Final Report Summary - FREYA (Fast Reactor Experiments for hYbrid Applications)
Executive Summary:
The "Fast Reactor Experiments for hYbrid Applications" or "FREYA" has been a five year collaborative project within the FP7-Fission-2010 call in support of the R&D implementation of the Strategic Research Agenda of SNE-TP. It had a global budget of 5 M€ and an EC contribution of almost 2.80 M€.
The project was built with 16 partners from national research institutes and universities and subdivided in 4 technical Work Packages, namely:
• WP1: ADS on-line reactivity monitoring methodologies
• WP2: Subcritical configuration for design and licensing of MYRRHA/FASTEF
• WP3: Critical configuration for design and licensing of MYRRHA/FASTEF
• WP4: Critical configuration for LFR
Next to the technical Work Packages, there was Work Package 5 that dealt with Education & Training whereas Work Package 1 dealt with the overall management of the project.
Eighteen deliverables are written next to 22 internal scientific reports. We would like to stress that following achievements have been realized in the course of the project:
• nine publications (in journals or in international conference proceedings);
• 23 oral presentations in conferences and workshops;
• contributions to seminars and lectures;
• internal FREYA meetings (Governing Board, Project Coordination Committee, Work Package meetings, task meetings and a mid-term review meeting);
• training of students through internships, 2 PhD and 2 post-doc positions related to the FREYA project;
• development of a dedicated website.
Building on the former activities that were achieved in FP5 MUSE (FIKW-CT-2000-00063) and in FP6 EUROTRANS (FI6W-516520), the further development of the methodology for on-line reactivity monitoring was extended in the FP7 FREYA project. For the FREYA experiments, a more appropriate fast neutron lead core of VENUS-F zero power facility as well as a new GENEPI-3C accelerator working in continuous and pulsed mode was used, therefore being much more representative for lead alloy cooled ADS. The experimental programme for the validation for on-line reactivity monitoring which was initiated within the GUINEVERE experiments in FP6 EUROTRANS was completed in FP7 FREYA.
To support the design of MYRRHA facility working in sub-critical mode and in critical mode, the reactor physics experiments were executed as a follow-up of the experimental programme for on-line reactivity monitoring. These experiments were carried out in the set of VENUS-F sub-critical and critical configurations that simulated sub-critical and critical MYRRHA cores details. These details in particular were In-Pile-Sections (IPS) for Mo productions and BeO reflector. In this case, the methods for on-line reactivity monitoring tested and chosen in Work Package 1 were used to obtain the reactivity of the sub-critical VENUS-F cores. The set of the corresponding VENUS-F MYRRHA critical mock-up cores were used as the reference for MSM method and for carrying out the core characterization measurements for computational tool tests.
For the development and licensing of the ALFRED Lead Fast Reactor design, the supporting reactor physics programme based on integral measurements in a VENUS-F zero power facility was accomplished in Work Package 4. The programme included measurements of spectral indices, axial traverses and void effect of the coolant.
A one week lab session covered the issues of the experimental methods used for the measurements of the main neutronic parameters for subcritical and critical cores on the basis of the experience gained in WP1-4 at the VENUS-F facility. A lab manual was prepared and implemented at the SCK•CEN site at the end of the FREYA Project.
Project Context and Objectives:
Project context and objectives
Work Package 1: ADS on-line reactivity monitoring methodologies
The purpose of this work package was to finalize the investigations of the VENUS-F sub-critical configurations for validation of the methodologies for on-line reactivity monitoring of ADS systems. To meet the Work Package's objectives the experimental strategy was as follows: to perform measurements with the GENEPI-3C source in the 3 different modes (pulsed, continuous, continuous with interruptions) for the basic SC1 sub-critical VENUS-F configuration (keff =0.957) and its variants obtained by modifying the Control Rods insertion. Three other configurations were also studied (SC2, SC3, SC4 with keff= 0.95 0.97 0.90) to enlarge the reactivity range for testing absolute measurements. The next step consisted in testing the robustness of the investigated techniques, i.e. their sensitivity to core composition or spatial effects: with this aim the source position was changed, and other materials were introduced in the reflector as an In Pile Section (mainly composed of polythene) and Stainless Steel assemblies in corners of the reflector. The following methods of sub-criticality monitoring were investigated: pulse neutron source - PNS (several variants, but all not applicable for industrial ADS but could be useful as additional reference together with MSM), Source Jerk (SJ), current-to-flux monitor and absolute source variation measurement techniques during programmed short beam interruptions – BTM. At the end, as to determine which precision is reachable for each technique, the reference reactivity values obtained with Modified Source Multiplication MSM) method were used in order to be compared to the results.
Finally, an analysis and review of the results of the ADS reactivity monitoring methods at Work Package 1 FREYA and also of previous related 5th and 6th Framework Programmes (MUSE, YALINA and GUINEVERE) have been carried out and a methodology for on-line reactivity monitoring in ADS were determined. This methodology also has been used in Work Package 2 of the FREYA project.
This Work Package was sub-divided in four separate tasks. The first was devoted to the subcritical configurations investigations with keff=0.957. The second task was dedicated to the core studies with keff-values in the region 0.90-0.97. The third task was related to the reflector effect and vertical position of the spallation source investigations. At the end of this Work Package, a description of a validated ADS on-line reactivity monitoring methodology was given.
Work Package 2: Subcritical configuration for design and licensing of MYRRHA
This Work Package had a strong interaction with the Central Design Team FP7 Project and later MYRRHA design team in the domain of design and licensing of the MYRRHA core.
Following the objectives for MYRRHA to be operated as a subcritical facility, an experimental programme in support of the design and licensing is needed. In this case, another core configuration than in Work Package 1 of the FREYA project has been chosen and assembled for the VENUS-F facility. Within the constraints of available fuel at the VENUS facility during the FREYA project, this core was as representative as possible of the MYRRHA core design features such as fuel/"coolant" volume fractions, fuel enrichment and neutron spectrum.
Based on the new fuel assembly design and on demand of the MYRRHA core design group, sub-critical VENUS-F core named SC8-3 simulated also in-pile-sections and BeO reflector of MYRRHA with graphite were assembled and investigated. The investigations included core characterization measurements, comparing the results of the methods of the sub-criticality monitoring having been chosen in Work Package 1 and measuring the reactivity effects related with safety aspects of the core such as water ingress and fuel agglomeration effects.
Work Package 2 of the FREYA project was divided in five separate tasks. The first was devoted to the VENUS-F new core definition simulating the subcritical configuration of MYRRHA design. The second task was dedicated to the core characterisation. The third one was related to measurements of different reactivity effects including effects coming from the introduction of experimental devices in the MYRRHA mock-up subcritical core. The fourth task was devoted to the implementation of subcriticality monitoring techniques, that were recommended in WP1. Finally, the last task of WP2 was dedicated to the transposition of the achieved result in VENUS-F to a system such as MYRRHA.
Work Package 3: Critical configuration for design and licensing of MYRRHA
Work Package 3 as well as Work Package 2 had a strong interaction with the MYRRHA design team in the domain of design and licensing of the MYRRHA core.
Following the objectives for MYRRHA to be operated as a critical facility, an experimental programme in support of the design and licensing is needed.
The fuel assemblies used to build VENUS-F critical cores to simulate MYRRHE critical were the same as in the WP2 since they are the same in the MYRRHA critical and sub-critical design.
Three core configurations characteristic of MYRRHA have been investigated within Work Package 3. They have been denoted as CC5, CC7 and CC8. The CC5 configuration simulates the “clean” MYRRHA core, i.e. only containing fuel and lead elements. The main difference with respect to the configurations previously investigated within WP1 is the introduction of a new type of fuel assembly type that contains Al2O3 in order to simulate the oxygen of the MOX fuel envisaged for MYRRHA. In the CC7 configuration, some graphite assemblies were introduced in order to simulate the MYRRHA reflector. Finally, in the CC8 configuration, two polyethylene elements are inserted to simulate the thermal In-Pile Sections (IPS) of MYRRHA, intended for Mo-99 production.
In all CC5, CC7 and CC8 configurations standard characterization core measurements have been accomplished. These are axial and radial flux distributions, spectral indices, control rods worth measurements and minor actinide fission rates ratios obtained with fission chambers. Different reactivity effects such as temperature effects void effects of the coolant were investigated too.
Simulation of all these design peculiarities and measurements of their reactivity effects of the MYRRHA critical core have been estimated by recent calculational tools. C/E comparison will help to reduce the design safety margins and will help to improve the reliability of the computational tool for the licensing of the design.
Work Package 3 of the FREYA project was divided in three separate tasks. The first was devoted to the VENUS-F new core definition simulating the critical configuration of MYRRHA design. The second task was dedicated to the core characterization measurements. The third one dealt with measurements of different reactivity effects.
Work Package 4: Critical configuration for LFR
This work package had a strong link to the previous FP6 project "ELSY", the FP7 project "LEADER" and ALFRED Lead cooled Fast Reactor design.
The ALFRED mock-up configuration was defined by performing a set of simulations of different ALFRED mock up candidates with ERANOS and MCNP computer codes. The ALFRED mock up that reproduces most accurately the ALFRED neutronics was chosen: it consists essentially in a reactor island at the core boundary made as a chessboard of inert (moderating Al2O3) and fuel assemblies.
The experimental campaign performed in the ALFRED mock-up was accomplished by measuring spectral indexes, axial traverses, control rod calibration and control rod worths. Finally, the void reactivity effect was experimentally measured simulating lead voiding in the ALFRED island. The comparison between all the experimental results performed versus corresponding simulations with ERANOS, MCNP and SERPENT code was also performed and quantified in terms of C/E values. This C/E comparison will help to reduce the design safety margins of the LFR and can significantly contribute to the LFR development.
Work Package 4 of the FREYA project was divided in three separate tasks. The first was devoted to the VENUS-F new core definition simulated LFR design. The second task was dedicated to the core characterisation. The third task was devoted to measurements of the reactivity effects and all C/E analysis in the LFR mock-up core.
Work Package 5: Training and Education
This Work Package had a strong link to all others FREYA Work Packages since it used the experience gained during the different WP's for training and education. The scope of activity of Work Package 5 included:
• Development of lab sessions contents,
• Organization of one week pilot lab session,
• Organization of a dissemination seminar.
The first task was devoted to the preparation of the contents of the training session. The contents of the lab session covered the issues of the experimental methods used for the measurements of the main neutronic parameters for subcritical and critical cores on the base of the experience gained in Work Packages 1 to 4 at the VENUS-F facility. These contents were prepared and distributed between the students one month before the lab session. In the second task of Work Package 5, a one week lab session has been implemented at the SCK•CEN site. The third task, dedicated to the organization of a Dissemination Seminar for a wider international audience on the results of the Project will be implemented at the end of 2016 together with the H2020 MYRTE WP5 kick-off meeting.
Project Results:
Please see document attached.
Potential Impact:
Potential impact (socio-economic and wider societal implications)
In the conclusions of Work Package 1, one notices that concerning the absolute measurement of reactivity during an ADS operation, i.e. during a beam interruption, we have highlighted the importance of spatial effects on detector kinetic responses when U5 fission chambers are used in the reflector (no effect is seen for a U5 fission chamber in the core, close to the source). These effects increase with the distance to the neutron source. They are mainly due to a delayed component of the flux coming from the core surroundings acting like a large reflector. Fortunately, reactivities deduced from such detectors can be corrected by applying correction factors computed with neutron transport code if the geometry around the core is correctly described. However, it was checked that these computed factors do not require a very detailed description of the reactor components (some parts can be homogenized) and are robust to the absolute reactivity value of the model within 1500 pcm. In these conditions the absolute reactivity measurements (during 2 ms beam interruptions) based on inverse point kinetics give results that are in perfect agreement with the reactivity values obtained from an MSM technique. However, if one wants to avoid the use of correction factors, we recommend the use of threshold fission chambers that are not sensitive to the low energy component of the neutron flux coming back from the reflector materials. Promising results were obtained with a U8 fission chamber. The relative monitoring of the reactivity thanks to the on-line measurement of the “current-to-flux” ratio in continuous beam phases was also validated when the neutron source intensity is taken instead of the beam current. Concerning reactivity assessment at the end of a core loading phase, good results were also obtained with area methods when spatial effects are taken into account, in case a pulsed source would be available. At last, on the basis of these results our recommendations for the beam interruption structure of the MYRRHA facility are to keep the possibility for durations in the 200 μs - 2 ms range, with frequencies lower than 250 Hz.
Based on the new fuel assembly design and on demand of the MYRRHA core design group, sub-critical VENUS-F core named SC8-3 simulated in-pile-sections and BeO reflector of MYRRHA with graphite was assembled and investigated in Work Package 2. The investigations included core characterization measurements, a comparison of the results of the methods of the sub-criticality monitoring and measuring the reactivity effects related with safety aspects of the core.
Based on the knowledge gathered during Work Package 2, the proposed techniques for MYRRHA sub-criticality monitoring are:
• Starting from the critical core – MSM method (insertion of a control rod and static counting with an external radioisotope source);
• During the start-up range (core loading and operation until 1% of full power) – source jerk in beam-trip mode as the main method and integral source jerk as an alternative method;
• During the intermediate and power range – current-to-flux as a relative method for quick detection of reactivity changes combined with source jerk in beam-trip mode as a method providing absolute value for periodical re-calibration of the current-to-flux technique.
A set of threshold detectors operating in pulse/fluctuation/current mode is proposed to be distributed in the MYRRHA reflector zone with the axial position in the core mid-plane. During the loading phase and start-up range, it is suggested to also use one in-core detector:
• Concerning nuclear data used in Monte Carlo and ERANOS codes, several clear trends have been observed;
• Calculations overestimate keff by 200-1100 pcm depending on o nuclear data library used;
• Calculation underestimate control and safety rod worth by 10-15% in case 6-group delayed neutron parameters are used and by 5-10% in case 8-group delayed neutron parameters are used;
• Calculated and experimental spectral indices show discrepancies for 238U, 240Pu, 242Pu, 237Np and 241Am to 235U indices;
• Measured and calculated spatial fission rate distributions are generally in a good agreement;
• Calculations can approximately predict the trend of the coolant void effect but they underestimate the experiment in absolute terms;
• The rate of the temperature effect calculated with MCNP is in agreement with the measured one.
Given the objectives for MYRRHA to be operated as a subcritical and critical facility, all outcome from Work Package 3 has provided a huge experimental data bank in support of the design and licensing of MYRRHA operating in critical mode. Experiments covered different areas such as neutron spectrum characterization for different core configurations and coolant void and temperature reactivity effects (important for the safety reports of a new installation). The extensive E/C comparison performed with both Monte Carlo and deterministic calculation methods results, and with different neutron data libraries, provided clear indications about the degree of performance of the current computational tool and data, together with the areas needed for further studies in support of the licensing of MYRRHA operating in critical mode.
The measurement campaign performed within FREYA Work Package 4, with the corresponding experimental results and their comparison with 3D simulation models for different computer codes, represent a useful set of data and models for the scheduled activities supporting the ALFRED LFR core design optimization and licensing phases.
After the FREYA training course “Measuring Reactivity in Fast-Neutron Systems” the students will be able to:
• Define the concept of the Monte Carlo method;
• Formulate the difference between the deterministic and stochastic approaches in reactor analysis;
• Define the concept of reactivity and its role in reactor analysis;
• Formulate the importance of the delayed neutrons in thermal and fast reactors;
• Define the concept of effective fraction of delayed neutrons in thermal and fast reactors;
• Formulate, solve and analyze the point kinetic equations.
In terms of skills:
• Select appropriate neutron detectors depending on the measurement purpose and detector characteristics;
• Determine the safety margins in terms of the fraction of delayed neutrons;
• Apply various methods and techniques to evaluate reactivity of nuclear setups;
• Evaluate uncertainty margins;
• Run a Monte Carlo code such as MCNP or Serpent;
• Participate in conferences and meetings devoted to criticality measurements;
• Discuss advantages and disadvantages of various reactivity evaluation techniques;
• Be members of expert groups on online reactivity monitoring.
List of Websites:
http://freya.sckcen.be/
Dr. Anatoly Kochetkov, Institute for Advanced Nuclear Systems
SCK•CEN, Boeretang 200, BE-2400 MOL, Belgium,
Email: anatoly.kochetkov@sckcen.be , Tel. +32 14 33 21 93
The "Fast Reactor Experiments for hYbrid Applications" or "FREYA" has been a five year collaborative project within the FP7-Fission-2010 call in support of the R&D implementation of the Strategic Research Agenda of SNE-TP. It had a global budget of 5 M€ and an EC contribution of almost 2.80 M€.
The project was built with 16 partners from national research institutes and universities and subdivided in 4 technical Work Packages, namely:
• WP1: ADS on-line reactivity monitoring methodologies
• WP2: Subcritical configuration for design and licensing of MYRRHA/FASTEF
• WP3: Critical configuration for design and licensing of MYRRHA/FASTEF
• WP4: Critical configuration for LFR
Next to the technical Work Packages, there was Work Package 5 that dealt with Education & Training whereas Work Package 1 dealt with the overall management of the project.
Eighteen deliverables are written next to 22 internal scientific reports. We would like to stress that following achievements have been realized in the course of the project:
• nine publications (in journals or in international conference proceedings);
• 23 oral presentations in conferences and workshops;
• contributions to seminars and lectures;
• internal FREYA meetings (Governing Board, Project Coordination Committee, Work Package meetings, task meetings and a mid-term review meeting);
• training of students through internships, 2 PhD and 2 post-doc positions related to the FREYA project;
• development of a dedicated website.
Building on the former activities that were achieved in FP5 MUSE (FIKW-CT-2000-00063) and in FP6 EUROTRANS (FI6W-516520), the further development of the methodology for on-line reactivity monitoring was extended in the FP7 FREYA project. For the FREYA experiments, a more appropriate fast neutron lead core of VENUS-F zero power facility as well as a new GENEPI-3C accelerator working in continuous and pulsed mode was used, therefore being much more representative for lead alloy cooled ADS. The experimental programme for the validation for on-line reactivity monitoring which was initiated within the GUINEVERE experiments in FP6 EUROTRANS was completed in FP7 FREYA.
To support the design of MYRRHA facility working in sub-critical mode and in critical mode, the reactor physics experiments were executed as a follow-up of the experimental programme for on-line reactivity monitoring. These experiments were carried out in the set of VENUS-F sub-critical and critical configurations that simulated sub-critical and critical MYRRHA cores details. These details in particular were In-Pile-Sections (IPS) for Mo productions and BeO reflector. In this case, the methods for on-line reactivity monitoring tested and chosen in Work Package 1 were used to obtain the reactivity of the sub-critical VENUS-F cores. The set of the corresponding VENUS-F MYRRHA critical mock-up cores were used as the reference for MSM method and for carrying out the core characterization measurements for computational tool tests.
For the development and licensing of the ALFRED Lead Fast Reactor design, the supporting reactor physics programme based on integral measurements in a VENUS-F zero power facility was accomplished in Work Package 4. The programme included measurements of spectral indices, axial traverses and void effect of the coolant.
A one week lab session covered the issues of the experimental methods used for the measurements of the main neutronic parameters for subcritical and critical cores on the basis of the experience gained in WP1-4 at the VENUS-F facility. A lab manual was prepared and implemented at the SCK•CEN site at the end of the FREYA Project.
Project Context and Objectives:
Project context and objectives
Work Package 1: ADS on-line reactivity monitoring methodologies
The purpose of this work package was to finalize the investigations of the VENUS-F sub-critical configurations for validation of the methodologies for on-line reactivity monitoring of ADS systems. To meet the Work Package's objectives the experimental strategy was as follows: to perform measurements with the GENEPI-3C source in the 3 different modes (pulsed, continuous, continuous with interruptions) for the basic SC1 sub-critical VENUS-F configuration (keff =0.957) and its variants obtained by modifying the Control Rods insertion. Three other configurations were also studied (SC2, SC3, SC4 with keff= 0.95 0.97 0.90) to enlarge the reactivity range for testing absolute measurements. The next step consisted in testing the robustness of the investigated techniques, i.e. their sensitivity to core composition or spatial effects: with this aim the source position was changed, and other materials were introduced in the reflector as an In Pile Section (mainly composed of polythene) and Stainless Steel assemblies in corners of the reflector. The following methods of sub-criticality monitoring were investigated: pulse neutron source - PNS (several variants, but all not applicable for industrial ADS but could be useful as additional reference together with MSM), Source Jerk (SJ), current-to-flux monitor and absolute source variation measurement techniques during programmed short beam interruptions – BTM. At the end, as to determine which precision is reachable for each technique, the reference reactivity values obtained with Modified Source Multiplication MSM) method were used in order to be compared to the results.
Finally, an analysis and review of the results of the ADS reactivity monitoring methods at Work Package 1 FREYA and also of previous related 5th and 6th Framework Programmes (MUSE, YALINA and GUINEVERE) have been carried out and a methodology for on-line reactivity monitoring in ADS were determined. This methodology also has been used in Work Package 2 of the FREYA project.
This Work Package was sub-divided in four separate tasks. The first was devoted to the subcritical configurations investigations with keff=0.957. The second task was dedicated to the core studies with keff-values in the region 0.90-0.97. The third task was related to the reflector effect and vertical position of the spallation source investigations. At the end of this Work Package, a description of a validated ADS on-line reactivity monitoring methodology was given.
Work Package 2: Subcritical configuration for design and licensing of MYRRHA
This Work Package had a strong interaction with the Central Design Team FP7 Project and later MYRRHA design team in the domain of design and licensing of the MYRRHA core.
Following the objectives for MYRRHA to be operated as a subcritical facility, an experimental programme in support of the design and licensing is needed. In this case, another core configuration than in Work Package 1 of the FREYA project has been chosen and assembled for the VENUS-F facility. Within the constraints of available fuel at the VENUS facility during the FREYA project, this core was as representative as possible of the MYRRHA core design features such as fuel/"coolant" volume fractions, fuel enrichment and neutron spectrum.
Based on the new fuel assembly design and on demand of the MYRRHA core design group, sub-critical VENUS-F core named SC8-3 simulated also in-pile-sections and BeO reflector of MYRRHA with graphite were assembled and investigated. The investigations included core characterization measurements, comparing the results of the methods of the sub-criticality monitoring having been chosen in Work Package 1 and measuring the reactivity effects related with safety aspects of the core such as water ingress and fuel agglomeration effects.
Work Package 2 of the FREYA project was divided in five separate tasks. The first was devoted to the VENUS-F new core definition simulating the subcritical configuration of MYRRHA design. The second task was dedicated to the core characterisation. The third one was related to measurements of different reactivity effects including effects coming from the introduction of experimental devices in the MYRRHA mock-up subcritical core. The fourth task was devoted to the implementation of subcriticality monitoring techniques, that were recommended in WP1. Finally, the last task of WP2 was dedicated to the transposition of the achieved result in VENUS-F to a system such as MYRRHA.
Work Package 3: Critical configuration for design and licensing of MYRRHA
Work Package 3 as well as Work Package 2 had a strong interaction with the MYRRHA design team in the domain of design and licensing of the MYRRHA core.
Following the objectives for MYRRHA to be operated as a critical facility, an experimental programme in support of the design and licensing is needed.
The fuel assemblies used to build VENUS-F critical cores to simulate MYRRHE critical were the same as in the WP2 since they are the same in the MYRRHA critical and sub-critical design.
Three core configurations characteristic of MYRRHA have been investigated within Work Package 3. They have been denoted as CC5, CC7 and CC8. The CC5 configuration simulates the “clean” MYRRHA core, i.e. only containing fuel and lead elements. The main difference with respect to the configurations previously investigated within WP1 is the introduction of a new type of fuel assembly type that contains Al2O3 in order to simulate the oxygen of the MOX fuel envisaged for MYRRHA. In the CC7 configuration, some graphite assemblies were introduced in order to simulate the MYRRHA reflector. Finally, in the CC8 configuration, two polyethylene elements are inserted to simulate the thermal In-Pile Sections (IPS) of MYRRHA, intended for Mo-99 production.
In all CC5, CC7 and CC8 configurations standard characterization core measurements have been accomplished. These are axial and radial flux distributions, spectral indices, control rods worth measurements and minor actinide fission rates ratios obtained with fission chambers. Different reactivity effects such as temperature effects void effects of the coolant were investigated too.
Simulation of all these design peculiarities and measurements of their reactivity effects of the MYRRHA critical core have been estimated by recent calculational tools. C/E comparison will help to reduce the design safety margins and will help to improve the reliability of the computational tool for the licensing of the design.
Work Package 3 of the FREYA project was divided in three separate tasks. The first was devoted to the VENUS-F new core definition simulating the critical configuration of MYRRHA design. The second task was dedicated to the core characterization measurements. The third one dealt with measurements of different reactivity effects.
Work Package 4: Critical configuration for LFR
This work package had a strong link to the previous FP6 project "ELSY", the FP7 project "LEADER" and ALFRED Lead cooled Fast Reactor design.
The ALFRED mock-up configuration was defined by performing a set of simulations of different ALFRED mock up candidates with ERANOS and MCNP computer codes. The ALFRED mock up that reproduces most accurately the ALFRED neutronics was chosen: it consists essentially in a reactor island at the core boundary made as a chessboard of inert (moderating Al2O3) and fuel assemblies.
The experimental campaign performed in the ALFRED mock-up was accomplished by measuring spectral indexes, axial traverses, control rod calibration and control rod worths. Finally, the void reactivity effect was experimentally measured simulating lead voiding in the ALFRED island. The comparison between all the experimental results performed versus corresponding simulations with ERANOS, MCNP and SERPENT code was also performed and quantified in terms of C/E values. This C/E comparison will help to reduce the design safety margins of the LFR and can significantly contribute to the LFR development.
Work Package 4 of the FREYA project was divided in three separate tasks. The first was devoted to the VENUS-F new core definition simulated LFR design. The second task was dedicated to the core characterisation. The third task was devoted to measurements of the reactivity effects and all C/E analysis in the LFR mock-up core.
Work Package 5: Training and Education
This Work Package had a strong link to all others FREYA Work Packages since it used the experience gained during the different WP's for training and education. The scope of activity of Work Package 5 included:
• Development of lab sessions contents,
• Organization of one week pilot lab session,
• Organization of a dissemination seminar.
The first task was devoted to the preparation of the contents of the training session. The contents of the lab session covered the issues of the experimental methods used for the measurements of the main neutronic parameters for subcritical and critical cores on the base of the experience gained in Work Packages 1 to 4 at the VENUS-F facility. These contents were prepared and distributed between the students one month before the lab session. In the second task of Work Package 5, a one week lab session has been implemented at the SCK•CEN site. The third task, dedicated to the organization of a Dissemination Seminar for a wider international audience on the results of the Project will be implemented at the end of 2016 together with the H2020 MYRTE WP5 kick-off meeting.
Project Results:
Please see document attached.
Potential Impact:
Potential impact (socio-economic and wider societal implications)
In the conclusions of Work Package 1, one notices that concerning the absolute measurement of reactivity during an ADS operation, i.e. during a beam interruption, we have highlighted the importance of spatial effects on detector kinetic responses when U5 fission chambers are used in the reflector (no effect is seen for a U5 fission chamber in the core, close to the source). These effects increase with the distance to the neutron source. They are mainly due to a delayed component of the flux coming from the core surroundings acting like a large reflector. Fortunately, reactivities deduced from such detectors can be corrected by applying correction factors computed with neutron transport code if the geometry around the core is correctly described. However, it was checked that these computed factors do not require a very detailed description of the reactor components (some parts can be homogenized) and are robust to the absolute reactivity value of the model within 1500 pcm. In these conditions the absolute reactivity measurements (during 2 ms beam interruptions) based on inverse point kinetics give results that are in perfect agreement with the reactivity values obtained from an MSM technique. However, if one wants to avoid the use of correction factors, we recommend the use of threshold fission chambers that are not sensitive to the low energy component of the neutron flux coming back from the reflector materials. Promising results were obtained with a U8 fission chamber. The relative monitoring of the reactivity thanks to the on-line measurement of the “current-to-flux” ratio in continuous beam phases was also validated when the neutron source intensity is taken instead of the beam current. Concerning reactivity assessment at the end of a core loading phase, good results were also obtained with area methods when spatial effects are taken into account, in case a pulsed source would be available. At last, on the basis of these results our recommendations for the beam interruption structure of the MYRRHA facility are to keep the possibility for durations in the 200 μs - 2 ms range, with frequencies lower than 250 Hz.
Based on the new fuel assembly design and on demand of the MYRRHA core design group, sub-critical VENUS-F core named SC8-3 simulated in-pile-sections and BeO reflector of MYRRHA with graphite was assembled and investigated in Work Package 2. The investigations included core characterization measurements, a comparison of the results of the methods of the sub-criticality monitoring and measuring the reactivity effects related with safety aspects of the core.
Based on the knowledge gathered during Work Package 2, the proposed techniques for MYRRHA sub-criticality monitoring are:
• Starting from the critical core – MSM method (insertion of a control rod and static counting with an external radioisotope source);
• During the start-up range (core loading and operation until 1% of full power) – source jerk in beam-trip mode as the main method and integral source jerk as an alternative method;
• During the intermediate and power range – current-to-flux as a relative method for quick detection of reactivity changes combined with source jerk in beam-trip mode as a method providing absolute value for periodical re-calibration of the current-to-flux technique.
A set of threshold detectors operating in pulse/fluctuation/current mode is proposed to be distributed in the MYRRHA reflector zone with the axial position in the core mid-plane. During the loading phase and start-up range, it is suggested to also use one in-core detector:
• Concerning nuclear data used in Monte Carlo and ERANOS codes, several clear trends have been observed;
• Calculations overestimate keff by 200-1100 pcm depending on o nuclear data library used;
• Calculation underestimate control and safety rod worth by 10-15% in case 6-group delayed neutron parameters are used and by 5-10% in case 8-group delayed neutron parameters are used;
• Calculated and experimental spectral indices show discrepancies for 238U, 240Pu, 242Pu, 237Np and 241Am to 235U indices;
• Measured and calculated spatial fission rate distributions are generally in a good agreement;
• Calculations can approximately predict the trend of the coolant void effect but they underestimate the experiment in absolute terms;
• The rate of the temperature effect calculated with MCNP is in agreement with the measured one.
Given the objectives for MYRRHA to be operated as a subcritical and critical facility, all outcome from Work Package 3 has provided a huge experimental data bank in support of the design and licensing of MYRRHA operating in critical mode. Experiments covered different areas such as neutron spectrum characterization for different core configurations and coolant void and temperature reactivity effects (important for the safety reports of a new installation). The extensive E/C comparison performed with both Monte Carlo and deterministic calculation methods results, and with different neutron data libraries, provided clear indications about the degree of performance of the current computational tool and data, together with the areas needed for further studies in support of the licensing of MYRRHA operating in critical mode.
The measurement campaign performed within FREYA Work Package 4, with the corresponding experimental results and their comparison with 3D simulation models for different computer codes, represent a useful set of data and models for the scheduled activities supporting the ALFRED LFR core design optimization and licensing phases.
After the FREYA training course “Measuring Reactivity in Fast-Neutron Systems” the students will be able to:
• Define the concept of the Monte Carlo method;
• Formulate the difference between the deterministic and stochastic approaches in reactor analysis;
• Define the concept of reactivity and its role in reactor analysis;
• Formulate the importance of the delayed neutrons in thermal and fast reactors;
• Define the concept of effective fraction of delayed neutrons in thermal and fast reactors;
• Formulate, solve and analyze the point kinetic equations.
In terms of skills:
• Select appropriate neutron detectors depending on the measurement purpose and detector characteristics;
• Determine the safety margins in terms of the fraction of delayed neutrons;
• Apply various methods and techniques to evaluate reactivity of nuclear setups;
• Evaluate uncertainty margins;
• Run a Monte Carlo code such as MCNP or Serpent;
• Participate in conferences and meetings devoted to criticality measurements;
• Discuss advantages and disadvantages of various reactivity evaluation techniques;
• Be members of expert groups on online reactivity monitoring.
List of Websites:
http://freya.sckcen.be/
Dr. Anatoly Kochetkov, Institute for Advanced Nuclear Systems
SCK•CEN, Boeretang 200, BE-2400 MOL, Belgium,
Email: anatoly.kochetkov@sckcen.be , Tel. +32 14 33 21 93
