Transcript IAEA-12/12/2005 Decay Heat in Nuclear Reactors  “ Decay Heat is the principal reason of safety concern in Light Water Reactors. It.

IAEA-12/12/2005
Decay Heat in Nuclear Reactors
 “ Decay Heat is the principal reason of safety concern in Light Water Reactors.
It is the source of 60% of radioactive release risk worldwide.”
Reactor at 3600 MW power -252 MW decay heat in operation and on shutdown.
-i.e. 7%
2% after 1 hour
1% after 1 day.
Failure to cool the reactor after shutdown results in core heating and possible
core meltdown i.e. Three Mile Island again!!
Present plants deal with this using active decay heat removal systems. If these
systems fail----------.
“It is of high importance to know precisely the amount of decay heat in order to
assess core and containment cooling strategy during an abnormal event.”
- Hence the reason for our meeting
IAEA-12/12/2005
Decay Heat in Nuclear Reactors
Sources of Decay Heat
- Unstable fission products which decay eventually to stable nuclei.
- Unstable Actinide nuclei produced in successive n captures in U and Pu fuel.
- Fission induced by delayed neutrons
- Reactions induced by spontaneous fission neutrons.
- Structural and cladding materials that are radioactive.
The 3rd and 4th of these are negligible and the last is usually not included.
 The codes used, such as ANS-5.1, model energy release from 235U, 238U, 239Pu
and 241Pu using sum of exponential terms with empirical constants. Some of the
input data are left to the discretion of the user to allow for differences in
power history, initial fuel enrichment and neutron-flux level.
two limiting cases are given-a single fission pulse and continuous, infinite operation
followed by an abrupt shutdown.
 Yoshida et al. show that all calculations underestimate the results of experiments
in the time range 300-3000 secs. Recent calculations suggest an overestimate
in range 3-300 secs.
IAEA-12/12/2005
Fission Products - Distribution
Mass Distribution-thermal fission of 235U
In the thermal fission of actinide nuclei about 550 fission product nuclei are produced
They have the characteristic double-humped mass distribution shown above
-This distribution is dictated by the well known shell closures in stable and near-stable
nuclei.
IAEA-12/12/2005
Super Heavies
Fewer than 300 nuclei
Proton Drip Line
Neutron Drip Line
Super Heavies
Super Heavies
Fewer than 300 nuclei
Fewer than 300 nuclei
Proton Drip Line
Proton Drip Line
Fission
Fission
Fragments
Fragments
Fission
Fission
Fragments
Fragments
Neutron Drip Line
Neutron Drip Line
Beta Decay and Reactor Decay Heat
To re-iterate
 Correct assessment of Decay Heat is important because it is needed for
a) Design of a safe power facility
b) Shielding for fuel discharges, fuel storage and transport flasks
c) Management of the resulting radioactive waste
What can we do to improve things?
Data required-cross-sections, fission yields, decay half-lives, mean beta
and gamma energies, neutron capture cross-sections and uncertainties
in these data.
Why are there gaps in the data? Is there reason to believe that we can
overcome the difficulties?
IAEA-12/12/2005
Nuclear Species that can be produced at ISOLDE
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Essence of Beta Decay
n p + e- + 
p + e- n + 
p
n + e+ + 
------Beta minus decay
------Electron capture
------Beta plus decay
Three-body process indicated by energy spectrum
and verified by measuring recoil and electron
momenta in coincidence.
Fermi Theory of Beta Decay.
-Assumes a Weak interaction at a point.
 = 2 | Vfi |2 (Ef)
where Vfi =  f*VI dv
and (Ef) = dn/dEf - no.of states in interval dEf
Fermi did not know the form of the interaction. Accordingly he assumed that it was
a point interaction
IAEA-12/12/2005
Essence of Beta Decay
Using Fermi’s Golden Rule we get the shape of the spectrum as
{
N(p)  p2(Q – Te)2 .F(Z/,p) .|Mfi|2 .S(p,q)
Shape factor
Statistical factor
Fermi Function
In Allowed approximation
N(p)

p2 .F(Z/,p)
(Q – Te)
Fermi-Kurie plots
Nuclear Matrix element
Essence of Beta Decay
One great advantage of studying beta decay is that we understand the interaction.
simplest form it takes is an allowed FERMI decay with J = 0, No parity change
However we also get fast transitions with J = 1, No parity change-GAMOW TELLER
Alowed GT selection rules J = 0,1 but 0 0, No change in parity.
στ
Gamow-Teller
Or
28
Fermi
2 p 1/2
1 f 5/2
2 p 3/2
1 f 7/2
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τ
28
τ
1 f 5/2
2 p 3/2
1 f 7/2
Essence of Beta Decay – Selection Rules
 Allowed Transitions(l = 0):Fermi J = 0, No parity change
Gamow-Teller J = 0,1, No parity change
Expansion of a plane wave
In angular momentum
Eigenstates.
First Forbidden(change of l = 1):Fermi J = 1, Yes parity change
Gamow-Teller J = 0,1,2, Yes parity change
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Essence of Beta Decay
Transition rate  = 0.693
t1/2
We introduce ft1/2  Const./ |Mfi|2
We get a variation in log10ft1/2 for two reasons
- the variation in the nuclear matrix element
- How forbidden it is i.e How large is the orbital angular
momentum change.
IAEA-12/12/2005
Essence of Beta Decay
The Future:- Has anything changed? Can we do better?
Three signs of hope for improvement.
1) Big upsurge in interest in exotic nuclei and their decays
2) Development of the IGISOL
3) Development of Total absorption Spectroscopy
IAEA-12/12/2005
J. Benlliure
Production techniques
 In-flight fragmentation
thin target spectrometer
gas cell
high-energy
low-energynucleus
nucleus
heavy projectile
 heavy projectile into a light target nucleus (projectile fragmentation)
 short separation+identification time (100 ns)
 limited power deposition
 Independent of Chemistry
 thinner targets (10% of range) and lower beam currents (1012 ions/s)
 beam is a cocktail of different nuclear species
Basis of Fragmentation studies at GANIL
J. Benlliure
Production techniques
 Isotopic separation on-line (ISOL)
thick target
ion source mass separator
light projectile
high-energy nucleus
post-acceleration
diffusion




light projectile into a heavy target nucleus (target spallation)
charged and neutral projectiles (n,g)
thick target (100% of range) and high beam current (1016 p/s)
high quality beams




long extraction and ionization time (ms)
chemistry dependent
target heat load
activation
Basis of SPIRAL
Production techniques
 Gamma/neutron converters
converter
e-, d
thick target ion source mass separator
low-energy
high-energy
nucleus nucleus
g, n
post-acceleration
diffusion
Basis of SPIRAL II
J. Benlliure
Production techniques
 Gamma/neutron converters(A variant of ISOL scheme)
thick target ion source mass separator
converter
high-energy nucleus
g, n
e-, d
post-acceleration
diffusion
 Two-step reaction scheme(ISOL + Fragmentation)
fission
ion source mass separator
light projectile
fragmentation spectrometer
post-acceleration
diffusion
J. Benlliure
• Available Beams
Regions of the Chart of Nuclei Accesible
with SPIRAL 2 beams
Primary beams:
 deuterons
 heavy ions
4. N=Z Isol+In-flight
6. SHE
5. Transfermiums
In-flight
2. Fusion reaction
with n-rich beams
1. Fission products (with converter)
3. Fission products (without converter)
8. Deep Inelastic Reactions with RNB
7. High Intensity Light RIB
IGISOL – Development of He Jet Technique
HeJRT Technique 1970s
IGISOL-R.Beraud(Lyons)
Applied at Jyvaskyla by
Beraud and Aysto
Advantages
- Chemistry Independent
- Ideal input to mass separator
but
- No Z discrimination unless some
other technique is used as well.
Note:-For our purposes important thing is that
it allows us to study refractory elements
The problem of measuring the β - feeding (if no delayed part.emission)
β+ ?
ZAN
•We use our Ge detectors to
construct the decay scheme
γ
•From the γ-balance we extract the
β -feeding
γ
Z-1AN+1
γ1
γ2
Consequence: Pandemonium Effect
Three unfavourable conditions
contribute to this effect:
• Very fragmented B(GT) at high
exc. energy
•Different gamma de-excitation
paths
•Very low intrinsic effciency of the
Ge detectors
Total Absorption spectroscopy
b-feeding
NaI
E2
g2
g1
g1
E1
g2
Ib
N
E2
Ideal case
Ex in the daughter
Essence of Beta Decay
The Future:- Has anything changed? Can we do better?
Three signs of hope for improvement.
1) Big upsurge in interest in exotic nuclei and their decays
2) Development of the IGISOL
3) Development of Total absorption Spectroscopy
IAEA-12/12/2005
Outline
 Introduction
- What is Nuclear Physics?
- Where are its frontiers?
- How does it relate to the rest of Physics?
How can we study nuclei?
- The need for beams of radioactive nuclei
- How can we produce RNBs?
Fragmentation and ISOL
The structure of nuclei
- The Goal- A unified theory
- The Challenges
- Symmetries
- Limits of Nuclear existence
- Haloes and skins
- New forms of collective motion
- ???????
The new opportunities-SPIRAL II – ISOL beams - High Intensity stable beams
GANIL-07/10/2005
Beta decay
 Three types of decay.
- n
p + e- + e
- p + e- p
n+
n + e+ + 
One of the earliest discoveries
1938 - Alvarez
1934 – Joliot-Curies
 Main characteristic – Cts. Energy distribution
FP Distribution
1. Fission products (with converter)
3. Fission products (without converter)
Fission
Fragments
Fission
Fragments
a

p
b
g
p
n
f2
f1
n
g
b

n
W.Catford
100Sn
48Ni
45Fe
Where is neutron drip-line ?
N drip-line maybe reached
N drip-line reached
E404aS : Identification of g-rays in the light
rare-earth nuclei near the proton drip-line
76Kr
v, M, Z, Q
+ 58Ni @ 328 MeV
g, g-g
p, a
VAMOS
- no condition
- beam ToF
- recoil ToF + DIAMANT
+ E - E
131Pm
613
547
407
454
326
237
273
159
N.Redon et al.
130Nd
DIAMANT gated
129Pr
Doppler corrected spectra
no gate
18+
12+
704
743
10+
18+ 16+
664
16+ 14+
613
Collaboration : IPN Lyon, Univ.Liverpool,
GANIL, CSNSM Orsay, CENBG Bordeaux,
ATOMKI Debrecen, Univ.York, Univ.Napoli,
TRIUMF
16+
14+
547
10+ 8+
10+
6+ 4+
130Nd
12+
14+ 12+
159
454
8+ 6+
2+ 0+
g-g-DIAMANT
4+ 2+
326
809
8+
6+
4+
2++
0
130Nd