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 3 rd and 4 th of these are negligible and the last is usually not included.  The codes used, such as ANS-5.1, model energy release from 235 U, 238 U, 239 Pu and 241 Pu 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 secs. Recent calculations suggest an overestimate in range 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 235 U

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  | V fi | 2  (E f ) where V fi =   f * V  I dv and  (E f ) = dn/dE f - no.of states in interval dE f  Fermi did not know the form of the interaction. Accordingly he assumed that it was a point interaction IAEA-12/12/2005

N(p)  p 2 (Q – T e ) 2.F(Z /,p).|M fi | 2.S(p,q) Essence of Beta Decay Using Fermi’s Golden Rule we get the shape of the spectrum as Statistical factor { Fermi Function Nuclear Matrix element Shape factor In Allowed approximation N(p) p 2.F(Z /,p)  (Q – T e ) Fermi-Kurie plots

1 f 7/ p 3/2 1 f 5/2 2 p 1/2 1 f 7/ p 3/2 1 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 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.

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 = t 1/2 We introduce ft 1/2  Const./ |M fi | 2 We get a variation in log 10 ft 1/2 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 (10 12 ions/s) beam is a cocktail of different nuclear species low-energy nucleushigh-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 (10 16 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 separator high-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 separator high-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 separator high-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-flight 5. 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+ ? ZANZAN Z-1 A N+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 Effect Very fragmented B(GT) at high exc. energy Very fragmented B(GT) at high exc. energy Different gamma de-excitation pathsDifferent gamma de-excitation paths Very low intrinsic effciency of the Ge detectorsVery low intrinsic effciency of the Ge detectors Three unfavourable conditions contribute to this effect:

Total Absorption spectroscopy 22 11 11 22  feeding E2E2 E1E1 E2E2 E x in the daughter II NaI N Ideal 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 Alvarez - p n + e – Joliot-Curies  Main characteristic – Cts. Energy distribution

1. Fission products (with converter) 3. Fission products (without converter) FP Distribution

Fission Fragments

n n p     p  n f2f2 f1f1 W.Catford

100 Sn 48 Ni 45 Fe

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 76 Kr MeV VAMOS - no condition - beam ToF - recoil ToF + DIAMANT + E -  E

         16 +  -  -DIAMANT 130 Nd Nd 131 Pm 129 Pr 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.