Analysis of design, verification and optimization of High intensity positron source (HIPOS) at HFR Petten

Transcription

1 Analysis of design, verification and optimization of High intensity positron source (HIPOS) at HFR Petten 1,2, K.Tuček 2, G.Daquino 2, L.Debarberis 2, A. Hogenbirk 3 1 International Atomic Energy Agency, P.O. Box 100, A-1400 Vienna, Austria 2 European Commission, Joint Research Centre, Institute for Energy, P.O. Box 2, NL-1755, ZG Petten, The Netherlands 3 Nuclear Research and consultancy Group (NRG), P.O. Box 25, NL-1755 ZG Petten, The Netherlands

2 Outline Introduction Design aspects Results Discussion and summary Conclusions

3 Main motivation: Introduction To stimulate positron based research and design new innovative facility at the RR; available as open-facility to the EU researchers. To build knowledge base multidisciplinary research based on positron annihilation principles (energy, applied physics, material science, etc.). To contribute to HFR Centre of Excellence initiative. To complement to existing research capabilities of HFR, mostly neutron. HIPOS International Scientific Workshop in North Holland (Bergen), November 2005 ERP launched in Q4/2005 project approved for 2 years with one year extension

4 Specific objectives: Introduction Individual tasks included: simulation, modeling and validation of the performance of (n, ) reaction, as well as effectiveness of concept for pair production reaction. This included full spectral analysis and design optimisation. Key objective of the HIPOS sub-task was to perform the feasibility study, which would demonstrate the capability of HFR to achieve very high intensity positron beam. HFR beam line site survey Pre-design of positron generator MCNP neutron-gamma source definition Experimental validation of neutron-gamma source MCNPX & GEANT4 calculations of positron production MCNPX calculations for optimization of design/material properties

5 Design aspects Positron generator incl. moderation stage Positron generator at HB9 reactions (n, ) and (,pairs) Positron source with expected intensity e+/s Modular concept of the source with possibility for it future upgrade Energy range 50 ev - 30 kev with floating potential

6 Design aspects HFR technological assessment Criteria Advantages Disadvantages Technology (++) Available on-site infrastructure (+) Low-vacuum technology (+) HV with support systems Operational conditions Installation Licensing (+) Operational time - 20 days (+) Off-time can be used to maintenance (+) Lower investments (++) Shorter installation time (+++) Short licensing process (no specific site-license!!!) (-) Limited technological area (-) UHV unavailable (some difficulties foreseen) (-) After each cycle 2 days off (-) Possible disturbance from other experiments/ higher background rate (-) Extra stabilization of environmental parameters (-) Difficult modification of any part of HFR technology/premises (-) Extra time for preparation and validation of DSR

7 Design aspects High-energy photons Position of HIPOS source Tungsten, platinum or nickel foil Thermal neutrons HFR reactor core Cadmium plate Two installations based on reactor e+ source concept are operational today, a few more under consideration (1) NEPOMUC and is installed at FRM-II reactor TU Munich (Germany), intensity of the positron generator is at level 10 9 e+/s of the slow-energy positrons (2) POSH, is installed at HOR reactor of Delft TU (NL), intensity of the positron generator is at level 10 8 e+/s of the slow-energy positrons. HIPOS concept Cadmium (n, gamma) concept was chosen (Cd booster for high energy gamma radiation). Tungsten (gamma, pair) conversion was investigated (W (110) orientation, good e+ affinity properties, + = -3.0 ev ) Multi stage system with electro-magnetic guidance system. W H 2O Cd

8 Experimental results A B (cm) f (cm -2 s -1 ) f (cm -2 s -1 ) Source term definition: Full characterisation of neutronics Neutron and gamma spectra of HB9 beam line NAA experimental validation Extra complication assessment done in the period of HEU to LEU fuel conversion period (cm) FIG. Distribution of (A) neutrons and (P) photons entering the plane surface of beam tube HB9 (flux values are given in cm -2 s -1 for a reactor power of 45 MW, dimensions along the axes are in cm).

9 Experimental results Energy-integrated fluxes for a reactor power of 45 MW at the planes bounding beam line HB9 for the three calculations Surface Analysis Neutron flux (cm -2 s -1 ) Photon flux (cm -2 s -1 ) Plane Cylindrical Initial WSSA SDEF Initial WSSA SDEF 9.58E+14 ± 3.35E E+13 ± 3.00E E+13 ± 6.66E E+13 ± 1.80E E+12 ± 1.84E E+13 ± 4.06E E+14 ± 8.93E E+14 ± 7.96E E+14 ± 1.53E EC14 ± 4.86E EC14 ± 4.84E EC14 ± 1.02E+11 There is a small difference between the initial analysis and the secondary analyses as far as the locations are concerned where the currents and fluxes were calculated. In the secondary analyses the cylindrical plane extends a bit more in the direction of the reactor, while the plane surface at the entrance of HB9 is a bit smaller in secondary analyses. This will lead to a somewhat larger value of the flux on the plane surface in the original analyses as compared with the secondary analyses.

10 Experimental results A B C D FIG. (A) Neutron and (B) photon flux of HB9 plane surface; (C) neutron and (D) photon flux of HB9 cylindrical surface (for a power of 45 MW).

11 -flux (s -1 ) n-flux (s -1 ) Experimental results MCNPX and GEANT-4 HFR neutron and photon source definition has been transformed into specific formats of MCNPX and GENAT4 codes. Comparison of results obtained by two Monte Carlo codes, MCNPX and GEANT4, for a simplified geometric set-up of the HIPOS. Characteristics of the HIPOS design based on Pt based converter/moderator were compared to the reference design, which uses tungsten. 1E18 1E16 1E14 1E12 1E10 1E-10 1E-8 1E-6 1E E18 Scoring surface of HB9 Scoring surface of HB9 Energy (MeV) n-plane n-cylind n-sum n-plane n-cylind n-sum 1E16 1E14 1E12 1E Energy (MeV)

12 Experimental results Gamma quanta multiplicity and spectrum energy for different data libraries investigated compared to HB9 beam total gamma source. Parameter/Library ENDL 92 JENDL 3.2 JENDL3.3 ENSDF HB9 Multiplicity [ /capture] Mean energy [MeV] Fraction of spectra above pair production threshold (E > MeV) [%] Multiplicity for HB9 beam tube gammas is obtained by comparing gamma current behind the Cd converter (thickness 2.5 mm) to the number of neutron captures in Cd. FIG. Comparison of gamma quanta emission from neutron capture in cadmium (ENSDF data correspond to thermal neutron capture in 113Cd)

13 Intensity Experimental results Because of this relative unsatisfactory quality of gamma production data in available MCNPX libraries (ENDL92, JENDL), the MCNPX code was for further studies modified to sample energy of exiting gammas from (n, ) reaction in Cd according to ENSDF data. The gamma quanta were assumed to be emitted isotropically. FIG. Comparison of gamma quanta emission from neutron capture in cadmium (ENSDF data correspond to thermal neutron capture in 113Cd), MCNPX vs. GEANT-4 1.2E E+15 MCNPX-DLC GEANT4 GEANT4-JENDL33 MCNPX-JENDL33 8.0E E E E E E E E E E E E+00 Energy (MeV)

14 Yield (e+/s) Experimental results 1E17 1E16 1E15 Scoring plane behind Cd surface Neutron-plane Neutron-cylind Photon-plane Photon-cylind Total Material issues of design, specifically thickness of cadmium converter: This part plays very important role not only in generation of highenergy gamma radiation It contributes to total yield of positrons. 1E Thickness Cd (mm) FIG. Total positron yield (current) behind the W converter in the direction of the beam tube axis (NOTE: for a 2.5 mm Cd thickness, a majority (50.2%) of positrons are created due to primary neutron source from the HFR)

15 Experimental results Burn up effect and further optimisation of the design Positron yields together with annual burn-up of 113Cd in the converter of the HIPOS system having different Cd thicknesses. A load factor of 80% is assumed for the calculation of 113Cd burn-up. Parameter/Cd thickness (mm) Positron yield behind W converter in the forward direction [10 13 e+/s] Cd mass (g) Cd burn-up (%/year) NOTE: the occurrence of 113 Cd in natural Cd is only at%. Hence, enriching Cd in 113 Cd offers an interesting opportunity to further prolong the interval between Cd converter replacements.

16 e + -flux (s -1 ) e + -flux (s -1 ) Experimental results 1E14 Scoring surface of W1 and W3 W1-plane W3-plane Spectrum of positrons behind the tungsten converter section 1 and 3 in all directions. 1E12 1.0E E+11 Scoring surface of W1 and W3 W1-plane W3-plane 1E10 6.0E E+11 1E Energy (MeV) 2.0E E Peak of the positron s spectrum behind the tungsten converter section 1 and 3 in forward direction of the beam tube axis. Energy [MeV]

17 Experimental results Burn up effect and further optimisation of the design Load factor of 80% is assumed for the calculation of 113Cd burn-up. Energy distribution of positrons behind the W converter peaks at about 1.1 MeV, with mean energy being 1.63 MeV. Fraction of positrons below 600 kev is 25.4%. Due to the self-absorption of positrons moving backwards (towards the reactor core), the resulting spectrum of positrons becomes more forward oriented along the beam tube.

18 Angular distribution (e+) Experimental results FORWARD BACKWARD neutron plane neutron cylind photon plane photon cylind Angular distribution of produced positrons from each particular source of origin (n-planar, n-cylindrical, -planar and -cylindrical), Angle of 0 represents forward direction of the beam tube axis.

19 Discussions Parameter/Design Reference design Thickness of W converter plate 12.5 m D2O moderator Platinum converter and moderators *Positron yield behind W converter [10 13 e+/s] *Positron yield behind W moderators [10 13 e+/s] Cd captures [10 15 /s] Cd burn-up [%/per year] Emission probability of thermalized positrons from tungsten plate is inversely proportional to the converter thickness. Therefore, a design variant with a 50% lower tungsten converter thickness (12.5 m) was also investigated (see table) to possibly further optimize yield of slow positrons. It was found that this design modification results in very similar integral positron yields ( e+/s vs e+/s) as for the reference design.

20 Discussions Thermal load is mainly due to the and e+/e-; fraction of energy deposited in the system due to neutron (scattering) is less than 1%. Ref. design, 2.90 kw is generated in Cd converter, while the highest power density (4.66 W/g) is in the first W converter/moderator unit. Parameter/Design Component Ref. design W-converter Pt-conver.& 12.5 m moder. Cd heating [W/g] W/Pt converter st W/Pt moderator Cd e+/e- heating [W/g] W/Pt converter st W/Pt moderator Total heat [kw] Cd W/Pt converter st W/Pt moderator NOTE: In Cd, a slight majority (51%) of heat generated is due to photons, while in W and Pt it is 58% and 55%, respectively.

21 Discussions Further thermalisation of neutron spectra by placing additional D2O moderator in front of the HIPOS in the beam tube, does not seem to increase neutron capture rates and subsequently gamma and positron yields (limited effect). Use of platinum instead of tungsten in both the converter and moderators results in slightly higher integral positron yields (Pt also holds a promise of having higher probability of emission of thermalised positrons, but its use is associated with economic penalties). Quite important factor of design is appropriate cooling system.

22 Conclusions The critical points of reactor-based positron source are the generation and re-moderation sections. Re-moderation part is sensitive to very precise optimisation of thickness according to the energy spectra since the positron intensity is reduced by factor 1000x. Careful consideration of all aspects the guarantee that presented HIPOS apparatus is designed to produce fluxes of slow positrons of the order of e+/s. The expected beam intensity seems to be relatively high, although it can decrease, due to different reasons, up to one order of magnitude, consistently with experience of RID Delft and FRM-II Munich.

23 Conclusions Final material concept of HIPOS can be summarized as following: (a) Cd structure with isotopic enrichment at level 99% of 113Cd will be clear advantage, this material modification will extend the operation lifetime to 10 years without any need for replacement of Cd-part of converter due to high burn-up effect (b) Replacement of aluminum structures by Zr-based alloys (like Zircalloy) due to safety aspects, since relatively high gamma heating. (c) H2O-based cooling system is proposed instead of D2O since the lower absorption in the latter will not contribute significantly to higher intensity of positrons, moreover there would be need for tritium separation system.

24 Conclusions Due to limited resources and time for this exploratory research, further R&D optimization of the design has not been carried out, however future studies should be focused on following areas: Thermal hydraulic calculations with optimization of coolant media (H2O, Ne, Xe, etc.) Study of new and more exotic positron moderators, as a cryogenic solid positron moderators, Analysis of modularity and integration concept into new research reactors.

25 Conclusions Finally, it was confirmed that HFR offers an unique opportunity for installation of such a novel facility due to outstanding characteristics of the reactor. Unfortunately, HFR had some problems (2009/2010) related to the ageing and corrosion of the primary components, there was unplanned shutdown for more than 1 year. Due to this situation all new projects were frozen, no possibility to proceed with installation of new experimental facilities. High intensity positron source is feasible and achievable. Results of this ERP can also help in design of new positron generators in other countries.

26 References [1], L.Debarberis, HIPOS - High Intensity Positron Source at HFR, EUR EN, ISSN (2009) [2] A. Zeman, K. Tuček, L.Debarberis, A. Hogenbirk, Nucl. Instr. And Meth. in Phys. Res. B (2011) accepted [3] C.Hugenschmidt et al., Nucl. Instr. And Meth. in Phys. Res. B 192 (2002) [4] H.Schut, Performance of the Delft high intensity positron beam POSH, in (ed.) Int. Sci. Work. - Application of high intensity positron beam techniques and digital lifetime positron spectroscopy in material science, The Netherlands, EUR-22182EN (2006)