Nuclear Energy Agency (NEA)
NEA/WKP(2023)3
June 2024
www.oecd-nea.org

NEA WORKING PAPER
Traditional and Novel
Measurement Methods for
Validation of Multi-Physics
Modelling and Simulation Tools
Report on Sub-Task 6 of EGMUP
Task Force 1
A Working Paper by the Expert Group on
Reactor Systems Multi-Physics (EGMUP)
Mathieu HURSIN
Bruno BIARD
Christophe DESTOUCHES
Benoit GESLOTJean Pascal HUDELOT
Laurent PANTERA
Guillaume RICCIARDI
Nicolas THIOLLAY

Nuclear Energy Agency

NEA/WKP(2023)3
Unclassified

English text only
17 June 2024

OECD Nuclear Energy Agency
STEERING COMMITTEE FOR NUCLEAR ENERGY

Traditional and Novel Measurement Methods for Validation of Multi-Physics
Modelling and Simulation Tools
Report on Sub-Task 6 of EGMUP Task Force 1
A Working Paper by the Expert Group on Reactor Systems Multi-Physics (EGMUP)

OECD NEA Working Papers should not be reported as representing the official views of themember countries of the OECD or its Nuclear Energy Agency. The opinions expressed andarguments employed are those of the author(s).
Authorised for publication by William D. Magwood, IV, Director-General, OECD Nuclear
Energy Agency.

Mathieu Hursin, PSI, Switzerland
Bruno Biard, Christophe Destouches, Benoit Geslot, Jean Pascal Hudelot, Laurent Pantera,
Guillaume Ricciardi, Nicolas Thiollay, CEA, France

JT03546322
OFDE

This document, as well as any data and map included herein, are without prejudice to the status of or sovereignty over any territory,
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2  NEA/WKP(2023)3
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TRADITIONAL AND NOVEL MEASUREMENT METHODS FOR VALIDATION OF MULTI-PHYSICS MODELLING AND SIMULATION TOOLSNEA/WKP(2023)3  3Foreword
The Nuclear Energy Agency (NEA) Nuclear Science Committee (NSC) mandated theExpert Group on Multi-Physics Experimental Data, Benchmarks and Validation(EGMPEBV) in 2014 to provide oversight of activities associated with the certification ofexperimental data and development of benchmark models for validation of multi-physics(MP) modelling and simulation (M&S) computational systems. In addition, this Expert
Group was seeking to establish appropriate processes and procedures for the use of data
and benchmark models for validation of M&S tools and data.
The EGMPEBV was organised into two task forces:
1. Evaluation of the status, expected needs, major challenges and priorities for thevalidation of MP M&S tools.
2. Providing best practices guidance for development of models for validation,
identifying needs for specific experiments with the intended purpose of validating
multi-physics M&S tools and data.
The EGMPEBV mandate expired in 2020 and its scope became part of the activities of theExpert Group on Reactor Systems Multi-Physics (EGMUP) supervised by the Working
Party on Scientific Issues and Uncertainty Analysis of Reactor Systems (WPRS).
This working paper has been developed to support the Task Force 1 activities in providing
an overview of the challenges with traditional and novel measurement methods forvalidation of multi-physics M&S tools.

TRADITIONAL AND NOVEL MEASUREMENT METHODS FOR VALIDATION OF MULTI-PHYSICS MODELLING AND SIMULATION TOOLS4  NEA/WKP(2023)3

Acknowledgements
The NEA expresses its sincere gratitude to Mathieu Hursin (PSI, Switzerland), who coordinated this working paper, and to Bruno Biard, Christophe Destouches, Benoit Geslot,
Jean Pascal Hudelot, Laurent Pantera, Guillaume Ricciardi and Nicolas Thiollay (CEA,
France) for their contributions.
Special thanks are also due to the members of the Expert Group on Multi-physics
Experimental Data, Benchmarks and Validation for their comments, review, and guidance.

TRADITIONAL AND NOVEL MEASUREMENT METHODS FOR VALIDATION OF MULTI-PHYSICS MODELLING AND SIMULATION TOOLSNEA/WKP(2023)3  5

Table of contents
List of abbreviations and acronyms ..................................................................................................... 7
Executive summary ............................................................................................................................. 10
1. Introduction ..................................................................................................................................... 12
2. Definition of a measurement........................................................................................................... 14
2.1. Reporting a measurement ........................................................................................................... 15
2.2. Determination of a reliable experimental uncertainty................................................................. 15
3. Process to develop an experiment dedicated to the validation of MP tools ................................ 17
3.1. Step 1: Determination of the validation requirements ................................................................ 17
3.2. Step 2: Expression of the modelling needs for code validation .................................................. 173.3. Step 3: Review of the existing measurements for the considered problem ................................ 18
3.4. Step 4: Selection of an existing solution or potential candidates to address the experimental
needs ........................................................................................................................................... 19
4. Demonstration of the methodology for PCMI .............................................................................. 21
4.1. Steps 1 and 2: Validation requirements and expression of modelling needs .............................. 21
4.2. Step 3: Review of the state-of-the-art of current instrumentation and experimental techniques 22
4.2.1. Gamma heating .................................................................................................................... 22
4.2.2. Nuclear fission power........................................................................................................... 25
4.2.3. Fast flux ................................................................................................................................ 27
4.2.4. Pellet swelling, cracking and densification .......................................................................... 27
4.2.5. Fission gas release ................................................................................................................ 29
4.2.6. Friction/bounding layer of gap ............................................................................................. 30
4.2.7. Cladding properties .............................................................................................................. 30
4.2.8. Influence parameters ............................................................................................................ 31
4.2.9. Electronic and data acquisition systems ............................................................................... 31
4.2.10. Scientific documentation .................................................................................................... 32
4.3. Step 4: Selection of an existing solution or potential candidates to address the experimental
needs ........................................................................................................................................... 32
4.3.1. Required improvements ....................................................................................................... 32
4.3.2. Recommendations for the future instrumentation and experimental techniques ................. 33
5. Conclusions ...................................................................................................................................... 40
References ............................................................................................................................................ 41

List of figures
Figure 1. Overall process to complete an experiment
Figure 2. Phenomena identification and ranking table (PIRT) decomposition of pellet-clad mechanicalinteraction (PCMI)
Figure 3. Novel measurement approaches for the phenomena associated with the cladding during PCMI
Figure 4. Schematic drawing of the SCK CEN gamma thermometer
Figure 5. Cross-section of the CALMOS-2 calorimetric probe (dimensions in mm)
Figure 6. Typical SPND/SPGD
Figure 7. Example schematic and photo of fission chambers
Figure 8. Representative activation foils and wires
Figure 9. Schematic of the CABRI Hodoscope

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23
24
24
25
26
27

TRADITIONAL AND NOVEL MEASUREMENT METHODS FOR VALIDATION OF MULTI-PHYSICS MODELLING AND SIMULATION TOOLS6  NEA/WKP(2023)3
Figure 10. Schematics of theTREAT Hodoscope
Figure 11. Pre- and post-neutron and gamma scanning
Figure 12. Different applications of the LVDT sensors

28
29
30

TRADITIONAL AND NOVEL MEASUREMENT METHODS FOR VALIDATION OF MULTI-PHYSICS MODELLING AND SIMULATION TOOLSNEA/WKP(2023)3  7

List of abbreviations and acronyms
ADC

Analogue to digital converter

BR1

Belgian Reactor 1 (Belgium)

BR2

Belgian Reactor 2 (Belgium)

CABRI

Research reactor (IRSN, France)

CALMAR

Spectrum adjustment code (CEA, France)

CALMOS

Calorimetric measurement system (OSIRIS, CEA, France)

CASL

Consortium for Advanced Simulation of Light Water Reactors (United States)

CEA

Atomic Energy and Alternative Energies Commission (France)

CROCUS

Zero-power research reactor (EPFL, Switzerland)

DNB

Departure from nucleate boiling

EGMPEBV

Expert Group on Multi-physics Experimental Data, Benchmarks and Validation
(NEA)

EGMUP

Expert Group on Reactor Systems Multi-Physics (NEA)

EGRFPExpert Group on Reactor Fuel Performance (NEA)

EPFL

Swiss Federal Institute of Technology (Switzerland)FC

Fission chamber

FPGA

Field programmable gate array

FWHM

Full width at half maximum

HANARO

High-flux advanced neutron application reactor (Korea)

HPGe

High-purity germanium detectors

ICSBEPInternational Criticality Safety Benchmark Evaluation Project (NEA)

IEA

International Energy Agency

IFE

Institute for Energy (Norway)

INL

Idaho National Laboratory (United States)

IRPhE

International Reactor Physics Experiments Project (NEA)

IRSN

Institute for Radiation Protection and Nuclear Safety (France)

JHR

Jules Horowitz Reactor (France)

LOCA

Loss-of-coolant accident

LVDT

Linear variable differential transformer

LTO

Long-term operation

LWR

Light water reactor

MADERE

Measurement applied to dosimetry for reactors experiment platform (CABRI,
IRSN, France)

TRADITIONAL AND NOVEL MEASUREMENT METHODS FOR VALIDATION OF MULTI-PHYSICS MODELLING AND SIMULATION TOOLS8  NEA/WKP(2023)3
MALIBURadiochemical Analysis of MOX and UOX LWR fuels irradiated to high burn-up
experimental programme (Belgonucléaire, Belgium)

MELODIE

Experiment for online cladding creep studies (OSIRIS, CEA, France)

MFC

Micro-fission chamber

MONACOMultichannel acquisition system for neutron and gamma measurements (CEA,
France)MOX

Uranium-plutonium mixed oxide fuelMP

Multi-physics

MPGD

Micro pattern gas detectors

M&S

Modelling and simulations

MTR

Materials test reactor

NEA

Nuclear Energy Agency

NEAMS

Nuclear energy advanced modelling and simulation programme (Department ofEnergy, United States)

NSC

Nuclear Science Committee (NEA)

NSRR

Nuclear safety research reactor (Japan)

NURESAFE

Nuclear reactor safety simulation platform project (European Commission)

OECD

Organisation for Economic Co-operation and Development

OSIRIS

Open pool research reactor (CEA, France)

OSLD

Optically stimulated luminescence detector

PARC

Phenomenon assessment and ranking chart

PCMI

Pellet-clad mechanical interaction

PIE

Post-irradiation experiment

PIRT

Phenomenon identification ranking table

PSI

Paul Scherrer Institute (Switzerland)

PWR

Pressurised water reactor

QOI

Quantity of interest

QPRT

Quantitative phenomena ranking table

RIA

Reactivity-initiated accident

SA

Sensitivity analysis

SPGD

Self-powered gamma detector

SPND

Self-powered neutron detector

ST

Sub task

TF

Task force

TLD

Thermoluminescent dosimeter

TREATTransient reactor test facility (INL, United States)

TRADITIONAL AND NOVEL MEASUREMENT METHODS FOR VALIDATION OF MULTI-PHYSICS MODELLING AND SIMULATION TOOLSNEA/WKP(2023)3  9

TRIPOLI

Monte Carlo radiation transport code (CEA, France)

TTL

Transistor-transistor logic

UOX

Uranium oxide

US

United States

WPRS

Working Party on Scientific Issues and Uncertainty Analysis of Reactor Systems
(NEA)

ZPR

Zero power reactor

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Executive summary
Novel modelling and simulations (M&S) capabilities in nuclear engineering offer
comprehensive insights into physical phenomena with unpreceded spatial and temporal
resolution and present new opportunities for the designer. The availability of dependable
numerical predictions and complex sensitivity analyses form the foundation for swift,
iterative design processes that lead to heightened safety margins and improved economics
for new designs. Additionally, these advanced M&S capabilities yield morecomprehensive, well-informed, and less conservative safety assessments of existing
designs to support, e.g. nuclear long-term operation (LTO) by lifetime extension, power
uprates of nuclear power plants, and higher fuel burn-up, which are key economic
improvement factors for the operation of the current fleet of nuclear power plants (IEA/
NEA, 2020).
While the pronounced benefits of novel M&S tools, bringing much improved spatial andtemporal resolutions, are evident, a validation challenge looms large. The heightened
precision inherent to these tools necessitates the concurrent availability of high-resolution
experiments for their validation process. However, there is a lack of both experimental data
and reliable estimates of their associated uncertainties, both of which are crucial for
validation purposes. Furthermore, it is often evident that a disconnect exists between themeasured physical quantity and the computed values, necessitating a complex
transformation process that can inadvertently introduce biases and additional uncertainties.
This underscores the imperative for additional experimental data and innovative
experiments.
The target audience of this working paper are experts in M&S of nuclear reactor systems,
experimentalists, operators of research facilities and data curators. The working paper
serves as input to the recommendation making process within the Expert Group on Reactor
Systems Multi-Physics (EGMUP).
The working paper provides recommendations on the design of novel experiments suited
to test multi-physics M&S tools for nuclear reactor systems. Thereby it does not aim to be
exhaustive and focuses on a specific validation domain and a specific challenge problem
to outline the design and decision-making process for providing optimum validation data.
The considered validation domains are pulsed reactors (CABRI/TREAT) as well as
Gen-II/III reactors, preferably pressurised water reactor (PWR), assuming steady-state andtransient events (normal operation and accident situations). The selected challenge problem
is the pellet cladding mechanical interaction (PCMI) in transient reactors. Indeed, PCMI ina reactivity-initiated accident (RIA) covers most of the domains considered by the
EGMPEBV, except chemistry, resonance/vibration or damages to the containment. The
types of MP codes to be validated are considered to be similar to the novel approaches
represented by the CASL, NEAMS and NURESAFE platforms (Turnsky and Kothe, 2016;
Sofu and Thomas, 2017; Chanaron, 2019), i.e. tightly coupled tools with high resolution in
space and in time.
The working paper outlines the conflict of increasing experimental needs and decreasing
funding for the experimental facilities, and provides recommendations on how to reducecost. The document proposes a methodology for defining validation requirements and fordesigning experiments based on Phenomenon Identification Ranking Table (PIRT) and
Quantitative Phenomena Identification and Ranking Table (QPRT) approaches. It
concludes that the cost of new experimental programmes can be reduced by supplementary

TRADITIONAL AND NOVEL MEASUREMENT METHODS FOR VALIDATION OF MULTI-PHYSICS MODELLING AND SIMULATION TOOLSNEA/WKP(2023)3  11

simulations to focus the experimental efforts on the most relevant measurements and to
gain better control of the relevant influence parameters. Better control leads to reduced
experimental biases and uncertainties to match the improved accuracies and resolutions of
the novel MP computational methods. It is stressed that the determination of themeasurement techniques and their associated measured parameters should be specified
based on an iterative process involving experimentalists and MP modellers.
The working paper illustrates the approach with an application to the modelling of PCMI
in pulse type reactors. It is concluded that the available measurement capabilities are notyet fully suitable for MP code validation, and gaps have been identified mostly in theavailability of online data and in the provision of improved spatial and temporal resolution.
The working paper provides several references to new measurement types and also
considers possible issues, e.g. related to radiation hardness. Single effects tests are still
considered as a useful input for MP validation work as they typically exhibit reduced
experimental uncertainties and fewer sources of potential measurement biases.
Additionally, the cost for single effects tests are typically lower than for MP experiments.
The working paper recommends a strategy in which electronic and data acquisition systems
for MP experiments are designed to acquire and store as much raw data as possible fromthe sensors to enable offline data processing capabilities such as analysing time correlations
between detectors, extending measurement ranges or correcting drifts in signals. A next
generation scientific documentation tool such as literate programming is recommended to
document the post-processing strategy within its source code.

TRADITIONAL AND NOVEL MEASUREMENT METHODS FOR VALIDATION OF MULTI-PHYSICS MODELLING AND SIMULATION TOOLS12  NEA/WKP(2023)3

1. Introduction

Novel modelling and simulations (M&S) capabilities in nuclear engineering offer
comprehensive insights into physical phenomena and present new opportunities for thedesigner. The availability of dependable numerical predictions and complex sensitivity
analyses form the foundation for swift, iterative design processes that lead to heightenedsafety margins and improved economics for new designs. Additionally, these advanced
M&S capabilities yield more comprehensive, well-informed, and less conservative safetyassessments of existing designs to support, e.g. nuclear long-term operation (LTO) bylifetime extension, power uprates of nuclear power plants, and higher fuel burn-up, which
are key economic improvement factors for the operation of the current fleet of nuclear
power plants (IEA and NEA, 2020).
On the nuclear reactor core level, the system behaviour is described by an interplay ofdifferent physical phenomena at various time and length scales, notably nuclear physics
and neutron transport (neutronics), thermal hydraulics, and material behaviour. Novel
M&S approaches for the reactor core simulations implement a fully coupled, so-called
multi-physics (MP) approach, which yield unprecedented spatial and temporal resolution.
There is still a challenge to validate the novel multi-physics simulation tools. A code withhigh-resolution requires also high-resolution experiments to validate its predictions andensure its stability. However, the availability of high-resolution datasets for the validationof the novel MP reactor core simulations is still limited. The deficiency lies in the absenceof both experimental data and reliable estimates of their associated uncertainties, both ofwhich are crucial for validation purposes. Moreover, one can frequently observe a
disconnection between the measured physical quantity and the computed quantities1,
necessitating a complex process to connect them. This process can introduce unwanted
biases and additional uncertainties. So there is a need for additional experimental input andinnovative experiments.
This working paper aims to provide recommendations on how to mitigate the conflict ofincreasing experimental needs and decreasing funding for the experimental facilities. The
question that will be addressed in this paper is: “What would be the best experiment tovalidate a novel high-fidelity MP modelling tool?”, and the main objective is to provide
guidance on how to select and design the experiments.
Exhaustiveness is out of the scope of this paper. The validation domain considered for thispaper are pulsed reactors (CABRI/TREAT) as well as Gen-II/III reactors, preferablypressurised water reactor (PWR), assuming steady-state and transient events (normal
operation and accident situations). The types of MP codes to be validated are considered tobe similar to the novel approaches represented by the CASL, NEAMS and NURESAFE
platforms (Turnsky and Kothe, 2016; Sofu and Thomas, 2017; Chanaron, 2019), i.e. tightly
coupled tools with high resolution in space and in time.
The main outcome of this working paper is a detailed methodology, which will beillustrated for a challenge problem. The challenge problem has been selected within thefocus areas identified by the Expert Group on Multi-Physics Experimental Data,
Benchmarking and Validation (EGMPEBV) in (NEA, 2016). It is the pellet cladding

1

An illustrative example is the keff parameter where voltage or current is measured and recorded, and a non-trivial process
is required to determine the required physical quantity.

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mechanical interaction (PCMI) in transient reactors. Indeed, PCMI in a reactivity-initiatedaccident (RIA) covers most of the domains considered by the EGMPEBV, except
chemistry (link with crud deposition), resonance/vibration (grid-to-rod fretting) or damages
to the containment. A similar approach should be carried out for each of the different
challenge problems defined in (NEA, 2016) to design the experiments required for MP
validation. Thereby, it will be required to make the difference between steady-state andtransient operation situations, or between safety-oriented or industrial optimisation topics.
The paper is organised as follows. In the second chapter, the concept of a measurement isreviewed so as to clarify what is involved when an experiment is designed. In the thirdchapter, the methodology involved to design a new experiment is sketched. Reference to
key bibliography is provided. Finally, the fourth chapter illustrates the methodology for thechallenge problem of PCMI in transient reactors such as CABRI or TREAT. Specifically,
to provide a picture of the state-of-the-art, the existing instrumentation of a pulsed reactor(CABRI as an example) is described first, listing all the quantities measured. Then theapproach to determine what should be the novel instrumentation for measurements insupport of MP validation is given.

TRADITIONAL AND NOVEL MEASUREMENT METHODS FOR VALIDATION OF MULTI-PHYSICS MODELLING AND SIMULATION TOOLS14  NEA/WKP(2023)3

2. Definition of a measurement

The measurement of a quantity is a very complex process that goes far beyond
instrumentation. The objective of the present section is to review the major steps of a suchprocess to identify the existence of potential weaknesses.
The overall process of performing an experiment is depicted in Figure 1 (Hudelot et al.,2018). It emphasises the fact that an experiment must not be resumed only to the choice ofan instrumentation and to the measurement of raw data, e.g. signals delivered by the
instrumentation, such as a current or a voltage. To transform the experimental raw data into
the target measured physical parameter and its associated uncertainty, a number of steps
are involved, which include the need of input data (e.g. technological data, nuclear data),
sometimes of calculated data to transform raw data into physical data, of calibration data,and of precise control of the boundary experimental conditions (e.g. environmental or
human factor parameters).
Figure 1. Overall process to complete an experiment

Source: CEA, 2023.

One of the major issues with the generation of experimental data is the physical data
determination. The physical data is the quantity that will be compared to a model prediction
during the validation process. The link between the raw measured data and the physicalinformation usually involves computation results based on models, which can potentiallybias the “measured” physical data. So models are required to generate data for validating
models, which is known as the Ouroboros paradox. In this setting, the degraded
performance of one component of a system degrades the performance of the whole system,
which, in turn, will further affect the degraded component behaviour.

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The generation of experimental data has, e.g. been illustrated in (Rais et al., 2018), which
deals with the measurement of control rod reactivity worth in the CROCUS zero powerreactor (ZPR). The paper describes the process to go from raw experimental data (e.g.
detector’s count rate) to the physical quantity to be simulated (e.g. the control rod worth).
The estimation of the control rod worth requires the use of parameters determined throughcalculation, which, in turn, alter the result of the measurement both in terms of accuracy
and precision. In the paper, the effect of kinetic parameters and nuclear data libraries on
the determination of the control rod worth was highlighted. The resulting experimental
uncertainty increased from less than a percent for the inverse reactor period to around 3.5%
for the control rod worth. Moreover, depending on the source of kinetic parameters, a biasof around 10% can be introduced in the experimental results. This work illustrates that evenfor experiments where the directly measured quantities (such as the inverse reactor period)
are accurately and precisely determined, the physical quantity of interest (reactivity worth)can be biased using simulated parameters during the conversion from raw to physical data.

2.1. Reporting a measurement
The reporting of a measurement requires that the experimental setup as well as the facility
itself are rigorously described. The description of an experiment includes the measurement
methods used and the results obtained for the parameters of interest, as well as methods
used to derive the physical parameters from the measurements. Experimental data include
values of parameters that are needed to completely determine the boundary conditions ofan experiment and that have been directly measured.
Care should be given to the reporting of the experimental information. The standard
deviation and the mean value of the measured quantity are both essential when considering
confidence intervals. Interested readers should refer to NEA (2019) for more information.

2.2. Determination of a reliable experimental uncertainty
It is generally agreed upon that estimates of physical data are useless if there is noknowledge of how reliable the estimates are. Therefore, reliable experimental uncertainties
need to be determined.
A large body of work by the international community has been dedicated to the compilationof experimental database for the validation of neutron transport simulation through theInternational Criticality Safety Benchmark Evaluation Project (ICSBEP) Working Groupand of the International Reactor Physics Experiment Evaluation (IRPhE) Project releasedby the NEA. In the framework of those activities, best practices were developed for theevaluation of experimental uncertainties. The present text is a summary of those practices,which are themselves based on three main references, the respective US and European
standards (ANSI, 1997; Bureau International des Poids et Mesures, 1999; ISO/IEC, 2008).
Even though the rigorous approach described in those references is used for neutrontransport related experimental data, the guidelines for the evaluation of experimental
uncertainty remain applicable to the other fields of nuclear engineering. It should be noted,
however, that some of the steps (sensitivity analysis and combination of uncertainties) may
be very difficult for the non-linear problems featuring important parameters with nonGaussian distribution. Such problems are few in the field of neutron transport but are very
likely to occur in the fields of thermal hydraulic or fuel performance.
Typically, uncertainties in measurements are of two types: there are aleatoric and epistemic
uncertainties. The first type of uncertainties is the aleatoric (or random) type. An example

TRADITIONAL AND NOVEL MEASUREMENT METHODS FOR VALIDATION OF MULTI-PHYSICS MODELLING AND SIMULATION TOOLS16  NEA/WKP(2023)3of random uncertainties is the fluctuations of the number of counts recorded by a neutron
detector.
The second type of uncertainty is the uncertainty of the epistemic uncertainty, also referred
to as systematic error. A systematic error occurs when a series of measured values areincorrect by approximately the same amount. An example of systematic error is the power
calibration of a research reactor. Unlike random uncertainty, repeated measurements of theparameter cannot reduce the systematic uncertainty. Those uncertainties are typically very
difficult to estimate. One way to do so is to base the estimation of systematic uncertainty
on the observed trends.
With respect to the inventory of the source of uncertainty, a rigorous approach needs to beundertaken to have an exhaustive reporting: each possible source of uncertainty in theexperiment should be reviewed and considered. An example of such a rigorous approachis provided in Table 1 of the ICSBEP uncertainty guide (NEA, 2019). Uncertainties of theactual measurands and also for additional parameters of interest need to be provided. The
parameters of interest are particular values of quantities that define the physical setup(dimensions, material compositions, masses, temperature, etc.) at the time of theexperiment.
When an uncertainty estimation is missing, then an approximate uncertainty of theparameter can be estimated based on the typical uncertainty of the considered parameter atthe experimental facility at the time of the experiments, information from the manufacturerof the measuring device, and personal experience. The basis of the uncertainty estimate
should always be explained. A prime example of this approach was used in the“Radiochemical Analysis of mixed oxide fuel (MOX) and uranium oxide (UOX) lightwater reactor (LWR) Fuels Irradiated to High Burn-up” (MALIBU) programme(Boulanger, 2007) where a 10% uncertainty was assigned to certain isotope concentrations
reflecting the spread of the measurements obtained at various institutions even if the
precision of the measurements at a given facility was better.
Comparison of model results to experimental datasets with realistic uncertainties finallyallows for revealing weaknesses in computational methods, reducing or eliminating them,and designing more accurate computational schemes for the future.

TRADITIONAL AND NOVEL MEASUREMENT METHODS FOR VALIDATION OF MULTI-PHYSICS MODELLING AND SIMULATION TOOLSNEA/WKP(2023)3  17

3. Process to develop an experiment dedicated to the validation of MP tools

The determination of the measurement techniques (both existing and novel) and theirassociated measured parameters for the “best experiment” should be specified based on an
iterative process with multi-physics (MP) modellers as a clear expression of the needs for
multi-physics validation is paramount to the proper use of experimental technique. In
particular, modellers should express what exactly the quantity they want to measure is, andexperimentalists should specify for each kind of measurement technique what the quantity
measured is, and what the derived quantities are that should be obtained (Oberkampf andRoy, 2010).
The methodology for developing experiments should be composed of four steps as
described in the following four dedicated subsections. It has for example been used todetermine the kind of instrumentation to be included in the test vehicles of TREAT. The
new instrumentation is developed to address the needs for validation of MP in a cheap andconvenient way, and it will become obvious that simulations are envisioned as a driver forthe experimental development.

3.1. Step 1: Determination of the validation requirements
A careful determination of the validation objectives is needed. Namely, what quantity needs
to be predicted by the MP tool during the validation process? The regulatory body andindustry are the usual end users of codes and simulation tools. As such, their needs willdefine the validation requirements.
A list of specific MP validation requirements including the Quantities of Interest (QOIs)
and their target accuracies are expected as input.

3.2. Step 2: Expression of the modelling needs for code validation
The modelling needs consist in a list of quantities to be measured with an associatedresolution and target accuracy in order to fulfil the validation requirements. They include alist of global and local parameters usually predicted by MP codes. Global parameters areeasy to instrument and measure but may not be relevant for MP validation purposes.
However, because many of the local parameters are difficult to measure directly,
calculations may be necessary to estimate local parameters based on global parameters.
This is a challenge in validation, as the use of calculated local parameters does not validate
a computational scheme.
Even though for controlled laboratory experiments, practically any parameter can bemeasured, the same measurement may not be possible in a MP validation experiment due
to the measurement conditions (pressure, temperature, radiation field, etc.). Moreover, theprecise knowledge of one parameter (e.g. clad temperature measured by thermocouple)
may increase the uncertainties on other parameters (e.g. pressure and velocity). Nonintrusive measurements (e.g. by laser) may improve this.
An approach relying on “all possible quantities” is not reasonable: to reduce the cost of thenext generation of experimental facilities, not all the predictable quantities should bemeasured and prioritisation is required. The required instrumentation should be developed
to measure the quantities needed for validation purposes as well as to estimate theiruncertainties.

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The key to define the modelling needs is an exhaustive literature review on the phenomenonof interest, considering existing resolution and associated uncertainty. Such a review willproduce Phenomenon Identification Ranking Table (PIRT) based diagrams. Figure 2 shows
the PIRT example for PCMI. The PIRT diagrams aim at describing the physical parametersof importance from a modelling perspective. As far as measurements are concerned,
together with the actually quantities of interest also all so-called “influence parameters”
should be determined. These parameters, including, for example, the temperature field andpressure, then also need to be measured and reported. The PIRT shows also what
measurement techniques could be applied to measure a quantity of interest for MP code
validation for a given physical phenomenon (see row 4 of Figure 2, e.g. gamma heating,fission power).
Figure 2. Phenomena identification and ranking table (PIRT) decomposition of pellet-cladmechanical interaction (PCMI)

Source: CEA, 2023.

3.3. Step 3: Review of the existing measurements for the considered problem
After determining the validation requirements and the modelling needs to fulfil thoserequirements, the next step is to summarise the existing techniques for a given problem.
Based on the PIRT diagram, the existing instrumentation in the facility of interest isreviewed for each physical parameter of importance as well as for the influence parameters.
The level of knowledge for each technique in application is assessed: spatial and time
resolution, limitations. From this assessment and given the modelling needs, the limitationsof the current experimental facility for MP code validation are determined.
Another method to determine which measurement requires improvement relies on thedetermination of a Phenomenon Assessment and Ranking Chart (PARC) through the input
of expert and non-expert participants via online questionnaire. The PARC method wastested for pellet-clad interaction modelling problems, but its outcomes are not fully
satisfactory as no real prioritisation of the various phenomena (and the associated QOI)
was achieved as all physics were ranked with approximately the same weight.

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3.4. Step 4: Selection of an existing solution or potential candidates toaddress the experimental needs
Once the limitations of the current measurement techniques for the requested QOI are
determined, the novel measurement techniques can be designed using Quantitative
Phenomena Identification and Ranking Table (QPRT) (Yurko and Buongiorno, 2012). If
the MP simulation tools can predict a lot of detailed local information, it is not the case forthe experimental data. To reduce the cost of the next generation of experiments targeting
MP validation, the required instrumentation should be developed using a QPRT approachto prioritise which quantity should be measured and with which accuracy.
The idea is to perform a sensitivity analysis (SA) of a given QOI with respect to the input
parameters using a simulation tool to gain a better understanding of the experimental
parameters to control or to measure, which is key to reach the QOI target accuracy. Suchan approach is used to determine the kind of instrumentation to be included in the test
vehicles of TREAT (Jensen, 2016).
It should be noted here that the SA process depends on the quality of the applied calculation
tools. When the tools exhibit a bias, which is not well understood (system codes forexample), then the SA process might lead to completely wrong results and the subsequent
experimental design might be flawed.
Figure 3 provides an example for the process of finding suitable measurement techniques
for a given phenomenon. The example addresses the “creep/corrosion/cracking/hydrides”
box of the PIRT diagram illustrated in Figure 2, and defines a list of quantities of interest
to be measured to describe the phenomena (determined through the QPRT analysis forexample). A few sensors and relevant measurement techniques are then proposed to
improve the measurement of such quantities. A detailed application of this process isprovided in Section 4.4.
Innovative techniques and instrumentation are not the only approach for better
experimental data. Several methods/techniques, such as combining techniques to measurea single parameter allows for enhanced confidence levels and identified biases. The next
generation of measurements looks at more detailed signal analysis which can, e.g. provide
higher order moments of a given signal or correlations between signals.
Finally, even though the goal of the present working paper is to describe the design
methodology for an MP experiment, the single physics experiments and specificallyseparate effect tests are also very useful. They are designed to isolate a phenomenon ofinterest: well-defined boundary conditions and/or improved instrumentation strategies canbe designed to optimise the validation of a specific model in an MP tool. Guided by PIRTand QPRT studies, the implementation of simpler separate effect tests can lead to cost
reduction. However, ultimately, a full MP experiment is still required to test the descriptionof the coupling of the different phenomena.

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Figure 3. Novel measurement approaches for the phenomena associated with the cladding during PCMI

Source: CEA, 2023.

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4. Demonstration of the methodology for PCMI

This chapter provides a demonstration of the general methodology for designing new MP
experiments as described in the previous chapters for validating PCMI in a pulsed reactor(e.g. CABRI and TREAT).
In this chapter, a technique is referred to as “state-of-the-art” when it is currently used inpulsed or material testing reactors. Some experimental techniques might already be readily
implemented in other domains. However, if they are not yet applied in the considered
reactors, such techniques are still categorised as “novel”.
To protect intellectual property, the description of measurement techniques will not include
technical details, and references to open literature are provided, if available. It is noted that
also the combination of “state-of-the-art” measurement techniques could lead to a better
determination of the parameters of interest with reduced uncertainty, to a determination ofquantities not yet measured, or to the identification of new phenomena. For example,
initiating phenomena for PCMI could be analysed with the help of acoustic signals
correlated with signals of pressure, temperature, flowrate and boiling detectors.

4.1. Steps 1 and 2: Validation requirements and expression of modellingneeds
The PIRT diagram illustrated in Figure 2 including a phenomena identification and ranking
table (PIRT) decomposition of the pellet-clad mechanical interaction (PCMI) is used as astarting point. The aim of this chapter is to illustrate the methodology and not to actuallydesign the experiments that one would need for the validation of an MP code for PCMI
analysis. So it is not a concern that Figure 2 is incomplete as, e.g. the thermal hydraulicrelated phenomena are missing, and not fully applicable to PCMI during a transient
experiment.
Based on the PIRT in Figure 2, the following physical parameters are considered ofimportance for PCMI:• gamma heating;• nuclear fission power;• fast flux;• pellet swelling, cracking and densification;• fission gas release;• friction/bounding layer of gap;
• cladding properties.
The modelling needs2 for the validation process requires online measurements withimproved spatial and temporal resolution of the previously mentioned quantities; to matchas closely as possible the resolution of novel MP codes.

2

Please note that a comprehensive overview on data requirements specific to fuel performance modelling, including PCMI,
and on determining the types of experimental data, and their sources, which can be used to meet these requirements iscurrently prepared by the NEA Expert Group on Reactor Fuels Performance (EGRFP). More information on EGRFP
activities is provided on the website: www.oecd-nea.org/jcms/c_12837.

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The influence parameters to be controlled include:• the material properties of the irradiated sample (e.g. history, composition);• the reactor operating conditions (e.g. coupling factor, power profiles, reactivity determination,
temperature coefficient);• the test device conditions (e.g. local temperature fields, flowrates and energy depositions).
The following sections review the status of current instrumentations and experimental
techniques to measure the above-mentioned physical parameters and the influenceparameters related to the test device conditions.

4.2. Step 3: Review of the state-of-the-art of current instrumentation andexperimental techniques
With the validation and modelling requirement set, the experimental techniques arereviewed to determine the limitations of the current experimental data for MP code
validation. For each box of the PIRT diagram in Figure 2, the related experimental
techniques at CABRI are listed in the following sections. A review paper (Kim et al., 2011)
addresses most of the state-of-the-art techniques in use in irradiation testing of fuels andmaterial and covers most of the experimental techniques listed below.
In the following, only sensors and experimental techniques are listed and discussed. As
already discussed in Chapter 2, a measurement consists also in data processing of theexperimental results including, e.g. time series or image processing steps. However, thissecond aspect will not be addressed in detail for the sake of conciseness.
It must also be kept in mind that some techniques have been developed specifically for agiven facility, and may not easily be transferred to other experiments or be transposed froma small to a large facility.
Derived quantities are not directly mentioned but must be kept in mind. For example, thecoupling factor (ratio of the driver core power to the tested rod or fuel assembly power) is
not directly measured, but derived from power, temperature, gamma heating and someother influence parameters. Therefore, even the most usual measurement and data tables
must be carefully checked and, if possible, enhanced.

4.2.1. Gamma heating
The gamma heating measurement aims at characterising the energy deposition in thesurrounding of the fuel sample. Gamma thermometers (see Figure 4) and differential
calorimeters (see Figure 5) have been developed to characterise the irradiation locations
but only in unperturbed conditions (Van Nieuwenhove and Vermeeren, 2020) (e.g. without
the sample to be irradiated and its container).
The thermometer principle is based on the measurement of the temperature difference
between the tip of the inner body and the coolant measured by a differential thermocouple(type K). The inner body (made of stainless steel) is heated by gamma rays and is thermallyinsulated from the outer housing by a gas layer (0.7 mm).
The differential CALMOS calorimeter device (Carcreff et al., 2018) developed for the open
pool research reactor (OSIRIS) measures the differential temperature difference between
the pedestal and the base of each cell and between the sample and the reference cells when
they are exposed to the same irradiation field. The achievable performances of this kind of
instrumentation with a measurement range from almost 11 W/g down to a tenth of mW/g
with a 5% precision.
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Due to their dimensions, this instrumentation cannot be inserted in the fuel element.
Therefore, gamma heating in the fuel sample itself needs to be inferred from non-perturbed
measurements noting that gamma heat deposit is roughly proportional to the Z of atomic
element of the target medium.
The TRIPOLI 4 Monte Carlo code provides accurate simulations of the gamma heating as
shown in (Carcreff et al., 2018). It can be noted that gamma heating is not a direct
measurement of the gamma fluence (rate) because it depends on the energy effectively
deposited in the medium by each photon.
Gamma flux could also be measured by ionisation chambers as well as self-powered
gamma detector (SPGD) as it was carried out at TREAT (Jensen, 2016; Bess et al., 2019).
A SPGD differs from self-powered neutron detector (SPND) by the nature of the emitter(Bismuth) sensible specifically to gamma photons (see Figure 6) (Villard et al., 2016).

Figure 4. Schematic drawing of the SCK CEN gamma thermometer

Source: SCK CEN, 2023.

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Figure 5. Cross-section of the CALMOS-2 calorimetric probe (dimensions in mm)

Source: CEA, 2023.

Figure 6. Typical SPND/SPGD

Source: CEA, 2023.

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4.2.2. Nuclear fission power
The determination of the energy produced at the location of the fuel sample is key to thedetermination of the energy deposited in the fuel. The next sections deal with the two typesof fission powers needed to be determined accurately: reactor power and test vehicle power.

Reactor power
The power released by fission reactions in the reactor drives the energy deposited in thefuel sample. The thermal power is related to the number of fissions using the mean energy
released by each fission. The macroscopic power of the reactor is classically monitored
using ex-core boron-type ionisation chambers. Those online measurement techniques andsensors are detailed in (NEA, 2000; Harrer and Beckerley, 1974).
The determination of the thermal power of a nuclear reactor, which is directly linked to thenumber of fissions per second, relies on three different techniques depending on its
effective power level and stability. For low reactor power, where no enthalpy measures are
possible (e.g. due to an insignificant temperature difference in between input and output
cooling fluid), the total number of fissions in the core is derived from online measurements
performed by calibrated fission or ionisation chambers. They give access to the fission rate
at the measurement location and, using neutronics simulations for mapping the fission
distribution over the reactor core, the total fission number occurring can be deduced.
Thermal reactor power is then derived from the fission rates considering the mean energy
per fission as well as the energy loss due to gammas and neutrons leaking from the core atthe periphery of the reactor.
A fission chamber (FC) is an ionisation chamber filled with a neutral gas and is composedof a cathode and an anode on which a well-characterised fissile material is deposited. The
fission reactions occurring in this material lead to the production of ions (fission products)
and to the formation of an electrical signal (see Figure 7).

Figure 7. Example schematic and photo of fission chambers

Source: CEA, 2023.

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Figure 8. Representative activation foils and wires

Source: CEA, 2023.

The absolute local flux level can be inferred from activation measurements, as it has been
performed at the Measurement Applied to DosimEtry for Reactors (MADERE) platform
for the CABRI reactor commissioning test (Lecerf et al., 2017) (see Figure 8) or frommeasurements using fission chamber calibrated for example in the BR1 neutron spectrum
benchmark (Lamirand et al., 2014).
When the reactor power is sufficiently high (>100 kW), the coolant is heated during thecirculation in the core and the absolute thermal power can be determined through enthalpy
balance measurement (e.g. flow and temperature measurements) between input and output
coolant flows. The relative change of the global power of the reactor is typically monitored
by boron chambers calibrated at some specific steady state of the reactor. Few steady-statemeasurements are carried out over the full power range envisioned during the test.
Determination of power and fluence rates at a specific location needs modelling and various
corrections to be derived with a good accuracy (Pantera et al., 2014).
The power follow-up in transients (up to 25 GW) consists in energy balance measurements(integration of thermocouple responses). The linearity of the ratio of energy balance tocount rates of boron chambers is established by showing repeatability over multiple pulses(Lecerf, et al., 2018).

Test vehicle power
The power deposited in the tested fuel sample is determined from the power produced in
the core during the pulse and the knowledge of coupling factors relating the power
deposition in the test vehicle to the core power. It is thus determined in a two-step approach:
1. An energy balance measurement is carried out in the test vehicle in steady state forthe core power below 25 MW. The underlying assumption is that steady-state andtransient neutronics conditions are the same.
2. The coupling factors are determined, which are measured as the ratio of energy
deposition core/energy deposition in sample in steady state (both CABRI and
TREAT). Energy deposition in the sample is determined by energy balance throughonline measurements. Dosimeter wires are used in TREAT for sample energy
deposition but they require post-irradiation experiment (PIE) analysis to determine
the coupling factors.
In some reactors (high-flux advanced neutron application reactor [HANARO] [Yang Noh
et al., 2018], OSIRIS [Loubiere et al., 2012]), a spatially resolved power distribution in thecore is determined through PIE by checking the activity of a few fuel rods by gamma
activation measurements, but such measurements are not performed in CABRI.
In addition, miniature fission chambers can be inserted in the test vehicle in an empty
location. They provide online fission power measurements, however, perturb the
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experimental conditions. So the measurements are not performed during the transient but
only in steady state.

4.2.3. Fast flux
Fast flux measurement is relevant for steady-state irradiation and not so much for transient
experiment in pulse reactor like CABRI. Nonetheless, spectral indices in CABRI aremeasured in steady state for a characterisation of fast flux.
In the Institute for Energy (IFE) Halden reactor (Solstad and Van Nieuwenhove, 2011),
SPNDs are used to determine the thermal flux. Modelling is used to infer the fast flux inthe sample based on the thermal flux measurements.

4.2.4. Pellet swelling, cracking and densification
The axial motion of fuel pellet stack in the test vessel is monitored online during transientswith the hodoscope, which is made of an array of fast-neutron detectors (proton recoil
detectors and 237Np fission chambers for CABRI). It allows measuring fuel distortion,elongation and relocation during transients. The spatial resolution of the hodoscope is notgood as it has only few pixels but it provides temporal information about fuel geometry
with a very good resolution of milliseconds. Depending on the measurement conditions,the hodoscope may detect a radial fuel motion as small as 0.2 mm, and an axial elongation
as small as 2 mm. Complex post-processing is needed to handle background noise.
Normalisation to fresh fuel is also needed.
In addition to the information provided by the hodoscope, experimental data is typicallyobtained before and after the test through ceramography. No experimental information on
the size of the sample is available during the ramp. This means that only a partial validationof the MP simulation can be done.
Figure 9. Schematic of the CABRI Hodoscope

Source: IRSN, 2023.

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The TREAT facility has also utilised in-pile high speed videography to directly image thebehaviour of nuclear fuel under transient conditions. The high speed video has provided
high value information on fuel motion and coolant voiding behaviours in dry and water
environments.
Moreover, the fuel sample is examined with gamma-scanning, X-ray tomography andradiography before and after the pulse. In CABRI (also in TREAT), neutron radiography
was performed in the past but is not available anymore. Progress has been made to improve
the acquisition rates and digital imaging versus film development. Gamma spectrometry
with high-purity germanium detectors (HPGe) focusing on 137Cs is made before and after
the test to create an exposure profile (Lemoine, et al., 2012). After the transient, changes inaxial profiles allow the determination of changes in geometry. Depending on thecollimation window, spatial resolution around 1 mm can be achieved.
Figure 10. Schematics of theTREAT Hodoscope

Source: INL, 2023.

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Figure 11. Pre- and post-neutron and gamma scanning

Source: CEA, 2023.

4.2.5. Fission gas release
Acoustic sensors have been used in the materials test reactor (MTR) OSIRIS to detectfission gas release (Lambert et al., 2011). The online determination of the gas composition
is inferred from modifications in the speed of sound following changes in the gas
composition. During the OSIRIS test, a 2% error was observed compared to othermeasurement techniques. The benefit of acoustic sensors is that they do not require specificfuel rods to be developed. Such sensors are sensitive to high temperatures and need to be
optimised for operating in reactivity-initiated accident (RIA) conditions.
Another approach to determine the fission gas release is to monitor the pressure in the fuelsample outside of the core and the gas flow out of the fuel element where the activity
composition is determined through gamma spectroscopy. The technique can be used forsteady-state and loss-of-coolant accident (LOCA) type transients. However, it cannot beapplied to RIA transient experiments because the transient is too fast and the gas transport
out of the core does not represent the actual kinetics of the physical phenomena.
The absolute pressure inside the test fuel rod can be measured using linear variable
differential transformer (LVDT) as shown in Figure 12 (Solstad and Van Nieuwenhove,2011).

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Figure 12. Different applications of the LVDT sensors

Source: Solstad and Van Nieuwenhove, 2011.

The fission gas release can also be assessed experimentally through post-irradiation drilling
techniques if the fuel is intact. As it is a post-irradiation technique, such a measurement
gives information about the equilibrium conditions, but not about the kinetic process. As aresult, only a partial validation of the MP simulation can currently be done.

4.2.6. Friction/bounding layer of gap
Currently, no measurement is available in CABRI and TREAT.

4.2.7. Cladding properties
A few cladding properties can be measured online including the axial dimension of the test
fuel sample using LVDT detectors. No measurements are available for the radial dimension
besides a diameter gauge test by IFE (Solstad and Van Nieuwenhove, 2011).
Moreover, through acoustic measurements with two microphones upstream anddownstream of the test pin, it is possible to detect cladding rupture (if any) during the pulse
experiment (Traore et al., 2017).
Similarly to the pellet properties of the fuel sample, non-destructive examinations arecarried out for the cladding before and after transients: a set of visual inspections as wellas metrology type measurements (diameter) using a confocal microscopy system, Eddy
current and ultrasonic testing for defaults checking and oxide thickness determination. Xray tomography (IRIS facility) with a spatial resolution around 400 microns is carried out
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as well. It should be noted that radial displacement instrumentation has not yet been
implemented, because it was proven to be too intrusive and disturbing for the
representativeness of the fluid flow. An online cladding dimension measurement has once
been carried out during the experiment for online cladding creep studies (MELODIE) using
diameter gauges (Guimbal et al., 2013).

4.2.8. Influence parameters“Influence parameters” are the physical parameters that affect the measurements of theprocesses of interest even though they are not strictly listed in the PIRT diagram in
Figure 2 (e.g. the temperature field and pressure). Such influence parameters should bemeasured online as best as possible and should be reported.

Temperature
The temperature of the cladding in the test vessel is determined with thermocouples
positioned on the surface of the cladding of the test fuel. To reduce the perturbation of theexperimental conditions due to the measurement, no welding is done, only a mechanicalcontact is obtained through the use of a specific device called “cage”. Thermocouples are
the state-of-the-art for temperature measurements. In the TREAT reactor, the temperatureof the experimental vehicle is remotely measured through a window using an infrared
pyrometer system (Jensen, 2016).

Flow condition in the test vehicle
The flow rate in the test vehicle is determined during the transient through flowmeters fortransients, turbine flowmeters for steady-state measurements, and with Pitot tubes. Boilingcan be detected through the use of piezoelectric ultrasonic sensors (one emitter and threereceivers). Finally, the temperature of the flow can be measured with thermocouple at agiven location facing the tested rod, but also upstream and downstream for thedetermination of the thermal balance in the test vehicle.

Pressure
Pressure measurements can be performed with LVDT. Counter-pressure sensors are usedin the reactor of the Atomic Energy and Alternative Energies Commission (CEA) (Lambert
et al., 2011), and piezoelectric sensors have been used for CABRI but have raised somereliability concerns. In the United States, in addition to the LVDT sensor, Fabry-Pérot
detectors are tested (optic based deformation measurement) for pressure measurement in
the gap (Jensen, 2016).

4.2.9. Electronic and data acquisition systems
Data acquisition systems are designed together with the detectors. Electronics should not
be the weak link of an acquisition chain. The sampling rate, which is based on frequency
bandwidth, data buffers size, and data transfer rate, and the acquisition input range must be
adapted to the physical phenomena at hand as well as to the experiment at stake.
Taking the case of online detection of neutrons, for which detectors deliver current pulses,
two types of acquisition systems are available depending on the flux range:•

At low flux, analogue or semi-digital acquisition systems can record and time stamp
individual events to eventually deliver an event rate (also called counting rate). A
standard (analogue) acquisition is based on a wide band current-to-voltage
preamplifier, followed by a shaping amplifier and a voltage discriminator, which

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issue transistor-transistor logic (TTL) pulses fed to digital counting module. This
process is reliable and robust, but limited by the dead time process to about six
decades.
•

For higher detection rates, detection events cannot be simply discriminated basedon their amplitude. Acquisition systems then focus on sampling the overall current
issued by the detector. This current is conveniently converted into voltage before
sampling. Because the detection process is Poissonian, it is known that mean andvariance (and more generally all the cumulants) of the current are estimators of thedetection rate. This detection mode, standardly based on measuring the current
mean over time using an electrometer, is limited to six or eight decades. Expanding
the number of decades can be done at the expanse of temporal resolution.

In the past 10 years, a wide range of data acquisition systems have been developed forfission chambers. For example, systems by the CEA and Mirion rely on three measurement
chains working in parallel on a single detector. A field programmable gate array (FPGA)
module is in charge of online data processing (rescaling and data conversion) andproducing a single flux estimator. These systems can cover up to 12 decades in neutron
flux.
Nowadays, reactor physics is still using one measurement chain for each detector, whereas
in particle physics, experimenters are used to integrated acquisition systems.
Neutron measurements, especially for pulse signals, deal with high frequency signals thathave to be conveyed to the acquisition system on very long coaxial cables. The resulting
signal to noise ratio is usually poor (around 10 or 20), sometimes impacting the quality or
reliability of the measurement itself. Therefore, electromagnetic compatibility issues must
be taken into account from the very beginning of the experiment design.

4.2.10. Scientific documentation
Measurement systems will be documented and operated following a quality assurance
process. Raw data will be stored with the complete identification of each sensor.
The raw data are typically analysed, sometimes automatically, to produce a report. Results
and reports will be archived in dedicated databases with meta data describing theexperimental conditions. As the physical support (magnetic storage) and operating systems
change periodically, databases need to be updated accordingly.

4.3. Step 4: Selection of an existing solution or potential candidates toaddress the experimental needs
4.3.1. Required improvements
Based on the outcomes of the previous steps of the analysis, the following improvements
should be addressed by the measurements of the novel MP experiments in order to validate
an MP tool:•

It appears that the amount of experimental data available for each transient test is rather
limited. Specifically, the number and type of online measurements should be extended
as to be able to validate the kinetics models. Indeed, a large amount of experimental
data is determined through pre- and post-irradiation analysis.

•

An improvement of the spatial and temporal resolution of the measurements is requiredas the MP codes typically lead to finer resolution; the computational results need to becompared to equivalent experimental data before to be trusted. Improvement in data

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acquisition systems (e.g. sampling rates, signal to noise ratio) would allow a better
temporal resolution with often the same sensor. Calibration and minimisation of the driftof sensor’s response have also to be addressed to lower measurement uncertainties.
•

The perturbation of the actual physical phenomenon by the sensor should be reduced byusing distant sensors or by reducing its size, improving its compactness.

•

Finally, the improvement of the data post-processing algorithms, in particular for
acoustic signal, documentation and recording procedures would improve theexperimental data treatment and could be helpful even for the re-analysis of pastexperiments.

4.3.2. Recommendations for the future instrumentation and experimental
techniques
Once the limitations of the state-of-the-art measurement techniques have been determined,
the experimental techniques in development or readily available in other domains arereviewed to find other solutions for meeting the MP code validation requirements. As
pointed out already in Chapter 2, experiments consist of more than only instrumentation
and require also, e.g. a collection of models, data, documentation, instrumentation andcalibration as well as post-processing tools. Consequently, improvements can not only beobtained with improved instrumentation (e.g. with the help of an improved sensor) but alsowith improved measurement protocols (e.g. by better calibration, or by a combined use ofmeasurement techniques).
The following paragraphs provide an overview of potential improvements by novel
measurement techniques for each of the boxes of the PIRT diagram depicted in Figure 2.
There is also complementary information available in (IAEA, 2013).
Noteworthy, many of the novel methods for inferring local parameters have been developed
for controlled laboratory environments. Their potential in MP validation experiments withrealistic power and irradiation conditions often still need to be demonstrated.

Gamma heating
Existing means to measure gamma heating rely on energy deposition and gamma flux
measurements: differential calorimeters (CEA), gamma thermometers (Norway, Belgium).
Such instruments may be relatively big and intrusive but are considered very reliable.
Miniaturisation has been identified as a progress priority, so measurement methods basedon miniature gas ionisation chambers (CEA) and self-powered gamma detectors (CEA,
Idaho National Laboratory [INL]) are now being investigated.
Delayed measurements of fine gamma flux profiles can be obtained using opticallystimulated luminescence (OSL) and thermoluminescent dosimeter (TLD) detectors (CEA)
even though such dosimeters are too sensitive for actual measurements during transients(Gruel et al., 2018; Le Guillou et al., 2017).
Miniaturisation of calorimeters with short response time has been investigated (CEA and
Aix-Marseille University) (Reynard-Carette et al., 2018) with the possibility to insert themin the experiment vehicle. These types of sensors could be used for fuel ramp experiments,
but not for RIA or LOCA type tests due to the very short time scale of the measurement.
Ongoing research is dedicated to estimate the impact of beta radiation on suchmeasurements to evaluate its contribution to the overall radiation energy deposition.

TRADITIONAL AND NOVEL MEASUREMENT METHODS FOR VALIDATION OF MULTI-PHYSICS MODELLING AND SIMULATION TOOLS34  NEA/WKP(2023)3
Finally, online simultaneous gamma and neutron measurements are required to decouple
the respective source of heat in the fuel sample. To this end, various sensors could be usedsimultaneously such as gamma thermometers, ultrasonic sensors, fission chambers w/o
deposit (current mode), SPNDs and SPGDs. Temperatures measurements would ideallycomplete this set of measurements.

Fission power
For fission power measurements, there exist axial, radial, steady-state and time-dependentmeasurement techniques: fission power measurements are performed in real-time withfission chambers (FCs), and slightly delayed for SPND. The activation dosimetry techniquegives a posteriori access to the precise evaluation of the integral number of fissions (e.g.
total fission deposited energy).
The raw quantity measured are typically the reaction rates or the radioactivity of a sample
at a given location. These raw data have to be corrected from gamma contributions such as
photo-fission, photo-activation, gamma contribution to the delivered current to infer thetrue neutron induced reaction rates. Inferring local neutron power from such a measurement
is not straight forward and includes normalisation of the local information to the thermal
power produced by the reactor using modelling scheme. Therefore, neutron and gamma
contributions should be measured simultaneously to enhance the precision of the local
power estimation.
The global power produced by a given reactor is complicated to determine. The thermalbalance method is the most appropriate and precise but it is not always possible for low
reactor power. Neutron and gamma online measurements and activation techniques forpost-irradiation calibration are used complementarily, and show-case the combined usageof several consistent experimental techniques to reduce the uncertainty on important
experimental parameters.
The methods presented above are based on the default assumption of a constant neutron
spectrum during the power pulse. This is not realistic because as the temperature increases,
the effective cross-sections change (Doppler effect) and modify the spectrum especially inthe epithermal energy range. Online measurements of the spectrum variation would
therefore provide an improved characterisation of the power pulse. This measurement
technique remains to be established, e.g. by using FCs or SPNDs sensitive to different
spectral domains and by combining these measurements using a spectrum adjustment codesuch as the CALMAR code (Grégoire et al., 2016).
There are studies dedicated to test the similarity of steady-state and transient coupling
factors, which is an important assumption to infer the fission power deposited in the fuelsample during transients.
Concerning modelling, the priority is given to the development of simulations of theexisting detectors to understand the sensor behaviour (sensitive material loading, gas
mixture and drift of the detector signal during the irradiation). The main challenge is to
reduce modelling uncertainty, which is mostly driven by nuclear data and geometricaluncertainties.
In order to access to local flux, detectors could be inserted within the fuel pellet for online
measurement. However, neutron flux measurements inside a fuel pellet, even with a 3 mmsize micro-fission chamber (MFC), are considered to perturb the local conditions too muchand to deteriorate the representativeness of the measurement.
Other techniques such as scintillators or optical fibres functionalised for dosimetry
applications could be investigated but their current sensibility and linear type of response

TRADITIONAL AND NOVEL MEASUREMENT METHODS FOR VALIDATION OF MULTI-PHYSICS MODELLING AND SIMULATION TOOLSNEA/WKP(2023)3  35

provide major drawbacks. However, recent studies on an optical fission chamber (Lamotte
et al., 2020) and progress in SiC technology (Coutinho et al., 2021) have to be taken into
account and could lead to significant progress in measurement techniques.

Fast flux
An accurate determination of the fast flux can be obtained by dosimetry techniques throughthe irradiation of a set of activation dosimeters with specific nuclear reaction thresholds. It
is recommended to apply neutron spectrum unfolding methods to renormalise the absolute
flux level and to evaluate the complete spectrum with realistic uncertainties.
For online fast neutron flux measurements during steady state and transients, FCs using
specific fertile deposits (232Th; 237Np; 242Pu) could be used. Those FCs have, however, avery low noise to signal ratio and a non-negligible gamma contribution, which can be
reduced by running the FC in Campbelling mode. In addition, raw data should be corrected
for thermal and gamma reaction contributions using thermal sensors or a dedicated neutron
shield should be applied. Micropocket FCs developed by INL (Jensen, 2016) and opticalFCs, which are promising sensors for this application should be investigated.
The hodoscope could also be used for time-dependent fast flux determination but the fastneutrons need to be discriminated from the thermal neutrons through signal postprocessing. Several sensor technologies could be tested:●

The use of (micro pattern gas detectors [MPGD] – Micromegas sensors for CEA)
could be envisioned as they were developed for giving an online neutron energy
spectrum evaluation.

●

Diamond detectors could be used for fast flux measurement. It was already used
for instance in the Ulysse reactor in CEA Saclay, or in the CROCUS reactor of the
Swiss Federal Institute of Technology (EPFL) (Hursin et al., 2018). Diamonddetectors are good radiation detectors due to their wide gap width and high charge
carrier mobility. These properties result in an excellent signal to noise ratio
together with a very fast response time. Diamond detectors are very robust and canwithstand large fluences without major degradation of their signal to noise ratio.

●

SiC based sensors are interesting because of their improved irradiation resistanceand of their increased energy sensitivity and larger dynamic ranges compared todiamond sensors.

●

ZnS based scintillators could be used together with wavelength shifting fibres toachieve finer spatial resolution (Vitullo et al., 2020).

The feasibility of operating such measurement techniques in the demanding environmentsof pulse reactors remains to be demonstrated as they have been developed for relatively
low flux levels.
Again, mixed sensor measurements can enhance the quality of the fast neutron flux
determination to mitigate issues induced by the demanding measurement conditions withhigh thermal and gamma flux.

Pellet swelling, cracking and densification
Local porosity measurements are currently only accessible through post-irradiation
analysis. However, there is also large interest in online measurement capabilities for
detecting and differentiating cracking and porosity changes and to differentiate bulk andlocal phenomena.

TRADITIONAL AND NOVEL MEASUREMENT METHODS FOR VALIDATION OF MULTI-PHYSICS MODELLING AND SIMULATION TOOLS36  NEA/WKP(2023)3
Acoustic instrumentation allows for active and passive interrogation techniques todetermine crack propagation. Those techniques are already used by the oil industry, arenon-intrusive, and suited for high-pressure and high-temperature conditions.
Micro to millimetre resolution can be obtained through the use of linear variable differential
transformer (LVDT) detectors. However, such detectors have large sizes and changes intemperature affect their measurements. Fabry-Pérot sensors have smaller size and only one
wire (instead of 4 or 5 for LVDT). Diameter gauges have been designed by IFE since 1960s,
but their application in pulse reactors is difficult because they cause a cold spot on thecladding, which might weaken the fuel and perturb the experimental conditions. Similarly,
Bragg grating based sensors are used in buildings to monitor structure integrities (e.g. forpipelines). However, optical fibres are sensitive to large fluences and their transmissivity
for visible light is degraded under irradiation since there is only a spectral window around
1μm with small attenuation. A pulse height based measurement design is not possible but
interferometry is still applicable and has already been tested in the OSIRIS (Cheymol etal., 2007) and BR2 (Brichard et al., 2007) facilities.
Novel ultrafast X-ray tomographic technics with 5 kHz scans could be used, too. Deporting
the X-ray source would allow using X-ray imaging under irradiation. Such technics could
be investigated to measure clad-pellet gap evolution (Banowski et al., 2018).

Fission gas release
Fission gas release kinetics needs the measurement of two parameters to be characterised:pressure and composition.
Pressure measurement inside the gap and its evolution with time could be obtained by
specific LVDT based sensors, as Japanese nuclear safety research reactor (NSRR)
researchers have developed for measuring pressure inside test fuel pins during very fasttransients (FWHM ~4 - 7 ms) (Fission Gas Dynamics programme) or through counterpressure sensor (CEA). These sensors are bulky and require several gas lines or electric
wires. Optical pressure sensors based on fibre optic properties or using a Fabry-Pérot
sensors (as developed by CEA (Cheymol et al., 2020) in replacement of the LVDTs) could
be advantageous. Classical direct pressure measurements based on the evolution of theamplitude of the ultrasonic signal cannot be applied because piezo-electric properties of thesensors are changing under irradiation.
Gas fission composition analysis could be performed in situ using a method of analysis of
the acoustic wave propagation in the gap. The method determines the respective amount ofHe and fission products from the acoustic wave velocity in the gap, which is proportional
to the molar mass of the gas. Post-irradiation measurements are also an option, and the slow
nature of the fission gas release does not require a high sampling frequency or time
resolution. The evolution of the elemental composition in Xe/Kr/He fission products could
be measured online, giving access to the kinetic process of the gas release. Online
measurement of molecular compositions during RIA may also be possible. Online in-coremeasurement by ultrasonic sensor was done in OSIRIS in 2011 (Lambert et al., 2011) for
steady-state/ramp experiment. The temperature is a key influence parameter to be acquired
during this measurement.

Friction/bounding layer of gap
Online measurements of gap profiles with acoustic interrogation (sonar like use) might bea candidate but two issues need to be resolved: development of a measurement device,
which is moving during the experiment without perturbing the experiment, and optimising

TRADITIONAL AND NOVEL MEASUREMENT METHODS FOR VALIDATION OF MULTI-PHYSICS MODELLING AND SIMULATION TOOLSNEA/WKP(2023)3  37

the acoustic coupling parameter (vibration, parallelism…) between cladding and the sensor
all along the measurement area.
The differential movement of cladding with respect to the fuel pellet in the axial direction
could be determined using LVDTs as performed by IFE (Solstad and Van Nieuwenhove,2011). Even though, Eddy current based techniques could show the existence of a gap, its
in-core implementation seems very difficult.

Cladding properties
The evolution of fuel dimensions (cladding length, densification) with exposure can bemeasured online using LVDTs as it is done in MTRs (HBWR, BR2, OSIRIS) and pulsed
reactors (CABRI, NSRR). It is worth noting that transient measurements are possible but
coils are sensitive to high radiation doses and electromagnetic compatibility effects.
Replacement of the LVDT technology by an optical technology such as Fabry-Pérot
sensors or dual Bragg grating measurements (one free fibre and one fibre is attached to thecladding being tested) have to be investigated as it allows miniaturisation of the sensors.
Attention is required for the response time of these systems, which have to be coherent withthe typical timescale of the observed physic phenomena.
The temperature of the cladding could be remotely measured using infrared thermometry
technique as already performed at the TREAT reactor (Jensen, 2016). CEA (Bouvry et al.,2017) and IRSN are also developing infrared pyrometry systems using fiber optics to be
applied respectively in Jules Horowitz Reactor (JHR) and CABRI. Infrared thermometry
implies measuring a high surface temperature (above 800-900°C) in a significantly cooler
surrounding media.
Acoustic methods for localisation and characterisation of clad cracking have to be further
developed since acoustic signals are very rich in temporal and frequency information linked
to different physics phenomena occurring in the fuel rod before and after the cracking(Traore et al., 2017). CEA is working on these techniques for future experiments as well as
on improving the modelling the path of the acoustic waves.
Thickness and thermal diffusivity of cladding and of oxide layer could be measured during
PIE with laser active pyrometry and phase sensitive modulation thermography (lock-in
thermography). The measurement method might be suitable for irradiation conditions but
further feasibility studies are required.

Influence parameters
The knowledge of the “influence parameters” listed below could be improved throughdedicated online measurements with improved spatial and temporal resolution.
Temperature
Bragg grating (e.g. at CEA) and ultrasonic measurements (e.g. in the United States) provide
fine and representative temperature profile measurements for the test pin. These
measurements are more representative of the cladding temperature than thermocouples andfar less intrusive.

Flow conditions in the test vessel
It may be possible to characterise the flow conditions in the test vessel through temperature
variation measurements and, especially, the thermal noise technique, which comprises
recording of temperature variations detected by different thermocouples at different
locations along the fluid flow (Por et al., 2003).

TRADITIONAL AND NOVEL MEASUREMENT METHODS FOR VALIDATION OF MULTI-PHYSICS MODELLING AND SIMULATION TOOLS38  NEA/WKP(2023)3
Electrical impedance tomography can be used to accurately measure the mean void
fraction. The spatial resolution of this method is low, but its temporal resolution can bevery high with up to 1 kHz sampling rate.
Existing flow visualisation techniques used in thermal hydraulics in the PANDA facility
could be adapted when possible (Paladino and Dreier, 2012). Particle image velocimetry
could be used for the measurement of two-dimensional velocity fields (Kapulla et al.,2014). With respect to the determination of the two-phase flow in the test vessel following
a departure from nucleate boiling (DNB) event, the following instrumentation could be
adapted (it remains to be seen if those methods are suitable for the space and conditions
constraints of the test vessel):●Wire-mesh sensors based on measurement of electrical conductance are used formeasuring two-phase flow parameters such as local void fraction, bubble velocity
etc. They have also been used for measurements in single-phase flows where a small
amount of tracer is added for studying fluid mixing. The typical data acquisition rate
is about 20 Hz (Prasser et al., 1998; Ylönen et al., 2011).

●

Cold neutron imaging: A non-intrusive imaging technique for non-transparent
channels has been developed using cold neutrons. The technique was successfully
applied for annular two-phase flow (Zboray and Prasser, 2013).

●

Near infrared film thickness measurement: An optical method, where near-infrared
light is used for the measurement of liquid film thickness using Beer-Lambart’s law(Mignot et al., 2018).

Finally, for high spatial-temporal resolution, distributed optical fibre sensors (Lomperski
et al., 2015) could be used provided that those experimental techniques can be adapted tothe pulse reactor type conditions.

Separate effect tests
In addition to the use of novel experimental techniques, another way to design MP
experiment is through a separate effect test.
As mentioned in Chapter 3, performing separate effect tests before the actual MP
experiment can help to reduce the overall cost of an MP experiment. Specifically, for the
PCMI challenge problem, the manufacturing of fuel rods to match given fuel conditions(designed structure, porosity, pre-strained cladding mechanically and chemically,
doping/artificial enrichment, hydrogen pickup, etc.) could be envisioned to produce
tailored and controlled separate effect tests. Those tests are needed to improve the model
performance according to pre-existing QPRT. Then more complex MP tests could bedesigned. Such an approach (several separate effect tests before the full MP test) could help
to reduce the cost of generating relevant experimental data for MP validation, and typicalbiases of such experiments due to neutronics modelling (power history) of the considered
fuel sample could be reduced. This approach is equivalent to using heated components inthermal-hydraulic experiments instead of nuclear heating.

Electronic and data acquisition systems
The MONACO system (Barbot et al., 2020), developed by CEA, is a suitable brick for MP
acquisition system for neutron flux monitoring. It is a new wide range acquisition system
for fission chambers dedicated to reactor physics experiments, which works in pulse,
current and Campbelling mode (fluctuation mode) and accommodate four simultaneousand synchronised channels now commercialised as MONACO 3 system.

TRADITIONAL AND NOVEL MEASUREMENT METHODS FOR VALIDATION OF MULTI-PHYSICS MODELLING AND SIMULATION TOOLSNEA/WKP(2023)3  39

Instrumentation for MP experiments should be developed with the aim to acquire as muchinformation as possible from the sensors, which is a strategy already successfully appliedin high-energy particle physics. This strategy becomes even more important given thelimited number of sensors in experimental vehicles. For example, for neutron flux
measurements one should not only acquire the counting rates, but also energies and timeof arrivals of experimental events. This strategy would enable new offline data processing
capabilities such as analysing time correlations between detectors, extending measurement
ranges or correcting drifts in signals.
For short experiments, like pulse reactor transients that last less than a second, thephilosophy would be to limit as much as possible online data processing (likediscriminating neutron pulses to produce TTLs) and, instead, acquire raw signals wheneverpossible similar to the analysis of acoustics sensors. It is already possible to digitise thewhole signal of neutron detector (taken at the output of the preamplifier) with minimum
loss of information. Of course, this type of “trigger-less” acquisition requires a large
amount of storage and requires fast analogue to digital converters (ADCs).

Scientific documentation
The next generation of scientific documentation tool should be used (literate programming,a technique coming from computational engineering) to have documentation embedded in
the post-processing code itself. The report could then be produced at the same time withthe data itself. Choices have to be made, justified and documented; processed data are
stored in a database with a coherent set of identification (meta) data.
The use of LaTeX or Markdown, a versioning system as well as the use of video recordings
is beneficial. This approach is also very good for knowledge transfer.

TRADITIONAL AND NOVEL MEASUREMENT METHODS FOR VALIDATION OF MULTI-PHYSICS MODELLING AND SIMULATION TOOLS40  NEA/WKP(2023)3

5. Conclusions

This working paper summarises a methodology to design multi-physics (MP) experiments
suitable for the validation of novel MP reactor core simulation codes. The method is
illustrated for PCMI. The outcomes in terms of potential instrumentations are not meant tobe exhaustive.
With respect to the status of the current state-of-the-art measurements in pulse type reactor,
it appears that the measurements are not fully suitable for MP code validation. Gaps havebeen identified mostly related to the need for online data with an improved spatial andtemporal resolution, but could partially be resolved with already existing techniques.
However, the feasibility and usefulness of the proposed techniques under irradiation has tobe demonstrated.
The determination of the measurement techniques (both existing and novel) and theirassociated measured parameters for the “best experiment” should be specified based on an
iterative process with MP modellers: a clear expression of the needs for MP validation isparamount. In particular, modellers should express what exact quantity they want tomeasure; the experimentalists should specify for each kind of measurement technique whatis the quantity measured, and what are the derived quantities that should be obtained,
including uncertainties. For example, the cladding surface temperature or the thermal
gradient within the cladding are not the same objects.
Finally, it should be stressed that although the goal is to validate MP simulation tools,
measurements addressing only a single physical phenomenon at a time are still extremely
useful as they allow reducing the experimental uncertainty and have better control on thepotential measurement bias.

TRADITIONAL AND NOVEL MEASUREMENT METHODS FOR VALIDATION OF MULTI-PHYSICS MODELLING AND SIMULATION TOOLSNEA/WKP(2023)3  41

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