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Nuclear Reactor Theory Intermezzo

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Nuclear Reactor Theory Intermezzo

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Nuclear Reactor TheoryIntermezzo

William D’haeseleer

William D’haeseleer

BNEN – NRT 2011-2012

Intermezzo:

One-Speed Diffusion Theory

of a Nuclear Reactor

See Duderstadt & Hamilton: § 5.III AD

§ 5. IV

[to a large extent to be studied independently]

William D’haeseleer

BNEN – NRT 2011-2012

Consider mono-energetic neutrons only

→ time-dependent neutron diffusion equation

Consider s as the neutron source due to fission

William D’haeseleer

BNEN – NRT 2011-2012

Consider steady state: critical system

slab geometry; thickness a

a

William D’haeseleer

BNEN – NRT 2011-2012

Solution:

Because of symmetry: C = 0

a

Second b.c.:

Non-trivial solution:n: odd integer

William D’haeseleer

BNEN – NRT 2011-2012

For a critical reactor, only 1-st “harmonic” remains:

a

Since:

“buckling”

For “a” very large, B1 very small, almost no buckle

William D’haeseleer

BNEN – NRT 2011-2012

Customary to designate

At any time in this case:

“Geometric Bucling”

William D’haeseleer

BNEN – NRT 2011-2012

Boundary conditions specify eigenvalues; constant A remains undetermined.

Must be found from overall power considerations

a

William D’haeseleer

BNEN – NRT 2011-2012

Sphere

and Φ finite everywhere

Solution:

William D’haeseleer

BNEN – NRT 2011-2012

Infinite cylinder

and Φ finite everywhere

Zero’s of J0; J0(xn)=0

Solution:

William D’haeseleer

BNEN – NRT 2011-2012

- Bessel functions J0, Y0
- Intermezzo on Bessel functions / following slides
–see also text “Bessel functions and their relatives” (GVdB & WDH)

William D’haeseleer

BNEN – NRT 2011-2012

Bessel Functions— Very elementary considerations —

William D’haeseleer

2007-2008

William D’haeseleer

BNEN – NRT 2011-2012

William D’haeseleer

BNEN – NRT 2011-2012

Solution of in rectangular coordinates

Take r as 1-D variable:

William D’haeseleer

BNEN – NRT 2011-2012

- Series expansion of Cosine and Sine functions

William D’haeseleer

BNEN – NRT 2011-2012

Solution of in cylindrical coordinates

Take r as 1-D variable:

(Note:

error in pdf document)

J0 and Y0 are Bessel functions of 0-th order

William D’haeseleer

BNEN – NRT 2011-2012

- Series expansions of zero-th order Bessel Functions

William D’haeseleer

BNEN – NRT 2011-2012

J0 behaves like a damped cosine

William D’haeseleer

BNEN – NRT 2011-2012

William D’haeseleer

BNEN – NRT 2011-2012

Asymptotic approximations (x sufficiently large)

For x large, each behaves as “damped” cos and sin

by sqrt(x)

William D’haeseleer

BNEN – NRT 2011-2012

Solution of in rectangular coordinates

William D’haeseleer

BNEN – NRT 2011-2012

Solution in cylindrical coordinates

Take r as 1-D variable:

(Note:

error in pdf document)

I0 and K0 are Modified Bessel functions of 0-th order

William D’haeseleer

BNEN – NRT 2011-2012

Modified Bessel Functions of 0-th order

William D’haeseleer

BNEN – NRT 2011-2012

Asymptotic approximations (x sufficiently large)

For x large, each behaves as “reduced” exponentials or hyperbolic functions

by sqrt (x)

William D’haeseleer

BNEN – NRT 2011-2012

Sphere / Infinite cylinder / Parallelepiped / Finite cylinder

William D’haeseleer

BNEN – NRT 2011-2012

William D’haeseleer

BNEN – NRT 2011-2012

Ref: Lamarsh NRT

William D’haeseleer

BNEN – NRT 2011-2012

Ref: Lamarsh NRT

Evolution of flux dependent upon

- geometry (diffusion; leakage)
- material composition (absorption; fission)

→ For arbitrary geometry & composition,

difficult to find Bg ; also in general:

Steady state only arises when reactor is critical;

i.e., whenk = 1

William D’haeseleer

BNEN – NRT 2011-2012

In general, k ≠ 1;

therefore apply a “trick” to find k = f (dimensions, material composition)

then set k = 1relationship between dimensions & material composition for criticality

!

William D’haeseleer

BNEN – NRT 2011-2012

≡ B²called“Material Buckling” Bm2

-- for k = 1 --

At this stage, k still unknown, since B² is not known

William D’haeseleer

BNEN – NRT 2011-2012

Note (1):

- suppose one writes
→ one could introduce f = fuel utilization factor

William D’haeseleer

BNEN – NRT 2011-2012

Note (2):Consider an infinite reactor

in such a reactor, Φ independent of position

(uniform throughout)

reduces to

William D’haeseleer

BNEN – NRT 2011-2012

Note (3):Consider now general case (finitereactor)

- Take now as a definition:
Then, source term

→ time-(in)dependent diffusion equation can be written as:

Time independent

Time dependent

William D’haeseleer

BNEN – NRT 2011-2012

Note (4):still finite reactor

- Time independent case:

Or for a critical reactor, k = 1

William D’haeseleer

BNEN – NRT 2011-2012

Reactor is critical if

Material Buckling

Geometric Buckling; B12=Bg2

for simple slab

“critical equation”

William D’haeseleer

BNEN – NRT 2011-2012

“critical equation”

determines requirement to have a critical reactor:

- material composition

- dimensions etc.

determines requirement to have a critical reactor:

- material composition

- dimensions etc.

Alternative expression of critical equation:

William D’haeseleer

BNEN – NRT 2011-2012

Physical meaning of critical equation

Consider a bare reactor of arbitrary geometry.

# of neutrons leaking out of the system

# of neutrons absorbed

William D’haeseleer

BNEN – NRT 2011-2012

Physical meaning of critical equation

Non-leakage probability*:

From:

William D’haeseleer

BNEN – NRT 2011-2012

*Note: symbols PL and PNL are used interchangeably and mean the same!

Physical meaning of critical equation

Interpretation of

Physical meaning of critical equation

Interpretation of

# of neutrons absorbed

Gives rise to a release of fission neutrons:

William D’haeseleer

BNEN – NRT 2011-2012

Physical meaning of critical equation

Physical meaning of critical equation

Of these neutrons, only a fraction PL does not leak out, and gives rise to absorption in the next generation:

Hence:

For a critical reactor:

William D’haeseleer

BNEN – NRT 2011-2012