RADIOACTIVE ISOTOPE SEPARATION AND ENRICHMENT

Presented By:Rishabh Tripathi

Roll No. 15115262Phd, NET

IIT Kanpur

INTRODUCTION

Why isotope separation and enrichment is needed?- Natural Uranium is 99.3% U238 with U235 the only fissile nucleas being 0.7%

- To improve concentration of U235 to appreciable amounts necessary to be used as efficient fuel in nuclear power plants enrichment is necessary

Grades of Uranium

a.) Slightly Enriched Uranium (SEU) U235 conc. is between 0.9% to 2% - fuel for PHWR

b.) Low Enriched Uranium (LEU) U235 conc. < 20% , mostly in the range of 3-5% - fuel for LWR

c.) Highly Enriched Uranium (HEU) U235 conc. > 20% - Weapons grade or used in fast neutron reactors / submarines

ENRICHMENT TECHNIQUES

Separation / Enrichment Techniques and Processes

Old Processes

a.) Gaseous Diffusion Process

b.) Electromagnetic Separation

c.) Thermal Diffusion

d.) Mass Diffusion

New Processes

a.) Gas Centrifuge

b.) Laser based Ionization (SILEX , ALVIS)

c.) Aerodynamic Separation

The only Uranium compound used as feedstock material for all above listed enrichment processes is UF6 (Uranium Hexafluoride) gas which is sufficiently volatile, solid at room temperature and sublimes at 56.5 deg C and remains in gaseous phase throughout the process. Can be oxidized to UO2 after enrichment to be used as fuel.

GASEOUS DIFFUSION

Gaseous Diffusion Process

Based on Graham's law of diffusion. Mathematically expressed as

It uses a cascade of separation chambers (STAGES) with semi-permeable membrane as diffusion barrier to cause partial enrichment of natural isotopic mixture as feed by differential separation by diffusion also termed as 'effusion'. Degree of separation depends on relative Molecular weights of components.

rate of diffusion 1Mol.Wt.

For two components

(rate of diffusion)1(rate of diffusion)2

M 2M 1=MU 238MU 235

DESCRIPTION

The separation factor () is mathematically given as

Also by material balance equations we have

Here xp & xt are the product and waste tail mass fractions respectively .

=rateU 235rateU 238

=M U 238M U 235= 352349=1.005=

x p(1 x t)x t(1 x p)

=overall

i=x ip(1x i

t)

x it (1x t

p)= for Stage i

=v (1x i)x i(1v)

, v=N U 235

N U 235+NU 238x i= feed concentration at stage iN i=molar flow rate of isotope

DESIGN LAYOUT

FEED (U235 = 0.72)

Product Stream (U235)

CASCADE OF 3 STAGES

HIGH PRESSURE SIDEc

LOW PRESSURE SIDEc

Depleted Phase from above stagecv

Enriched Phase from Below stagecv

STAGE 1

STAGE 2'

STAGE 2

FEED (U235 = 0.72)

DIFFUSER STAGE

BARRIER (Membrane)

v p1

p2

F,xf,xi

P,xp

T,xt

MATERIAL BALANCE

F=P+T

Fx f=Px p+Tx t

FP=x px tx f xt

TP=x px fx fx t

Material Balance (for entire cascade)

Overall

Component Balance

Feed to Product Ratio

Tail to Product Ratio

Material Balance (for any stage i)

Overall

Component Balance

Tail to Product Ratio

t i+1+ pi1=p i+t i

t i+1x ti+1+ pi1x p

i1=pix pi +t ix t

i

t i+1p i1

=x pi1x t

i

xti+1x t

i

when i=0 ,t 0= f 0=F

CASCADE LAYOUT

TAIL STREAM(T , xt)

ENRICHED PRODUCT STREAM (P, xp)

FEED (F , xf)

ENRICHING SECTION

STRIPPING SECTION

Stage 1

Stage 1

Stage 2

Stage n

Stage 2

Stage m

Np = sum(1:n)

Nw = sum(1:m)

ADVANTAGES & DRAWBACKS

Advantages

- Continuous Operation

- Tail composition can be strictly controlled based on product and waste requirements

- Multiple feeds of different compositions can be admitted at different stages of the cascade

Drawbacks

- Very High energy requirements due to compressor and cooler assembly. 56% of energy consumption is due to compressor operation.

- Low rate of Product formation, Large SWU , Large no. of process units required . For instance to obtain 2-3% concentration of U235 as product a total of about 4500 stages are required.

- Scale of Operation is very high due to large size of Diffuser and Compressor units.

GAS CENTRIFUGE

> A gas centrifuge is a device that performs isotope separation of gases. A centrifuge relies on the principles of centrifugal force accelerating molecules so that particles of different masses are physically separated in a gradient along the radius of a rotating container. A prominent use of gas centrifuges is for the separation of uranium-235 from uranium-238.

> The gas centrifuge process uses a large number of rotating cylinders in series and parallel formations. Based on the principle of Buoyant density centrifugation . It uses the concept of buoyancy to separate molecules in gas or solution. Also known as density gradient centrifugation.

> In addition, if one creates a thermal gradient in a perpendicular direction by keeping the top of the rotating column cool and the bottom hot, the resulting convection current carries the lighter molecules to the top while the heavier ones settle at the bottom, from which they can be continuously withdrawn.

> The Zippe-type centrifuge is a gas centrifuge designed to enrich the rare fissile uranium isotope Uranium-235 out of the mixture of isotopes found in naturally-occurring uranium compounds. The isotopic separation is based on the slight difference in mass of the isotopes.

DESCRIPTION

Separation factor () for gas centrifuge is given as

Hence separation factor for Gas centrifugation process for U Isotopes is bigger than that for gaseous diffusion process

The Total length of Gas centrifuge to carry out fuel enrichment is given as

=x p(1x t)x t(1 x p)

=e(M U 238MU 235)

2(Rc2r 2)

2RT

M U 238MU 235=3 g / g mole

for Rc=68 cm , r=0 , T=300K ,=4050k rev /s

=1.03

H=2ln(

Rcr)

Do(1)2 V (x)

DESIGN LAYOUT

U-238 MovementU-235 Movement

Outlet for U-235 enriched phase

Inlet for Feed

Outlet for U-235 Depleted Phase

A GAS CENTRIFUGE

U-238 MovementU-235 Movement

Outlet for U-235 enriched phase

Inlet for Feed

Outlet for U-235 Depleted Phase

ZIPPE CENTRIFUGE

HEAT ADDITION

ADVANTAGES & DRAWBACKS

Advantages

-Lesser Power Consumption as compared to gaseous diffusion process. Only 2-2.5% of Total power consumption of gaseous diffusion plant

-Continuous process, higher yields, less separative work units

-Higher separation efficiency

-Lower scale of operation

-Height of column independent of absolute radius of centrifuge bowl

Drawbacks

- Despite large separation factor the no. of units required is large for a given peripheral velocity

- Longer centrifugation columns are needed for low rotational speeds for a given degree of separation

COST ANALYSISa.) SEPARATIVE WORK UNITS (SWU)

It is the amount of separation done by an isotope enrichment process.

Proportional to the total input (energy / machine operation time) and to the mass processed..

The same amount of separative work will require different amounts of energy depending on the efficiency of the separation technology. A function of the concentrations of the feedstock, the enriched output, and the depleted tailings (xf, xp and xt).

Measured in Separative work units SWU or kg SW.

Amount of energy in KW consumed per kg of mass processed per unit time.

*V(x) is known as separation potential

b.) Mass of Natural Uranium (NU)

It is the mass of Natural uranium needed to yield a desired mass of enriched uranium. Since SWU proportional to F*P*xp/xt and NU proportional to F*xt then for a given xp and F we see, SWU * NU = kP k is const

Net Overall Cost = Cp*P + Ct*T - Cg*SWU Cf*F

Hence based on the cost of feed and Product streams the overall cost of enrichment is determined and specific isotope separation technology is chosen.

WORK SWU=PV ( x p)+TV (x t)FV ( x f )

V ( x)=(12x )ln (1xx

)

OLD TECHNOLOGIES

A.) Mass Diffusion Separation of isotopes is effected through diffusion of the light isotope (U235) of the feed gas mixture into a condensable vapor (steam or some separating agent vapor) at higher rate than diffusion of the heavy isotope (U238). Separation efficiency lower than gaseous diffusion process.

B.) Thermal Diffusion - This process utilizes the transfer of heat across a thin liquid or gas to accomplish isotopic separation. Lighter U235 Molecules diffuse towards a hot surface , Heavier U238 Molecules diffuse towards a cold surface.

C.) Electromagnetic Isotope Separation Metallic U235 is vaporized , and then ionized to positively charged ions. Cations are accelerated and are separated under the effect of electromagnetic field. Device known as Calutron and used for Fuel Enrichment. High Energy cost.

Drawbacks

- Low Separation Efficiency, High Input Energy Cost, Low Productivity

- High Volume of Radioactive Waste

NEW TECHNOLOGIES

NEW SEPARATION / ENRICHMENT TECHNIQUES

A.) Laser Isotope Separation (SILEX Separation of Isotopes by Laser Excitation ; AVLIS Atomic Vapor Laser Isotope Separation ; MLIS Molecular Laser Isotope Separation)

- U235 atoms in UF6 gas is preferentially excited by lasers and selectively ionized and isotopes separated under the effect of magnetic field

B.) Aerodynamic Nozzle Process - Uses H2 or He gas as diluent or Carrier gas for UF6 gas stream. Passed through nozzle at supersonic speed with enhanced centrifugal forces acting on high velocity gas stream . The light fraction obtained thus gets enriched in U235 isotope while the heavier fraction gets depleted.

Advantages

- Low Energy Inputs , Lower Capital Costs and Lower Tails Assays

- Higher Separation Factor as compared to previous technologies, High Energy Efficiency and Low Volume of Radioactive Waste.

CONCLUSION

ENRICHMENT TECHNIQUE

YEAR 1960 1990(% usage)

YEAR 1990 2012(% usage)

YEAR 2012 2018(% usage)

GASEOUS DIFFUSION

55 - 65 33 - 40 2 - 3

GAS CENTRIFUGE

10 - 12 40 - 50 90 - 93

LASER ISOTOPE SEPARATION

0 2 - 3 6 - 7

OTHERS 25 - 30 5 - 6 0

REFERENCES

a.) OECD Nuclear Energy Agency (2010). Nuclear Energy Today. OECD Publishing.

b.) Raymond L. Murray "Introduction to Nuclear Engineering" Chap. 3 Pg 70 85. 2nd Edition Prentice Hall India

c.) Manson Benedict and Thomas Pigford "Nuclear Chemical Engineering Chap. 12-14 Pg 626 914. 2nd Edition McGraw- Hill Publications

d.) Glassstone Samuel Elements of Nuclear Reactor Theory Chap. 2-4 Pg 16-60. Ninth Edition Prentice Hall Publications

THANK YOUQ & A

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