Low energy linear acceleratordriven subcritical assembly

Transcription

1 UNIVERSIDAD SIMÓN BOLÍVAR LABORATORIO DE FÍSICA NUCLEAR Low energy linear acceleratordriven subcritical assembly Eduardo D. Greaves 1 and Laszlo Sajo-Bohus 2 Universidad Simón Bolívar. Apartado 89000, Caracas 1080 YV, Venezuela 1 egreaves @yahoo.com 2 sajobous@gmail.com Technical Meeting on Molten Salt Reactors Vienna, Austria 31st Oct-4 Nov. 2016

2 Contents Conceptual system Background Theoretical Experimental Results Proposal Conclusions

3 Conceptual system Radiotherapy electron linear accelerator operating at 18 MeV Electron beam strikes Tungsten Target inside subcritical MSR Produces Bremsstrahlung photons and neutrons in the target Produces criticality in the MSR

4 Background Inspiration of the idea: Measurement of patient neutron doses during radiotherapy High energy electrons from linac strike Tungsten target and produce bremstrahlung and neutrons Previous work: Barrera et al. (México, 2015 ) Neutron fluence on the treatment couch measured with 10 B+CR-39 Nuclear track detector and simulation with Monte Carlo. Passive detectors results are shown by squares. Martín-Landrove et al. (2015)

5 Theoretical Tamii, A. et al. Eur.Phys.J. A50 (2014) 28 arxiv: Photonuclear reactions: (γ, n), (γ, 2n), electronuclear (e, e n) or (γ, p) gamma induced fission, (γ, fis). i σtot γ, n = σi ( γ,) i i σ( γ, i) = σγ (,n) + 2 σγ (,2n) + νσγ (,fis) Total cross section: ( ) Where i ν =neutron multiplicity (for actinides fission ~ 2.2.)

6 Photon spectra of radiotherapy linacs From: Secondary neutron spectra from modern Varian, Siemens, and Elekta linacs with multileaf collimators Rebecca M. Howella et al (2010)

7 The neutron yield Y(k) per incident photon and energy group k, for target and PADC-detector-(C I2 H l8 O 7 ) elements, assuming a parallel beam of photons, is given by: Y( k) = f (k) t ρσ(k) 1 j i where ρ j is the atomic density, σ j ( k ) is the cross-section for (γ, n) reaction of the j-th isotope and t is the target thickness. f 1 (k) takes into account photon attenuation in thick targets j Photoneutron yields calculated for passive detectors. Adapted from Allen and Chaudhri (1988)

8 Possible photon reactions on U and Th target. U nat = 99.3% ( 238 U) + 0.7% ( 235 U); Th nat is assumed pure 232 Th. 238 U + γ 236 U +2n + γ 235 U + n fission fragments (A1, A2) 238 U + γ 237 U +n + γ 236 U + n + γ 235 U + n fission fragments (A1, A2) 238 U + γ 237 U +n 237 Np +β - + γ 235 Np +2n + e 235 U fission fragments (A1, A2) 232 Th + γ 231 Th + n; 232 Th + γ fission fragments (A1, A2)

9 Photo fission cross section for 238 U and 232 Th (Adapted from Caldwell et al 1980) Upper curve has ad-hoc value of 100 added for visibility

10 Experimental LINAC Varian 2100 accelerator at the La Trinidad, GURVE-Clinic, Caracas, Venezuela Operated at 18MV for 66 min Irradiation of simulated fuel samples.

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12 Targets samples of simulated fuel

13 Neutron-Proton (n-p) Scattering in CR B (n, Alpha) (Few n-p scattering in CR-39 ) 10 B (n, Alpha) + Th, U nat, (γ n) + B nat (Alpha, n) + Th, U FISION fragments

14 Examples of micrographs of the etched tracks seen as digitalized images.

15 Example of micrograph of the etched tracks of a FISSION

16 Assembled simulated target fuel samples 238 U (99.3%) U (0.7%) + 10 B (97.6%) + (C I2 H l8 O 7 ) 238 U (99.3%) U (0.7%) + 10 B (20%) + 11 B(80%) + 10 B (97.6%) + (C I2 H l8 O 7 ) 238 U (99.3%) U (0.7%) Th + 10 B (20%)+ 11 B(80%)+ 10 B (97.6%) + (C I2 H l8 O 7 ) 232 Th + 10 B (20%) + 11 B(80%) + 10 B (97.6%) + (C I2 H l8 O 7 ) 10 B (20%)+11B(80%) is used as a boric acid compound

17 CR39 Detector Etching (6N NaOH 70 o C, 6h)

18 Track analysis

19 Reaction 232 Th (γ, fis) Results ( 232 Th) Reaction rate per gamma flux unit Σ(σ*Φ) Mass (g) +/- 0,1mg Number of nuclei Detector number Reaction rate in the detector (1/s)

20 Results (U nat ) Reaction rate per gamma flux unit Reaction Σ(σ*Φ) Mass (g) +/- 0,1mg Number of nuclei Detector number Reaction rate in the detector (1/s) U nat (γ, fis)

21 Proposal Cheap reliable radiotherapy linear accelerator

22 Proposal FLiBe: 7 LiF 72 mol % BeF 2 16 mol % ThF 4 10 mol % UF 4 2 mol % Temperature: o C ρ =1,93 g/cc Subcritical Fuel Furukawa s ADMSR

23 Conclusions We show the production of neutrons in the accelerator tungsten target We show the production of neutrons in thorium and uranium targets. We show the production of fission in thorium and uranium targets. We show the feasibility of use of radiotherapy accelerators for photodriven subcritical assemblies.

24 Conclusions We demonstrate that passive nuclear track detectors with 10 B converter are effective and economical means to estimate fission and neutron production rates. We conclude that further work must be done modeling a subcritical assembly driven by a radiotherapy linear accelerator

25 Gracias