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IAEA-12/12/2005
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
Decay Heat in Nuclear Reactors
IAEA-12/12/2005
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.
Fission Products - Distribution
In the thermal fission of actinide nuclei about 550 fission product nuclei are produced
IAEA-12/12/2005
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.
Mass Distribution-thermal fission of 235U
Proton Drip Line
Neutron Drip Line
Super Heavies
Fewer than 300 nuclei
Proton Drip Line
Neutron Drip Line
Super Heavies
Fewer than 300 nuclei
Fission
Fragments
Fission
Fragments
Proton Drip Line
Neutron Drip Line
Super Heavies
Fewer than 300 nuclei
Fission
Fragments
Fission
Fragments
IAEA-12/12/2005
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
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*VI 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
N(p) p2(Q – Te)2 .F(Z/,p) .|Mfi|2 .S(p,q)
Essence of Beta Decay
Using Fermi’s Golden Rule we get the shape of the spectrum as
Statistical factor{
Fermi FunctionNuclear Matrix element
Shape factor
In Allowed approximation
N(p)p2 .F(Z/,p)
(Q – Te)
Fermi-Kurie plots
1 f 7/228
2 p 3/21 f 5/22 p 1/2
1 f 7/228
2 p 3/21 f 5/2
στ Gamow-Teller
Or τ Fermi
στ
τ
Essence of Beta Decay
IAEA-12/12/2005
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 changeHowever 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.
Essence of Beta Decay – Selection Rules
IAEA-12/12/2005
Allowed Transitions(l = 0):-
Fermi J = 0, No parity change
Gamow-Teller J = 0,1, No parity change
First Forbidden(change of l = 1):-
Fermi J = 1, Yes parity change
Gamow-Teller J = 0,1,2, Yes parity change
Expansion of a plane wave In angular momentum Eigenstates.
Essence of Beta Decay
IAEA-12/12/2005
Transition rate = 0.693t1/2
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.
Essence of Beta Decay
IAEA-12/12/2005
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
Production techniques
J. Benlliure
In-flight fragmentation
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
low-energy nucleus high-energy nucleus
heavy projectile
thin target gas cell spectrometer
Basis of Fragmentation studies at GANIL
Production techniques
J. Benlliure
Isotopic separation on-line (ISOL)
light projectile into a heavy target nucleus (target spallation) charged and neutral projectiles (n) 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
light projectile
thick target
diffusion
ion source
post-acceleration
mass separatorhigh-energy nucleus
Basis of SPIRAL
Production techniques
J. Benlliure
Gamma/neutron converters
low-energy nucleus
e-, d
thick target
diffusion
ion source
post-acceleration
mass separatorhigh-energy nucleus
converter
, n
Basis of SPIRAL II
Production techniques
J. Benlliure
Gamma/neutron converters(A variant of ISOL scheme)
Two-step reaction scheme(ISOL + Fragmentation)
e-, d
thick target
diffusion
ion source
post-acceleration
mass separatorhigh-energy nucleus
converter
, n
light projectile
fission
diffusion
ion source
post-acceleration
mass separator fragmentation spectrometer
2. Fusion reaction with n-rich beams
1. Fission products (with converter)
4. N=Z Isol+In-flight5. Transfermiums In-flight
3. Fission products (without converter)
Primary beams: deuterons heavy ions
Regions of the Chart of Nuclei Accesible with SPIRAL 2 beams
Regions of the Chart of Nuclei Accesible with SPIRAL 2 beams
7. High Intensity Light RIB
6. SHE
8. Deep Inelastic Reactions with RNB
•Available Beams
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
Z-1AN+1
γ
γ
γ2γ1
•We use our Ge detectors to construct the decay scheme
•From the γ-balance we extract the β -feeding
Consequence: Pandemonium EffectConsequence: Pandemonium Effect
• Very fragmented B(GT) at high Very fragmented B(GT) at high exc. energyexc. energy•Different gamma de-excitation Different gamma de-excitation pathspaths•Very low intrinsic effciency of the Very low intrinsic effciency of the Ge detectorsGe detectors
Three unfavourable conditions contribute to this effect:
Total Absorption spectroscopyTotal Absorption spectroscopy
2
1 1
2
feedingE2
E1
E2
Ex in the daughter
I
NaI
N
Ideal caseIdeal case
Essence of Beta Decay
IAEA-12/12/2005
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
Outline
GANIL-07/10/2005
Introduction - What is Nuclear Physics? - Where are its frontiers? - How does it relate to the rest of Physics?
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
How can we study nuclei? - The need for beams of radioactive nuclei - How can we produce RNBs? Fragmentation and ISOL
Beta decay
Three types of decay. - n p + e- + e One of the earliest discoveries
- p + e- n + 1938 - Alvarez
- p n + e+ + 1934 – Joliot-Curies
Main characteristic – Cts. Energy distribution
1. Fission products (with converter)
3. Fission products (without converter)
FP Distribution
Fission
Fragments
Fission
Fragments
n
n
p
p
n
f2f1
W.Catford
100Sn
48Ni
45Fe
Where is neutron drip-line ?
N drip-line maybe reached
N drip-line reached
E404aS : Identification of -rays in the light rare-earth nuclei near the proton drip-line
p,
, -v, M, Z, Q
76Kr + 58Ni @ 328 MeV
VAMOS
- no condition- beam ToF- recoil ToF + DIAMANT + E - E
159
326
454
547
613
664
704743
809
2+
0+
4+
2+
6+
4+
8+
6+
10+
8+
12+
10+
14+
12+
16+
14+
18+
16+
--DIAMANT 130Nd
130Nd
18+
16+
10+
14+
2+
0+
8+
6+
4+
12+
130Nd131Pm129Pr
159 32
6
454
547
613
237 27
3
407 DIAMANT gated
no gate
Doppler corrected spectra
Collaboration : IPN Lyon, Univ.Liverpool,GANIL, CSNSM Orsay, CENBG Bordeaux,ATOMKI Debrecen, Univ.York, Univ.Napoli,TRIUMF
N.Redon et al.