Transport of Neutrons and Photons in Construction Parts of VVER‑1000 Reactor

1. Transport of Neutrons and Photons in Construction Parts of VVER‑1000 Reactor Michal Košťál PhD thesis Department of experimental reactor physics at LR-0, Research Center Řež Czech Technical University in Prague Faculty of Nuclear Sciences and Physical Engineering Department of Nuclear Reactors

2. The objects of PhD thesis and supporting references • Compilation of the calculation model for neutron and photon transport in VVER-1000 transport benchmark (with prospect of calculations in biological shielding) • Determination of neutron emission density, across the reactor core and assessment of link between neutron emission density and fission density • Determination of neutron emission spectra of various fuel pins • Estimation of related uncertainties • Estimation of sensitivity to the selection of specific nuclear data library • Estimation of sensitivity to the selection of specific transport model (in case of Fe and H2O)

3. VVER-1000 benchmark • Radial full scale VVER-1000 transport benchmark (RPV, baffle, barrel) • Baffle is not is not unruffled - milled cooling holes in vertical and in horizontal plane as well • For simulation of the water density reduction displacer is used • RPV consist of four 5cm steel blocks, the first one consist of 1cm of stainless (RPV cladding simulator) and 4cm low alloy steel

4. LR-0 Reactor • Light water moderated zero-power reactor • Maximal nominal power 1 kW, thermal neutron flux density~ 1013 n.m-2 s-1 • Core in Al tank, inner diameter 3500 mm, thickness 16mm, height 6600 mm • Power control realized by means of moderator level change or control-cluster position • Demineralized water with or without diluted boric acid is used as moderator • Dismountable fuel elements • VVER type fuel, length of pins is shortened (125cm) with regard to LR-0 construction

5. Upper view onVVER-1000 coreinside LR-0

6. The mock-up construction allows to determine the fluxes in its various parts. Measuring points • 4 points in reflector • In front or water • Behind 5cm, • Behind 10cm • Behind 15cm • 5 points in positions • In front of RPV • In ¼ of RPV • In ½ of RPV • In ¾ of RPV • Behind RPV

7. Pin power distribution • Radial profile • Fission density ~ ( generally not proportional to emission density) • Model verified on keff results, being 0.99462 (ENDF/B VI.2.)

8. Various position incident neutron spectra • Different properties of steel causes considerably harder spectra near baffle than in other regions • The neutron spectra vary across the core

9. Variations in fission products and energy generation

10. Various position neutron emission spectra • Only small variations between corner pin emission spectra and inner pin emission spectra • both are similar with Watt emission spectra for 235U and thermal neutron

11. Comparison with diffusion approach • There are considerable discrepancies between both • Possible reasons of such discrepancies • Incorrect boundary conditions (i.e. approximation of full core, but benchmark is just 1/6 of VVER-1000 core • Peripheral regions (near baffle) seems to be reflection of innacuracies from diffusion approach

12. Fuel pins selection for C/E comparison • Selection reflects the pins with expected discrepancies • The experimental uncertainties prevail in C/E uncertainty • Peripheral pins uncertainty unanswerable problem in this selection – power density in center (As-27) ~20x higher than in periphery (As-4) and reasonable doses must be ensured Power determined by means of La-140 fission product activity measurement Experiment realized 16 days after irradiation – enough time for setting of La-Ba equilibrium

13. Pin power density C/E • Selection of pins in positions with expected discrepancies • near the core and baffle (1 – 31) • assemblies corners (32 – 46) • near lateral reflector (47 – 52) • Comparison of symmetrical pins used for verification of experiment • Near baffle, better agreement with MCNP than with MOBY DICK • diffusion approximationinsufficiency appears in the boundary regions (high neutron flux gradient, different material boundary • Near water gap (corner pins, near lateral reflector pins), both MCNP and MOBY DICK results in similar agreement with experimental values

14. Axial profile of power density C/E • Discrepancies in distant grids locations

15. Neutron fluxes in reflector

16. Neutron fluxes in RPV

17. Transport model effect • H2O • keff • Slight variations if used • ENDF/B VII & S(α, β) results closer to experiment • Fe • Photon flux density (18cm Fe) • Notable variations if used • ENDF/B VII & S(α, β) results closer to experiment

18. Nuclear data library effect - fuel • Only slight variations • Except ENDF/B VI.2 discrepancies less than related uncertainties • Best C/E agreement CENDL 3.1 • Only ENDF 6 calculations differ from experiments more than related uncertainty

19. Nuclear data library effect – Fe (18 cm slab) • Neutrons (thick layers) • Most notable discrepancies (4–7 MeV) for JENDL 4 and TENDL 2009 • Photons • Most notable discrepancies (>7MeV) for JEFF 3.1 and TENDL 2009

20. Thank you for your attention

21. Published results • Thermal scatter treatment of iron in transport of photons and neutrons, M. Košťál, František Cvachovec, Bohumil Ošmera, Wolfgang Hansen, Vlastimil Juříček, Annals of Nuclear Energy, Volume 37, Issue 10, October 2010, pp 1290–1304 • The Pin Power Distribution in the VVER-1000 Mock-Up on the LR-0 Research Reactor, M. Košťál, V. Rypar, M. Svadlenkova, Nuclear Engineering and Design, Volume 242, January 2012, pp 201– 214 • Determination of AKR-2 leakage beam and verification at iron and water arrangements, M. Košťál, F. Cvachovec, J. Cvachovec, B. Ošmera, W. Hansen Annals of Nuclear Energy, Volume 38, Issue 1, January 2011, pp 157-165 • Calculation and measurement of neutron flux in the VVER-1000 mock-up on the LR-0 research reactor, M. Košťál, F. Cvachovec, V. Rypar, V. Juříček: Annals of Nuclear Energy, 40 (2012), pp 25–34, • The Power Distribution and Neutron Fluence Measurements and Calculations in theVVER-1000 Mock-Up on the LR-0 Research Reactor, Košťál, M., Juříček, V., Novák, E., Rypar, V., Švadlenková, M., Cvachovec, in press, ISRD-2011, Bretton woods, USA • Transport of neutrons and photons through iron and water layers, Košťál, M., Cvachovec, F., Ošmera, B., Noack, K., Hansen, W.,. Proceedings of the 13th International Symposium on Reactor Dosimetry, Ackersloot, Netherlands. pp. 269 – 279 Results send for review: • Neutron and photon transport in Fe with the employment of TENDL 2009, CENDL 3.1., JENDL 4 and JENDL 4 evolution from JENDL 3.3 in case of Fe, M. Košťál, F. Cvachovec, J.Cvachovec, B. Ošmera, W. Hansen, Nuclear Engineering and Design • Thermal neutron transport in the VVER-1000 mock-up on the LR-0 research reactor, Nuclear Engineering and Design, M. Košťál, V. Juříček, J. Milčák, A. Kolros • The criticality of VVER-1000 mock-up with different H3BO3 concentration, M. Košťál, V. Rypar, V. Juříček, Progress in Nuclear Energy

22. Influence of power distribution on results • The variation are smaller than related uncertainties => Diffusion approximation power density may be used in following transport calculations

23. 3He reaction rate attenuation • In RPV simulator of VVER-1000 • In RPV simulator of VVER-1000 with PE liner

24. Pin power measurement • La-140 – 1596keV (fraction 0.954) • Long irradiation time => long decay time => many measured pins • Sr-92 – 1383keV ( fraction 0.9) • Short irradiation time => short decay time => few measured pins