© V.H. Sanchez-Espinoza et al., Published by EDP Sciences, 2025

1. Introduction

Experimental research is a key pillar to enhance knowledge and perform code validation. In addition, the construction and operation of experimental facilities equipped with state-of-the-art measurement devices to catch the physical phenomena of interest may be in some cases very costly. Nevertheless, key experimental investigations are mandatory in nuclear engineering which is highly regulated to assure robust safety features at high standards in the safe design and operation of nuclear reactors. The safety demonstration in the frame of a licensing process or during the regular supervision of the operation is based on numerical tools validated for the reactor design of interest and it is complemented by dedicated experiments. In view of the increased interest on the deployment of Generation 3 and 3+ Light Water Reactors (LWR) and of water cooled (WC) Small Modular Reactors (SMR) in the EU [1] and worldwide [2], it is of great interest to do a systematic screening of the experimental research programs across Europe, specifically dedicated to severe accident phenomena, nowadays running. Based on such studies, strategic recommendations to the stakeholders can be done to keep high level of knowledge needed to assess the safety features of any reactor design going to be built in the EU in the next years. In the EU, new initiatives were started e.g., in the European User Facility Network (OFFERR) with the goal to optimize the use of existing experimental facilities and open opportunities for research groups without experimental infrastructure to take profit of existing ones and to take part or even perform experiments according to their needs [3].

Hence, the Horizon Europe SEAKNOT project [4, 5] that started in 2022 aims to contribute to the knowledge preservation and dissemination about severe accident research in Europe. To achieve these goals, the following objectives are defined: (1) perform a critical analysis of the existing knowledge on severe accidents, (2) identify the needs for future experimental research required to understand the severe accident progression required for the optimization of mitigative measures aimed to minimize the radiological consequence of core meltdown accidents and (3) disseminate and improve the knowledge of young researchers on reactor safety regarding both analytical and experimental investigations on relevant severe accident domains. These goals can be translated in the following specific objectives:

• Elaborate a Phenomena Identification Ranking Table (PIRT) on severe accidents considering the available EU-capabilities on severe accident experimental research (Work Package 1),

• Elaborate a validation data base directory by developing a comprehensive mapping of the existing and future experimental European activity (2023–2030) (Work Package 2),

• Build a Severe Accident Infrastructure NETwork (SAINET) of the European research infrastructure on severe accident for the time horizon 2023–2030 and identify the severe accident phenomena addressed including ATFs, mitigation actions, and accident-resistant instrumentation including SMRs (Work Package 3), and

• Promote mutually beneficial collaborations between the experimental facilities and the various European partners such as universities, regulatory bodies, and industry (Work Package 4).

The Work Package 3 is focused on two tasks:

• Mapping of Severe Accident Experimental Facilities (MAPEX): The goal here is to identify and list the experimental facilities devoted to severe accident research still in activity inside Europe (within SEAKNOT-partners, EU-institutions in countries partners of SEAKNOT, and in EU-institutions not participating in SEAKNOT).

• Establishing a Severe Accident Infrastructure NETwork (SAINET): The goal here is to build SAINET infrastructure to gather main players in Europe on the field of severe accident research based on the outcome of the mapping activity and able to answer to future experimental needs defined in Work Package1 and 2.

This paper describes the main outcomes of the critical review performed for MAPEX considering facilities of different stake-holders (universities, research centers, industry, TSOs) in the European Union focused on severe accident research by means of a questionnaire. Valuable information was collected such as type of facilities, their main characteristics in term of operation conditions, severe accident domain, instrumentation, use of data, and appropriateness of data for code validation, operational team, critical issues as funding, retirement of operational/scientific team, future work program, and current funding frameworks. Based on it, a European research platform for experimental investigations can be proposed.

2. Methodology

For the purpose of gathering information for the MAPEX, European experimental facilities under operation on severe accident topics and still operational until 2022/2023 were identified within the SEAKNOT-consortium and outside it. Future experimental activity between 2023 and 2030 has been assessed, too. In this approach, attention has been paid to the topics of the experimental programs devoted to single or coupled phenomena that may take place in all phases of a severe accident progression in LWR and SMRs.

To achieve these goals, the following methodology has been applied:

• Building of a questionnaire for the experimental mapping activity (see appendix 1)

• Identification of test facilities in SEAKNOT-Partners involved in Work Package 3,

• Identification of test facilities in SEAKNOT-Partners not involved in Work Package 3,

• Identification of test facilities in institutions of EU-Countries partners of SEAKNOT but not belonging to the SEAKNOT-consortium,

• Identification of test facilities in institutions of EU-Countries not partners of SEAKNOT.

According to this methodology, the following steps [6] were carried out to draw conclusions about the assessment of the capabilities of the European severe accident experimental facilities: Identify institutions and contact persons to answer the questionnaire. For it, four target groups of institutions were considered: Group-1: SEAKNOT-Partners involved in the Work Package 3, Group-2: SEAKNOT-Partners not involved in Work Package 3, Group-3: Institutions in EU-Countries partners of SEAKNOT but not beneficiaries, and Group-4: Institutions in EU-Countries not partners of SEAKNOT.

Identify key-persons in all the four-target groups to answer the questionnaire: Evaluate the questionnaire regarding e.g., Total number of test facilities, mapping of experimental facilities with SA research domain, and identify experimental facilities under risky situation regarding funding, research team, and perspective. More details of the methodologies can be found in [6].

Table 1 shows the number of requests sent out and the number of responses/facilities identified by evaluating the questionnaire.

One first challenge was concerning the number of answers. As it can be seen in Table 1, SEAKNOT partners have answered but less than one third of organizations outside of SEAKNOT have answered to the questionnaire. Nevertheless, it can be considered that the mapping of European Severe Accident activities is relatively complete because the main severe accident facilities in activity were belonging to SEAKNOT consortium.

3. Quantitative evaluation of the questionnaire

The synthesis hereafter is structured around the following topics of relevance for SEAKNOT: main phenomena addressed by research programs of experimental facilities, main severe accident phases covered by programs of experimental facilities, reactor types targeted by experimental programs, experimental programs addressing ATF, experimental programs addressing SMR-issues, experimental facilities without a defined research topic for 2024–2030, and experimental programs under critical conditions (man-power shortage, knowledge loss, etc.).

3.1. Experimental facilities operated by SEAKNOT-Partners and their age

In Figure 1, the number of severe accident experimental facilities operated by European institutions partners of SEAKNOT is shown. Geographic repartition in Europe is not uniform: more than 50% of these experimental facilities are still operating in 3 main European countries: France (CEA, IRSN), Sweden (KTH) and Switzerland (PSI). The other countries are less involved in experimental activities whereas a lack of experimental data has been identified by the SEAKNOT partners [4].

Fig. 1. Number of facilities per institution- SEAKNOT-WP3.

The time life of experimental facilities is also an important parameter. In the frame of SEAKNOT assessment activity of severe accident facilities still in operation, it has been asked to SEAKNOT partners to provide insights into the average age of the experimental facilities (Fig. 2). Furthermore, an overview of the facilities expected to remain operational until 2030 for future R&D activities has been also assessed.

European severe accident facilities are relatively old: the majority of the test facilities were constructed between the eighties and end of the nineties, meaning that their average age is around 30 years. The maintenance of old facilities are crucial but very expensive, meaning possible closure by 2030.

3.1.1. Severe accident current and future research programs

Based on the information collected, an important aspect was to identify in which current and future research programs the facilities are involved or not. This information is essential for assessing the capability of European experimental facilities to meet both current and future (2030) research needs, derived from SEAKNOT PIRT activities (WP1) and to propose future European programs in order to answer to nuclear safety challenge for SMRs and new mitigation tool for nuclear energy in Europe.

Table 2 lists the facilities with the current and future severe accident programs in which the facilities are involved. The majority of European severe experimental facilities have some domestic and international programs, but almost no European program activities. But, some experimental facilities at IRSN (France), PSI (Switzerland) and KIT (Germany) have no program planned for the period 2025–2030, meaning in most unfavorable situation the mothballing of the facilities, leaving them vacant while continuing to cover maintenance costs. European experimental expertise in the field of severe accident has taken several years and has been very costly for European community whereas it can be lost rapidly if any European program in the 2025–2030 is proposed.

3.1.2. Severe accident fields and topics addressed by the experimental facilities

Based on the information collected, an important aspect was to identify the actual research topics of test facilities. This information will be useful for identifying relevant topics for which only a limited number of experimental facilities are available. In Table 3, the experimental facilities are listed with their respective severe accident research fields and topics provided in the questionnaire. Almost all severe accident topics, identified in the frame of SEAKNOT project, are covered as it can be seen in Table 3. But it will be only at the end of the PIRT activity of SEAKNOT that it will be possible to assess if European facilities can answer to all experimental needs in the field of severe accident for the period 2025–2030.

Experiment facilities are focused on quite diverse research fields and mainly related to LWRs including CANDU reactors and water-cooled SMRs. It is important to note that no specific tests for SMR are devoted to severe accident conditions, except the ones dedicated to passive heat removal systems. The tests devoted to mitigation are related to source term and containment issues.

3.1.3. Experimental facilities and scaling

Table 4 lists the different facilities along with their operator, facility name, associated severe accident research fields, and the scaling information.

For severe accident studies and nuclear safety, scaling effect is crucial to be able to have reliable data for extrapolation to reactor case. The majority of the test facilities are devoted to PWRs, and BWRs, some to VVER, and CANDU (current generation 2 European fleet). SMRs activity is also concerned, but no specific facility is strictly devoted to severe accident studies for SMRs. The majority of the facilities are categorized as of medium and large-scale. Only few of them are of small scale mainly to perform analytical studies. Based on this classification, 21 facilities are categorized as “large-scale”, 18 as “mid-scale” and 3 as small-scale facilities. However, this declared categorization by the facility technical staff, should be technically supported in case of future application for SMR SA investigation, whatever the design.

3.2. Experimental facilities operated by SEAKNOT-Partners not involved in WP3

The SEAKNOT-partners not involved in the WP3- have also answered to the SEAKNOT questionnaire (Fig. 3). Five facilities are operated by two German institutions, namely Becker Technologies (BT) and Research Center Jülich (FZJ). Figure 4 shows the starting dates of the operation of the facilities operated by two German institutions. These facilities are recently operated (after 2000) except for REKO.

Fig. 2. List of facilities with the year of commissioning of the SEAKNOT-WP3.

Fig. 3. List of facilities with the year of commissioning of the SEAKNOT-WP3.

Fig. 4. List of facilities with the year of commissioning of the SEAKNOT-partners not involved in SEAKNOT WP3.

Table 5 summarizes the current research fields and topics addressed by the five facilities. The main topics of severe accident research are devoted to containment phenomena during severe accidents relevant for LWRs and SMRs. Some activities are planned till 2030 mainly focused on CFD approach and SMRs application (Tab. 6).

4. Summary and conclusions

In the frame of the European project SEAKNOT (Severe Accident research and KNOwledge management) project, an analysis and mapping of European severe accident research facilities currently under operation has been performed. Through a general questionnaire, SEAKNOT partners have been able to answer about their current and future (2025–2030) experimental activities in the field of severe accidents including LWRs and SMRs. The questionnaires evaluated comprise data of 59 facilities.

A systematic evaluation of the provided information was performed, which included the description of the facilities, current and future research focus, main severe accident phenomena addressed, reactor types considered. In this context, particular attention was given to identify research programs addressing new topics such as ATF and SMRs. The information collected and the evaluation performed pave the way for the mapping of European severe accident facilities under operation and it allows to identify experimental needs in the severe accident research for LWRs and SMRs until 2030.

In 2025, the main topics interesting severe accident studies LWRs and SMRs are covered by several experimental facilities belonging to SEAKNOT consortium. 22 experimental facilities do not have concrete “research plan” established yet for the period 2024–2030 which is a bad indicator for research in nuclear safety.

European experimental expertise in the field of severe accident has taken several years and has been very costly for European community. This level of excellence, recognized worldwide, can be lost rapidly if any European program in the 2025–2030 takes into account the field of severe accident for the current and future nuclear fleet.

Acknowledgments

The authors would like to acknowledge the work and thank all the participants of SEKNOT project funded by the European Union under the grant Agreement No. 101060327. Views and opinions expressed are those of the Authors only and do not necessarily reflect those of the European Union or EUROATOM. Neither the European Union nor the granting authority can be held responsible for them.

Funding

The authors thank the Horizon 2020 SEAKNOT project SEKNOT project funded by the European Union under the grant Agreement No. 101060327.

Conflicts of interest

Authors VHSE, PP, and LEH certify that they have no financial conflicts of interest (e.g., consultancies, stock ownership, equity interest, patent/licensing arrangements, etc.) in connection with this article.

Data availability statement

Data associated with this article cannot be disclosed due to legal reason of the different owner of the data.

Author contribution statement

VHSE: Writing, Editing, Conceptualization, Formal Analysis, Methodology, Data Curation, Investigation, PP: Reviewing, Conceptualization, Investigation, Reviewing, LEH: Funding acquisition, Reviewing, Methodology.

References

Appendix A SEAKNOT questionnaire for the mapping of severe accident facilities in Europe

1. Is your Institution operating an experimental facility in 2023 on the following severe accident areas?

• a)

  In-vessel    □

• b)

  Ex-vessel   □

• c)

  Source term  □

• d)

  Containment  □

• e)

  Other     □

2. Test facility description in 2023

(In case of several facilities, please provide below information separately for each facility)

¹ Critical competences for the next 5 years: The experimental activity will be stopped, person with critical experimental competences will be retired and not replaced, the experimental activity will be reoriented, others.

3. Other topics

All Tables

All Figures

	Fig. 1. Number of facilities per institution- SEAKNOT-WP3.
In the text

	Fig. 2. List of facilities with the year of commissioning of the SEAKNOT-WP3.
In the text

	Fig. 3. List of facilities with the year of commissioning of the SEAKNOT-WP3.
In the text

	Fig. 4. List of facilities with the year of commissioning of the SEAKNOT-partners not involved in SEAKNOT WP3.
In the text