Management prior to disposal

While the above highlights the need for a clear end-point (permanent geo­logical disposal), political will and public support, much radioactive waste management must be done prior to disposal. Radioactive waste manage­ment approaches vary from country to country. However, a key aspect is to know what waste you have. A national inventory must be collected as is done in the UK (NDA, 2010a) and all other countries.

Figure 1.8 shows a flowchart for solid radioactive waste management prior to disposal, i. e., pre-disposal (Ojovan, 2011). Figure 1.9 reveals that all activities concerned with radioactive waste are conventionally divided into pre-disposal and disposal stages.

Disposal is the final step in managing radioactive wastes whereas pre­disposal includes activities such as decommissioning, pre-treatment, treat­ment, conditioning, immobilisation, storage and transport. While various disposal options are available, it is most likely that immobilised wastes will be disposed of in GDFs of one sort or another.

Waste management requires a series of steps:

• pursuing opportunities for waste minimisation

• re-use and recycling

• waste treatment

• packaging

• storage

• transport and then final disposal where required.

This waste hierarchy indicates the preferred options in the managing of waste where disposal is very much the last option; it can be represented as in Fig. 1.10 .

Waste minimisation is a process of reducing the amount and activity of waste materials to a level as low as reasonably achievable. Waste minimisa­tion is now applied at all stages of nuclear processing from power plant design through operation to decommissioning. It consists of reducing waste generation as well as recycling, reuse and treatment, with due

for both primary wastes from the original nuclear cycle and secondary wastes generated by reprocessing and clean-up operations. Waste minimisa­tion programmes were largely deployed in the 1970s and 1980s. The largest volume of radioactive waste from nuclear power production is LLW. Waste minimisation programmes have achieved a remarkable tenfold decrease of LLW generation over the past 20 years, reducing LLW volumes to approxi­mately 100 m3 annually per 1 GW(e).

Recycling means recovery and reprocessing of waste materials for use in new products. Recycled waste can be substituted for raw materials reducing the quantities of wastes for disposal as well as potential pollution of air, water, and land resulting from mineral extraction and waste disposal. However, recycling has certain limitations when applied to radioactive materials. Due to their inherent radiation, radionuclides are much more difficult to recover from contaminated materials. Recovery usually pre­sumes concentration of species into a smaller volume even though this may result in more dangerous materials. Waste radionuclides recovered from contaminated materials are difficult to recycle in new devices or com­pounds. Hence even materials which contain large amounts of radioactive constituents (e. g., SRS) often are immobilised (conditioned) and safely stored and disposed of rather than recycled.

image012

image13

1.9 Schematic of radioactive waste management activities.

 

1.10 The waste hierarchy used in the UKs decommissioning programme.

 

image14"

One example of recycling in the nuclear industry is of spent fuel. There are 435 currently operating NPPs in 30 countries which produce 368.2 GWe. A typical NPP generating 1 GW(e) produces annually approximately 30 t of SF. The annual discharges of spent fuel from the world’s power reactors total about 10,500 tonnes of heavy metal (t HM) per year and the total amount of SF that has been discharged globally is approximately 334,500 tHM (Bychkov, 2012). During use, only a fraction of fuel is burnt, generating electricity but also forming transmutation products that may poison it. After use, the fuel elements may be placed in storage facilities with a view to permanent disposal or be reprocessed to recycle their reus­able U and Pu. Most of the radionuclides generated by the production of nuclear power remain confined within the sealed fuel elements. Currently only a fraction of SF is reprocessed in countries such as France and the UK, although countries with large nuclear power programmes such as Russia and China plan to significantly increase the reprocessing capacity (Table 1.7). Also the US is reviewing the approach to open nuclear fuel cycle con­sidering reprocessing as a viable option.

Despite the complexity of such a process, recycling of fissile elements (U, Pu) from SF results in a significant reduction of toxicity of the radioactive wastes (Fig. 1.11 ).

Another potential example of recycling in the nuclear industry is of mili­tary grade Pu, much of which is stockpiled in the US, Russia and the UK; a legacy of the Cold War. Since 1972, world production of plutonium has exceeded demand for all purposes. The total world plutonium inventory is not reported but a rough calculation indicates at least 2,000 metric tonnes at the beginning of the twenty-first century. It is technically possible to convert this material into a mixed U/Pu oxide (MOX) reactor fuel so that it can be used to generate energy in a suitable nuclear reactor. MOX nuclear fuel consists either of UO2 and PuO2 either as two phases or as a single phase solid solution (U, Pu)O2 (Burakov et al., 2010). The content of PuO2 may vary from 1.5 to 25-30 wt% depending on the type of nuclear reactor. Whereas most efficient burning of plutonium in MOX can only be achieved in fast reactors, it is currently used in thermal reactors to provide energy, although the content of unburnt plutonium in spent MOX fuel remains significant (>50%).

Key aspects of waste management are to reduce the hazards associated with wastes and the volume of the waste material. Hazard can be reduced substantially by converting highly mobile liquid or gaseous wastes into stable solid forms using the techniques indicated in Figs 1.8 and 1.9. Immobilisation reduces the potential for migration or dispersion of con­taminants including radionuclides. The IAEA defines immobilisation as the conversion of a waste into a waste form by solidification, embedding or

Table 1.7 Spent fuel recycling capacities, tonnes per year (Bychkov, 2012)

Country

Site

Plant (reactor type SF)

Start of operation

Capacity

Actual

Planned

China

Lanzou

RPP (LWR)

2008

50

50

CRP (LWR)

2020

800

France

La Hague

UP2-800 (LWR)

1994

800

800

UP3 (LWR)

1990

800

800

India

Trombay

PP (Research)

1964

60

60

Tarapur

PREFRE1 (PHWR)

1974

100

100

Kalpakkam

PREFRE2 (PHWR)

1998

100

100

PREFRE3A (PHWR)

2005

150

150

Tarapur

PREFRE3B (PHWR)

2005

150

150

Japan

Tokai-mura

PNC TRP (LWR)

1977

90

90

Rokasho-mura

RRP (LWR)

2012

800

800

Russia

Chelyabinsk

RT1 (WWER-440)

1971

400

400

Krasnoyarsk

RT2 (WWER-1000)

2020

1500

UK

Sellafield

B205 (GCR)

1967

1500

THORP (LWR/AGR)

1994

900

900

Total

5 ,900

6,700

LWR: light water reactor; PHWR: pressurised heavy water reactor; WWER: water- water energy reactor; GCR: gas cooled reactor; AGR: advanced gas cooled reactor. Source: Bychkov (2012).

encapsulation. It facilitates handling, transportation, storage and disposal of RAW. Another term closely linked with immobilisation is conditioning.

Treatment of primary RAW includes operations intended to benefit safety and economy by changing the waste characteristics. Three basic treat­ment objectives are:

• volume reduction

• removal of radionuclides

• change of physical state and chemical composition.

As seen in Figs 1.8 and 1.9 , such operations include: incineration of com­bustible waste or compaction of dry solid waste (volume reduction); evapo­ration, filtration or ion exchange of liquid waste streams (radionuclide removal); and neutralisation, precipitation or flocculation of chemical species (change of composition). The waste volume reduction factor (VRF) of a treatment process is defined as the ratio of initial volume of the treated waste V0 to the final volume after treatment V/. VRF = VJVf. The higher the VRF, the more efficient is the treatment process. However, volume reduction inevitably leads to concentration of radionuclides which may impact on the safety and economics of the process. Treatment may lead to

image15"Time, years

1.11 Relative radiotoxicity of SF and resulting HLW on reprocessing and recycling. FP, fission products; MA, minor actinides. The time required to achieve the initial toxicity of uranium ore is significantly reduced on recycling and transmutation of MA.

several types of secondary RAW such as contaminated filters, spent resins and sludges. After treatment, depending on the radionuclide content in the waste, it may or may not require immobilisation.

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