Behler, M.

Dagan, R.

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KIT – University of the State of Baden-Wuerttemberg and National Research Center of the Helmholtz AssociationKarlsruhe Institute of Technology www.kit.eduAnalyse von Kritikalitätsbedingungen im BE- Lagerbec kenR. Dagan1, M. Behler 21 Karlsruhe Institute of Technology, Institute for Neutron Physics and Reactor Technology, P.O. Box 3640, 76021 Karlsruhe, Germany2 GRS, Nuclear Fuel Department, 85748 Garching, Germany2Layout of unit 4 SFP3Layout of representative Rack⚛ Conservative approach: ⚛ reflective boundary conditions ⚛ water at bottom and top of the fuel pins⚛ Reference case: Fresh BWR fuel 4Layout of fuel element⚛ 4wt % U235 enrichment (yellow pins)⚛ 3.4wt % U235 enrichment and 5 wt% in Gadolinium (green) pins⚛ Clad Zircaloy 4⚛ Stainless Steel type 1.4568 ( Norm DIN  EN10027-1/2 (X7 CrNIAl 17-7) Cell pitch:    159.5 mm (see Fig. 3) Leg height:    184.2 mm Leg weight (8 legs):  3 kg each, Stainless steel Base plate thickness:   12.7 mm Base plate weight of rack: 78 kg, Stainless steel Rack height (above base plate):  4293 mm Rack steel thickness:   2.5 mm Rack Weight (above base plate): 2000 kg, Stainless steel Radius of opening in base plate per FA:  48.26 mm (3.8 inch diameter) Absorber material:    none Between the steel walls of the rack is empty space, which is filled with water (8.5 mm in outer walls (13.5 mm total wall thickness) and 35 mm in long inner walls (40 mm total wall thickness)).  5Impact of Racks steel walls on criticality⚛ All calculations with reflective boundaries⚛ 4 longitudinal walls:  Keff=0.7707⚛ 2 Longitudinal walls: Keff =0.78638⚛ No inner longitudinal walls:    Keff =0.80234for all calculations41 10σ −= ×6Impact of void with reflection at the boundary (I)⚛ Reference calculations with fresh fuel were donewithout inner longitudinal walls:  Keff: 0.8023⚛ With 94% void in the spaces  between the fuel pins and normal  density within the pin:Keff=0.91107⚛ 94% void over all the  rack , including the fuel elementsKeff : 0.56327Impact of void without reflection at the boundary ( II)⚛ The reference calculations without inner longitudinal walls: (with reflection)           Keff: 0.8023⚛ Void only in the inner spaces between the rows  of the fuel elementswithout reflection:          Keff=0.7875(less by :   Δkeff ≈ 0.13 for the full reflection case) (former page)8Change of criticality due to burn up of Gdloaded BWR fuel elementFuel Element of about 14GWd/MTU exhibit up to Δkeff ≈ 0.15 Geometrical model of fuel assembly and rackTwo geometrical models analyzed (2d + 3d): Infinite lattice of single fuel assemblies(simplified model) Infinite lattice of fuel racks(still simplified but more realistic)Two different fuels considered: Fresh fuel Burn up of 13 GWd/tHM(considered Nuclides: 234, 235, 236, 238U,238, 239, 240, 241, 242Pu, 237Np, 241, 243Am, 109Ag, 133Cs,153Eu, 155, 157Gd, 95Mo, 143, 145Nd, 103Rh, 101Ru,147, 149, 150, 151, 152Sm, 99Tc, 16O)9Geometrical model of fuel assembly and rack Different enrichments per fuel pin considered Assumed distance between racks: 10cm in x and 6 cm in y Reflective boundary conditions10model „BE“model „rack“yxkeff calculationsNeutron multiplication factor keff of considered models: Reactivity increases with burnup, maximum at reactor conditions at about13 GWd/tHM, increase in keff:   Δkeff ≈ 0.12(from OECD/NEA  Expert Group BUC Phase IIIc Benchmark) Realistic model „rack“ results in a keff well below the administrative limit of 0.95 Model „BE“ more conservative ⇒ structure of the rack important to keep keff below 0.9511keff,BE keff,rack2d 3d 2d 3dFresh fuel 0.8504 0.8481 0.8014 0.799413 GWD/tHM 0.9797 0.9773 0.9232 0.9212keff calculationsVariations of water density between 0.05 g/cm3 and 1.0 g/cm3 : Scenario: Heating up of the water12keff calculationsVariations of the rack separation in y between 0.0 cm and 20.0 cm: Scenario: steel looses its stability due to heating up, legs and spacers between racks deform and racks „topple down“ („domino effect“): Approximation: reducing the distance betweenracks 13dkeff calculationsVariations of the fuel assembly separation in y inside the racks between 0.0 cm and 3.5 cm:14dab1514C ProductionPhysical formation of 14C in fuel assemblies byneutron capture reactionsternary fission in the fuelduring reactor operationO 	 n → O ∗ → C  HeO n,α CN 	 n → N ∗ → C  pN n,p CC 	 n → C ∗ → C  C n, Cternary fissionin LWR fuel1.7×10−6 per thermal 235U fission [1]1.8×10−6 per thermal 239Pu fission [1][[1] Neeb (1997) The radiochemistry of nuclear power plants with light water reactors. de Gruyter, Berlin. // Nucl. EngineeringInternational (2003) vol. 48, no. 590, Fuel design data.16Results: inventory analysisexperimentally obtained results for 14C, 55Fe and 125Sb are in good agreement with calculationsThe build up of C14 was linear to the N14 concentration about 1000 Bq/gr per 1ppm N14.C/E 137Cs inventory is different by factor 117→ The  precipitation of volatile (light blue) 137Cs on the inner cladding surface during operation can not be taken into account in the  MCNP calculations.radionuclide 14C 55Fe 137Cs 125Sb[Bq/(g Zyr-4)]Experimental 3.7(±0.4)×104 1.5(±0.2)×105 3.4(±0.3)×106 2.4(±0.2)×105calculated 3.2×104 1.3×105 2.9×104 2.6×10517Conclusions⚛ With the conservative consideration it is seen that the sub criticality is well below the limit of 0.95  (~0.7-0.80)⚛ Expected void should in principle decrease the sub criticality due to lack of moderation. ⚛ The Gd impact is worth about 0.15 in criticality, at maximal conservative condition .⚛ Only fuel elements , all of which, with the maximal Gd worth combined with (unrealistic)⚛ with dedicated location of void between the fuel elements ⚛ or compaction of the racks to  a “reactor like” configurationmight lead to super criticality. ⚛ Release of Activated nuclides is unavoidable and should be consideredfor any fuel deformation scenario. 18Acknowledgment⚛ this project has received funding from the Euratom 7th FP under Grant agreement N° 604965”.

Analyse von Kritikalitätsbedingungen im BE-Lagerbecken

https://publikationen.bibliothek.kit.edu/1000062261/3947101

Analyse von Kritikalitätsbedingungen im BE-Lagerbecken

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