Neutron Capture and Waste Transmutation

Transcription

1 Neutron Capture and Waste Transmutation J.L. Tain Instituto de Física Corpuscular C.S.I.C - Univ. Valencia Euro Summer School on Exotic Beams Valencia, September 4-12, 2003

2 Layout of the lectures Transmutation of radioactive waste, ADS & Nuclear Data ( J. Benlliure) Neutron capture: theory and practice J.L. Tain Neutron Capture and Waste Transmutation 2

3 Nuclear Waste, Transmutation, ADS & Nuclear Data

4 Nuclear power plants in the world: 441 plants in operation, 32 under construction UE Total: 141 J.L. Tain Neutron Capture and Waste Transmutation 4

5 Nuclear electricity generation: Total: Installed: 359 GW(e) Produced: 2574 TWh (81.7% availability) Share: World average: 17.3% EU average: 33.6% J.L. Tain Neutron Capture and Waste Transmutation 5

6 Nuclear Fission Reactors Most reactors in use are of Light Water Reactor type, either Boiling Water Reactors or Pressurized Water Reactors: Fuel: UO 2, 2-4% 235 U Moderator-Coolant: H 2 O n-spectrum: thermal CHAIN REACTION n 235 U n 235 U PWR n fission fragments Reactors operate at CRITICAL POINT (neutron balance) which must be kept: REACTOR CONTROL J.L. Tain Neutron Capture and Waste Transmutation 6

7 Nuclear waste generation: The loaded fuel is transformed during the energy generation process: n U fi 3n I + 99 Y; 134 I(52min) fi 134 Xe. FISSION Fission Product 99 Y(1.5s) fi 99 Zr(2s) fi 99 Nb(15s) fi 99 Mo(66h) fi 99 Tc( y) n U fi 239 U; CAPTURE 239 U(23.5min) fi 239 Np(2.4d) fi 239 Pu( y) Actually a complex set of reactions will take place TRansans-Uranics n Pu fi 240 Pu( y); n Pu fi 241 Pu(14.4y) fi 241 Am(432y) Minor Actinides J.L. Tain Neutron Capture and Waste Transmutation 7

8 Partial reaction chain of the U-Pu cycle: J.L. Tain Neutron Capture and Waste Transmutation 8

9 As a consequence an inventory of long lived highly radioactive isotopes builds up in a reactor High Level Waste A 1GWe (3GWth) Pressurized Water Reactor, producing 7TWh (80% avail.), burns 1 ton/year fissile material. Loaded with 27.3 Ton of 3.5% enriched UO2 (954kg 235 U) produces after a burn-up of 33GWd/ton (~1 year): 266 kg Pu (156kg 239 Pu) 5% 4,352% 20 kg MA 946 kg FP ( 63kg long-lived FP) and still contain 280kg 235 U plus 111kg 236 U Masas relativas al Uranio 4% 3% 2% 1% 0% Fragmentos de Fisión 1,003% 0,062% 0,097% 0,002% Neptunio Plutonio Americio Curio J.L. Tain Neutron Capture and Waste Transmutation 9

10 The hazard they represent can be measured by the evolution of their radio-toxicity: J.L. Tain Neutron Capture and Waste Transmutation 10

11 One possible strategy to reduce the hazard of HLW in the long term is provided by transmutation (and/or incineration): Actinides: Fission: n Am( y) fi FF s + n s Capture (+ Decay) + Fission: n Am fi 244 Am(10.1h) fi 244 Cm; n Cm(18.1y) fi FF s + n s Long-lived FP: Capture: n + 99 Tc( y) fi 100 Tc(15.8s) fi 100 Ru. Is it possible to use the same reactions that create the waste to destroy it? The key point is the separation of the different components from spent fuel (or partitioning) J.L. Tain Neutron Capture and Waste Transmutation 11

12 Neutron induced reactions: strong energy dependence 235 U J.L. Tain Neutron Capture and Waste Transmutation 12

13 and isotope dependence: Fission: Capture: Therefore there are several possible (from physics point of view) solutions to the problem of burning HLW J.L. Tain Neutron Capture and Waste Transmutation 13

14 For example, the burning of TRU is favoured by fast n spectrum... In any case a surplus of n are needed to transmute J.L. Tain Neutron Capture and Waste Transmutation 14

15 The scientific considerations, together with technological, military and political considerations has lead to the proposal of different schemes for the management of HLW. A currently considered scheme is the double strata scenario: J.L. Tain Neutron Capture and Waste Transmutation 15

16 Accelerator Driven Systems: A nuclear reactor with a subcritical core which uses an accelerator to produce the neutrons necessary to maintain the chain reaction: Advantage: less dependence on fuel composition For example: a very high intensity ~1 GeV proton accelerator producing neutrons by spallation on a molten Pb or Pb/Bi target which acts also as the coolant, with solid actinide fuel. J.L. Tain Neutron Capture and Waste Transmutation 16

17 An ADS can also be utilized for energy production and if based on Th-U fuel cycle, will generate a reduced amount of TRU... ThU-cycle fissile fertile UPu-cycle Energy Amplifier J.L. Tain Neutron Capture and Waste Transmutation 17

18 Transmutation of TRU: set of reactions Present in nuclear wastes Medium Half-Life (<100 años) Short Half-Life (< 30 dias) High A actinides Thermal and Fast Fission Fast Fissión Low Fission Cross Section TRU Transmutation Scheme Fast Spectrum Av. Flux Intensity (n/cm2/s) 3,00E+15 Cm242 Cm243 Cm244 Cm245 Cm246 Cm247 Second 1 Time Unit α / SF α / EC/ SF α / SF α / SF α / SF α Hour / 6.2E /0.29/ 5.3E / 1.35E / 6.1E / 3E Day ,446 29,068 18, , , ,494 Year 3E+07 18,130 2,798 6,257 2,922 16,459 64,7% 8,0% 65,2% 11,4% 44,6% Am241 Am242 Am242m Am243 Am244 α / SF β / EC IT / α / SF α / SF β / EC 100 / 3.77E / /0.46/1E / 3.7E / 4E-2 432,225 0, , ,922 0,001 3,652 17,792 1,844 4,892 44% : 44% 13,1% 8,4% 87,0% Pu238 Pu239 Pu240 Pu241 Pu242 Pu243 α / SF α / SF α / SF β / α α / SF β 100 / 1.9E / 3.1E / 5.7E / 2.45E / 5.5E , , ,805 14, ,707 0,001 4,220 3,477 9,033 2,688 11,354 6,775 37,5% 19,4% 54,8% 14,2% 61,1% 30,6% Np237 Np238 Np239 Pu239 Symbol & Mass α / SF β β α / SF Decay modes 100 / 2E / 3.1E-10 Branching ratios ,095 0,006 0, ,608 Half-Life 4,332 15,928 Ln(2)/(σφ) 3,477 Absorption-Half-Life 81,5% 13,1% 19,4% (n,γ)/absoption J.L. Tain Neutron Capture and Waste Transmutation 18

19 Transmutation of TRU: new fuel compositions J.L. Tain Neutron Capture and Waste Transmutation 19

20 Challenges in the field of ADS: Waste separation (or partitioning) methods Design of a high power accelerator, the spallation target and their coupling Study of reactor core behaviour and transmutation rates Need for new or improved accuracy nuclear data: proton spallation reaction: n yield, residues ( J. Benlliure) neutron induced reactions: fission, capture, (n,xn), on actinides, fission products and structural materials J.L. Tain Neutron Capture and Waste Transmutation 20

21 An example of poorly known reaction J.L. Tain Neutron Capture and Waste Transmutation 21

22 another example of not so poorly known J.L. Tain Neutron Capture and Waste Transmutation 22

23 J.L. Tain Neutron Capture and Waste Transmutation 23

24 Neutron radiative capture: Theory and practice

25 Neutron reactions at low energies A neutron is absorbed to form a compound nucleus : n + A Z fi A+1 Z* which lives for a short time and decays: A+1 Z* fi n + A Z (elastic) A+1 Z* fi A+1 Z* + g (radiative capture) Other contributions: A+1 Z* fi A1 Z 1 * + A2 Z 2 * + xn (fission) A+1 Z* fi n + A Z* (inelastic) A+1 Z* fi A+1-x Z* + xn (n n multiplication) J.L. Tain Neutron Capture and Waste Transmutation 25

26 The CN formation probability is higher for certain neutron energies E n corresponding to quasi-bound or virtual states: resonances A E R = S n + E n S n : neutron separation energy of CN ( <10MeV ) A+1 fi level separation D 0 ~ 1 ev 100 kev Life-time «Energy-width: G G = G n + G g + G f + (s = s n + s g + s f +...) G ~ 1 mev 100 kev J.L. Tain Neutron Capture and Waste Transmutation 26

27 Shape of neutron cross-section 1/v: thermal G < D0, G > DE: resolved resonance region (RRR) log s resolution G < D0, G < DE: unresolved resonance region (URR) G > D 0 : overlapping resonances 1/v resolved unresolved overlapping log E n In the RRR region, s is described using the R-Matrix formalism, in one of its usual approximations. In the URR region, average s are described by Hauser-Feshbach statistical theory It is a parametric approach since nuclear theory cannot predict the values. Experimental information is strictly necessary. J.L. Tain Neutron Capture and Waste Transmutation 27

28 Single Level Breit-Wigner Formalism: (n,g) For l-capture into an isolated spin J resonance at E R : D = h/ g J = s g (E) = pd2 g J G n G g (E-E R ) 2 + ¼G 2 2mE (neutron wavelength) 2J + 1 2(2I + 1) (spin stat. factor; I - l ± ½ J I - l + ½ ) G(E) = G n (E) + Gg + ; (FWHM) s g (barn) n+ 197 Au, l=0, J=2 E R = 4.9 ev G n = 15.2 mev G g = mev G n (E) = E/E R G n (E R ), l=0 E n (ev) and the channel radius R c J.L. Tain Neutron Capture and Waste Transmutation 28

29 E L A S T I C C A P T U R E s g = SLBW formalism: elastic and capture cross sections 4p s = g J [ sin 2 F + 1 cos 2F l l + G n x sin 2F l G 1 + x ] 2 G 1 + x 2 k 2 4p k 2 g J 2 x = (E-E R ) G potential resonant interference G n G g G n 1 G 1 + x 2 k = 2mE / h P G n = (E) l G n (E R ) P (E l R ) g J = 2J + 1 2(2I + 1) 238U (n,g) S E R = E R + (E l R ) - S l (E) G n (E R ) 2P (E l R ) P 0 = F 0 = k R C = r, S 0 = 0 (n,n) P = l r2 P / ((l- S l-1 l-1 )2 + P 2 l-1 ) S = l r2 (l-s ) / ((l- S l-1 l-1 )2 + P 2 l-1 ) - l F = F l l-1 tan(p l-1 / (l -S l-1 )) J.L. Tain Neutron Capture and Waste Transmutation 29

30 From the analysis of experimental data on capture, total, fission, cross-sections, resonance parameters are obtained for every nucleus. All the information is combined, cross-checked for consistency, etc, in a process called evaluation until a recommended set of parameters is obtained. This information is published in a Evaluated Nuclear Data File using an accepted standard format (ENDF-6) There exist several files: Neutron Reaction Data BROND-2.2 (1993, Russia) CENDL-3 (2002, China) ENDF/B-VI.8 (2002, US) JEFF-3.0 (2002, NEA+EU) JENDL-3.3 (2002, Japan) J.L. Tain Neutron Capture and Waste Transmutation 30

31 Measurement of (n,g) cross-sections s g (E) = Number of capture reactions Number of target nucleus per unit area x Number of neutrons of energy E N g s g (E) = N n n T N g n T [at/barn] N n (E) Needs: sample of known mass and dimensions count the number of incident neutrons of energy E count the number of capture reactions but there are a number of experimental complications J.L. Tain Neutron Capture and Waste Transmutation 31

32 Neutron Beams Need to span a huge energy range: 1meV 100MeV Since neutrons cannot be accelerated, they have to be produced by nuclear reactions at certain energy and eventually decelerated by nuclear collisions (moderated) Energy determination: kinematics of two-body reaction mechanical selection of velocities ( chopper ) Time Of Flight measurement Sources: Radioactive Nuclear detonations Reactor Light-ion accelerator Electron LINACS Spallation E n = ½ m n L 2 t 2 J.L. Tain Neutron Capture and Waste Transmutation 32

33 ForschungsZentrum Karlsruhe Van de Graaf 7Li-target 7 Li(p,n) 7 Be, Q= MeV E p ~ 2MeV fi E n ~ 5-200keV Rate: 250kHz, Dt = 0.7ns 30keV > Thresh. J.L. Tain Neutron Capture and Waste Transmutation 33

34 Geel Electron LINear Accelerator (g,n), (g,f) on U (bremsstrahlung) E e ~ 100MeV, I e ~ mA Rate: Hz, Dt ~ ns J.L. Tain Neutron Capture and Waste Transmutation 34

35 GELINA n yield vs. E e Bare U-target Polyethylene Moder. 1 MeV NEUTRON SPECTRA U-TARGET & H 2 O MODER. H2O Moder. H2O MODERATOR 1 ev J.L. Tain Neutron Capture and Waste Transmutation 35

36 CERN neutron Time Of Flight p-spallation on Pb target E p = 20GeV, I p = 7x10 12 ppp Rate: 2.4s -1, Dt = 14ns n_tof J.L. Tain Neutron Capture and Waste Transmutation 36

37 n_tof Entrance to the target PROTON BEAM LINE p-intensity pickup p current monitor n beam tube Water cooled Pb target NEUTRON BEAM LINE shielding shielding J.L. Tain Neutron Capture and Waste Transmutation 37

38 n_tof 2nd collimator Bending magnet shielding BEAM DUMP n monitor Sample changer EXPERIMENTAL AREA n monitor J.L. Tain Neutron Capture and Waste Transmutation 38

39 The characteristics of the spallation-moderation process and the collimators in use determine the neutron beam parameters: intensityenergy distribution, energy resolution and spatial distribution. n_tof TARGET: 80x80x60 cm 3 Pb + 5cm H 2 O MODERATION: ξ = Æ ln(e i /E f ) 1+ (A-1) 2 ln 2A A-1 A+1 ( x(h)=1, x(pb)=0.01 ) SPALLATION PROCESS: ~600 n/p Intensity distribution J.L. Tain Neutron Capture and Waste Transmutation 39

40 The statistical nature of the moderation process produces variations on the time that a neutron of a given energy exits the target assembly n_tof time vs. energy Resolution Function 5eV DE n 2 DL 2 E = 2 2 L n Dt 2 t 2 Beam energyresolution 180keV RF Dt beam also important: time spread of beam J.L. Tain Neutron Capture and Waste Transmutation 40

41 n_tof The collimation system determines the final number of neutrons arriving to the sample an its spatial distribution Neutrons on sample Beam profile 4cm 10-5 J.L. Tain Neutron Capture and Waste Transmutation 41

42 Neutron Intensity Monitoring: Reaction: n + 6 Li fi t + a Si detectors n_tof Si Si Si Si 200mg/cm2 on 3mm Mylar J.L. Tain Neutron Capture and Waste Transmutation 42

43 Techniques for radiative capture detection: Detection of the capture nucleus Activation measurements Irradiation: A(n,g)A+1 A+1 radioactive with suitable T 1/2 Measurement of characteristic g-ray of known I g with Ge detector Detection of g-ray cascade n Total Absorption Spectrometers Total Energy Detectors Moxon-Rae Detectors Pulse Height Weighting Technique J.L. Tain Neutron Capture and Waste Transmutation 43

44 Define: e gi : total efficiency for g-ray of energy E gi e p gi : peak efficiency for g-ray of energy E gi E C E g1 Then: total efficiency for cascade: m g e C = 1 - P (1 - e gi ) i=1 E g2 E g3 peak efficiency for cascade: e p C m g = P ep gi i=1 If e p gi = 1, "i ep C = e C = 1 Total Absorption Spectrometer If e gi «1 & e gi = ke gi, "i m g m g e S e gi = k S E gi = k E C i=1 i=1 Total Energy Detector J.L. Tain Neutron Capture and Waste Transmutation 44

45 n_tof Total Absorption Calorimeter 40 BaF 2 crystals DW/4p = 95% 6% J.L. Tain Neutron Capture and Waste Transmutation 45

46 Deposited energy (from Karlsruhe 4p BaF 2 detector) e = 95% BACKGROUND REDUCTION BaF 2 (n,g) contamination J.L. Tain Neutron Capture and Waste Transmutation 46

47 n_tof TAC The good energy resolution and detector granularity makes feasble the measurement of fisioning nuclei Deposited Energy Multiplicity 245Cm(n,g) 245Cm(n,g) (Monte Carlo simulation D. Cano - CIEMAT) J.L. Tain Neutron Capture and Waste Transmutation 47

48 Total Energy Detectors Bi-converter Moxon-Rae type detectors: The proportionality between efficiency and g-ray energy is obtained by construction: C-converter (g,e - ) converter + thin scintillator + photomultiplier g e - Maximum depth of escaping electrons increases with E g e g / E g Mo-converter But proportionality only approximate (need corrections) Not much in use nowadays Bi/C-converter J.L. Tain Neutron Capture and Waste Transmutation 48 E g

49 Total Energy Detectors Pulse Height Weighting Technique: The proportionality between efficiency and γ-ray energy is obtained by software manipulation of the detector response (Maier-Leibniz): If R ij represents the response distribution for a γ-ray of energy E γj : i max S R ij = e gj i=1 E g = 2MeV R ij E g = 9MeV it is possible to find a set of weighting factors W i (dependent on energy deposited i) which fulfil the proportionality condition (setting k=1): W i i max S W i R ij = E gj i=1 for every E γj J.L. Tain Neutron Capture and Waste Transmutation 49

50 How accurate can be the weighting function? Fix experimental WF 56 Fe sample dependent 56 Fe Using GEANT Monte Carlo generated responses with full description of setup (including sample): s sg 2% J.L. Tain Neutron Capture and Waste Transmutation 50

51 Detectors: C 6 D 6 liquid scintillators Advantage: low neutron sensitivity e g = 3-5% Also detector dead material is important Optimized BICRON and surrounding materials HOME MADE: C-fibre cell No cell window No PMT housing J.L. Tain Neutron Capture and Waste Transmutation 51

52 Acquiring the data: full train of detector pulses Digitizer: 8bit-500MS/s FADC + 8MB memory On-line zero suppression Dt = 2ns E n down to 0.6eV 0.5 MB/pulse/detector TOF & E C6D6 from pulse-shape fit of PMT anode signal J.L. Tain Neutron Capture and Waste Transmutation 52

53 N g s + (E) = Yield: Y (E) = N g g /N n (E) g n T N n (E) Sample effects: Analysing the data Self-shielding: Y g (E) = (1 - e )s n T s(e) g (E) s(e) shielded Multiple scattering correction: elastic collision(-s) + capture Singlescattering Thermal (-Doppler) broadening 58Ni(n,g) E R =136.6keV T = 300K Beam effects: Resolution Function J.L. Tain Neutron Capture and Waste Transmutation 53

54 Use a R-Matrix code as SAMMY to fit the data and extract the parameters: E R, G g, G n, 56Fe(n,g) E R = 1.15keV Y = G g G T broad. RF singlescattering double scattering 197Au(n,g) E R = 4.9eV J.L. Tain Neutron Capture and Waste Transmutation 54

55 Slow neutron capture nucleosynthesis (s-process) Pb-Bi isotopes: termination-point of s-process & ADS target-coolant J.L. Tain Neutron Capture and Waste Transmutation 55

56 Preliminary data on Pb-Bi capture 209Bi(n,g) 800 ev 2.3 kev 3 kev 4.4 kev 5 kev 209 Bi(n,γ) 12 kev 20 kev l = 0 l = Pb (n,γ) 354 kev 527 kev n_tof s g lower for l=0 802 ev 2310 ev 5112 ev ev ev ev 3348 ev 4456 ev 6286 ev 6524 ev 9708 ev 9760 ev ev ev ev ev J.L. Tain Neutron Capture and Waste Transmutation 56

57 Bibliography: Nuclear Engineering, R.A. Knief, Taylor & Francis Ltd., 1992 Hybrid Nuclear Reactors, H. Nifenecker et al., Prog. Part. Nucl. Phys. 43 (1999) The Elements of Nuclear Interaction Theory, A. Foderaro, MIT Press, 1971 Neutron Sources for Basic Physics and Applications, Ed. S. Cierjacks, NEA/OECD, Pergamon Press, 1983 Neutron Radiative Capture, Ed. R.E. Chrien et al., NEA/OECD, Pergamon Press, 1984 Evaluation and Analysis of Nuclear Resonance Data, F.H. Froehner, NEA-OECD, JEFF Report 18, 2000 J.L. Tain Neutron Capture and Waste Transmutation 57