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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

Jonghwa Chang jhchang@kaeri.re.kr

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 

• Fuel, Fuel Road

– Generate heat by fission & transfer it to coolant

– Keep active fission products in the cladding during intended core life

– 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)

– 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

– Used in SFR( EBR-II, IFR, PRISM, KALIMER)

Fuel Types ‐ Ceramics

• Uranium Dioxide - UO 2

– Most widely used fuel material

– High Melting Point : ~ 2800 o C

• Higher Thermal Conductivity than UO 2

• Good Fuel for Fast Reactor

• Reacts with Water : Not used in LWRs

• U Carbide - UC 2

• 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/cm 3

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

– High melting point : ~ 2700 o C

• candidate fuel for fast reactor

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

+ Low thermal cross section but poor corrosion resistant+ Used in pure form in water operated at low temperature

• 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– More expensive than SS, Fabrication cost higher since welding done in inert atmosphere

– 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

– 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

– Inert Gas, Good corrosion Resistant, Stable to Radiation

– Good Thermal Conductivity as a gas

Properties of Reactor Coolants

• 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

Number

Average Collisions ( 2MeV to 1eV ) Absorption (Thermal Neutron) H

14.5 20 70 92 121 1747

0.332 0.00053

0.0092 0.0034 0.00027 7.59

• 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

• Solid Moderator , thus can not be used for Coolant

– Strong Corrosion Resistant

• Control Rod Materials

– B4C, Ag-In-Cd, Hafnium etc

 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

 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

 Also used in fast reactor : reactivity requirement less sever

• 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

: Type 316 for Upper End Fitting, Tube, Lower End Fitting

• 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

– NDT(nil-ductility temperature) rising appreciably with neutron irradiation,

so that shielding from fast neutron and gamma ray may be required

Nuclear Plant

Fossil Plant

Energy resource Potential energy

of water C + O 2  CO 2 + Heat U + n  A + B + 2.5n

+ heat Molecule (Atomic)

Energy produced from

Moving Distance

[kg/cm 2 ]

Steam Temperature [°C]

Revolution per min [RPM]

 Safety

• Radioactivity confinement

• Health impact to operator / public

• 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 )