Plasma Processing of Nuclear Wastes

Plasma Processing of Nuclear Wastes Mike Wise, Chief Metallurgist, Tetronics Ltd The Role of Radiochemistry in the Sentencing and Disposal of Radioact...
Author: Andrea Morton
25 downloads 1 Views 2MB Size
Plasma Processing of Nuclear Wastes Mike Wise, Chief Metallurgist, Tetronics Ltd The Role of Radiochemistry in the Sentencing and Disposal of Radioactive Wastes Royal Society of Chemistry 7th April 2008

Plasma Processing • • • • • • • •

Tetronics Ltd. What is plasma? Plasma devices Cold crucible melting Slag design Vitrified products Vitrification v cementation Caesium volatility

Tetronics Ltd. • • • • • • • •

Name from Tetra and Ionics Founded in 1964 by three local businessmen Privately owned company: Tetronics Holdings 2004 Main business is the application of plasma as a clean heat source Two spin off companies, PRL and APP Now located in Swindon 50 employees www.tetronics.com

What is Plasma? • A plasma is an ionised gas comprising ions, molecules, electrons, photons and atoms either in their ground states, or in various states of excitation: the fourth state of matter. • Overall, a plasma is electrically neutral. • A plasma is electrically conducting, its resistance being a function of its length, temperature and composition. • The power (temperature) of a plasma is determined by its length (voltage), the type of gas used (voltage) and the applied current, which can be readily controlled.

Plasma Formation As a gas is heated, collisions become more energetic and occur with greater frequency. At about 2,000 K molecules dissociate into their constituent atoms. At about 3,000 K the collision energies cause outer shell electrons to be ejected resulting in a plasma containing free electrical charges. Weakening of bonds

Collision and breaking of bonds Cathode Transfer of electrons Anode Electrons

1,000K

3,000 ~4,000K

• •

Electrically conductive Intense heat and light

Plasma 10,000K

-

Types of Plasma • There are two main types of plasma: •



cold plasmas, like those in fluorescent tubes, where the electron temperature is much higher than that of the ion temperature thermal plasmas, like welding arcs, or the sun, where the electron and ion temperatures are similar

Effect of Temperature and Pressure on Plasma

Te electron temperature

Tg ion temperature

Types of Thermal Plasma Devices Transferred Arc Torch Non-transferred Arc Torch gas

gas

cathode

_

+

nozzle

Graphite Electrode gas

_ +

+

+

Can be anodic or The arc is transferred from the torch The arc attaches to an internal cathodic. No (cathode) to the workpiece (anode). anode, but projects beyond it. cooling required, Very efficient. (TIG Welder). More complex, less efficient. but they erode. Anode torches are also available (Gas Heater, Plasma Spray Torch) (e.g. for clean melting).

Tetronics Plasma Systems • Tetronics use transferred arc devices, which means that a return electrode is required when a single, transferred-arc plasma device is used • For conducting melts the return path is via an anode in the base of the furnace • For non-conducting melts, such as glasses or wastes, Tetronics use a twin electrode system in which the plasma is generated in free space above the melt • This means that no return electrode is required in the base of the crucible and plasma restart with graphite electrodes is simply achieved by a touch start

Examples of Transferred Arc Plasmas

70 mm cathode torch argon plasma ~650 mm long; 1000 A, 450 kW

Twin-torch coupled argon plasma; anode left, cathode right; 400 A, 30 kW

Some Plasma Facts • Think of it as a high-temperature electric flame (10,000 ºC) • It is controllable, clean and has a low gas volume • Thermal plasmas can be generated by water-cooled torches or graphite electrodes • For waste processing, graphite electrodes are practical as they are cheaper and do not require water cooling • Argon is the most commonly used plasma torch gas but, for waste processing with graphite electrodes, nitrogen can be used (cheaper) • Graphite electrodes wear by erosion but can be continuously replaced (as in the steel and aluminium industries)

Cold Crucible Containment • •





For clean melting, and for nuclear waste processing, the melt is contained in a water-cooled, ‘cold-wall’, copper crucible. In cold crucible melting, the melt is contained in a thin solid layer of its own composition (a skull) and so is not contaminated by the cold crucible, and vice versa. This means that there are no problems of contamination of the containment vessel by the absorption of radionuclides (as there are with refractory-lined, ‘hot-wall’ vessels). The cold crucible does not wear, and the non-volatile, inorganic fraction of the melt composition remains essentially constant, thereby allowing omnivorous solvent melts (slags or fluxes) of the desired chemistry, melting point and fluidity to be designed.

ILW Vitrification: Schematic Plasma System Ar, N2, O2, H2O Off-gas

Feed system

Off-gas cleaning

Clean gas discharge

Ar

Ar Cathode

Anode Secondary waste return

Coupling zone Copper cold crucible

Reaction chamber Discharge

Prototype Twin-Electrode Cold-Crucible Furnace Feeder

Plasma Electrodes

Water Cooled Copper Crucible

Off-Gas Equipment Furnace Support Frame

Vitrification of ILW Sludges (1) •

~6000 t of ILW sludges are stored in B30 pond and SIXEP at Sellafield: • Sludge #2 - B30 Magnox Legacy Sludge; 1761 t • Sludge #3 - SIXEP Sludge Mg(OH)2 ILW; 2000 t • Sludge #4 - SIXEP Sand/Clino ILW; 1900 t



• • •

Sludges #2 and #3 are mainly magnesium hydroxide, from the corrosion of Magnox cans, and sludge #4 is a mixture of sand and clinoptilolite, a zeolite ion exchange medium. The main radionuclides present are Sr90 and Cs137 plus some fuel residues. The current technology is to add cement to the sludges then pour the resultant concrete into RWMD containers for storage. Vitrifying the sludges, however, would reduce the volume of material to be stored, so requiring fewer containers and less storage volume, with consequent cost savings.

Vitrification of ILW Sludges (2) • • •







Vitrification also oxidises all of the constituents thereby avoiding some cement encapsulation problems (organics; Mg + H2O→H2 evolution). The vitrified product is stable and chemically inert; it’s a glass . A problem with high-magnesia content sludges is that that they need additions of fluxing agents in order to produce slags with melting points and viscosities that enable easy casting. (MgO; M.P. 2831 °C) Hence, target slag compositions were designed using MgO-Al2O3-SiO2 ternary phase diagrams to optimise these requirements whilst incorporating high levels of magnesia with a wide band of tolerance. Simulant slags were made up using magnesium hydroxide/carbonate, silica, alumina, sodium hydroxide solution and clinoptilolite (as appropriate) according to the chemical analyses supplied. These were doped with Sr and Cs at ~100 times the sludge analysis and ceria was added to simulate the actinide content.

Sludge #3 Waste-Form Design and Results Species

Species

Analysis%

Analysis%

H2O

40.0

Na2O

0.17

Mg(OH)2

48.0

MgO

20.90

UO2

7.5

Al2O3

18.30

Al2O3

0.5

SiO2

54.55

cellulose

4.0

K2O

0.06

CaO

0.29

TiO2

0.79

Analysis%

Fe2O3

0.79

MgO

20.8

La2O3

0.20

Al2O3

23.4

CeO2

4.76

SiO2

51.1

Total

100.81

CeO2

4.7

Total

100.0

Total

55% SiO2

100.0

Sludge #3 analysis Species

Target composition

18% Al2O3

21% MgO

Ternary phase diagram suggests the slag to be near the cordierite-protoenstatite-forsterite eutectic with a liquidus temperature ~1365 °C

Analysed waste-form composition of highmagnesia simulant sludge

Sludge #3 Result: Homogeneous, Stable Product

This X-ray diffraction pattern of the final waste-form of sludge #3 shows that none of the lines from the original feedstock phases is present confirming that complete reaction has occurred. 40% water content; 49% flux addn. to sludge 27% volume reduction

Because of the rapid cooling effect of the cold crucible, the waste-form has solidified as a glass rather than as a crystalline solid. This is confirmed by the X-ray diffraction pattern. Density of sludge = 1400 kg m-3 Density of glass = 2690 kg m-3

Self Fluxing • •

The addition of fluxing agents could be avoided by mixing together sludges of appropriate compositions. The following data refer to the processing of a mixture of simulant sludges: sludge #3, with a high magnesia content, and sludge #4, sand/clinoptilolite, in the proportions in which they could arise.

Species

Analysis%

Species

Analysis%

Species

Analysis%

H2O

40.0

H2O

27.9

H2O

12.4

Mg(OH)2

48.0

Mg(OH)2

5.0

CaO

1.8

UO2

7.5

SiO2

8.0

Al2O3

10.9

Al2O3

0.5

NaOH

9.1

SiO2

64.0

cellulose

4.0

clinoptilolite

50.0

NaOH

2.9

Total

100.0

K2O

1.6

Total

100.0

Sludge #3 high magnesia 16 m3/day

Sludge #4 sand/clinoptilolite 20 m3/day

Clinoptilolite analysis

Blending Sludges #3 and #4 for Self Fluxing Species

50.9% SiO2

Feed composition of mixed simulant sludges #3 and #4

7.6% Al2O3

24.6% MgO

Analysis%

Na2O

9.0

MgO

24.6

Al2O3

7.6

SiO2

50.9

K2O

0.9

CaO

1.5

Fe2O3

0.9

CeO2

4.7

others

0.5

Total

100.6

Analysed waste-form composition of mixed simulant sludges (major elements)

Mixed Sludges #3 and #4: Product

This X-ray diffraction pattern of the final waste-form shows it to consist of forsterite and glass. Forsterite was not present in the start material, and none of the lines from the original feedstock phases is present, confirming that complete reaction has occurred.

Because of the higher melting point and wider melting range, the waste-form has solidified as a semi-crystalline solid. This is confirmed by the X-ray diffraction pattern. Density = 2690 kg m-3

Cementation v Vitrification To illustrate the scale of these reductions, comparisons between cementation and plasma vitrification for 10 volumes each of sludge #2 and sludge #4 are shown below.

Density of sludge #2 ~1300 kg m-3 Density of glass = 2640 kg m-3 60% water content; 32% flux addition to sludge 64% volume reduction

Density of sludge #4 = 1600 kg m-3 Density of glass = 2340 kg m-3 30% water content; no flux addition to sludge 59% volume reduction

Plasma Solution - Cost Cementation

Plasma



Unit cost of packaging (Drums and Stillage)

£10,000

£10,000



Cost of Packaging

£300,000,000

£65,000,000



50 year life cycle cost for one store

£200,000,000

£200,000,000



Cost for Interim Stores

£923,000,000

£200,000,000



Cost of one transport flask

£750,000

£750,000



No of flasks per store (Export 2 per day)

12

12



Cost of Flasks (Capex)

£42,000,000

£9,000,000



Nominal cost of single flask round trip

£1,000

£1,000



Cost of Transport to Store & to Repository

£60,000,000

£13,000,000



Total Cost up to Repository Gate

£1,325,000,000

£287,000,000

ILW Vitrification Summary •

• •





Heating in the twin arc plasma furnace drives off the water, oxidises the carbonaceous materials and converts the inorganic solids into a homogeneous melt On cooling, this forms an inert, stable waste-form of higher density and of lower volume than the original sludge For example, 100 kg of Magnox simulant sludge plus fluxes produced 73.6 kg of waste-form; the density of the settled sludge was ~1300 kg m-3 and the density of the waste-form was 2640 kg m-3 resulting in a net volume reduction of 64%. A net reduction in volume was obtained for all of the simulant sludges processed, the value of which varied from 27% to 64% depending on the magnesia content of the sludge and the amount of flux required. Therefore, by using plasma processing the number of storage containers and, hence, repository costs, would be reduced dramatically compared with using cement encapsulation.

Caesium volatility • Melts containing alkali metals will suffer evaporative loss depending on the temperature to which they are raised. • This is a particular problem for melts containing 137Cs and other volatile radionuclide isotopes, particularly if high intensity heat sources such as electron beam, lasers or plasma are used. • The loss of volatiles from a melt can be minimised by appropriate design of the melting system and the slag chemistry.

Potential Advantages of Tetronics’ Process • • • •

• •

The Tetronics process uses twin graphite electrodes with indirect plasma heating and no plasma hot spot. Twin graphite electrodes enable a plasma ‘touch start’. The cold crucible process avoids cross contamination. The high silica content, low basicity, of Tetronics’ melts should be a significant factor in retaining caesium and other network formers, such as strontium. Silica rich slags suffer less volatile loss than borosilicate glasses at equivalent viscosities. Silica rich slags are much more viscous than borosilicate glasses at equivalent temperatures, which should reduce volatile losses.

Suggest Documents