1 Gy.
• Roentgen equivalent man (rem) - one rad of x-rays
  or ?-rays.
• Sievert (Sv) - 1 Sv 100 rem.

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EFFECTS OF IONIZING RADIATION

• The amount of tissue damage due to exposure to
  ionizing radiation depends on
• The intensity and energy of the radiation.
• The distance between source and object being
  irradiated.
• Degree of shielding provided by matter between
  source and object.
• The duration of the exposure.
• The type of radiation.
• The tissue being irradiated.

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PHYSIOLOGICAL EFFECTS OF WHOLE-BODY IRRADIATION
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From Faure (1998)
Nuclear fuel cycle starting with the mining and
milling of uranium ore leading to their use in
nuclear power reactors in the U.S.
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TYPES OF NUCLEAR WASTE

• High-level nuclear waste (HLW) - Generates
  significant decay heat. Spent fuel or products
  from the immobilization of highly active liquid
  wastes arising from fuel reprocessing. Typically
  in range 1016-1018 Bq t-1.
• Spent unreprocessed fuel (SURF) - The uranium
  fuel remaining after use and not reprocessed to
  recover 235U.
• Low-level nuclear waste (LLW) - Less than 4?109
  Bq t-1 ?-activity and lt12?109 Bq t-1
  ?-?-activity, but more active than 4?104 Bq t-1.
  Wide range of materials from fuel fabrication,
  reprocessing and reactor operation, and from
  outside nuclear power industry.

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Decay rate of reprocessed high-level waste and
spent unreprocessed fuel rods. The data pertain
to a pressurized water reactor generating 33 GW
day tonne-1 after five years of cooling. From
Faure (1998).
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Heat output from SURF, HLW and cladding hulls as
a function of time. The data pertain to a
pressurized water reactor generating 33 GW day
tonne-1 after five years of cooling. From Faure
(1998).
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Water-dilution volumes for radionuclides in spent
fuel discharged from a 1-GW(e) pressurized water
reactor as a function of decay time. From
Langmuir (1997).
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Water-dilution volumes for radionuclides in spent
fuel reprocess-ing wastes, generated by operating
a 1-GW(e) pressurized water reactor for one year,
as a function of decay time. From Langmuir (1997).
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Relative toxicity hazard (relative to 0.2
uranium ore) of ingest-ing a given weight of HLW
or spent fuel and the same weight of various
metal ores. From Langmuir (1997).