Nuclear EnergyNuclear Energy NUCLEAR FUELNUCLEAR FUEL

Dr. Ayesha Mohyuddin

Nuclear fuel is any material that can be

consumed to derive nuclear energy. The

most common type of nuclear fuel is

fissile elements that can be made to

undergo nuclear fission chain reactions

in a nuclear reactor

The most common nuclear fuels are

235U and 239Pu. Not all nuclear fuels

are used in fission chain reactions

Nuclear FuelNuclear Fuel

A nuclear fuel pellet contains about 4 g of fuelIt produces the same amount of energy as a

ton of coal or 150 gallons of gasolineIt’s fairly cheap - $3 per pellet (compare to 150

gallons of gasoline!)It produces no greenhouse gases, nor VOCs,

nor NO, nor SO2

It does not rely on petroleumSpent fuel pellets emit radioactivityThe leftover residue is “toxic” ...

Nuclear FuelNuclear Fuel

Radiation is the result of an unstable atom decaying to reach a stable state. Half-life is the average amount of time it takes for a sample of a particular element to decay half way. Natural radiation is everywhere—our bodies, rocks, water, sunshine. However, manmade radiation is much stronger. There are currently 37 radioactive elements in the periodic table—26 of them are manmade and include plutonium and americium (used in household smoke detectors).

RadiationRadiation

Types of RadiationTypes of Radiation

There are several different kinds of radiation: alpha radiation: Alpha radiation is the release of

two protons and two neutrons, and normally occurs in fission of heavier elements. Alpha particles are heavy, positively charged and cannot penetrate human skin, but are hazardous if ingested

beta radiation: Beta radiation are electrons and Beta particles can penetrate the skin, but not light metals.

gamma rays: Gamma rays is a type of electromagnetic radiation which is left over after alpha and beta are released and include X-rays, light, radio waves, and microwaves.

neutron emission..

Penetration of Radioactive particlesPenetration of Radioactive particles

Effects of RadiationsEffects of Radiations Ions are created with the passage of the alpha, beta &

gamma Radiations. The effect of radiation is on a cellular level—changing its functionality (causing cancer or inherited birth defects) or killing it.

Depending on the information source, radiation doses are measured in rems or sievert, where 100 rem = 1 sievert.

An exposure of 100 Sv will cause death within days, 10-50 Sv will cause death from gastrointestinal failure in one to two weeks, and with an exposure of 3-5 Sv will cause red bone marrow damage half of the time. Severe affects consist of burns, vomiting, hemorrhage, blood changes, hair loss, increased susceptibility to infection, and death. With lower levels of exposure symptoms are cancer (namely thyroid, leukemia, skin cancers etc.), but also include eye cataracts. The radiation can also affect DNA causing mutations that change individuals’ genes and can be passed on to future generations. The current occupational dose recommended by the International Commission for Radiological Protection is 50 mSv per year. The average radiation dose per year for non-nuclear workers is about one mSv.

Types of Nuclear FuelsTypes of Nuclear Fuels The different nuclear processes will use different types

of fuel. In general terms:Fission reactions will use fissile heavy elementsFusion will use fusible light elementsThe convection point is iron. Elements heavier that iron

will have higher mass than its parts and yield energy when they break apart. Lighter elements that iron will have less mass than its parts giving away energy when they fusion.

Very unstable substances will not be useful, they need to be stable under normal conditions but become unstable when bombarded with neutrons. Special isotopes able to generate a chain reaction are chosen as nuclear fuel.

Fission Nuclear FuelsFission Nuclear Fuels The known fissile materials are:Uranium-233Uranium-235Plutonium-238Plutonium-239Plutonium-241Neptunium-237Curium-244

The most often used fuels are Uranium-235 and Plutonium-239; they become instable when bombarded by slow (also known as "thermal") neutrons. They are not easy to find or produce materials, and the process to generate them is usually the most expensive part of the process. Thorium-232 is also fissile but it needs fast moving neutrons to start the chain reaction.

Uranium-235Uranium-235 The most often isotope of Uranium found in Nature is

U-238U-235 is only found in low proportions (0.71%).U-235 is

created from U-238 via isotope separation.The critical mass for an unreflected sphere of U-235 is about 50 kg (17 cm of diameter).

Fission Process: One slow neutron strikes a U-235 atom; the result is U-236.U-236 is highly unstable and it fissions. There are twenty different fission processes, the products masses always add up 236.

Example: U-235 + 1 neutron -> 2 neutrons + Kr-92 + Ba-142 + ENERGY

Uranium: Nuclear FuelUranium: Nuclear Fuel Uranium is usually mined similarly to

other heavy metals—under ground or in open pits—but other methods can also be used. After the uranium is mined it is milled near the excavation site using leaching processes.

Nuclear Fuel CycleNuclear Fuel Cycle

Uranium mining

and millingConversion

Fabrication

Enrichment

ReactorSpent fuel

storageWaste disposal

Reprocessing

Step 1: MiningStep 1: MiningThere are three main methods:

Underground mining (40%)

Open pit mining (29%)

In Situ Leaching-ISL (16%)

by-products give 15%

Uranium MiningUranium Mining

Uranium is usually mined similarly to other heavy metals, ore samples are then drilled and analyzed. The uranium ore is extracted by means of drilling and blasting. Uranium concentrations are a small percentage (2 - 0.1% U) of the rock that is mined, so tons of tailings waste are generated by the mining process that emit radon.

Therefore tailings are placed underground or capped

Open Pit MiningOpen Pit Mining

Ranger Pit Number 1, Northern TerritoryAll of the material removed from this hole

Tunnels are dug into the earth, where ore is extracted

The ore is crushed into a powder, then soaked in a lake. The impurities precipitate and the rest is dried by heat.

Lake uses an intense amount of water

Much of harmful environmental effects reported

Underground MiningUnderground Mining

In Situ Leaching (ISL)In Situ Leaching (ISL)This method is used because there are reduced

hazards to the employees of the mines, it is less expensive, and there are no large tailings deposits.

However, there are also several significant disadvantages including ground water contamination, unknown risks involving the leaching liquid reacting to the other minerals in the deposit, and an inability to restore the leaching site back to natural conditions after the leaching process is done.

PROCESS: Wells are drilled into aquifers, the water is removed, and a solvent, such as hydrogen peroxide, is pumped in. The peroxide dissolves the uranium, and the solution is pumped back up. An ion exchange system causes the uranium to precipitate in the form of UO42H2O (uranium peroxide)

In Situ LeachingIn Situ Leaching

Crushing & Concentration of OreCrushing & Concentration of OreThe ore is first crushed into smaller bits, then it is sent through a ball mill where it is crushed into a fine powder. The fine ore is mixed with water, thickened, and then put into leaching tanks where 90% of the uranium ore is leached out with sulfuric acid. Next the uranium ore is separated from the depleted ore in a multistage washing system. The depleted ore is then neutralized with lime and put into a tailings repository.

YellowcakeYellowcake

Meanwhile, the uranium solution is filtered, and then goes through a solvent extraction process that includes kerosene and ammonia to purify the uranium solution. After purification the uranium is put into precipitation tanks—the result is a product commonly called yellowcake.

Yellow cakeYellow cake

The dissolved uranium solution, including other metals, is then treated with amines dissolved in kerosene to selectively separate the uranium, which is then precipitated out of solution using ammonia, forming Ammonium di-uranate, or "yellowcake"

TransportationTransportation

In the final processes the yellow cake is heated to 800˚Celcius which makes a dark green powder which is 98% U3O8. The dark green powder is put into 200 liter drums and loaded into shipping containers and are shipped overseas to fuel nuclear power plants.

Leaders of Uranium Mining (2000)Leaders of Uranium Mining (2000)Canada 10,682

Australia 7,578

Niger 2,895

Namibia 2,714

Uzbekistan 2,350

Russia (est) 2,000

Kazakhstan 1,752

USA 1,456

South Africa 878

China (est) 500

Ukraine (est) 500

Czech Republic 500

India (est) 200

France 319

others 422

Total world 34,746

company tonnes U

Cameco 7218

Cogema 6643

WMC 3693

ERA 3564

Navoi 2400

Rossing 2239

KazAtomProm 2018

Priargunsky 2000

Step 2: Conversion ChemistryStep 2: Conversion ChemistryTo enrich uranium it must be in the gas form of UF6. This is called conversion. The conversion diagram shown here is from Honeywell. First the yellow cake is converted to uranium dioxide through a heating process (this step was also mentioned in the mining process). Then anhydrous hydrofluoric acid is used to make UF4. Next the UF4 is mixed with fluorine gas to make uranium hexafluoride. This liquid is stored in steel drums and crystallizes.

Step 3: Uranium EnrichmentStep 3: Uranium EnrichmentNatural U is 0.72% whereas power plants use 3-

5% enriched.235U only fissile nuclide Natural Uranium-only 1 atom of 235U in 140

atoms of 238U

Enrichment ProcessesEnrichment ProcessesA number of enrichment processes have been

demonstrated in the laboratory Gaseous diffusion - most common Gas centrifuges - 9 countries Aerodynamic separation - too expensive Electromagnetic separation Laser

Only two, the gaseous diffusion process and centrifuge process, are operating on a commercial scale

In both of these, UF6 gas is used as the feed materialMolecules of UF6 with U-235 atoms are about one

percent lighter than the rest, and this difference in mass is the basis of both processes

Large commercial enrichment plants are in operation in France, Germany, Netherlands, UK, USA, and Russia, with smaller plants elsewhere

Enrichment: Centrifuge Process Enrichment: Centrifuge Process vacuum tubes, each

containing a rotor one to two metres long and 15-20 cm diameter.

rotors are spun rapidly, at 50,000 to 70,000 rpm

heavier molecules with U-238 increase in concentration towards the cylinder's outer wall

there is a corresponding increase in concentration of U-235 molecules near the centre.

enriched gas forms part of the feed for the next stages, depleted UF6 gas goes back to the previous stage (cascade)

very high speeds:, outer wall spinning cylinder 400 and 500 metres per second= 1 million times the acceleration of gravity

Enrichment: Centrifuge ProcessEnrichment: Centrifuge Process

Enrichment: Gaseous Diffusion Enrichment: Gaseous Diffusion ProcessProcess involves forcing UF6 under pressure through a

porous membranesas 235U molecules are lighter than the 238U

molecules they move faster and have a slightly better chance of passing through the pores in the membrane

the UF6 which diffuses through the membrane is thus slightly enriched, while the gas which did not pass through is depleted in 235U

this process is repeated many times in a series of diffusion stages called a cascade

enriched UF6 is withdrawn from one end of the cascade and depleted UF6 is removed at the other end

the gas must be processed through some 1400 stages to obtain a product with a concentration of 3% to 4% 235U

Enrichment: Gaseous Diffusion Enrichment: Gaseous Diffusion ProcessProcess

The large Tricastin enrichment plant in France (beyond cooling towers). The nuclear reactors in the foreground provide power for it.

Step 4: Step 4: Nuclear Fuel Nuclear Fuel FabricationFabrication

UF6, in solid form in containers, is heated to gaseous form, and the UF6 gas is chemically processed to form LEU uranium dioxide (UO2) powder

this powder is then pressed into pellets, sintered into ceramic form (fuel pellets)

pellets are then loaded into zirconium alloy tubes that are afterwards hermetically closed (fuel rods). For every tonne of Uranium in the fuel, up to 2 tonnes of Zirconium alloy are needed for the tubes

rods are constructed into fuel assembliesfuel assemblies are made with different

dimensions and number of fuel rods, depending on the type reactor

Pellets and Fuel RodsPellets and Fuel Rods

Fuel AssemblyFuel Assembly

Plutonium-239Plutonium-239 Plutonium is very rare in nature.For military purposes, it is obtained processing

Uranium-238 in breeder reactors.It has a reasonably low rate of neutron emission due to

spontaneous fission.It is usually contaminated with Plutanium-240 which is

more unstable (4%-7% of plutanium-240 is considered bomb-grade).

The critical mass for an unreflected sphere of Plutonium is 16 kg.

Fission process: When Platinium-239 absorbs a slow neutron it becomes Platinium-240, which decays fast via different processes emitting at least two neutrons.

There are around 80 generations in the chain reaction. The whole process takes 0.8 microseconds.

MOX FuelMOX Fuel Plutonium, made in power reactors and from dismantled

nuclear weapons, is a valuable energy source when integrated in the nuclear fuel cycle

Over one third of the energy produced in most nuclear power plants comes from plutonium. It is created there as a by-product.

'MOX' is derived from 'mixed oxides', and refers to reactor fuel made from a mixture of plutonium and uranium oxide

For use in a light water reactor, the proportion of plutonium is about 5%. This is a similar fissile content as low enriched uranium fuel

MOX is formed into ceramic fuel pellets, extremely stable and durable, and which are sealed in metal (usually zirconium) tubes, which in turn are assembled into fuel elements

In most cases a part of the reactor core can be loaded with MOX fuel elements without engineering or operational modifications to the reactor

Plutonium is radiologically hazardous, particularly if inhaled, so must be handled with appropriate precautions

MOX FuelMOX Fuel

Plutonium-MOX economyPlutonium-MOX economy

Use of MOX fuel (part plutonium) in nuclear reactors to prolong uranium supplies◦presence of plutonium leads to increased risk

of proliferationPotential for move to ‘Generation IV’

reactors completely fuelled by plutonium◦even greater proliferation risk

MOX Fuel: Glove BoxesMOX Fuel: Glove Boxes

A glovebox (or glove box) is a sealed container that is designed to allow one to manipulate objects where a separate atmosphere is desired. Built into the sides of the glovebox are gloves arranged in such a way that the user can place their hands into the gloves and perform tasks inside the box without breaking containment. Part or all of the box is usually transparent to allow the user to see what is being manipulated.

MOX Fuel: Glove BoxesMOX Fuel: Glove Boxes

Uranium-233Uranium-233 U-233 is not found naturally.It is obtained from Thorium-232 in nuclear reactors.The fissile properties of U-233 are similar to U-235 and

Pu-239.Fission Process: When U-233 absorbs a neutron, it

becomes U-232.U-232 has the property of emitting gamma radiation

(neutrons) at levels higher than weapon-grade plutonium-239.

Nuclear Fuel: A BRIGHTER INSIGHT FOR Nuclear Fuel: A BRIGHTER INSIGHT FOR FUTUREFUTURE