W.Gelletly

1. Beta Decay & Reactor Decay Heat W.Gelletly Physics Department,University of Surrey IAEA-12/12/2005

2. 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

3. 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

4. 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

6. Fission Fragments Fission Fragments Fission Fragments Fission Fragments

7. 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

8. Nuclear Species that can be produced at ISOLDE IAEA-12/12/2005

9. Essence of Beta Decay ------Beta minus decay ------Electron capture ------Beta plus decay • n p + e- +  p + e- n +  p n + e+ +  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

10. Essence of Beta Decay N(p) p2 .F(Z/,p) (Q – Te)  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 Fermi Function Statistical factor Nuclear Matrix element In Allowed approximation Fermi-Kurie plots

11. 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 τ Fermi 2 p 1/2 στ 1 f 5/2 1 f 5/2 2 p 3/2 2 p 3/2 28 28 1 f 7/2 1 f 7/2 τ IAEA-12/12/2005

12. Essence of Beta Decay – Selection Rules  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. IAEA-12/12/2005

13. 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

14. Essence of Beta Decay The Future:- Has anything changed? Can we do better? • Three signs of hope for improvement. • Big upsurge in interest in exotic nuclei and their decays • Development of the IGISOL • 3) Development of Total absorption Spectroscopy IAEA-12/12/2005

15. J. Benlliure low-energy nucleus high-energy nucleus Production techniques • In-flight fragmentation thin target gas cell spectrometer 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

16. J. Benlliure high-energy nucleus Production techniques • Isotopic separation on-line (ISOL) thick target ion source mass separator light projectile 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

17. low-energy nucleus high-energy nucleus Production techniques • Gamma/neutron converters ion source converter thick target mass separator g, n e-, d post-acceleration diffusion Basis of SPIRAL II J. Benlliure

18. high-energy nucleus Production techniques • Gamma/neutron converters(A variant of ISOL scheme) ion source converter thick target mass separator g, n e-, d post-acceleration diffusion • Two-step reaction scheme(ISOL + Fragmentation) fragmentation spectrometer fission ion source mass separator light projectile post-acceleration diffusion J. Benlliure

19. Regions of the Chart of Nuclei Accesible with SPIRAL 2 beams • Available Beams Primary beams:  deuterons  heavy ions 6. SHE 4. N=Z Isol+In-flight 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

20. 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

21. The problem of measuring the β - feeding (if no delayed part.emission) β+ ? • We use our Ge detectors to construct the decay scheme • From the γ-balance we extract theβ -feeding ZAN γ γ Z-1AN+1 γ1 γ2

22. 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

23. Total Absorption spectroscopy b-feeding E2 NaI g2 E1 g1 g1 g2 Ib N E2 Ex in the daughter Ideal case

24. Essence of Beta Decay The Future:- Has anything changed? Can we do better? • Three signs of hope for improvement. • Big upsurge in interest in exotic nuclei and their decays • Development of the IGISOL • 3) Development of Total absorption Spectroscopy IAEA-12/12/2005

27. 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

28. 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

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

30. Fission Fragments Fission Fragments

31. Ways to study Atomic Nuclei final states decay reactions a incident beam b n g p p n f2 f1 n g b n n W.Catford

32. 100Sn 48Ni 45Fe

33. Where is neutron drip-line ? N drip-line maybe reached N drip-line reached

35. E404aS : Identification of -rays in the light rare-earth nuclei near the proton drip-line 76Kr + 58Ni @ 328 MeV , - v, M, Z, Q p,  • VAMOS • - no condition • - beam ToF • recoil ToF + DIAMANT • + E - E

36. 326 4+ 2+ 6+ 4+ 454 8+ 6+ 2+ 0+ 547 159 10+ 8+ 12+ 10+ 613 14+ 12+ 664 16+ 14+ 18+ 16+ 704 743 809 N.Redon et al. 273 326 130Nd 131Pm 129Pr 237 159 454 DIAMANT gated 547 407 613 Doppler corrected spectra no gate 18+ --DIAMANT 130Nd 16+ 14+ 12+ 10+ 8+ Collaboration : IPN Lyon, Univ.Liverpool, GANIL, CSNSM Orsay, CENBG Bordeaux, ATOMKI Debrecen, Univ.York, Univ.Napoli, TRIUMF 6+ 4+ 2+ 0+ 130Nd