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.