Calibration of JET Neutron Detectors at 14 MeV neutron energy

Transcription

1 Calibration of JET Neutron Detectors at 14 MeV neutron energy Paola Batistoni, ENEA, Frascati (Italy) EUROfusion WPJET3 Project Leader Neutron Group User s Meeting, NPL, UK,

2 Contributors CCFE: K. Keogh, Z. Ghani, A. Milocco, L. Packer, D. Plummer, S. Popovichev, B. Price ENEA: M. Angelone, P. Batistoni, S. Loreti, M.Pillon IPPLM: S. Jednorog, E. Laszynska JSI: A. Cufar, I. Lengar, L. Snoj KIT: A Klix VR: S.Conroy, H. Sjostrand JEU: A. Peacock And JET Contributors# # See the Appendix of F. Romanelli et al., Proceedings of the 25th IAEA Fusion Energy Conference 2014, Saint Petersburg, Russia Page 2

3 JET and its evolution since Clean, metal-walled JET, Easy man access 1994 Inside JET T, Be in V.V. walls & air >2011 ITER-like wall T, Be in V.V. walls & air Delicate divertor Difficult man access JET 1985 JET 1991 March During Shutdown Page 3

4 Introduction Accurate calibration of JET neutron detectors is required Measure fusion performance and plasma ion parameters Reduce uncertainty margin in the use of the JET neutron budget Provide accurate neutron yields for neutronics benchmark experiments Periodic calibrations are needed as the machine configuration changes in time Neutron calibration in ITER <10% accuracy required (fusion power, neutron yield, tritium accountancy) Very challanging because of the large difference between the available calibration sources intensity (<10 10 n/s) and the plasma neutron source intensity (up to n/s) JET neutron calibrations provide very important (unique!) tests of the calibrating procedure to be adopted in ITER, and guidance to reduce the related uncertainties. Page 4

5 JET Schedule JET approved schedule DTE2 neutron budget: 1.7 x neutrons New JET 2020 proposed schedule Page 5

6 Technological Exploitation of JET DT operations Scope Experiments, collection of data and analyses during DT operations at JET in technological areas that are relevant to ITER and DEMO Objective Validate codes, methods, data and assumptions used in the preparation of ITER operation and plant management, and relevant to DEMO design. JET DT Technology Project (EUROfusion WP JET3) First complete and consistent fusion nuclear case for a tokamak Page 6

7 14 MeV Neutron calibration at JET JET neutron monitors Fission chambers ( 238 U, 235 U pairs) Last calibrated in 1989 Activation system (8 Irradiation Ends) Neutron MeV successfully obtained in 2013 using a 252 Cf neutron source overall uncertainties < 10% target New MeV planned in 2016: Target accuracy < 10% (as in ITER) Benchmark ITER neutron calibration Page 7

8 Fission Chambers: 252 Cf Central Ring Scans Oct 8 Oct 6 KN2 3U I.E. Oct 2 NBI 8 NBI 4 ILA

9 Need of a calibration at 14 MeV? KN1 - Fission chambers Sean Conroy MCNP model Response function of 235 U fission chambers is relatively flat But the transparency of the machine to neutrons changes with neutron energy It has been estimated that the response of 235 U fission chambers can increase by 15% from 2.5 to 14 MeV neutrons Original curve by Swinhoe and Jarvis (MORSE calculations), 1983 S. Conroy s recent MCNP calculations Page 9

10 Need of a calibration at 14 MeV? KN2 - Activation system Response function is given by the activation reaction used For DD plasmas the 115 (n,n ) 115m In is used 115m In Half life τ 1/2 = h Energy of γ-ray emitted E γ = 336 kev For DT plasmas options are given by 56 Fe(n,p) 56 Mn, τ 1/2 =2.57 h, E γ =1369 kev 28 Si(n,p) 28 Al, τ 1/2 =2.24 min, E γ =1779 kev 63 Cu(n,2n) 62 Cu, τ 1/2 =9.67 min, E γ =511 kev 93Nb(n,2n)92Nb τ 1/2 =10.2 days E γ =934.5 kev Though KN2 irradiation layout (and its MCNP model do not change) the response function & detecting conditions change from 2.5 MeV to 14 MeV neutrons Page 10

11 14 MeV neutron calibration 14-MeV Neutron Generator (NG) Max neutron emission rate: n/s Calibration/characterization required Monitoring of source intensity required TiT/D target 14 MeV Neutron Generator D + /T + beam Length 70 cm, weight 3 kg Si-diode Activation foils Diamond Detector NG deployed inside JET vessel by remote handling system Layout for in-vessel calibration mainly driven by remote handling limitations and safety requirements Page 11

12 Layout for in-vessel calibration Pre Amp 8 Activation foils (Fe, Al, Nb) Silicon detector Diamond detector NG HVPS & CU Digitalizer Neutron Generator with monitoring detectors Same assembly at NPL and inside JET vessel Neutron Generator deployed in vessel by MASCOT arms Page 12

13 NG 14 MeV neutron source characteristics NGs provide inherently anisotropic neutron sources, the energy spectrum depends on the emission angle in the lab system The neutron source characteristics are altered by the presence of the NG body due to neutron scattering on the NG materials An accurate calibration and characterization of the NG model is neede An accurate model of NG is needed Page 13

14 MCNP models of NG Just the target (too thin to see) Target + copper cup Target + cooling unit ENEA-JSI neutron source subroutine Originally developed for FNG ENEA (M. Pillon) and then further developed by JSI (A. Milocco) DT kinematics + slowing down of D beam in Ti MCNP model according to the specification of the supplier Full model Page 14

15 Comparison total flux vs angle Anisotropy of neutron emission from NG Calculated neutron spectra from NG (D+ T, E beam = 100 kev) Page 15

16 5.5x10-4 Comparison energy spectrum vs angle 0 5.0x x x x x x x x x x10-5 DT 0 DT 30 DT 60 DT 90 DT 120 DT 150 DT 180 TD 0 TD 30 TD 60 TD 90 TD 120 TD 150 TD 180 Calculated neutron spectra from mixed beam NG D + T, E beam = 100 kev and T + D, E beam = 100 kev A About 20% of neutrons at E < 13 MeV Page 16

17 NG Calibration & characterization - Strategy Characterize/calibrate NG at a standard neutron facility Total neutron emission in 4π (<±5%) Angle energy distribution of emitted neutrons Use well characterized detectors Use redundant measurements Monitor the NG yield during the in-vessel calibration Relate the monitor detectors signals to absolute yield measured at neutron facility Use monitoring detector in same configuration at neutron facility and in the invessel calibration Validate the DT neutron source routine + MCNP model of NG Validate MCNP model at standard neutron facility Use MCNP calculations to relate NG in-vessel calibration to plasma operations Page 17

18 Monitoring detectors Activation foils Fe, Al, Nb (S. Jednorog, IPPLM) Redundant active detectors SC diamond, Si-diode Cross check for their stability At NPL they are calibrated, i.e. there is a relation between their integral signal and the NG absolute yield In vessel they provide absolute values of neutron produced during each irradiation interval Neutron generator Redundant passive detectors Fe, Al, Nb activation foils Same reactions to be used in KN2 during in-vessel calibration and later with DT plasmas At NPL they are calibrated, i.e. there is a relation between their activity and the NG absolute yield Removed at the end of every day Diamond Detector (M. Pillon, ENEA) Si-diode (S. Popovichev, JOC) Diamond PHS measured at FNG (M. Pillon) Page 18

19 NG calibration & characterization at NPL NG Calibration / characterization planned at NPL in Nov. 9 20, 2015 Monitoring detectors Diamond (ENEA), Si Diode (JOC), Nb, Fe, Al foils (IPPLM) Calibration / Characterization detectors Fe, Al foils (NPL) High energy yield > 6-7 MeV Long Counters (NPL) Total yield > E> 6-7 MeV Diamond (CNR - Milan) Energy spectra vs angle NE213 (KIT) Energy spectra vs angle Page 19

20 NPL Long Counter measurements NPL Long counters have flat energy response function (similar to 235 U fission chambers) uncertainty for neutron fluence at 15.5 and 17 MeV is 2.4% and 3% respectively in a well characterised fields L. Packer, Z, Ghani NPL low scatter cell, 14.8 MeV point source, 2m NG Spectral width may lead to larger uncertainties Page 20

21 Support MCNP analyses Effective centre of Long Counter for NG energy spectrum at given angles Background contribution Neutron scattered by NG body + mechanical support background subtraction using the shadow cone method Scattering interference due to the presence of many detectors + shadow cones on the floor? L. Packer, Z, Ghani Activation of NG and dose rates Page 21

22 SPARE SLIDES

23 Effect of the NG materials The NG in MCNP model prepared according to the specification of the supplier has only 2.2 kg while the mass of the actual NG is 5 kg Large part of the model is simply modelled as oil (~45% of the mass) and the electronics is not modelled 51 Stainless steel (SS) was put in cells: 80 (empty), 55 (oil) and 51 (oil). SS in cell 80: SS with densities 0.8 g/cm3, 4.0 g/cm 3 and 8.0 g/cm 3 was put into the previously empty cell Mass of the model: 2.4 kg, 3.2 kg and 4.2 kg SS + oil in cells 55 and 51: Mixtures of oil and SS were put in the cells 20 w% SS, 80 w% oil g/cm 3 50 w% SS, 50 w% oil g/cm 3 90 w% SS, 10 w% oil g/cm 3 55 Mass of the model (variation in cell 55): 2.4 kg, 2.9 kg, 5.4 kg Mass of the model (variation in cell 51): 2.2 kg, 2.3 kg, 2.8 kg 80 P. Batistoni Neutron Aljaz Cufar User's Club SAMM Meeting, NPL, June UK, Page 23

24 Effect of the NG materials 51 For these angles the total flux is almost unchanged 55 Cells 80 and 55: The effects are mostly negligible because relatively small fraction of neutrons are emitted into the part of space where the SS was added 80 Cell 51: The effects are small but visible Flux at ~120 is almost unchanged

25 Simulation of measured neutron spectra at NPL Simulated neutron spectra from the NG. (Aljaz Cufar) Simulated diamond spectrometer response at different angles (M. Tardocchi)