Page 1 of 18 MIRTE: AN EXPERIMENTAL PROGRAM DESIGNED TO TEST THE REACTIVITY WORTH OF SEVERAL STRUCTURAL MATERIALS N. Leclaire ∗ , I. Duhamel and F.X. Le Dauphin Institut de Radioprotection et de Sûreté nucléaire (IRSN) BP17 92262 Fontenay-Aux-Roses, France [email protected], [email protected], [email protected]J. Piot Commissariat à l’Energie Atomique et aux Energies Alternatives Centre de Valduc - Bâtiment 010 - 21120 Is-Sur-Tille - France [email protected]I. Villetard de Laguerie AREVA, Direction de la Recherche et de l’Innovation Tour AREVA 92084 Paris La Défense Cedex - France [email protected]F. Lespinasse 1/7, rue Jean Monnet 92298 Châtenay-Malabry Cedex - France [email protected]J. B. Briggs INL 2525 Fremont Street P.O. Box 1625 Idaho Falls, ID83402 - USA [email protected]ABSTRACT In the framework of its missions on assessment of the criticality hazard, IRSN decided in 2005 to perform a new critical experimental program dealing with various structural materials to address the need of criticality calculation codes and nuclear data validation. In 2007, the program evolved into an international collaboration involving IRSN, the French industrial group AREVA, the French National Radioactive Waste Management Agency ANDRA (Agence Nationale pour la gestion des Déchets Radioactifs) and the United States Department of Energy (DOE). Phase-1 of the MIRTE Program, which is now finished, consisted of 43 sub-critical approaches extrapolated to critical conditions using the neutron amplification method. The 43 experiments included reproducibility and “reference”, also called replacement experiments. These experiments are being used to evaluate the eventual bias due to the tested structural materials. A second phase, called MIRTE 2, was planned to test other kinds of materials in the same configurations or with very small adaptations. Key Words: Critical experiment, Interaction, MIRTE 1, MIRTE 2, Reflection, Structural Materials ∗ Corresponding author: 33 1 58 35 91 66
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MIRTE: AN EXPERIMENTAL PROGRAM DESIGNED TO TEST THE REACTIVITY WORTH OF SEVERAL STRUCTURAL MATERIALS
N. Leclaire∗, I. Duhamel and F.X. Le Dauphin
Institut de Radioprotection et de Sûreté nucléaire (IRSN) BP17 92262 Fontenay-Aux-Roses, France
In the framework of its missions on assessment of the criticality hazard, IRSN decided in 2005 to perform a new critical experimental program dealing with various structural materials to address the need of criticality calculation codes and nuclear data validation. In 2007, the program evolved into an international collaboration involving IRSN, the French industrial group AREVA, the French National Radioactive Waste Management Agency ANDRA (Agence Nationale pour la gestion des Déchets Radioactifs) and the United States Department of Energy (DOE). Phase-1 of the MIRTE Program, which is now finished, consisted of 43 sub-critical approaches extrapolated to critical conditions using the neutron amplification method. The 43 experiments included reproducibility and “reference”, also called replacement experiments. These experiments are being used to evaluate the eventual bias due to the tested structural materials. A second phase, called MIRTE 2, was planned to test other kinds of materials in the same configurations or with very small adaptations.
The MIRTE program is being carried out at the CEA (Commissariat à l’Énergie Atomique et aux Énergies alternatives) Valduc Centre on the Apparatus B assembly (see Figure 1), which is a sub-critical experimental facility that has been used for more than 40 years by IRSN for criticality experimental programs.
The aim of the program is to test different kinds of structural materials in reflected and
interacting configurations. The structural materials to be tested have been chosen based on priority partner validation needs, material availability and fabrication costs.
The program is composed of two main phases, MIRTE 1 and MIRTE 2. The experiments involved water-moderated low-enriched U(4.738%)O2 rod lattices with thermal energy spectra. The experiments have been designed to provide a highest possible reactivity worth of the structural material and to ensure a credible and low experimental uncertainty.
MIRTE 1, which was performed from December 2008 to June 2010, consisted of forty-three sub-critical approaches extrapolated to critical conditions using the neutron amplification method. It included reference experiments (without structural materials) for bias estimation [ 1] and reproducibility experiments for uncertainty evaluation. Three types of experimental assemblies were constructed (see Figure 2):
• A single lattice of UO2 rods reflected on its four lateral sides with aluminum or glass (SiO2) walls,
• Two interacting lattices of UO2 rods separated by a single test screen composed of iron, nickel, zirconium, aluminum, lead, copper or concrete with varying water contents (3%, 6%, 9%) with thicknesses ranging from 5 cm to 30 cm,
• Four interacting lattices of UO2 rods separated by thin cruciform plates composed of copper, nickel, iron, and titanium with thicknesses less than 2 cm.
An identification nomenclature was assigned to these three kinds of configurations. The first identifier stands for the number of rod arrays (1A for reflected configurations, 2A for large interacting configurations or 4A for interacting configurations separated by thin plates). The second identifier indicates the test material or the replacement material: water (called “eau” in the IDs) or air. The third identifier gives its thickness in millimeters. A fourth identifier is used for repeatability (Rv) or reproducibility (Rb: S1, S2, S3 or S4 type) experiments.
A first evaluation of the experimental results that highlights potential biases for some materials and configurations has already been performed. Moreover, titanium experiments were reviewed and approved for inclusion into the ICSBEP Handbook in 2011 [ 2]. Other experiments being subject to a 7-year confidentiality agreement, a complete description in ICSBEP format will not be available before the end of 2017.
Figure 1. View of the Apparatus B facility
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MIRTE: an experimental program designed to test the reactivity worth of several structural materials
In 2010, the MIRTE 1 funding partners expressed their satisfaction and their desire to pursue their collaboration with IRSN for a second phase of the MIRTE program, referred to as MIRTE 2. Experiments that could adapt simultaneously to the experimental device in place, to the partners needs and any specific technical issues, and to the Valduc facility schedule were proposed. This program has been split into three different steps: MIRTE 2.1 to improve the accuracy of some MIRTE 1 experiments, MIRTE 2.2 focused on testing new materials and MIRTE 2.3 for configurations that require slight modifications to the device.
Figure 2. Schematic views of the experimental devices
The status and results of the MIRTE 1 program will be the subject of the first part of this paper. Emphasis will focus on fabrication tolerances and measurements efforts to achieve low uncertainties, which are prerequisite to the derivation of accurate biases due to nuclear data libraries and criticality codes. Design studies, sensitivity analyses, as well as the status of MIRTE 2 program will also be discussed.
2 EXPERIMENTAL CONFIGURATIONS
2.1 Experimental device The experiments are sub-critical approaches, with an upper limit on the multiplication factor of
1-β/10, extrapolated to critical. The sub-critical approach, driven by two Am-Be neutron sources, is carried out by increasing the amount of water in the core. The experimental tank is equipped with a measurement-needle that follows the free upper level of water and provides the water height.
The neutron amplification technique is used in this approach: neutron multiplication is determined by at least three BF3 counters, which provides the count-rate, M, for each measured value of water height. The 1/M method is used to extrapolate sub-critical water-height measurements to the critical height.
The array of rods is placed on a stainless steel pedestal inside a parallelepiped tank. A picture of the support pedestal is given in Figure 3.
The three experimental devices are designed to allow easy modification and replacement of the structural material test plates. The grids maintaining the UO2 rods are designed to allow a square lattice pitch of 1.6 cm for all the configurations, leading to a thermal energy spectrum.
2.2 Different types of configurations
2.2.1 Reflected configurations Reflected configurations consist of one UO2 rod lattice reflected on all four sides by walls. The
four walls are maintained in position by four aluminum devices. The lower and upper grids are designed for each reflected configuration in order to ensure a distance of 0.8 cm (the half of the lattice pitch value) between the center of the outermost rods and the inside surface of the test screens. Figure 4 presents one of the performed experiments with 200-mm-thick aluminum walls (MIRTE 1).
Figure 4. Aluminum reflected configuration
2.2.2 Interacting configurations with large screens The experiments described in this section involve two UO2 rods lattices separated by a large
screen with a thickness ranging from 5 cm to 30 cm. The experimental device is presented in . It consists of two movable aluminum (AG3) baskets composed of two grids pierced by
10 x 62 holes to allow the rods to be installed in a rectangular lattice. The grids are designed to ensure a distance of 0.8 cm between the center of the outermost rods and the external surface of the test screen. Thus, no gap is considered between the outer edge of the grid maintaining the rods and the test screen.
Figure 5
Figure 5. Experimental device for interacting configurations with thick separator, which are not presented on this picture
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2.2.3 Interacting configurations with thin screens
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Figure 6 presents the experimental device, which is composed of four movable aluminum baskets. The square grids are pierced by 15 x 15 holes. A dedicated aluminum device was manufactured to ensure the positioning of the plates. As for the previous configurations, a distance of 0.8 cm between the center of the outermost rods and the surface of the cruciform plates is considered.
Figure 6. Experimental device for interacting configurations with thin cruciform plates, which are not presented on this picture
2.2.4 Reference experiments Similar critical configurations, referred to as reference or replacement experiments were
performed without the test screens, which can be replaced either by water or by an aluminum box filled with air. Reference experiments enable users to determine a more accurate bias due to structural materials [ 3].
Design of the reference experiments of the MIRTE 1 program was optimized using the sensitivity capabilities of the TSUNAMI-3D module of the SCALE5.1 package. These reference experiments allow more accurate quantification of the effect of the test material, because the configurations are nearly the same and the main difference between the two critical water height measurements is due only to the test material itself, even if the size of the arrays is not exactly the same.
2.2.5 Reproducibility experiments
Reproducibility experiments were performed to better assess uncertainties such as rod sampling, rod positioning, or screen positioning. Within the framework of the MIRTE 1 program, the following fifteen repeatability/reproducibility experiments were proposed:
• Repeatability experiments (Rv), which consists of a new sub-critical approach after water draining without any change in the configuration. These kinds of experiments will give information about the uncertainties on the water height measurement, the extrapolation method and the temperature.
Four kinds of reproducibility experiments (Rb), which consist of a new sub-critical approach after:
• Water draining and removal of the experimental device (support pedestal, screens and lattices…) without any change in the device (S1), allowing estimating the uncertainty on the rods positioning due to the gap between grid holes and rods,
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• Water draining, removal of the experimental device and moving the lattice baskets (S2), which highlights the uncertainty on lattice positioning,
• Water draining, removal of the experimental device (support pedestal, lattices…) and changing the rods (S3) in order to estimate the uncertainty on rod positioning,
• Water draining, removal of the experimental device, changing of the rods (other sample) and shifting of the test screens (S4) to evaluate uncertainty in screen positioning.
2.3 Fuel rods The fuel rods used in MIRTE experiments are the UO2 rods available in the Valduc facility.
Those rods are well characterized since a measurement campaign was launched in 1995 to more accurately quantify the pellet and cladding dimensions and compositions.
They are composed of low-enriched UO2 pellets (4.738 wt. % 235U) clad with Zircaloy-4. The 90-cm-high fuel column is composed of about 1.495-cm-long pellets. The overall rod length is 102 cm. Each rod has an outer radius of 0.475 cm, an inner radius of 0.418 cm and the fuel pellet radius is 0.395 cm.
2.4 Experimental uncertainties An important objective of the MIRTE program was to provide feedback to nuclear data
evaluation and criticality code package validation. Integral experiments must provide very accurate experimental data in order to achieve that objective.
Therefore, the program was designed in order to minimize the experimental uncertainties as much as possible. This was done through:
• A judicious choice of the water critical height (around 80 cm versus 90 cm for the fissile column height),
• The enforcement of strong constraints in terms of screens dimensions, planarity, parallelism and chemical composition,
• The choice of a 1.6-cm square pitch for the UO2 rods array that corresponds to optimum moderation,
• The use of 1260 well-characterized UO2 rods (dimensions and composition of pellets and claddings are well known thanks to the measurement campaign initiated in 1995),
• And the realization of repeatability/reproducibility experiments, which helped with the evaluation of some experimental uncertainties.
3 MIRTE 1 PROGRAM
3.1 Status of the program The first experiment was performed on December 2008 and the last took place at the end of June 2010. The date of each experiment was chosen after close collaboration with Valduc team considering optimization of availability of Apparatus B, costs and technical requirements such as minimizing corrosion of metallic screens. A total of 43 sub-critical approaches were realized. They are summarized in . Table 1
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MIRTE: an experimental program designed to test the reactivity worth of several structural materials
Table 1. MIRTE 1 experiments: configurations and screens dimensions*
Configurations Experiments Test Material Thickness (mm)
Length (mm)
Height (mm)
Array geometry
1A-Al-200 1A-Al-200-Rv 200 472 1000
1A-Al-050 50 322 1000 1A-Al-050-Rv
Aluminum
50 322 1000
17 x 17
1A-Verre-200 1A-Verre-200-Rb (S4) Glass 200 472 1000 18 x 18
Reflected configurations
(1 array reflected by 4
screens) R1A-Eau-200 Water >200 NA NA 17 x 17 2A-Zr4-100 Zircaloy 100 560 1000 7 x 35 2A-Al-300 Aluminum 300 992 1000 8 × 62 2A-Ni-200
2A-Ni-200-Rb (S2) Nickel 200 688 1000 9 x 43
R2A-Eau-050 50 NA NA 7 x 34 R2A-Eau-100 Water 100 NA NA 9 x 39
R2A-Air-200(Fe) 8 x 55 R2A-Air-200(Ni)
AG3 Box 200 880 1000 9 x 43
2A-Béton3-300 Concrete with 3 % water 8 x 55
2A-Béton6-300 Concrete with 6 % water 9 x 40
2A-Béton9-300 Concrete with 9 % water
300 992 1000
9 x 45
2A-Cu-050 Copper 50 720 1000 8 x 45 2A-Pb-200 200 720 1000 7 x 45 2A-Pb-050
2A-Pb-050-Rv Lead 50 608 1000 6 x 38
Interacting configurations
with large screens
(2 arrays separated by 1
screen)
2A-Fe-200 Iron 200 880 1000 8 x 55 4A-Fe-003 3 331 9 x 9 4A-Fe-020 Iron 20 444 1000 12 x 10 4A-Ni-003
4A-Ni-003-Rb (S2) Nickel 3 363 1000 10 x 9
4A-Ti-010 10 402 1000 11 x 10 4A-Ti-005
4A-Ti-005-Rv 4A-Ti-005-Rb (S2) 4A-Ti-005-Rb (S4)
Titanium 5 365 1000 10 x 10
R4A-Eau-020 20 NA NA 9 x 8 R4A-Eau-010 10 NA NA 9 x 8 R4A-Eau-005
* These dimensions correspond to specification values
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3.2 Test screens The large screens have the shape of parallelepiped blocks pierced by two holes on the top in
order to allow the screen shifting (see Figure 7). To ensure their positioning on the support pedestal for interacting configurations, they were also pierced by two holes on the bottom.
Cruciform plates were made of two thin metallic plates pierced at their bottom by a 500-mm long groove in the middle. They are then assembled as a cruciform device to be installed between the four UO2 arrays (see Figure 8).
The screens materials were chosen in close collaboration with the funding partners. The aim was to test both the screens in configurations similar to those encountered in industrial settings and to test the nuclear data (capture or scattering cross sections) in thermal spectra. The list of materials and their dimensions is given in Table 1.
Preliminary calculations were made to fix tolerance constraints on thicknesses, planarity, parallelism and perpendicularity of the different sides of the screens to ensure their positioning and lower the experimental uncertainties. In addition, TSUNAMI-3D calculations [ 5] were performed to optimize the thickness against the sensitivity to capture or scattering cross sections.
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3.2.1 Fabrication of the screens
Figure 8. Picture of the 3-mm iron plates for interacting configuration
Figure 7. : Picture of the 50-mm copper screen for large interacting configurations
3.2.2 Geometrical issues Prior to the experiments, screen dimensions were measured with a high level of precision. For
metallic, concrete, and glass screens, the method consisted of 3D measurements with respectively laser tracker, 3D machine and FARO arm (see F ). These techniques allowed determination of the thickness, length and width with a very low uncertainty (0.015 mm for the 1σ systematic uncertainty) and thus lowering the experimental uncertainties. The principle consisted in making a mesh of at least 50 measurements at different locations of the screens.
igure 9
In addition to these techniques, more traditional devices such as the micrometer called “palmer” were used for thin screens to measure the thickness and the dimensions of the grooves.
MIRTE: an experimental program designed to test the reactivity worth of several structural materials
Figure 9. 3D measurements of concrete screen with FARO arm and glass screen with 3D machine
3.2.3 Chemical issues Strong constraints were also imposed on the composition of the screens to achieve the lowest
possible experimental uncertainties. Implied was a near perfect understanding of the fabrication process. This understanding was especially important for manufacture of the concrete screens, for which the final water content was variable (3%, 6% or 9%).
Screens with 3% and 6%-water-content were obtained by drying 9%-water content screens. As a result, studies were done to investigate the homogenization of the drying process, as illustrated in
. On the left hand side figure, one can see the decrease of water content in concrete with drying time. On the right hand side figure, the profile of the water content in the screen is given versus drying time. For the fabrication of the 3%-water-content screen, it was particularly important to let the water homogenize throughout the concrete before drying at 70°C under drying bell jar.
Figure 10
Drying Homogenization
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Another issue was linked to some metallic screens (copper, iron), for which corrosion occurred
after the screen was immersed in water. The screen had to be wiped and dried after each experiment and the water was analyzed to eventually detect traces of corrosion.
Figure 10. Drying process of concrete screens
In addition, the chemical composition of all screens was systematically analyzed. Different techniques were proposed depending on the desired impurity contents, on the element to be quantified and on the screen nature. For most screens, ICP AES and ICP MS (Infrared Coupled Plasma Atomic Emission/Mass Spectrometry) techniques were used, the detection limit of the impurities being close to 10 ppm by weight of material. For scattering materials such as aluminum or zirconium, it was proposed to use a much more precise technique, GDMS (Glow Discharge Mass Spectrometry), which detects traces of impurities as low as 0.01 ppm by weight of material. These new analyses allowed demonstration that some impurities (Gd, Hf, Cd, B) which were historically assumed to be present in the structural material were in fact absent and thus allowed reduction of the overall uncertainty for zirconium, aluminum and lead screens.
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Moreover, the bulk density of each screen was measured using the pycnometry technique, which gave a high level of precision (<0.05 g/cm3 for the 1σ random uncertainty).
3.3 keff results
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igure 11 igure 12 igure 13
Calculations were performed at IRSN using the multi-group route APOLLO2-MORET 4 (CRISTAL package [ 4]), the MORET 5 continuous energy code (under validation) using JEF2.2, JEFF3.1, and ENDF/B-VII.0 libraries, the KENO-V.a [ 5] multi-group code using the ENDF/B-VI.7 library, the TRIPOLI-4 continuous energy code [ 6] with the JEF2.2 library and the MCNPX2.6 continuous energy code with the ENDF/B-VII.0 library. A comparison was systematically done between the calculated keff (C) and the experiment keff (E). Due to confidentiality issues, the results reported in F , F , and F are the differences of keff between each code and MORET 5 code using the JEF2.2 library. The aim was to test the physical models of the different codes developed by the partners and in use in the international community, the approximations of the multi-group codes, and the use of different kinds of libraries to test the nuclear data.
3.3.1 Code effect The main tendencies related to codes are the followings:
• Generally, a good agreement (C/E) is obtained with TRIPOLI-4, MORET 5 and MCNPX2.6 for configurations with and without screens,
• A slight tendency of overestimation is observed with APOLLO2-MORET 4 for configurations without screen; it can be explained by the physical models (pij calculation and self-shielding options),
• An additional tendency of overestimation is observed with APOLLO2-MORET 4 for configurations with aluminum, lead, iron, titanium, concrete and nickel; it can be explained by the multi-group treatment of the material nuclear data,
• A good agreement (C/E) is observed for zirconium in a large interacting configuration,
• A tendency to underestimate keff is shown with APOLLO2-MORET 4 for copper in a large interacting configuration,
• A general tendency to underestimate keff is also observed with KENO-V.a; this tendency can be attributed to the ENDF/VI.7 cross sections of uranium.
3.3.2 Library effect MORET 5 (pre-release version undergoing testing for validation) calculations were performed
using the ENDF/B-VII.0 library for all isotopes and various libraries (JEF2.2, JEFF3.1 and ENDF/B-VII.0) for the material of interest. Some library effects have been identified for aluminum as a reflector and for zirconium, lead, and aluminum in large interacting configurations. Given that the keff is sensitive to nuclear data (capture and scatter), those calculations highlight the discrepancy between the three libraries and enable calculation of the potential bias associated with the material nuclear data.
It should be noted that the nuclear data of aluminum and copper are identical in the JEFF3.1 and ENDF/B-VII.0 libraries.
4.1 Origin and different steps The reason for a second phase of the MIRTE program was both the additional needs of the
partners in terms of validation and the satisfaction of funding partners.
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It was planned to split the MIRTE 2 program into three phases:
• MIRTE 2.1, which began in January 2011, was composed of new experiments following the feed back of MIRTE 1 evaluation (new reproducibility experiments, new reference experiments, new critical approach with less rods in order to increase the critical water height) involving several configurations identical to MIRTE 1,
• MIRTE 2.2 will begin in 2012 and will consist of experiments with new materials such as resins (BORA or VYAL-B), Manganese, Chromium, Molybdenum, Chlorine, and Rhodium using the same experimental device as for MIRTE 1,
• MIRTE 2.3 could begin in 2013 and would consist of experiments that involve slight modifications of the experimental device; it will necessarily end before 2014 because of the Valduc Apparatus B refurbishment.
4.2 MIRTE 2.1 program: experimental issues
4.2.1 Low critical heights The exploitation of the MIRTE 1 program was the opportunity for IRSN to highlight some
potential improvements.
In fact, for some experiments, calculations with JEF2.2 library showed an underestimation of about 500 pcm. It was the case for R2A-Eau-050 reference experiment in a large interacting configuration. It was first thought that the underestimation observed for this experiment was due to its low critical height. As a consequence, the sub-critical approach was first repeated; then, a new experiment was designed in which a few rows of UO2 rods were removed, thus increasing critical water height. The same underestimation was obtained for the two experiments. The MIRTE 1 feedback was that, even if the reactivity worth slope was higher for low critical heights (around 60 cm), the MIRTE 1 experiments were sufficiently precise not to have an impact on keff. The “room return” effect could then be discarded. The underestimation was thus to be attributed to the code or libraries. After investigating nuclear data, it was shown based on calculations with MORET 5 and MCNPX that the scattering data in the JEF2.2 library was responsible for the underestimation.
4.2.2 Rod sampling effect
The calculation of copper and titanium reproducibilities in thin interacting configuration highlighted a potential rod sampling effect. For these experiments, 400 rods were chosen randomly amongst 1260. It seemed that the choice of the sample had an impact on the resulting critical height. Three additional experiments were then proposed using each of the three different sets of rods selected from the 1200 available rods. The experiments included configurations with thin cruciform screens of both titanium and copper. .
A 4 mm discrepancy on the water critical height (corresponding to a reactivity worth of 50 pcm) was obtained for one set of rods for both 4A-Cu-005 and 4A-Ti-005. It appears that the three samples are not statistically equivalent. It may be necessary to clearly identify each rod and its position in rods lattices for future programs.
4.2.3 Neutron counters positioning effect Additional feedback from the MIRTE 1 program was the existence of a potential neutron
counters positioning effect. In fact, in MIRTE 1, several critical approaches were performed for 1A-Al-050 experiment, with BF3 neutron counters, more or less sensitive, positioned either just against the aluminum screen or 170 mm away. The BF3 counters (2.5-cm diameter) are composed of either copper of aluminum tubes depending on their sensitivity and a BF3 wire at the middle of
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the tube. A 2-cm discrepancy was observed on the critical height. It was not clear whether the discrepancy could be attributed to the neutron counter position or to an eventual experimental bias. However, this effect was judged interesting for the international community and for the assessment of previous criticality programs. New experiments were then proposed in which the distance between the outermost row of UO2 rods of the lattice and the neutron counters was varied. Those experiments are still in progress. The first results tend to show that a potential effect (around 100 pcm if the counter is positioned next to the screen instead of 15 cm away) is obtained for 1A-Al-050 reflected configuration.
4.3 MIRTE 2.2 experiments
4.3.1 Needs The MIRTE partners expressed their needs in terms of structural materials (elements present in
industrial configurations or elements to be validated). The following materials were identified: Mo, Mn, Cr, Rh, Cl, BORA and VYAL-B resins.
In a first step, the MIRTE design team ensured that experiments involving the structural materials were not available in the ICSBEP Handbook or in other international criticality programs. It was decided to have them only in interacting configurations to test mostly their capture cross-sections and to a lesser extent their scattering cross-sections.
4.3.2 Design of configurations Different configurations were designed for each material of interest. The objective was to
perform experiments that are most representative of industrial configurations, regarding the reactivity worth and the sensitivity profiles. Similar to MIRTE 1, the experiments were designed with the aim to obtain a critical height close to 80 cm. The lattice sizes and material thicknesses were determined using APOLLO2-MORET 4 and TRIPOLI-4 with data libraries based on JEF2.2. The retained configurations are given in Table 2. In addition to that task, the TSUNAMI-3D sequence of SCALE 5.1 package was used to optimize sensitivities. As shown in Figure 16 for molybdenum, an intermediate screen thickness (10 mm) can lead to the maximum of sensitivity to the material.
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igure 17
As for MIRTE 1, having experiments with and without test material allows a comparison of the reactivity sensitivity coefficients. For MIRTE 2, the same reference experiments as for MIRTE 1 are used. As illustrated in F , it can be inferred that nuclear data feedback is possible since the sensitivity to the capture and scattering of molybdenum (configuration with structural material) is one order of magnitude higher than the reactivity sensitivity coefficient to fission of 235U and capture of 238U (sensitivity to the reactivity between the configuration with and without structural material).
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Figure 16. Optimization of the design of molybdenum experiments
Figure 17. Comparison to replacement experiments for molybdenum
Sensitivity difference
for 238U capture
Mo scattering Sensitivity difference
for 235U fission
Mo capture Sensitivity difference
for 235U capture
Mo Capture 4A-Mo-010
Mo Capture 4A-Mo-005 Mo Capture
4A-Mo-030
MIRTE: an experimental program designed to test the reactivity worth of several structural materials
4.3.3 Fabrication of screens and safety report At the present time, the design team and experimentalists are studying the feasibility of the
configurations in terms of screens manufacturing and costs. For some materials such as rhodium, the cost can be the determining parameter; for others, like manganese, the chemical form and the physical-chemical interaction between the screen and water need to be investigated. All of these concerns are discussed with the suppliers.
A safety report written in parallel to the feasibility studies describes the retained configurations.
Table 2. Configurations of interest for MIRTE 2.2 program
Test Materials Configurations Thickness (cm) Purpose
Chromium 4 arrays separated by 1 cruciform metallic screen 0.5 to 3 Encountered in steel
Molybdenum 4 arrays separated by 1 cruciform metallic screen 0.5 to 3 Reprocessing plants
Manganese 4 arrays separated by 1 cruciform metallic screen with or without external box 0.5 to 3 Encountered
in steel
BORA 4 arrays separated by 1 cruciform screen with or without external box 0.5 to 2 -
VYAL-B 4 arrays separated by 1 cruciform screen with or without external box 0.5 to 4 -
4 arrays separated by 1 cruciform PVC screen 1 Chlorine 2 arrays separated by 1 parallelepipedic PVC screen 2 to 8 PVC and wastes
Rhodium 2 arrays separated by 1 parallelepipedic box containing rhodium in solution 4 Reprocessing
plants
4.4 MIRTE 2.3 experiments
4.4.1 Dimensioning of MIRTE 2.2 configurations for thicker screens Other configurations that require slight modifications of the experimental device are planned in
this third phase of the MIRTE 2 program. For instance, for thicker BORA resins, it is foreseen to manufacture larger grids. The list of experiments is still open and depends on MIRTE 2.2 results.
4.4.2 Study on neutron spectrum hardening
One of the requests of the MIRTE partners was a shift of the sensitivity profiles towards the epithermal energy range. A working group was launched between IRSN and ORNL (Oak Ridge National Laboratory) to look for the design of such configurations. Several kinds of configurations were tested for that purpose:
• Pitch of the UO2 rods lattice reduced,
• Rods placed in sleeves of test material,
• Material surrounded by a lattice of UO2 rods,
• Use of HTC rods (mixed UO2-PuO2 rods),
• Alternated array of UO2 and molybdenum rods,
• Use of one oblong array of UO2 rods between two screens of test material.
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igure 18Examples of the proposed configurations are given in F and Figure 19. It can be seen in Figure 20 that the best configuration, regarding the sensitivity to molybdenum scatter in epithermal and fast energy range, is one oblong array of UO2 rods reflected by two screens.
Figure 19. Configuration with one oblong UO2 rods lattice reflected by a material on both sides
Figure 18. Configuration with Mo sleeves
Some configurations could be tested within MIRTE 2.3. IRSN has recently proposed a conceptual design for experiments that could be conducted in experimental facilities at Sandia National Laboratories (SNL) and in Apparatus B at CEA, Valduc. The design allows incremental spectrum hardening from thermal to lower intermediate range and, at the same time, to model accidental scenarios with water density variation, or mist for criticality safety assessment. The design has been proposed for Thermal Epithermal eXperimental Program (TEX) at the TEX feasibility meeting.
Mo scattering (4A-010)
Mo capture (2A-100)
Epithermal Energy range
Mo scattering (1A oblong)
Mo capture (1A oblong)
Mo scattering (2A-100) Mo capture
(4A-010)
Figure 20. Shift of the sensitivity profile to the epithermal energy range – example for molybdenum experiments
MIRTE: an experimental program designed to test the reactivity worth of several structural materials
5 CONCLUSIONS
The MIRTE international program initiated at Valduc facility in late 2008 was focused on testing reactivity effects of various structural materials in different configurations. It is designed to provide a highest possible reactivity worth of the structural material and to ensure a credible and low experimental uncertainty. The first phase involving a total of 43 experiments, called MIRTE 1, ended in June 2010. Since then, an evaluation of the program with different codes and libraries was performed. It allowed identifying various tendencies for the different materials. An ongoing work of evaluation of biases associated to the structural materials is in progress. Four experiments involving titanium and their references will be available in the 2011 ICSBEP Handbook.
A second phase of the MIRTE program, called MIRTE 2, began in January 2011 with the realization of some MIRTE 1 reproducibilities and neutron counters positioning experiments (MIRTE 2.1). About 15 experiments have already been performed and this phase should be ended in late 2011.
To answer the needs of IRSN partners and industrialists, new experiments using the same device and the same rods but different materials (Mn, Cr, Mo, Cl, BORA and VYAL-B resins) have been designed. Suppliers have already been contacted to discuss the fabrication of some structural materials. Also, a comparison between the sensitivity to the capture and scattering of the structural material in MIRTE 2 experiment and the reactivity sensitivity coefficient (sensitivity to the reactivity between MIRTE 1 reference experiment and MIRTE 2 experiment with material) to 238U (capture) and 235U (fission) was realized. It used the advanced capabilities of SCALE 5.1 code system (TSUNAMI). It was shown that no additional reference experiment was needed. Under authorization of the safety authorities, these experiments could begin no later than early 2012.
Further investigations focused at hardening the neutron spectrum and therefore implying modifications of the experimental device (change of the pitch…) have been considered. It seems, however difficult to significantly harden the spectrum without major modifications. Such experiments were proposed for feasibility study. They can be conducted at the Sandia National Laboratories (USA) within the TEX program [ 7] in 2014-2019. MIRTE 2.3 should concern slight modifications of the device, and could begin in 2013 and end no later than in 2014.
6 ACKNOWLEDGMENTS
Grateful acknowledgements are here expressed to AREVA, ANDRA and DOE, the funding partners of the program and to Tatiana Ivanova for her cautious review. The authors wish also to thank the experimentalists of the Valduc facility for their availability and their professionalism.
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