Reactivity monitoring using the Area method for the subcritical VENUS-F core within the framework of the FREYA project N. Marie 1, G. Lehaut 1, J.-L. Lecouey 1, A. Billebaud 2, S. Chabod 2, X. Doligez 3, F.-R. Lecolley 1, A. Kochetkov 4, W. Uyttenhove 4, G. Vittiglio 4, J. Wagemans 4, F. Mellier 5, G. Ban 1, H.-E. Thyébault 2, D. Villamarin 6 1 Laboratoire de Physique Corpusculaire de Caen, ENSICAEN/Univ. de Caen/CNRS-IN2P3, France 2 Laboratoire de Physique Subatomique et de Cosmologie, CNRS-IN2P3/UJF/INPG, France 3 Institut de Physique Nucléaire d'orsay, CNRS-IN2P3/Univ. Paris Sud, France 4 StudieCentrum voor Kernenergie-Centre d Etude de l Energie Nucléaire, Belgium 5 Commissariat à l Energie Atomique et aux Energies Alternatives, DEN/DER/SPEX, France 6 Centro de Investigationes Energeticas MedioAmbientales y Tecnologicas, Spain On behalf of the FREYA collaboration Reactivity monitoring using using the Area the method, Area method, Nathalie N. Marie Marie TCADS-2. TCADS. Nantes, France 21-23 21-23 May May 213 213 1
Introduction Results obtained for Pulse neutron Source experiments (PNS) analysed with the Area method used to estimate the reactivity of 4 sub-critical levels of VENUS-F reactor, k =,96 One of the investigated techniques for ADS (Accelerator Driven Systems) on-line reactivity monitoring which has to be validated in the program of the FP7 FREYA project (Fast Reactor Experiments for hybrid Applications ) Facility hosted at the SCK-CEN site in Mol (belgium) Is the result of the coupling of the VENUS-F reactor with the GENEPI-3C accelerator Reactivity Reactivity monitoring using the the Area method, N. N. Marie TCADS. TCADS-2. Nantes, Nantes, France 21-23 France May 21-23 213 May 213 2
The GENEPI-3C accelerator The GEnérateur de NEutrons Pulsé et Intense delivers a 22-keV deuteron beam which hits a TiT target located at core center the 14-MeV neutrons produced via fusion reactions T(D,n) provides = the external neutron source can be operated in : continuous mode pulsed mode continuous mode with 1 s, 2 ma peak current periodic short beam interruptions source intensity is 1-2 x1 6 Reactivity Reactivity monitoring using the the Area method, N. N. Marie TCADS. TCADS-2. Nantes, Nantes, France 21-23 France May 21-23 213 May 213 3
The VENUS-F reactor The VENUS-F reactor fast, zero-power, lead-moderated, sub-critical reactor 12x12 grid surrounded by stainless steel (ss) casing can receive assemblies : ss casing outer reflector Pb reflector 93 Fuel : Pb+U (3% enrich.) PEAR rod with small reactivity worth = -136 5 pcm VENUS-F reactivity can be changed by moving: 2 control rods (CR) mm (fully inserted) 6 mm (fully retracted) 24 mm; 479.3 mm 4 sub-critical configurations studied 4 CR heights; derived from SC1 configuration : 479.3 mm, k ~.96, r/b ~ -5.3 $ Study the neutron flux : 1 Fission Chambers (FC) with various 235 U deposit masses: 1 g for CFUL 1 mg for RS 1 mg for CFUF 1 mg for CFUM 4
The reference reactivity values Test performances of Area method Reactivity of each subcritical configuration determined first by analysing other experiments with MSM method (Modified Source Multiplication) Results extracted with MSM technique = reference reactivity values : Configuration SC1/CR=mm SC1/CR=24mm SC1 SC1/CR=6mm Height of CRs (mm) MSM reactivity ($) 24 479.3 6-6.35 ±.27-5.92 ±.25-5.3 ±.23-5.9 ±.22 used as benchmark 5
Principle of the Area method Neutron population evolution over time, after a source neutron pulse Neutron point kinetics (NPK) N( t) N e r b t b e r r b 2 r b t one-delayed n group approximation fast component prompt neutrons slow component delayed neutrons b A p N A d N r b r( r b ) prompt surface delayed surface r $ A A p d r b Area method based on analysis of time-dependent count rates of FC 6
Construction of Pulsed Neutron Source (PNS) histograms detector count rates acquired after each pulse During PNS experiments, FC time responses measured while GENEPI-3C in a pulsed mode associated PNS histograms : Source histogram : f = 2 Hz time elapsed after pulse 1 million of pulses PNS spectra normalized to same maximum ( FC responses corrected for dead time ) 7
Typical PNS histograms Shapes depend on the detector positions After t=, sharp increase time needed for neutrons to be transported from source emission point to the FC position after maximum, rates decrease more or less rapidly 1,5 ms «fast component» decay of prompt neutrons smoother decrease for FC in reflector or nearby the SS casing sign presence of spatial ects, not predicted by NPK source neutron injection above 2 ms, constant level reached, called delayed neutron level L d CFUL658 8
Origine of the slow component Slow component = of the contributions from previous pulses of the delayed neutrons emitted by the precursors Construction of L d 2 neutron pulses 2 peaks each black curve = of neutrons just injected, + all the other neutrons still remaining in the reactor and originating from the previous pulses simulations with point kinetic equations At long times, the neutron delayed components flatten precursor contribution tends to equilibrium 9
Estimate of the delayed neutron level f =2 Hz PNS experiments should NOT be performed at f > 5 Hz 1
Results with Area method Procedure applied to fission rates measured 1 FC f = 2 Hz 4 subcritical levels of SC1 changing CR heights Dispersion of results 3 different groups identified 11
Results with Area method only one in core CFUF34 gives same reactivity as the reference one PNS histogram has similar shape as the one predicted by NPK All other reactivities underestimated 12
Results with Area method core-reflector interface in corners of ss casing in inner part of reflector Similar reactivities underestimated compared to the reference one 13
Results with Area method far away from core in outer part of reflector outside ss casing Area Method fails at providing the correct r in PNS histogram, L d not reached A d determined uncorrectly -r underestimated 14
Improvement of the Area method Spatial ects could explain dispersion among reactivity values necessary to evaluate correction factors varying with the position in the reactor, to be applied to experimental results performe Monte Carlo (MC) simulations of neutron pulses (MCNP 5.1.4) 1. Create MCNP input file with a simplified geometry of VENUS-F save computing time 15
Improvement of Area method 2. MC correction factors : true value distorted value corrected reactivity value 16
Corrected reactivity values Final results Effect of correction negligible for CFUF34 Except for FC in outer reflector corrected values all compatible with reference reactivities 17
Corrected reactivity Averages Discarding FC in outer reflector calculate averages of the corrected reactivities CR height (mm) 6-5.9 ±.3-5.9 ±.22 479.3-5.26 ±.3-5.3 ±.23 24-5.91±.5-5.92 ±.25-6.31 ±.5-6.35 ±.27 Remarkable agreement 18
Conclusions Present the reactivities extracted with the Area method for 4 different subcritical levels of the VENUS-F reactor drived by the GENEPI-3C accelerator The dispersion observed among the reactivity estimations pointed out that space ects bias the results and that they must be accounted for Expose the method used to compute, by means of simulations with MCNP, the spatial correction factors Except for two fission chambers located inside the outer lead reflector, all the corrected reactivity values were compatible and in good agreement with the reference values previously estimated with the MSM method. Thank you for your attention. 19
Condition for neutron precursor equilibrium Before analysing the data with the Area method check neutron precursors in equilibrium estimate when this condition is verified? PNS histograms simulated reactivity values extracted by applying the Area method as for the experimental fission rates input value > 2.1 5 pulses PNS spectra superimposed get back input reactivity value < 2.1 5 pulses permanent regime not reached reactivity inferred distorted Nb pulses added for PNS construction simulations with point kinetic equations Experimental PNS histograms with > 1 million pulses 2
Estimate of the delayed n level f =2 Hz PNS experiments should NOT be performed at f > 5 Hz time intervals between pulses shorter L d saturation NOT reached f limite lower when FC farther away from core 21
Estimate of prompt and delayed surfaces reactivity value statistical uncertainty systematic uncertainty Its total uncertainty 22
Principle of the Area method Neutron population evolution over time, after a source neutron pulse N( t) N e r b t b e r r b 2 r b t one-delayed neutron group approximation Reactivity Effective delayed neutron fraction r b k 1 k, ˆ F dk, Fˆ Average decay constant i i b i bi i Effective neutron generation time 1, v, Fˆ 23