In high temperature gas-cooled reactors (HTGR), also known as fast reactors, graphite is utilized as the moderator of the nuclear reaction. The graphite is either used as part of the structural materials for the reactor
|
Steps
|
SF
|
ILW
|
LLW
|
Tailings
|
Comments
|
|
Mining and milling
|
|
|
|
65,000
|
In terms of radiation doses and numbers of people affected, uranium mining has been one of the most hazardous steps in the nuclear fuel chain, disproportionately impacting indigenous communities.
|
|
Conversion
|
—
|
—
|
32-112
|
—
|
Besides airborne and waterborne uranium, hazards include chemicals such as hydrofluoric acid, nitric acid, and fluorine gas.
|
|
Enrichment
|
|
|
3-40
|
|
Typically buried at dump sites with a high risk of leaching radionuclides into the groundwater. Waste is contaminated with polychlorinated biphenyls (PCBs), chlorine, ammonia, nitrates, zinc and arsenic.
|
|
Fuel fabrication
|
"
|
"
|
3-9
|
"
|
Because fuel fabrication does not involve the production of liquid waste, its effects are mainly restricted to workers and are on the same order as for workers in the reprocessing sector.
|
|
Reprocessing and vitrification
|
not
applicable
|
not
applicable
|
not
applicable
|
not
applicable
|
Wastes from reprocessing, together with spent fuel, contain more radioactivity than any other waste in the fuel cycle. Phenolic and chlorinated compounds are produced in large amounts due to the use of decontamination reagents such as CCI4 together with phenolic tar (Gad Allah, 2008).
|
|
Reactor
operations
|
—
|
22-33
|
86-130
|
—
|
Boiling water reactors have considerable emissions of radioactive noble gases.
|
|
Spent fuel storage and encapsulation
|
|
2
|
0.2
|
|
Considerable quantities of "low-level" waste are created due to fission products leaking into the spent fuel pools from cracks in the fuel cladding (Choi eta/., 1997).
|
|
Spent fuel final disposal
|
26
|
—
|
—
|
—
|
Insufficient treatment can cause continued exposure to environment and local population.
|
|
Decommissioning
|
—
|
9
|
333
|
—
|
Most of the radioactivity from reactor decommissioning waste is in a relatively small volume of intensely radioactive material.
|
|
Totals
|
26
|
33-44
|
457-624
|
65,000
|
|
|
|
 |
|
Steps
|
SF
|
HFW
|
IFW
|
FEW
|
Tailings
|
Comments
|
|
Mining and milling
|
|
|
|
|
50,060
|
Mill tailings account for over 95% of the total volume of the radioactive waste from MOX-OT processing cycle. This does not include mine wastes. Many tailings sites all over the world remain unremediated and/or neglected and pollute ground and surface water with radioactive and non-radioactive toxic substances.
|
|
Conversion
|
—
|
—
|
—
|
25-86
|
—
|
Besides airborne and waterborne uranium, hazards include chemicals such as hydrofluoric acid, nitric acid, and fluorine gas.
|
|
Enrichment
|
|
|
|
3-25
|
|
Typically buried at dump sites with a high risk of leaching radionuclides into the groundwater. Waste is contaminated with polychlorinated biphenyls (PCBs), chlorine, ammonia, nitrates, zinc and arsenic.
|
|
Fuel fabrication
|
|
|
13
|
7.4-12.5
|
|
Because fuel fabrication does not involve the production of liquid waste, its effects are mainly restricted to workers and are on the same order as for workers in the reprocessing sector.
|
|
Reprocessing and vitrification
|
|
2-4
|
17-39
|
8016-8037
|
|
As in FUE-OT system, wastes from reprocessing, together with spent fuel, contain result in the highest risk. The waste is high inorganic content. There is a particularly high risk of further contamination through accidents of storage facilities at the reprocessing plant.
|
|
Reactor
operations
|
—
|
—
|
22-33
|
86-130
|
—
|
Boiling water reactors have considerable emissions of radioactive noble gases.
|
|
Spent fuel storage and encapsulation
|
|
|
0.3
|
0.03
|
|
As in the FUE-OT system, large quantities of "low-level" waste are created due to fission products leaking into the spent fuel pools from cracks in the fuel cladding. Fisson products are trapped in resins in filters, which then become "low-level" waste in the United States and intermediate level waste in Europe.
|
|
Spent fuel final disposal
|
26
|
—
|
—
|
—
|
—
|
Insufficient treatment can cause continued exposure to environment and local population.
|
|
Decommissioning
|
—
|
—
|
10.1
|
315
|
—
|
Most of the radioactivity from reactor decommissioning waste is in a relatively small volume of intensely radioactive material.
|
|
Totals
|
26
|
2-4
|
62-95
|
8452-8615
|
50,060
|
|
|
|
|
|
Table 15.3 Carbon-14 production mechanisms and cross-sections
|
Target isotope
|
Mechanism
|
Thermal cross-section (barns)
|
Isotopic abundance (%)
|
|
14N
|
14N(n, p)14C
|
1.81
|
99.6349
|
|
12C
|
12C(n, y)14C
|
n/k
|
n/k
|
|
13C
|
13C(n, y)14C
|
0.0009
|
1.103
|
|
17O
|
17O(n, a)14C
|
0.235
|
0.0383
|
Source: International Union of Pure and Applied Chemistry (IUPAC), 1984. n/k: Not known.
|
core vessel or as fuel containment elements in the form of pebbles. The graphite used from natural sources contains non-carbon impurities within the carbon matrix. Among these impurities are oxygen and nitrogen from entrapped air, cobalt, chromium, calcium, iron, and sulfur (Khripunov et al, 2006). Upon exposure to high neutron flux, most of the impregnated impurities are expected to transmute to unstable radioactive forms. For example, experimental exposure of graphite in nuclear reactors have shown that the stable forms of oxygen, nitrogen, and C-12 are converted to radiocarbon-14 (C-14) as shown in Table 15.3.
The radioactive fission products are created within the fuel grains and migrate through grain boundaries and then through microscopic cracks in the graphic matrix (Fig. 15.2). Most of the fission products are entrained in the matrix — a small proportion escapes through the outer layers into the gas phase. The challenge of reprocessing involves the separation of the metallic radionuclides from the graphite matrix and reducing the amount of C-14. Impurities in the fuel itself include: (1) metallic fission products (Mo, Tc, Ru, Rh, and Pd) which occur in the grain boundaries as immiscible micron to nanometre-sized metallic precipitates (e-particles); (2) fission products that occur as oxide precipitates of Rb, Cs, Ba, and Zr, and (3) fission products that form solid complexes with the UO2 fuel matrix, such as Sr, Zr, Nb, and the rare earth elements (Kleykamp, 1985; Shoesmith 2000; Buck et al., 2004; Bruno and Ewing, 2006).
