Reporting on nuclear safety

Oversight reports of spent nuclear fuel final disposal facility

STUK carries out inspections of the spent fuel encapsulation and disposal facility in Olkiluoto in accordance with the inspection programme during construction. STUK publishes tertial reports on the regulatory oversight of the plant under construction.

Oversight reports for year 2025

Tertial report 1/2025, January–April

At the end of 2021, Posiva submitted an application for an operating licence for a final disposal facility for spent nuclear fuel to the Ministry of Economic Affairs and Employment. The Ministry originally asked STUK to deliver its statement by the end of 2023. Since it was not possible to issue the statement within the original schedule, the deadline was extended. The assessment work is still going on, and STUK has been given additional time and it will issue its statement by 31 December 2025, if possible. The completion of the statement depends on when STUK has all the material required for preparing the safety assessment.

Once Posiva has demonstrated that the operation of the plants and the safety of the final disposal meet the requirements, the safety assessment can be completed. If Posiva’s future material updates do not cause any further investigation, it is possible that the inspection of the operating licence application materials will be completed well before the end of the year.

During the first third of 2025, STUK has only received individual material updates to Posiva’s operating licence application material. In connection with the requests for clarification, Posiva has asked STUK for additional time to respond to the requirements. STUK expects to receive the remaining material updates during the second third of the year.

Once the operating licence application material has been processed, STUK will prepare a safety assessment, which will be appended to STUK’s safety statement. The Government can only grant an operating licence for Posiva’s encapsulation and final disposal facility if STUK's safety statement supports it.

During the first third of 2025, STUK made a decision requiring Posiva to investigate the situation of the systems and plant-level test runs. STUK has not yet received most of the test run reports of safety-classified systems. Therefore, it has not been possible to assess the test runs as a whole. The phasing of test runs also needs to be clarified.

As part of the safety assessment, STUK assesses the readiness of Posiva’s organisation to start operating the final disposal facility. STUK has continued the enhanced supervision of the Posiva organisation, which it started in autumn 2024, to ensure that the operations meet the set safety requirements and prerequisites. In addition to enhanced supervision, STUK has adopted event investigation methods as a supervisory measure to deepen its understanding of the observations it has made about the organisation and the underlying factors. With event investigation methods, STUK also aims to identify areas for development that can be utilised in the planning, targeting and development of supervision as well as in updating regulations and STUK’s regulations and guidelines.

STUK’s monitoring of supervision areas, with the main aspects of monitoring described below for the period between the beginning of January and the end of April 2025.

STUK is moving from three-monthly reporting to semi-annual reporting in the reporting of the supervision of the final disposal facility. The first semi-annual report is expected to be published in the last third of 2025.

Construction and commissioning

In the first third of 2025, STUK continued the supervision of the commissioning of Posiva's encapsulation and final disposal facility. STUK has reviewed commissioning-related documents and been on site to carry out supervisory activities.

With the test runs of the nuclear facility’s systems, Posiva ensures that the systems function as planned. In particular, STUK supervises safety-classified systems, whose operation is important for nuclear and radiation safety. Through plant-level tests on the entire facility, Posiva demonstrates that the various systems and functions work as a whole as planned in normal operating situations and that the facility can be used in accordance with the operating instructions.

Most of test run reports of safety-classified systems have not yet been submitted to STUK, so it has not yet been possible to assess the test runs as a whole. STUK can assess the commissioning once the reports in accordance with the requirements have been submitted to it.

Instructions and operation of the plant

STUK has processed Posiva’s administrative instructions and the operating and alarm procedures as well as the instructions related to emergencies that are needed when the operation of the plants begins. STUK has also overseen the validation of the instructions. The instructions are still being drawn up or updated and their validations are still pending.

STUK has previously submitted a request for clarification on Posiva’s operational limits and conditions (OLC). STUK expects Posiva to update the material in accordance with the request for clarification in May 2025. OLC are part of the operating licence application and must be approved before STUK can complete the safety assessment.

Radiation measurements and emergency response arrangements

The commissioning of radiation and radioactive release measurement systems has progressed, but matters related to the management of releases of Posiva’s nuclear plants are still unfinished. STUK is awaiting clarification of these matters when Posiva issues updates of operating licence application materials to STUK. In addition, STUK has received for processing applications for deviating from the requirements of the YVL Guides concerning technical barriers to the spread of radioactive substances. STUK cannot issue a favourable safety assessment of the encapsulation and final disposal facility and Posiva cannot start to operate the plant before the aforementioned matters are taken care of appropriately.

With regard to emergency response arrangements, STUK has required Posiva and TVO to update their shared emergency plan. STUK is processing the updated emergency plan.

Posiva's trial run of final disposal

Posiva completed the trial run for the encapsulation plant at the beginning of February 2025. According to current information, Posiva will continue the trial run with an underground part in autumn 2025. STUK will inspect the underground part of the trial run in the same way as the encapsulation plant.

STUK made many observations about the readiness of Posiva’s guidelines during the inspection of the trial run of the encapsulation plant. STUK also made one requirement regarding the traceability of decision-making. STUK has reviewed its supervision experiences and no significant need for changes to the planned supervision has been identified.

Natural and technical release barriers at the final disposal facility

Posiva must demonstrate by means of procedure tests that the methods of manufacture, installation, construction and use of the planned release barriers for radioactive substances are suitable for the intended use and meet the required quality and safety requirements.

During the year third, Posiva submitted several reports of the procedure tests of the final disposal capsules to STUK for processing. STUK has identified matters in the submitted materials that need to be clarified further. STUK is awaiting the required additional clarifications and the procedure test report for the extruded copper tube for processing during the summer.

The caplsule’s system description material and the related changed technical requirements have been approved by STUK, which will hopefully reduce uncertainties related to production qualification.

STUK expects to receive system descriptions of the final disposal holes and the bentonite buffer in the near future. Once they have been approved, STUK can process the engineering implementation plans for these systems.

With regard to tunnel backfilling, Posiva has completed the procedure installation test, and its reporting is expected to be processed in the near future. In addition, STUK has agreed with Posiva on a start-up readiness inspection of the production of tunnel backfill material, after which Posiva can start manufacturing material for the first final disposal tunnel.

There have been delays in the manufacture and commissioning of the installation equipment, which is why the procedure tests for the installation of the bentonite buffer and canister have been postponed to the autumn.

Successful procedure tests demonstrate that the release barriers can be manufactured and installed as planned. In its safety assessment, STUK assesses the results of the methodological tests as part of the overall assessment of the safety of the final disposal solution.

Nuclear material regulation

The nuclear material safeguards reports submitted by Posiva around the end of the year are sufficiently accurate and STUK has submitted the required reports to the International Atomic Energy Agency, IAEA, based on them.

The installation work of the IAEA's and the European Commission’s monitoring equipment is almost complete and the final installations can be completed during 2025.

The reporting practices required for monitoring and the secure transfer of data have been discussed with the IAEA, the European Commission and Posiva, and data transfer methods will be developed further this year.

Security arrangements

Posiva has provided STUK with materials related to the implementation of security arrangements. STUK has approved the materials, but issued some requirements for them. However, Posiva has not yet provided all the material related to the implementation of the security arrangements, so STUK is not yet able to issue a positive safety assessment of the security arrangements.

Posiva still also has certain open security arrangement requirements related to commissioning, the fulfilment of which STUK will verify when Posiva has made progress in the deployment of security arrangements. In addition, STUK has planned the upcoming security arrangement inspection for the spring.

Organisational oversight

Posiva has implemented an extensive management system and safety culture self-assessment and updated its safety culture action plan. STUK uses these assessments to assess Posiva’s ability to identify management and safety culture development needs and how Posiva will take the necessary measures. STUK also participated in a safety culture seminar organised by Posiva. In the seminar, STUK presented related regulatory requirements and expectations for the licencee's safety culture. STUK has emphasised to Posiva that scheduling pressures must not affect the careful processing of safety matters and the quality of operations.

In the inspection, STUK has assessed Posiva's learning from internal experiences and investigation of events. In the inspection, STUK found that Posiva has increased its resources for operating experience activities, but has not yet carried out event investigations related to nuclear and radiation safety. STUK required Posiva to take care of the development of operating experience competence as well as the assessment and development of operations as required by the regulations. STUK also noted that there is room for improvement in the processing of human and organisational factors in connection with observations and deviations.

In the inspection, STUK assessed the readiness of Posiva’s procurement operations and supplier control for the production phase and the identification of development needs. Posiva’s resource estimates were found to be appropriate and will be updated after the start of production. Posiva is prepared for the procurement of spare parts, but the work has progressed at a slower pace than expected. Appropriate guidelines are used in supplier control, and Posiva develops the control to be more targeted. STUK intends to supervise the development of spare parts management and supplier control. In its supervision, STUK continues to pay attention to the adequacy of the resources and expertise needed to ensure safety, which is particularly important as Posiva moves towards the operation phase. In the spring inspection, STUK will assess Posiva’s resource planning and general procedures for ensuring competence, in which Posiva utilises TVO Group’s resources and competence in many ways. There is a relatively established practice between the companies to reserve and acquire the necessary skills and resources. The procedures for managing external resources also seem appropriate. In the future, STUK considers it important that competence areas and related resource needs that are significant for long-term safety are identified and that possible risks related to the availability of resources are assessed. STUK supervises Posiva's ongoing project to secure competence, in which an analysis is used to identify task-specific competence requirements and to ensure the necessary training and competence development in a timely manner.

Assessing long-term safety

STUK has worked on completing the review of the safety justification for demonstrating long-term safety submitted as part of the operating licence application. At the same time, STUK has been preparing an inspection report on the safety justification and safety assessment texts concerning long-term safety. In addition, as part of the review of the safety justification, STUK and external experts have been preparing the last supplementary emissions, transmission and consequence analyses of radionuclides. In the first third of 2025, STUK assessed Posiva’s responses to the request for clarification submitted in the second third of 2024. In addition, at the end of the first third of 2025, STUK received the last response to the request for clarification concerning the impacts of the design changes on long-term safety. In April, STUK submitted the first request for clarification concerning the safety justification to Posiva in 2025. The request for clarification includes questions concerning probabilistic and consequence analyses, analyses with different transit assumptions and different timings.

Oversight reports for year 2024

Triannual report 3/2024, September–December

At the end of 2021, Posiva submitted an application for an operating licence for a final disposal facility for spent nuclear fuel to the Ministry of Economic Affairs and Employment. The Ministry originally asked STUK to deliver its statement by the end of 2023. Since it was not possible to issue a statement within the original schedule, an additional period lasting until the end of this year was agreed a year ago. As the assessment work is still ongoing, STUK proposed to the Ministry in December that it deliver the statement by 31 December 2025, if possible. The Ministry approved STUK’s proposal. The completion of the statement depends on when STUK has all the material required for carrying out the safety assessment.

Once Posiva has demonstrated that the operation of the plants and the safety of the final disposal meet the requirements, the safety assessment will be completed promptly. If Posiva’s future updates to the material do not cause any further investigation, it is possible that the work will be completed well before the end of the year.

During the final third of 2024, STUK received updated operating licence application materials from Posiva and continued processing them. STUK also supervised the commissioning of Posiva's nuclear facility and the preparations for its use. On 30 August, Posiva started a trial run of final disposal at the encapsulation plant to test the final disposal operations without spent nuclear fuel. This test continues during 2025.

Overall, STUK has progressed well in processing the operating licence application, but the material is still being reviewed. The processing has been delayed more than expected due to deficiencies observed in the material, updates to the materials connected to Posiva’s plant changes and the extent of the needed updates. STUK received a number of updated materials from Posiva during the final third of the year. STUK has also submitted a new request for clarification concerning the operating licence application material and is awaiting responses to some previous requests for clarification.

STUK has paid special attention to the operations of Posiva’s organisation. As part of the safety assessment, STUK assesses the readiness of Posiva’s organisation with regard to the operation phase. STUK has enhanced supervision to ensure that the organisation meets the defined safety requirements and prerequisites.

Once the operating licence application material has been processed, STUK will prepare a safety assessment, which will be attached to STUK’s safety statement. The Government can only grant an operating licence for Posiva’s encapsulation and final disposal facility if STUK's safety statement supports it.

STUK cannot issue a favourable assessment before it has approved Posiva’s final safety report, the operational limits and conditions, the safety justification concerning long-term safety and has determined under its supervision that the operations of Posiva’s organisation meet all the requirements set for it.

STUK’s supervision consists of several areas, with the main aspects of supervision described below for the period between the beginning of September and the end of December 2024.

Construction and commissioning

During the last third of 2024, STUK supervised the manufacture, installation and commissioning of the systems needed at Posiva’s encapsulation and final disposal facilities. The encapsulation and final disposal facility requires lifting and transfer equipment to transfer spent nuclear fuel and the transfer container and canister containing it. STUK has reviewed the updated construction plans of these lifting and transfer equipment.

With testing the nuclear facility’s systems, Posiva ensures that the systems function as planned. STUK pays particular attention to the systems that are safety-classified, which means that their operation is important for nuclear and radiation safety. Tests on the entire facility demonstrate that the various systems and functions work as a whole as planned in normal operating situations and that the facility can be used in accordance with the operating instructions. STUK is processing the report prepared regarding the test of the encapsulation plant. Thus far, STUK has only received test operation reports of safety-classified systems for one system, and these reports do not provide a sufficient basis to assess the trial runs as a whole. The commissioning assessment will be carried out once the required reports have been delivered to STUK.

The number of processed system materials has decreased in the last third of the year, and most of the necessary documents have already been delivered to STUK. There are likely some matters to be clarified regarding the system materials and STUK has not been able to approve Posiva’s final safety report, which is one of the materials of the operating licence application.

Instructions and operation of the plant

STUK prepared a request for clarification on the operational limits and conditions. STUK expects Posiva to update the material in early 2025 in accordance with STUK’s request for clarification. The operational limits and conditions are part of the operating licence application and must be approved before STUK completes the safety assessment.

STUK has also processed Posiva’s administrative instructions and the operating and alarm procedures as well as the instructions related to emergencies that are needed when the operation of the plants begin. STUK has overseen the validation of the instructions carried out in connection with the plant’s commissioning tests and trial run of final disposal. The instructions are still being drawn up or updated and their validations are still pending.

Radiation measurements and emergency response arrangements

The commissioning of radiation and radioactive release measurement systems has progressed, but matters related to the management of releases of Posiva’s nuclear plants are still unfinished. STUK cannot issue a favourable safety assessment of the encapsulation and final disposal facility and Posiva cannot start to operate the plant before these matters are taken care of appropriately.

With regard to Posiva's emergency response arrangements, STUK has required Posiva and TVO to update their shared emergency plan.

Posiva's trial run of final disposal

Posiva started the trial run of final disposal at the end of August, and it progressed according to plan during the first three canisters. STUK oversaw the trial run and made several observations. According to the observations, Posiva improved its operations during the trial. After three canisters, Posiva decided to take a technical break of approximately two months, during which technical changes were made and operations developed. The trial was continued before Christmas with a fourth canister. The trial run of the encapsulation plant will continue until early 2025. The trial run of underground operations is expected to start later in 2025.

Natural and technical release barriers at the final disposal facility

Posiva’s challenges to meet its defined technical requirements have continued, but progress has been made in the reassessment of Posiva’s own technical requirements. During the autumn, STUK has reviewed some of Posiva’s redefined requirements, but based on the material, STUK has not been able to assess the impact of the changes on safety as a whole. STUK is still waiting for materials related to the technical changes made by Posiva.

STUK has approved the first report on procedure tests on the canister, which concern the manufacture of the base and lid. Delivery of the related structural plan is expected next. STUK has also suspended some processing and made requests for clarification regarding incomplete and poor-quality materials.

With regard to tunnel backfilling, STUK has approved the procedure test plan for the installation, but is still awaiting an updated system description. The procedure test for the installation of tunnel backfilling is expected to take place in early 2025.

The system description of the bentonite buffer and the structural plan for installation have not been submitted to STUK. STUK suspended the processing of the procedure test plans for the installation of the bentonite buffer and canister because significant deficiencies and inconsistencies were observed in the material during the preliminary inspection. The plan has not yet been resubmitted, which may delay the completion of the procedure test.

STUK has carried out partial structural inspections in the final disposal tunnels and inspected the final disposal holes used in the trial run of final disposal and the conformity of the final disposal tunnel. Otherwise, Posiva’s rock construction work is mainly focused on levelling the bottom of tunnels by crushing.

Nuclear material regulation

Some of the safeguards and the reporting practices required for monitoring have been tested during Posiva's trial run.

Security arrangements

Due to future material deliveries and inspections related to commissioning, the conclusion of the safety assessment regarding security arrangements cannot be given yet.

STUK has processed the security arrangement material related to the transfer of spent nuclear fuel and supervised the transfer operations related to Posiva’s trial run. STUK has also carried out an annual inspection, which focused especially on the physical security arrangements, considering the preparations for commissioning.

Posiva still has certain open security arrangement requirements related to commissioning, the fulfilment of which STUK will verify when Posiva has made progress in the deployment of security arrangements.

Organisational oversight

STUK has overseen the organisation of Posiva’s independent self-supervision and the development of procedures. Posiva intends to learn more about self-supervision for future operations during the trial run. From STUK’s point of view, there are still scheduling pressures evident in Posiva’s operations that may cause them to progress even if the plans or requirements of the nuclear sector have not been fully taken into account. This has been observed during supervision, for example, as unfinished matters. In addition, deficiencies have been observed in the approval processes of the key operating licence materials as well as in the systems-related materials delivered during the autumn.

STUK has emphasised that Posiva must specify in more detail what kind of nuclear safety culture it aims to achieve for the operation phase. In addition, Posiva must specify concrete measures for developing the safety culture. STUK has also observed slowness in the implementation of some development measures, which have been determined based on observations made on the basis of external assessments.

In its supervision, STUK will continue to pay attention to the adequacy of the resources necessary to ensure competence and safety. STUK has enhanced supervision, particularly in those areas where additional information and evidence are required for the preparation of the safety assessment.

Assessing long-term safety

STUK has continued the review of the safety justification for demonstrating long-term safety submitted as part of the operating licence application.

During the final third of the year, STUK assessed responses that Posiva submitted to STUK to the request for clarification sent in the second third. STUK is still awaiting responses to two requests for clarification concerning the buffer and tunnel backfilling and the impact of the design changes on long-term safety.

As part of the review of the safety justification, STUK has continued to carry out emissions, transmission and consequence analyses of radionuclides. These are related to the assessment of the biosphere transmission and doses of radionuclides.

Posiva is expected to answer the remaining questions during the winter and spring 2025. Once STUK has received the answers to the questions it has posed, it can finalise its assessment of long-term safety.

Triannual report 2/2024, May–August

During the second tertial of 2024, STUK received updated operating licence application materials from Posiva and continued processing them. STUK also supervised the commissioning of Posiva's nuclear facility and planned supervision during its use. At the end of the tertial, Posiva started a trial run at the encapsulation plant to test the final disposal operations without spent nuclear fuel.

STUK will draw up a safety assessment at the end of the processing of the licence application material, and this assessment will be appended to STUK’s safety statement. STUK’s favourable statement on safety is a prerequisite for the Government to grant an operating licence for Posiva’s encapsulation and final disposal facility.

Overall, STUK has progressed well in processing the operating licence application, but the material is still being reviewed. STUK received a number of updated materials from Posiva at the beginning of September. Without these materials, the processing cannot be completed. STUK is still waiting for some material. The scope of the materials and the processing of necessary updates have delayed the processing of the application material more than expected.

The Ministry of Economic Affairs and Employment of Finland, as the licensing authority, originally, requested STUK’s statement by the end of 2023, but STUK asked for additional time until the end of 2024, as STUK is still processing the licence application material. STUK’s supervision consists of several areas, with the main aspects of supervision described below for the period between the beginning of May and the end of August 2024.

Construction and commissioning

STUK has supervised the manufacture, installation and commissioning of the systems needed at Posiva’s encapsulation and final disposal facilities. The encapsulation and final disposal facility requires lifting and transfer equipment to transfer spent nuclear fuel and the transfer container or canister containing spent fuel. STUK has reviewed the construction plans of this lifting and transfer equipment and carried out structural inspections to ensure that the equipment is installed in accordance with the approved plans and methods.

The number of processed system materials has decreased in the last third of the year, and most of the necessary documents have already been delivered to STUK. Tests on the entire facility demonstrate that the various systems and functions work as a whole as planned in normal operating situations and that the facility can be used in accordance with the operating instructions. STUK has reviewed the plans for trials concerning the plant’s operations and supervised the implementation of trial use during the summer.

Instructions and operation of the plant

Before STUK can make a favourable safety assessment of the encapsulation and final disposal facility and Posiva can start to operate the plant, STUK must also approve the plant’s technical specifications (operational limits and conditions). STUK is currently processing the updated material. STUK has also reviewed Posiva’s administrative instructions and the operating and alarm procedures as well as the instructions related to emergencies that are needed when the operation of the plants begins. STUK has monitored the validation of the instructions carried out in connection with the plant’s commissioning tests. Some of the instructions and their validations are still in progress or being updated.

Radiation measurements and emergency response arrangements

The commissioning of the radiation and radioactive release measurement systems has progressed. Posiva has made changes to some of the release measurement devices, so the materials still need to be updated. STUK has also required Posiva to supplement the licence application material with regard to the measurement and monitoring of releases. Clarifications concerning these issues were received from Posiva in September.

STUK has required Posiva and TVO to update their shared emergency plan. The precautionary action zone and emergency planning zone defined for the Posiva nuclear plants correspond to those of TVO. The environmental impacts of possible accidents in the vicinity of the Posiva plants are significantly less than the environmental impacts of a serious reactor accident at the nuclear power plant.

Posiva's trial run of final disposal

Posiva started the trial run at the end of August. In connection with the trial run, STUK conducts an inspection during construction to assess the operation of the Posiva organisation, the functionality of the instructions and the cooperation of the various systems. STUK’s objective is to test the methods of monitoring during the operation phase in the trial run so that monitoring during the operation phase is smooth and timely. The trial run will last several months and it is first carried out at the encapsulation plant, after which it will continue at the final disposal facility. The inspection will end at the end of the trial run.

Natural and technical release barriers at the final disposal facility

Posiva has not fully met the technical requirements set for the manufacture and installation of natural and technical release barriers. Posiva has submitted plans concerning the solving of these problems to STUK, and STUK awaits final reports on the methodological tests for manufacture during the autumn. Due to this, Posiva has made and will be making changes to its own requirements. The changes also affect other systems, which must be taken into account when making the change. This has delayed the delivery of materials to STUK. During the summer, STUK received, among other things, reports on the methodological tests for the manufacture of the canister base and lid and the bentonite buffer.

STUK expects Posiva to proceed with methodological tests for the installation of technical release barriers during the autumn. These tests must be completed before Posiva starts the trial run underground at the final disposal facility. In the summer, STUK received installation methodological test plans for the tunnel backfilling, and the remaining plans are expected in early autumn.

Nuclear material regulation

The International Atomic Energy Agency IAEA and the European Commission have continued the installation of international nuclear safeguards monitoring equipment at Posiva's encapsulation plant. STUK established these in its inspections. The installation work is almost completed, and the final installations can be carried out during autumn or next spring. Posiva's trial run has no significant impact on the schedules for installing monitoring equipment.

In connection with an installation visit by the IAEA in August, it was noted that two verification seals jointly installed by the IAEA and the European Commission had been broken during the summer. Due to the incident, some of the sealed devices must be replaced with new ones. As a result of the incident, STUK has requested Posiva to submit a special report on regulatory control of nuclear safeguards to STUK and the European Commission.

The reporting practices required for monitoring and the secure transfer of data have been discussed with the IAEA, the European Commission and Posiva. Discussions will continue and data transfer methods will be developed during 2024.

Some of the safeguards and the reporting practices required for monitoring are intended to be tested during Posiva's trial run.

Security arrangements

Posiva has continued to deliver materials to STUK in response to STUK’s requirements. However, Posiva still has some pending requirements related to the commissioning. Security arrangement material related to the transfers of spent nuclear fuel has also been submitted to STUK for processing. As Posiva prepares itself for the trial run, STUK plans its own security arrangement inspections during the trial run to verify that Posiva’s operations comply with the instructions and plans.

Organisational oversight

In its supervision, STUK has paid attention to the organisation and procedures of Posiva’s self-monitoring independent of regulatory supervision, and STUK monitors the implementation of the practical procedures for self-monitoring during the trial run. Posiva intends to learn more about self-monitoring for future operations during the trial run.

STUK has observed that Posiva does not always have sufficient resources, for example, for the operating experience activities and for promoting training. The aim of the operating experience activities is to identify or initiate events investigations in order to improve radiation and nuclear safety. In its supervision, STUK pays attention to the adequacy of the resources necessary to ensure competence and safety in the future.

Posiva has updated its management system and its key documents as it moves to the production phase organisation. STUK has assessed and monitored Posiva’s organisational change procedures, change safety assessments and change management. STUK also assesses and monitors the safety culture expectations of the Posiva production phase and their promotion.

From STUK’s point of view, Posiva’s urgent schedule has been emphasised recently, for example, at the start of the trial run. The trial run was started even though some of the plant’s systems are still being tested and not all of Posiva’s personnel have been trained. This has also been reflected in STUK’s supervision as unfinished matters and partially inadequate document deliveries and quality assurance-related activities. STUK will continue to monitor Posiva’s operations both during the trial run and as part of normal monitoring.

Assessing long-term safety

STUK has continued the review of the safety justification for demonstrating long-term safety submitted as part of the operating licence application.

In the second third of the year, STUK submitted a request for clarification to Posiva, which presented the remaining questions related to the demonstration of the long-term safety of the final disposal. The emphasis is on assessing the effects of design changes on long-term safety.

Posiva is expected to answer the remaining questions during the autumn. Once STUK has received the answers, it can continue carrying out the inspections.

Triannual report 1/2024, January–April

During the first tertial of 2024, STUK continued to process the operating licence application of Posiva, supervised the commissioning of the nuclear facility and planned the supervision to be carried out during its use.

As a conclusion to the processing of the operating licence application material, STUK is drawing up a safety assessment, which will be appended to the safety statement prepared on the operating licence application. STUK’s favourable statement on safety is a prerequisite for the Government to grant an operating licence for Posiva’s encapsulation and final disposal facility.

Overall, STUK has progressed well in processing the operating licence application, but the material is still being reviewed. Due to the scope of the material and the processing of updates STUK required of Posiva, the processing in its entirety has taken longer than anticipated.

The Ministry of Economic Affairs and Employment (MEAE), as the licensing authority, originally requested STUK to issue a statement by 31 December 2023, if possible. At the end of 2023, STUK asked MEAE for additional time until the end of 2024 to provide a statement, as STUK is still processing the licence application material.

STUK’s supervision consists of several areas, with the main aspects of supervision described below for the period between the beginning of January and the end of April 2024.

Construction and commissioning

STUK has supervised the manufacture, installation and commissioning of the systems needed at Posiva’s encapsulation and final disposal facilities. The encapsulation and final disposal facility requires lifting and transfer equipment to transfer spent nuclear fuel and the transfer container or canister containing spent fuel. STUK has reviewed the construction plans of this lifting and transfer equipment and carried out structural inspections. The structural inspections verify that the structure has been manufactured and installed in accordance with the structural plans and approved manufacturing and installation methods.

The number of the above-mentioned system materials being processed by STUK is continuously decreasing, as most of the documents have already been delivered to STUK.

The commissioning of the overall plant began in April with trials of the final disposal facility's ventilation systems, and the trials will continue throughout the summer. The purpose of the trials is to determine that the various systems in the facility as a whole function as designed and that the facility can be operated in accordance with the instructions. STUK reviews the plans for the trials concerning the operation of the overall plant, supervises the trial operation and the validation of instructions in connection with the trials.

Instructions and operation of the plant

STUK has reviewed, among other things, Posiva’s administrative instructions and the operating and alarm procedures that are needed when the operation of the plants begins. STUK is still waiting for several sets of operating instructions to be completed and delivered to STUK so that STUK can assess the procedures presented in the operating instructions in their entirety. STUK will assess the status of Posiva’s instructions for use before the actual use of the Posiva plants begins.

Radiation measurements and emergency response arrangements

The commissioning of the radiation and radioactive release measurement systems started in February 2024. Posiva still needs to make changes to some of the release measurement devices. STUK has also required Posiva to supplement the operating licence application material with regard to the measurement and monitoring of releases. Answers to these questions are expected at a later date from Posiva.

STUK is working on an updated emergency plan. Posiva’s plan is shared with TVO Group. With regard to the state of preparedness, the precautionary action zone and emergency planning zone defined for the Posiva nuclear plants correspond to those of TVO. The consequences of possible accidents in the vicinity of the Posiva plants are significantly less than the consequences of a serious reactor accident at the nuclear power plant.

Posiva’s trial run of final disposal

In the trial run of final disposal, Posiva tests the operation of the final disposal facility with testing fuel that does not include any spent nuclear fuel. The purpose of the test is to practise the transfer of spent nuclear fuel to the encapsulation facility, encapsulation at the encapsulation facility and canister transfer and final disposal at the final disposal facility. Prior to the trial run of final disposal, all systems have been tested and tests have been carried out at the plants for the operation of the overall plant. The trial run test is carried out with Posiva’s own specifications, and only a few regulatory requirements apply to the test.

STUK has planned monitoring of the trial run and expects to have the planning completed during the early summer. In the trial run, STUK assesses the operation of the Posiva organisation, the functionality of the instructions and the seamless cooperation of the various systems. STUK’s objective is to test the methods of monitoring during the operation phase in the trial run so that monitoring during the operation phase is smooth and timely.

Natural and technical release barriers at the final disposal facility

Posiva faces challenges with constructing the disposal holes according to Posiva's accuracy requirements. The disposal holes are where the final disposal containers are ultimately placed. STUK monitors the situation and assesses the potential safety impact of the problem as Posiva delivers materials to STUK for review.

Progress has been made in the qualification of the production methods of the final disposal canister, clay components and other technical release barriers preventing and retarding the release of nuclear waste into the environment. As part of the production qualification, STUK has participated in the supplier assessments carried out by Posiva as an observer. In terms of the production of clay components, the qualification has been completed. STUK has received qualification materials for the production methods of the canister base, lid and bottom welding in the spring. With regard to the production of copper pipes for the final disposal canister, Posiva has faced challenges in producing canisters that meet the set requirements. At the end of April, STUK received Posiva’s material on how Posiva is progressing in qualifying the production of copper pipes.

STUK expects to receive material regarding the qualification of the installation of the final disposal canister and clay components during the summer.

Nuclear material regulation

The safeguards supervision performed by STUK shows that Posiva has fulfilled its obligations in relation to safeguards of nuclear materials. The International Atomic Energy Agency IAEA and the European Commission have continued the installation of international nuclear safeguards monitoring equipment at Posiva's encapsulation plant. The installations will continue in 2024. Some of the safeguards and the reporting practices required for monitoring are intended to be tested during the Posiva cooperation test.

In March, the IAEA carried out a supplementary inspection visit to the Posiva plants. The European Commission and STUK participated in the visit.

Security arrangements

Posiva has made progress in implementing security arrangements. Posiva has also provided STUK with various materials relating to the security arrangements for processing, which can be used to assess the adequacy of the security arrangements. Posival still has some pending requirements that it needs to consider before commissioning. STUK has monitored the effectiveness and adequacy of the security arrangements by reviewing documents and conducting field inspections.

Organisational oversight

In its monitoring, STUK has paid attention to deficiencies in Posiva’s quality inspection operations and their handling. STUK required that Posiva assesses the adequacy of the current procedures in the nonconformity management audit. In addition, STUK required that Posiva ensure that all procedures are adequately instructed and pay attention to the maintenance of the skills and qualifications of the quality inspection personnel.

STUK has monitored the development of Posiva's use-phase organisational preparedness and the licensee’s own monitoring of preparedness. Posiva has developed procedures for establishing use-phase organisational preparedness. STUK monitors the application of procedures, including the development of safety culture. STUK has found some shortcomings in Posiva’s extensive sets of material and has required Posiva to supplement the material. Based on the monitoring, STUK sees need for improvement in the assessment of the impact of changes in larger concepts and, in part, in the management of the whole. In addition, STUK’s monitoring has revealed some resource-related challenges. STUK will continue to monitor the adequacy of Posiva’s resources.

Assessing long-term safety

STUK has continued the assessment of the safety justification for demonstrating long-term safety submitted as part of the operating licence application. At the beginning of the year, STUK submitted a request for clarification to Posiva, in which it required background documents relating to the replacement of the buffer material surrounding the final disposal canister. During the processing of the operating licence application, Posiva has made design changes to the release barriers, and STUK must also assess the design changes made with regard to long-term safety. This has an impact on the processing time of STUK’s operating licence application material.

As part of the review of the safety justification, STUK has also continued to conduct emissions, transmission and consequence analyses of radionuclides as part of the assessment of the biosphere transmission and doses of radionuclides.