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• Good Moderator for Reactors used Natural Uranium as Fuel due to Large Moderation Ratio (No Uranium Enrichment). • Used for Moderator at High Temperature[r]

4 Nuclear Reactors

‐ fission reactor: generate power by chain reaction of neutron fission

‐ fusion reactor: generate power by nuclear fusion

 sustain fission chain reaction in the Reactor Core

* sustain fusion condition by plasma ignition, laser implosion

 remove generated heat by Coolant (fluids or solid)

‐ water

‐ metal (sodium, lead)

‐ molten salt (florides)

‐ gas (helium, CO2)

 geneate power by heated Coolant

‐ rotate motor‐generator, screw drive

‐ direct use of heat

‐ direct thermo‐electric conversion

 maintain system integrity

‐ system monitoring

‐ component strength

‐ fluid purity

 limit release of radioactivity to environment

Nuclear  Reactor Components

Components of a Reactor:

• Fuel:    Provide Fissile Materials to continue Fission Reactions and  

Produce   Nuclear Energy

• Moderator: Moderate Fast Neutrons to Thermal Neutrons for Higher 

Fission Reactions 

• Control Materials : Control Fission Reaction in a Nuclear Reactor

• Coolant:

• Protect overheating of Fuel

• Generate Steam utilizing Heat from Fuel 

• Structural Materials:

• Fix Reactor Components at appropriate Position 

Nuclear Fuel

• Fuel, Fuel Road

– Generate heat by fission & transfer it to coolant

Requirements for Nuclear Fuel

– Nuclear Property

• Large Fission Cross section

• Small Parastic Absorption Cross section. – Physical/Chemical Property

• Large Energy Density • High Melting Temperature • High Thermal Conductivity

• Chemically Stable (with coolant, in particular) • Easy to Manufacture

• Mechanical properties (Radiation resistant)

 Dimensional changes

 Densification

Fuel Types ‐ Metallic

• Uranium Metal

– Poor mechanical properties & susceptible to radiation damage + , ,  phase changes ; brittle

+ dimensional instability and swelling – Density high: ~ 18 g/cm3

– High thermal conductivity (20~30 W/m/K) but low melting temperature (~1000oC)

– Chemically reactive with air, water and hydrogen + Corrosion by water

• Plutonium Metal

– phase changes (with variable thermal expansion coefficients) – Low melting point and thermal conductivity

– Chemically unstable and biologically hazardous • Uranium Alloys : U-Zr or U-Pu-Zr

– Less radiation damage and corrosion improved resistance –  Phase to be retained with addition of alloying materials – Zr raises Melting temperature of U-Pu

– Zr improves fuel cladding compatibility by suppressing inter-diffusion – Eutectic between Metal fuel and clad

Fuel Types ‐ Ceramics

• Uranium Dioxide - UO2

– Most widely used fuel material

– High Melting Point : ~ 2800oC

– Thermal Conductivity : ~ W/m/K

– Density : ~ 10.5 g/cm3

– Chemically stable

Chief limitation is swelling caused by fission gas, which markedly increase above critical burnup.

• U Carbide - UC

• Higher Thermal Conductivity than UO2

• Good Fuel for Fast Reactor

• Reacts with Water : Not used in LWRs

• U Carbide - UC2

• Even Higher Thermal Conductivity than UC

• Used in HTGR

• Reacts with Water : Not used in LWRs

• Not used in Fast Reactor : Contains More Carbon (Moderator)

• U nitride – UN

– High density : ~ 14.3 g/cm3

– High thermal conductivity : ~15 W/m/K

– High melting point : ~ 2700oC

Clad

• Function:

– Protect Fuel from Chemical Reaction or Corrosion – Contain Fission Products in clad Material

– Enforce Mechanical Strength of Fuel Rod • Requirements:

– High Mechanical Strength at High Temperature – High Thermal Conductivity

– Small Neutron Absorption Cross section • Clad Materials

– Zr Alloy :Used in most water reactors

– Stainless Steel : Mostly used in Fast Reactors

+ Strong Strength and Corrosion Resistant, but High Neutron Absorption Cross section

+ Ferretic steel( such as HT9, ODC) being developed to improve strength – Al Alloy : Used in Research Reactors

Zr Clad

• Zr Alloy

– Zr contains 0.5-3.0 % of hafnium

– High Corrosion resistant and Small Absorption Cross section

– Sudden increase of corrosion rate with impurities(eg., nitrogen, aluminum) in Zr and breakaway of oxode film

– Zircaloy-2 (1.5% tin, 12% Fe, 0.09% Cr and 0.05% Ni) better performance with breakaway, but embrittlement problem caused by hydrogen produced by

reaction with water

– Zircaloy-4(elimination of Ni from Zr-2) improve embrittlement phenomena – Also used for fuel assembly grid or duct in LWR

– Zr alloys exhibit significant creep at temperatures and stresses encountered in PWR design

– At high temperatures, Zr react with steam to release hydrogen : Zr + 2H2O → ZrO2 + 2H2

Coolant

• Coolant Requirements:

– Thermal Characteristics: High Specific Heat, Thermal conductivity, – High Boiling Temperature but low melting temperature

– Nuclear Characteristics: Small neutron Absorption Cross section

– Chemically Stable with Structural Materials Clad, Moderator and Good Corrosion Resistant

– Low pumping power

– Cheap and Easy to handle • Coolant Materials

– Water (H2O)

• Not very High Temperature at high Steam Pressure

• Emit Induced Radioactive Materials, thus required Shielding Materials

( )

n p 

   

16 16

Coolant

• Sodium (Na)

– Large Thermal Conductivity, not so corrosive to stainless steel – Small absorption cross section, Low cost

– Chemical reaction with Water and Air – Induced Radioactivity :

– High melting point (98 oC) : NaK better but expensive • Pb or Pb-Bi

– Pb: Hi melting temperature(327 oC), Pb-Bi :melting temperature 125 oC, – For Pb-Bi, radioactive polonium-210 produced

– High density: Large pumping power required, Seismic concern – Corrosive with structures

• Helium Gas (He)

– Inert Gas, Good corrosion Resistant, Stable to Radiation – Good Thermal Conductivity as a gas

• CO2

– Cheap and Easy to Handle – Stable to Radiation

Na  n  Na   (1.368, 2.754 M eV )

23 24

Moderator

• Function:

– Moderate Fast Neutron produced from Fission Reaction to Thermal Neutrons • Requirements:

– High Scattering Cross section and Small Absorption Cross section – Materials with Large Atomic Number Density and Small Mass Number

Atom Mass

Number

Average Collisions

Moderator

• Moderator Materials – Water

• Used in Most Commercial Power Reactors

• Relatively Large Absorption Cross section  U enrichment required • Also Used for Coolant

• Cheap and Easy to Handle – Heavy Water (D2O)

• Good Moderator for Reactors used Natural Uranium as Fuel due to Large Moderation Ratio (No Uranium Enrichment)

• Very Expensive (Heavy water: 0.015% of water) • Large mean free path, large volume of tank required – Carbon (C)

• Good Moderator for Reactors used Natural Uranium as Fuel due to Large Moderation Ratio (No Uranium Enrichment)

• Used for Moderator at High Temperature

Non‐Fuel Core Components

• Control Rod

– controls fast reactivity change so that the reactor could be kept in control : power change or steady state

• Burnable Absorber

– to compensate the long term reactivity change in the core • Neutron Source

Control Rod

• Function:

– Absorb Neutrons and Control Fission Reactions in a Reactor • Requirements:

– High Neutron Absorption Cross section – Easy Manufacturing

– Strong Corrosion Resistant • Control Rod Materials

Control rod

 Ag-In-Cd : Used in PWR

 Alloy made of ~ 80% Ag, 15% In, 5% Cd

- σγ(Cd) ~ 2,450 barns in the thermal neutron range - σγ(Ag) ≫1 in the intermediate neutron range

 Alloy of Ag and Cd + Small amount of indium (In) :

improve the absorption characteristics in the low epithermal region

 Reasonable corrosion resistance to hot water, encapsulated in SS tubes

 B4C : Used in BWR ,HTGR and SFR

 Various forms depending upon reactor types

 High thermal absorption cross section of 10B ~ 3,800 barns

 Issues of swelling and heat generated in the reaction of B(n,α)Li

Structural Materials

• Materials for Reactor Internals

: Type 304SS is used for most parts • Core Support Barrel

• Upper Guide Structure Assembly • Core Shroud Assembly

• Materials for Control Elements Drive Mechanism (CEDM)

A Motor Housing Assembly

: SA182 F347 for Upper End Fitting : ASME code cage N-4-11 for Motor

Housing

: SB166 for Lower End Fitting B Upper Pressure Housing

Structural Materials

• Function

– Fix Reactor Components at appropriate Position

– Protect Release of Fission Products from a Nuclear Reactor • Structural Materials

– Corrosion Resistant : Inconel Alloy, Zircaloy , Stainless Steel – Mechanical Strength : Inconel, Stainless Steel, Zircaloy

– Low Neutron Absorption : Zircaloy, Stainless Steel, Inconel • Carbon Steel

– High strength, low cost

– Low resistance to corrosion by high-temperature water or steam – Used for pressure vessel with corrosion-resistant SS coated

Nuclear Power Plant

Nuclear Plant

Fossil Plant

Reactor

Boiler

Turbine Generator Steam

Steam

Energy Resources for Electricity Generation

Item Hydro Power Fossil Power Nuclear Power

Energy resource Potential energy

of water C + O2CO2 + Heat

U + n A + B + 2.5n + heat

Molecule (Atomic)

mass 18 12 235

Energy produced from

same mass 3 E-05 1 2.6 E+05

Moving Distance

of molecule 100 m 10E-10 m 10 E-15 m

Force Gravitation Binding energy of

atom Nuclear Force

Plant Steam Pr [kg/cm2]

Steam Temperature [°C] Revolution per [RPM] Efficiency [%] Remark Nuclear

Plant 60~70 270 1,800 33 PWR Thermal

 Safety

• Radioactivity confinement • Decay heat removal

• Fuel melt / damage

• Health impact to operator / public

 Economics

• Construction cost

• Operation, Maintenance cost • Fuel cost

• Life Cycle Cost ( waste, decommisioning )

• compare with alternatives (fossil, solar, wind, etc.)

• electricity transmission, fuel transport, ultimate cooling (sea side, river side, arid area), waste handling

• environmental cost ( land area use ) • social cost

 Waste management • Used fuel

• Final repository

• Low/Intermediate level waste

Considerations on Nuclear Reactor Project

 Etc

• Non-proliferation policy (diversion to weapon)