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Annals of Nuclear Energy 101 (2017) 312321
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Annals of Nuclear Energy
journal homepage: www.elsevier .com/locate /anucene
The neutronics scheme adopted for the HELIOS irradiation experiment inthe High Flux Reactor Petten
http://dx.doi.org/10.1016/j.anucene.2016.11.0170306-4549/ 2016 The Authors. Published by Elsevier Ltd.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Corresponding author.E-mail address: firstname.lastname@example.org (E. DAgata).
E. DAgata a,, C. Dderlein a, H. Tsige-Tamirat a, M. Oettingen b, R. Mutnuru ca European Commission, Joint Research Centre, Institute for Energy and Transport (JRC-IET), P.O. Box 2, NL-1755 ZG Petten, The NetherlandsbAGH University of Science and Technology, al. A. Mickiewicza 30, 30-059 Krakow, PolandcNuclear Research and Consultancy Group (NRG), P.O. Box 25, NL-1755 ZG Petten, The Netherlands
a r t i c l e i n f o
Article history:Received 18 May 2016Received in revised form 26 October 2016Accepted 11 November 2016Available online 28 November 2016
a b s t r a c t
A dedicated neutronics computational scheme is a crucial tool in the preparation and the post-analysis ofan irradiation experiment, as it defines the key parameters of the experiment. The complex and verydynamic environment of a Materials Testing Reactor (MTR) core, on the other hand, represents a chal-lenging object for neutronics calculations. Within the framework of the HELIOS experiment conductedat the High Flux Reactor (HFR) in Petten (The Netherlands) in 2009 under the European ProjectEUROTRANS, a new neutronics scheme has been developed with the objective of finding the optimumbalance between accuracy and calculation performance. This scheme is based on the Monte-Carlo tech-nique and employs a two-level approach by separating the problem into overlapping spatial and tempo-ral domains. The underlying physical phenomena are presented, together with elements of validationwhich gives a further insight into the effects governing the neutron fluxes in the core. 2016 The Authors. Published by Elsevier Ltd. This is anopenaccess article under the CCBY-NC-ND license
The preparation and accurate interpretation of the results of anirradiation experiment are important steps of an irradiation cam-paign. They requires a precise knowledge of the conditions of theirradiation conditions, particularly as it concerns the neutronenergy spectrum and the fission source distribution, to better pre-dict the behaviour of the experiment during the irradiation andunderstand exactly, at the end of the campaign, how the irradiatedmaterial behaved. The neutronics evaluations necessary to obtainthese data are complicated by the particularity of the core andthe operations of a MTR. Besides the high degree of materialheterogeneity and geometrical complexity within the core compo-nents, it is the dynamic core loading and the consequential fre-quent change in the environment of the irradiation device thatneed to be taken into account (insertion, extraction of control rods;reshuffling every cycle of the fuel elements, addition of otherexperiments in the core, etc. . .). Furthermore, the composition ofthe irradiated samples can evolve significantly over the durationof the irradiation, which may extend over several months. Thispaper presents the neutronics scheme developed for the interpre-tation of the HELIOS irradiation experiment, which has been con-ducted in the HFR reactor in Petten between April 2009 and
February 2010. After a short description of the HFR reactor andthe experiment in Section 2 and 3, the Monte-Carlo based schemeis presented in Section 4. Section 5 summarises the results of thecalculations and presents first elements of the experimental vali-dation. The conclusions and the outlook on further developments,round up the presentation. Finally, the new neutronics computa-tional scheme here presented for the HELIOS experiment, maybeconsidered as a general procedure and it can be adapted to anykind of irradiation experiments when detailed knowledge aboutirradiation conditions are available.
2. The High Flux Reactor in Petten
The High Flux Reactor (HFR) in Petten is a tank-in-pool typeirradiation reactor, cooled and moderated by light water and oper-ated at a power of 45 MWth. It is in operation since 1961 and useslow-enriched U3Si2 fuel since 2006. Besides performing a widerange of technological irradiations (Ftterer et al., 2009), the HFRis one of the major European production facility of radioisotopesfor medical applications (May and Moss, 2008).
The reactor core is composed of rectangular sub-assemblies,arranged in an 8 by 7 array (see Fig. 1), 62.0 by 74.0 cm wide.The core contains 33 standard fuel assemblies (7.2 8.0 cm),which consist each of 20 fuel plates (plate/meat thickness1.71/0.76 mm, gap width 1.5 mm, see Fig. 2) in an aluminium cas-ing. Cadmium wires with a diameter of 0.5 mm are fixed on the
Fig. 1. Schematic cross section of the HFR core with indication of cardinaldirections.
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outside of the casing as burnable poison next to each end of thefuel plates, in order to decrease the power peaking at plate extrem-ities. The active core height is 60 cm.
The core reactivity is controlled by six control assemblies, asshown in Fig. 1. The active (absorbing) part of these assembliesconsists of a rectangular arrangement of cadmium plates with anAl cladding within a AlMgSi frame (see Fig. 2). A follower arrange-ment of 17 fuel plates is fixed below the absorbing part and entersthe active core zone when the control element is withdrawn inupward direction. The 17 remaining core positions are taken upby irradiation devices and fuel rigs or aluminium filler pieces whenunoccupied. The core is surrounded on three sides (north, east andsouth) by a beryllium reflector (thickness 8.0 cm). Other irradiationpositions are located on the west side of the core, in the Pool SideFacility. Samples can be irradiated there at different distance fromthe core with the possibility to retrieve them without the necessityto stop the reactor or wait for the end of the cycle. The experimen-tal equipment of the core is completed by beam tubes, pointing tothe north, south and east sides of the core (cf. Fig. 1).
The neutron source within the core, as in the G7 position wherethe HELIOS irradiation took place, amounts to about 8.41013n/cm2/s for the fast flux (E > 0.82 MeV) and 7.01013 n/cm2/s forthe thermal flux (E < 0.625 eV). The more precise determinationof the neutron fluxes in the location where the fuel contained in
Fig. 2. Fuel assembly (left) and control assem
HELIOS is placed, and their variation in time, is object of the newcalculation scheme, which is described in the following section.
3. The HELIOS experiment on Minor Actinide transmutation
Minor Actinides (MA) contribute to a large part of the radiotox-icity of spent nuclear fuel. The transmutation of these long-livedisotopes, among which 241Am is the most prominent member, isan attractive option for the reduction of mass and radiotoxicityof nuclear waste. The technical feasibility of transmutation usingMA burning fuels has already been extensively established (NEA,2006). It has however become evident in the recent EFTTRA exper-iments (Neeft et al., 2003) that the swelling of MA containing fuel,due to the trapping of the helium produced in the transmutationprocess of 241Am, is a key issue of this approach. The HELIOS exper-iment (DAgata et al., 2009), performed within the EUROTRANSproject of the EURATOM 6th framework program, investigates inthis context the influence of irradiation temperature andmicrostructure on gas release and fuel swelling in Uranium freefuels. A set of five samples (see Table 1) were used to investigatethe influence of these parameters by testing matrices with differ-ent microstructures and degrees of porosity. Samples 3 and 5 weredoped with Plutonium to assure a high sample temperature (up to1500 C) from the beginning of the irradiation and to favourthereby the gas release. Samples 2 and 3 were also equipped withthermocouples (TCs), inserted in a central hole on the samplematerial.
The samples consisted of 60 mm high stacks of cylindrical pel-lets, respectively annular pellets for the samples carrying the cen-tral thermocouple, with a diameter of 5.45 mm and were enclosedin gas-tight test pins with an inner (ID) and outer diameter (OD) of5.65 and 6.55 mm, respectively. The test pins were stacked withinmolybdenum shrouds which were placed in two different sampleholders (ID 26.5 mm, OD 28.5 mm), named HELIOS1 and HELIOS2(see Fig. 3) made of Stainless Steel 316. The sample holders werefilled with sodium, to enhanced the thermal bonding and thusimprove the heat removal from the test pins. Around the sampleholders there is a channel of the thimble of the experiment whichis surrounded from the water of the first cooling system. The spacebetween the outer wall of the sample holder and the inner surfaceof the channel is connected to a dedicated gas conditioning unitand filled by helium-neon gas mixture. By modifying the composi-tion of the gas mixture during the irradiation, the heat transferbetween the holder and the cooling water can be changed, adjust-ing thereby the temperature in the sodium and the samples.
For experimental and safety purposes, each sample holder wasequipped with 23 more thermocouples located at different axialpositions within suitable grooves made in the Mo shroud. In termsof neutronics instrumentation, each sample holder was carrying 4gamma scanning wires and one flux detector (FD) per pin, embed-ded in the molybdenum shroud. These flux detectors (see Fig. 4)consisted of small pieces of wires of different materials (seeTable 2), each one sealed in a small quartz tube and stacked in a
bly (absorber part, right) cross sections.
Table 1Main characteristics of the HELIOS samples used for neutronic calculations.
Sample no Composition Density [g/cm3] Theoretical Density (TD) Measured [%] Final Density based on TD [g/cm3] Mass [g] Volume [cm3]
1 MgO + Am2Zr2O7 0.66 91.50 3.96 5.54 1.402a (Am,Zr,Y)O2 0.70 92.60 5.91 7.71 1.313a (Pu,Am,Zr,Y)O2 0.41 0.74 89.70 5.95 7.77 1.314 (Am,Zr,Y)O2 + Mo 0.69 94.10 8.84 12.38 1.405 (Pu,Am)O2 + Mo 1.24 0.30 94.00 9.99 13.98 1.40
a Central thermocouple.
Fig. 3. Schematic view of HELIOS sample holders.
Fig. 4. Configuration of the flux detectors (FDs).
Table 2Flux detector (FD) materials.
FD material Approx. mass [mg]
NiCo (1 wt% Co) 0.07Fe 0.7Ti 1.0Nb 0.75
1 The vertical position was not significantly changed during the HELIOS irradiation,xcept for cycles 5 and 6 where the rig was lifted by 34 cm.
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stainless steel tube (2.0 mm, length 25 mm). The FDs werelocated next to each fuel sample, with the steel tube fitted into acorresponding groove in the molybdenum tube, as to measure
the local irradiation flux of the sample. The detectors have beenanalysed by gamma spectroscopy after termination of the experi-ment (Mutnuru, 2010) to determine the fluence rate within thetest samples for their analysis.
The HELIOS 1 and 2 sample holders were placed in the positions2 and 3, respectively, of a standard QUATTRO rig (see Fig. 5), posi-tions 1 and 4 being filled with aluminium dummies. The rig takesthe place of a fuel assembly in the HFR core (see below) andassures the sample holders to be cooled by the cores coolant flow.The sample holders could be moved vertically during irradiation byup to 150 mm by means of the Vertical Displacement Unit, in orderto compensate for changes in vertical flux profile1. After each reac-tor cycle, which lasted between 22 and 31 days, the rig was rotatedhorizontally by 180 to reduce the influence of radial flux gradients.
The irradiation of the HELIOS experiment lasted 240 equivalentfull power days. After a cooling period, the sample holders wereextracted from the rig and disassembled. The gamma scanningwires and the flux detectors underwent gamma-spectroscopy
Fig. 5. The QUATTRO fuel rig (South orientation).
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analysis at the local laboratory of the Nuclear Research and Consul-tancy Group (NRG). Some of the irradiation samples have beenanalysed in the same laboratory, others have been examined inthe hot cells of the Institute for Transuranium Elements (ITU) ofthe Joint Research Centre (JRC) of European Commission.
4. The calculation scheme
4.1. Challenges and calculation strategy
The preparation of an irradiation experiment requires as preciseas possible estimates of the fluxes expected during the experimentin order to assure the achievement of the test objectives and to sat-isfy the required safety conditions. At the same time, the calcula-tion tool needs to be flexible, easy to use and fast enough toperform a large number of parametric assessments. Besides theseoperational constraints, the neutronics calculation is challengingbecause of the complex and extremely heterogeneous geometryinvolved and the dynamic environment within the core. Thechanges in the samples environment that need to be taken intoaccount are as follows:
a) change of orientation of the test rig between the cycles or itsdisplacement to another core position,
b) change in the composition of the neighbourhood from cycleto cycle (e.g. different test rigs in adjacent core positions,burn-up of neighbouring fuel elements and theirreshuffling),
c) control rod movement during the cycle,d) axial movement of the sample holders during the cycle,e) depletion of the sample materials during the cycle.
The Monte Carlo code MCNP5 with the JEFF3.1 nuclear datalibraries (Monte Carlo Team, 2003) has been chosen for thisscheme as it allows for a detailed modelling of complex geometriesand it is one of the codes used (and validated) to perform neutroniccalculation at HFR. However, the need to integrate over small vol-umes (sample diameter: 5 mm) within a large system (core diam-eter: 1 m) would require a high number of particle histories to besimulated in a direct, straightforward approach. The long calcula-tion times which would ensue are incompatible with its utilisationas a design and optimisation tool. The scheme presented heresolves this dilemma by applying a two steps procedure, where afirst calculation at core scal...