Nuclear waste management

Transcription

1 Nuclear waste management Permanent staff : G. Ban, F-R. Lecolley, J-L. Lecouey, G. Lehaut, N. Marie-Nourry, J-C. Steckmeyer Emeritus : J-F. Lecolley PhD student: H-E. Thyebault Collaborations : LPSC Grenoble, IPHC Strasbourg, SCK.GEN Mol (Belgique), CEA Cadarache, CEA/IRFU Saclay, GANIL Caen Introduction Over the past two years, in the framework of the GUINEVERE experiment and FREYA program we have contributed to experiments carried out at the lead moderated VENUS-F fast core at critical as well as at subcritical configurations. The first coupling of the GENEPI-3C accelerator with the VENUS-F core in the subcriticalconfigurationwasachievedonoctober12 th,2011andthenintensivelystudied.forthefirsttime, Pulsed Neutron Source measurements(pns) and short continuous beam interruptions were performed in a lead subcritical fast core. The second part of our activities was devoted to the development of an original experimental device in the framework of the FALSTAFF project whose objective is the study of fission using the neutron source facility (NFS) at SPIRAL2. 41

2 The GUINEVERE experiment The GUINEVERE (Generator of Uninterrupted Intense NEutrons at the lead Venus REactor) experiment [GUI] is dedicated to feasibility studies for Accelerator Driven Systems (ADS) which are envisaged in partitioning and transmutation strategies. It aims at providing a zero power experimental facility to investigate sub-criticality on-line monitoring procedures and to validate simulation tools. These issues are of major importance in view of the achievement of a future powerful ADS such as the MYRRHA project [MYR]. The GUINEVERE facility is hosted at the SCK CEN site in Mol (Belgium) and consists in the coupling of the fast VENUS-F reactor to a neutron source provided by the GENEPI-3C accelerator(fig. 1). Figure 1 : Overview of the GUINEVERE facility (courtesy SCK CEN) The fast VENUS-F reactor consists of square fuel assemblies (FA) composed of a 5x5 pattern mixture of fuel and solid lead rodlets, the latter acting as a fast system coolant. Radially and axially the fissile zone is surrounded by lead reflectors. The outer side length of a FA is 80 mm. The fuel is 30% 235 U enriched metallic uranium provided by CEA. The FA are arranged in a cylindrical geometry (~800 mm in diameter, 600 mm in height). The VENUS-F core is equipped with six safety rods, two control rods and one absorbent rod (PEAR rod). Figure 2 shows cross sections of the fast VENUS-F reactor. Figure 2 : Left diagram: detector location and cross section of the critical core configuration CR0 including safety rods (violet, SR), control rods (red, CR), fuel assemblies (violet), lead (yellow), PEAR rod (green) and stainless steel (grey) Right diagram: cross section of the subcritical core configuration SC1 including accelerator insertion channel (empty central zone). The GENEPI-3C accelerator [GUI] provides neutrons by T(d,n) 4 He fusion reactions. It accelerates deuteron ions to the energy of 220 kev and guides them onto a tritiated target located at the VENUS-F core center. This provides a quasi-isotropic field of about 14 MeV neutrons. In a first step, a critical configuration called CR0 was loaded and experimentally characterized[cr0]. In a second step, a sub-critical configuration called SC1 (k eff ~ 0.96) was obtained by replacing the four central FA by the device devoted to the accelerator pipe hosting (Fig.2). 42

3 Estimate of the reactivity of SC1 configuration using the MSM method The MSM (Modified Source Multiplication) technique allows one to connectreactivities ρ 1 and ρ 2 oftwodifferent subcriticalconfigurations through count rates C 1 and C 2 measured in the configuration 1 and in the configuration 2 with the same neutron source and the same detectors: C1 ρ 2 = f MSM ρ1 C Where ƒ MSM is the MSM correction factor which accounts for the difference of neutron source importance and detector efficiency between the two configurations. If detector efficiency and spatial effects are the same in both configurations, the MSM correction factor is equal to one. If these assumptions are made, the so-called ASM (Amplified Source Multiplication) estimate of reactivity is obtained. 2 TheASM/MSM method hasbeen used to estimatethereactivity ofthe so-called SC1 configuration of the VENUS-F core. First a slightly subcritical configuration, named hereafter CR0, was created by dropping the PEAR rod (PElletised Absorber Rod) when the VENUS-F was at critical (CR0 configuration). Its reactivity was determined by analyzing the time evolution of detector count rates during the PEAR rod drop. This slightly subcritical level served as a reference configuration for applying MSM methods. Second, an Am-Be source emitting neutrons per second was introduced in a reflector slot, and static count rates were measured in several fission chambers, for configurations CR0 and SC1. In order to determine the reactivity of CR0, several rod drop experiments using the rod drop system PEAR were carried out at three different reactor powers (2, 4 and 8 W). Using the KEG program [KEG], data sets were analyzed via direct reactor point kinetics (DPK) and inverse point kinetics (IPK) [DPK-IPK]. The set of delayed neutron parameters used was calculated with an ERANOS model [ERA] of the VENUS-F critical configuration. Figure 3 : Example of rod drop at 2 W. The figure shows the number of counts recorded by one of the fission chamber (CFUL detector) 43

4 Table 1 : PEAR rod worth obtained using DPK and IPK for analyzing the evolution of count rates in the fission chambers. Detector name ρ(cr0 ) by DPK ($) ρ(cr0 ) by IPK ($) CFUL ± ± CFUL ± ± RS ± ± RS ± ± RS ± ± Table 1 shows the results obtained for the reactivity (in $) of CR0 for several fission chambers. The average reactivity and its standard deviation were calculated for both analysis methods and finally after calculating the weighted mean and the weighted standard deviation, the reactivity of CR0 was found to be ρ(cr0 ) = ± $ After completion of the PEAR rod drop experiments, the Am-Be source was inserted in the reflector and static count rates were registered in the several fission chambers for both configurations CR0 and SC1. Table 2 : MSA and MSM reactivity of the SC1 configuration calculated from the fission chambers count rate ratios. Name C(CR0 ) / C(SC1) ρ(sc1) MSA ($) f MSM ρ(sc1 ) MSM ($) CFUL ± ± ± ± CFUL ± ± ± ± CFUL ± ± ± ± RS ± ± ± ± RS ± ± ± ± RS ± ± ± ± RS ± ± ± ± Table 2 shows the count rate ratios measured with seven fission chambers as well as the extracted values for the MSA reactivity of the SC1 reactor configuration. As expected from the large flux shape difference between the CR0 and SC1 configurations, results for reactivity are very scattered, ranging from $ to $. Obviously some MSM corrections must be calculated in order to obtain consistent results for all the detectors. MSM correction factors were calculated using the Monte Carlo simulation code MCNP [MCNP]. The geometry of the CR0 and SC1 configurations were derived from the MCNP input file provided by SCK CEN [UYT]. Results are gathered in Table 2. MSM factor values range from 1.015(22) to 1.840(39). As expected, the larger correction is applied to the closest detector to the source. As can be seen, once the MSM correction has been applied, all the detectors give statistically compatible values for ρ(sc1) MSM. The final estimate for ρ(sc1) MSM and its uncertainty was obtained by calculating the weighted mean and the weighted standard deviation of the reactivity values given by the seven fission chambers: which leads to ρ(sc1) MSM = ± $ k eff (SC1) MSM = ± pcm 44

5 if one assumes β eff = pcm (ERANOS calculations) with an uncertainty of 10 pcm. The reference measurement of SC1 subcritical level configurations by ASM/MSM method with rod drop will help to estimate the reliability of the other methods of reactivity determination which could be applied in industrial ADS facilities. These methods (PNS, Source Jerk and others) are currently under investigation in the GENEPI-3C-driven subcritical VENUS-F core in the framework of the FREYA Project [FREYA]. Pulsed neutron source measurements The current-to-flux technique was proposed to be combined to absolute reactivity measurements in order to establish a complete online reactivity measurement procedure for ADS [MUSE]. The absolute reactivity values are foreseen to be deduced from dynamical measurements requiring source variations. The study of the techniques used to analyse such measurements is one of the purposes of the GUINEVERE program. As a first step, to evaluate their accuracy, they have to be applied in Pulsed Neutron Source conditions for a given reactivity and their results will be compared to the reference value given by the MSM method. Several methods were investigated: the Area method [AREA] which is based on the separation of two components in the total time response of a detector after a PNS injection: the prompt time response and the delayed neutron contribution,thek P method [KP] which isbased on thedetermination of the inter-generation neutron lifetime distribution. In figure 4 are shown time spectra obtained from PNS measurements performed in the SC1 configuration with various detectors. Figure 4 : PNS measurements for various detectors in SC1 configuration (not normalized) 45

6 Table3showspreliminaryresultsfortheareamethod(AM)andthek P method(kp) for the RS detectors. Table 3 : Reactivity of the SC1 configuration calculated from PNS measurements (AM = Area Method, KP = k P method). Name ρ(sc1) AM ($) ρ(sc1) KP ($) RS ± RS ± RS ± RS ± When compared to the results obtained with the MSM method, the values of reactivity inferred with the area method are very similar since the maximum relative difference equals 4%. The differences between the values determined with both methods might be narrowed when correction factors due to possible spatial effects will be estimated and taken into account. With the k P method, the result dispersion is as large as 30%. The possible causes for this dispersion can be many: dead time in fission chambers, spatial effects and modeling inaccuracy are suspected to be the main ones. Improvement of each of these aspects is under study. The FALSTAFF Project The FALSTAFF project aims at providing highly constraining data to significantly improve the description of the fission process. More specifically the goal is to measure the neutron multiplicity as a function of the fragment characteristics (mass, nuclear charge and kinetic energy) in neutron-induced fission of specific actinides in the MeV range. This experiment is particularly rich since it probes numerous aspects of fission, as the scission deformation, the influence of single-particle structure, the dissipated energy and the energy sharing between the two fragments. In order to keep good detection efficiency, neutrons are not detected, but their multiplicity is deduced from the measured fragment masses before and after their evaporation. New developments on microscopic calculations and the future generation of nuclear reactors are two of the main motivations for new experimental programs devoted to the study of fission. Ionization chamber with scintillating gas In order to minimize energy straggling of fission fragments, one possibility is to use ionization chamber (IC) not only to identify and measure the energy of the fission fragments but also to measure their velocity via the well-known time-of-flight technique. However the time response of an IC does not reach the resolutions required for a good determination of the fission fragment velocity. That is the reason why we have developed an IC filled with scintillating gas and coupled to a pair of photomultipliers through transparent windows. The light emitted by the gas provides the stop signal for the time-of-flight measurement. The IC with scintillating gas is now available and a program to determine its time resolution with several gases under various conditions(electric field, pressure ) will start in

7 [GUI] A. Billebaud et al., The GUINEVERE Project for Accelerator Driven System Physics, Proceedings of Global 2009, Paris, France (September 6-11, 2009). [MYR] H.A. Abderrahim et al., MYRRHA Technical Description, Technical Report for the OECD MYRRHA Review Team, SCK CEN, Belgium(2008). [CR0] W. Uyttenhove et al., Experimental Results from the VENUS-F Critical Reference State for the GUINEVERE Accelerator Driven System Project, Proceeding of the Int. Conf. on Advancements in Nuclear Instrumentation, Measurement Methods and their Application, ANIMMA, Ghent, Belgium(June ). [KEG] Program KEG: Kinetics with Eight Groups, J.-L. Lecouey, LPC Caen [DPK-IPK] K. O. Ott and R. J. Neuhold, Introductory Nuclear Reactor Dynamics, American Nuclear Society, ISBN: [ERA] M. Carta, private communication, June 2011 [MCNP] MCNP -A General Monte Carlo N-ParticleCode, Version 5, LA-ORNL, RSICC LA-UR , Los Alamos National Laboratory(2003) [UYT] W. Uyttenhove, MCNP VENUS-F core model, version 3.1, October 26, 2010 [FREYA] FREYA collaboration, FP [MUSE] MUSE collaboration, 5th EURATOM FP-Contract#FIKW-CT Deliverable #8: Final Report (2005) [AREA] N.G. Sjostrand, ArkivförFysikBand 11 nr 13, 233 (1956) [KP] F. Perdu et al., Prog. Nucl. Energy, 42:107 (2003) 47