industrial energy management chapter 4 : nuclear power plants

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Industrial Energy Management

Chapter 4 : Nuclear Power Plants

Jun.-Prof. Benoît Fond, G-10/R-119

[email protected]

mailto:[email protected]

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Intro

April 2007 : 30 countries operate 449 nuclear

reactors for electricity generation. 60 new reactors

under construction

11 % of world electricity production is nuclear energy

(40% coal, 22.2% gas, 16.5 % Hydro)

Nuclear Power worldwide

Billion kWh as of 2016

Source:

nuclear energy institute

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Layout

1. Physical principles of nuclear power

• Atomic structure and radioactivity

• Fission reactions

• Criticality

2. Topology of nuclear power plants

• Classification

• Pressurized water reactors

• Boiling water reactors

• Fast neutron reactors

3. Nuclear Waste treatment

4. Risks

Nuclear Power plants

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Atom (10-10m): Core (10-15m) + electron cloud

Binding energy : Due to forced between particles, forming atoms from

constituent protons, neutrons, and electron releases energy. Mass is

turned in energy -> Mass defect

The mass of the atom is smaller than the mass of separated particles :

E=mc2

Atomic structure and radioactivity

Physical principles of nuclear power

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A(mass number)=Z(atomic

number)+N(number of neutron)

From mass we can calculate mass

defect.

Uranium has two natural isotopes or nuclides:238U (99.3%) and 235U (0.7%)Table of chemical element

Same Z

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Valley of Stability

Spontaneous radiative decay :

• Alpha : 𝑍𝐴𝑋 → 𝑍−2

𝐴−4𝑌 + 24𝐻𝑒

• Beta+ : 𝑍𝐴𝑋 → 𝑍−1

𝐴𝑌 + 𝑒− + തν

• Beta- : 𝑍𝐴𝑋 → 𝑍+1

𝐴𝑌 + 𝑒+ + ν

𝑁 𝑡 = 𝑁0𝑒−𝑡/𝜏 = 𝑁02

−𝑡/𝑡1/2

Half-life 238U: 4,5 109 years235U: 7,2 108 years14C: 5,7 103 years

Radioactivity

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Binding energy per nucleon

Fission reaction :

An heavy nuclide split into two smaller

ones

~0.85 MeV per neutron -> 200 MeV

per nuclide

Chemical reaction e.g. C oxidation ->

a few eV

Fusion reaction :

Two light nuclide form a bigger one

e.g. 2H + 3H -> 4He + 1n 17.6 MeV

Fission

Fusion

Fuel Energy per kg of

fuel

235U (fission) 23.000.000 kWh

2H + 3H (fusion) 96.000.000 kWh

C 9 kWh

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Physical principle of Nuclear Power

1n + 235U -> X + Y + 2,4 n + …

Reaction needs “excitation energy” : fission

energy barrier (for deformation prior to fission)

kinetic energy of n + binding energy of n to

nuclide neutron

Binding energy of n to 235U is much higher and 238U rarely fissions

Energy release 200 MeV

82% kinetic energy of X and Y

11% neutron, beta, gamma radiation

7 % later radiative decay

Neutron emission

Prompt neutrons (99%, immediately)

Delayed neutrons (0.7%, during later radiative

decay)

Net neutron growth -> Possibility of chain

reaction -> Bomb

Fission reactions

1%

Source : Energie,

electricité et

nucléaire, G.

Naudet and P.

Reuss, EDP

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• Every fission produces on average v

(rapid) neutron

• Every emitted neutron has probability w

to cause fission of heavy nuclide

-> Multiplicity factor 𝑘 = 𝑤 × 𝑣 effective

neutron multiplication factor

From N fissions, kN new fissions can be

obtained

k<1 subcritical : the reactor stops

k=1 critical : steady state – normal operation

k>1 supercritical : reaction amplifies –

reactor start or bomb

Criticality

Physical principle of Nuclear Power

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Physical principles of nuclear reactors

v is fixed, so we must control w probability of

emitted (rapid) neutron to trigger new fissions

What happens to a (rapid) emitted neutron ?

• It collides with another atom

(thermalisation) and becomes slower

• It is absorbed by another nuclide :

• Fertile capture : Fission (235U)

• Sterile capture : No Fission (238U)

• It leaves the reactor

With 235U, the probability of capture of thermal

neutron is high

• We must allow thermalisation. To slow

down neutron, a moderator material is

used. It must be have light nuclide for

efficient diffusion, but not capture neutron

How to change k ?

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Size of the reactor (leak)

Poisons and absorbers :

Indium and Cadmium on control rods to capture neutron.

Boric acid (liquid)

Fission product poisons : 135Xe and 149Sm

Consumable poison : B, Gd

Temperature : negative effect of temperature on criticality is needed for

reactor safety

Doppler effect on 238U : increase capture (+)

Water dilation : decrease scattering (-) and thermal neutron capture (+)

How to change k ?

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For prompt neutron, average dt = 25 microseconds. Rate : 40000/s

If k=1.0001, after 1s, kn=55 (+ 5500%)

Some neutrons (0.7%) are emitted from fission products after radioactive

decay. Decay takes 11 s

So for 1000 neutrons, 7 have a dt of 11 s and 993 a dt of 25 microsecond, so

on average, dt is 0,08 s

If k=1.0001, n=12.5 after 1s, kn= 1,0013 (+0.13%)

Must slower response.

The role of delayed neutron


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Topology of nuclear reactors

Light nuclides for efficient deceleration

Low probability of neutron capture. High density to reduce deceleration

distance.

Normal water : Very good moderator (+++), but high capturing rate.

Requires enriched uranium -> Mostly used. Pressurized water reactor (66%),

Boiling water reactors (23%, e.g. Fukushima)

Heavy water (2H2O) : Good moderator (++), and low capturing rate ->allow

natural uranium -> CADUX Heavy water reactor (5% Canada)

Graphite : Average moderator (+), and low capturing rate ->allow natural

uranium

Cheap and high thermal properties. -> 6% UNGG (France), Magnox (UK),

RBMK (USSR, Tchernobyl)

Moderator

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Combustible: Natural Uranium, Enriched Uranium, Plutonium, MOX (Plutonium

and natural Uranium).

Moderator Medium : Water, Heavy water (2H2O), Graphite, None (Rapid neutrons)

Heat transfer Medium : Pressurized water, Boiling water, Heavy water, CO2, He

Cladding material

Absorbers

Main elements of nuclear reactors


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The Fermi Pile (1942, Chicago)

Natural Uranium and Graphite

Geiger counter

Reads neutron flux Moves control rod

Fermi & Co

reads Geiger counter

Safety :

Heavy absorbing bar

Cadmium Salt solution

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• Enriched Uranium (3-4% 235U) as Fuel

• Water as moderator

• Water as working fluid

• 3 Water Loops

• Primary circuit, 150 bar,

~280-320°C, liquid only

Moderator and rod

cooling

• Working fluid, 70 bar

~280° C (Water/steam)

• Condenser cooling

water

Pressurised water reactor


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Each fuel rod is sheathed with Zircalloy

Moderator flows in between

Empty rods for control

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Pressurised water reactor components

Reactor vessel primary cooling pump boilerPrimary water loop


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Boiling water reactors

Two water loops only. Primary water at 70 bar

Boiling and moisture separation in reactor


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Enriched Uranium Oxide (~2-3%)

Control rod at bottom

Steam liquid cyclone separator at top

Advantage :

Simpler (only two loops)

Disadvantages :

Bigger reactor (less dense)

Radioprotection : Water is radioactive after

neutron capture (16N) so turbine casing must be

protected from radiation

Boiling water reactors

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Heavy water reactors

Moderator (Heavy water) is not pressurized, but circulated to prevent

heating

Working fluid (Heavy water) flows around rods in pressure tubes

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Heavy water reactors

Little neutron capture -> run

on natural Uranium

No pressure on reactor pool

-> light construction

Mainly developed by

Canada

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Neutron capture by 238U produces plutonium after beta - radiation

239Pu is fissile upon capture of rapid neutron, and produces more neutrons.

Neutron excess in chain reactions

->Rapid neutron reactor without moderator, with 239Pu/238U as fuel

Pu fission -> emission of rapid neutron:

-> Captured by 238U-> produces Pu : Breed more Pu that it consumes

-> Captured by 239Pu-> more fission

Liquid sodium used as heat carrier, little neutron diffusion, little neutron

capture, large temperature range as liquid

Fast neutron reactors (breeder)


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Fast neutron reactors (breeder)

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Nuclear fuel cycle and waste

Fuel cycle and waste

Specificities :

• Very high energy density, muss smaller mass to handle

• Possibility for regeneration in breeder reactor

• Fuel still emit heat after use for years due to radioactive decay of fission

products

• Fuel is not fully exhausted after use in reactor.

Nuclear Fuel production:

Uranium Ore mining

Chemical reaction -> UF6 -> Separation 235U and 238U for enrichment

Uranium Oxide Formation or MOX (Enriched Uranium and Platinum)

Yearly consumption ~70,000 tons, Reserves ~50 years for regular 3% 235U

fuel. Outlook : Breeder and Thorium

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• Fuel waste

retreatment

possible (239Pt, 238U and 235U)

• Highly radioactive

wastes

• Gamma and

neutron radiation

are most

dangerous

Waste

Fuel cycle and waste

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Risk

Dissemination of radioactive products

Exposure to radioactivity on plant components

Accidents :

• Core Meltdown (Fukushima and Three Miles Island), cooling issues after

emergency shutdown, plus valve failures or H2 explosion ->dissemination

• Supercritical reaction (Tchernobil) - > Explosion

Risk

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References

R. L. Murray, Nuclear Energy : An Introduction to the Concepts,

Systems, and Applications of Nuclear Processes, BH

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industrial energy management chapter 4 : nuclear power plants

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