Sustainable Lifespan Re-Extension Management of Energy Facilities: Economic Assessment and Decision-Making Model for Phased Decommissioning

Abstract

This study proposes a decision-making model based on the economic assessment of phased decommissioning of energy facilities, specifically focusing on a nuclear power plant (NPP). The objective of the research is to develop and validate an economic assessment methodology for comparing immediate and deferred dismantling strategies for a 1000 MW NPP unit. For economic justification, a comparison of the economic expenses is proposed based on the accumulation of radioactive waste, safety activities, and labour costs for the two options. The methods employed include a multifactorial analysis based on expert assessments, considering economic expenses related to radioactive waste accumulation, safety activities, and labour costs. Criteria with differences exceeding 10% for quantitative indicators and fundamental differences for qualitative indicators were deemed significant; each criterion’s acceptability was weighted accordingly. The key results show that deferred dismantling is economically preferable; the total score for deferred dismantling exceeds that of immediate dismantling by approximately 10% (14.16 points vs. 15.86 points). A comparison of block schedules for decommissioning, dynamics of labour costs, and annual volumes of reprocessed radioactive waste for the baseline and optimised deferred dismantling options shows that both options meet the continuity condition of the ‘active’ stages. At the same time, the optimised option demonstrates significant advantages in the uniformity of labour costs and workload of radioactive waste treatment plants during dismantling. The activities at the stage of power unit decommissioning are proposed to be carried out within the licence framework for its operation by the organisational and technical solutions to ensure safety during operation. The deterministic consequences and risks will align with the safety assessment, which will be determined based on the latest analysis results, taking into account sustainable operation.

1. Introduction

Nuclear power plants (NPPs) are an important component of the energy infrastructure, providing a stable supply of electricity with low carbon emissions. Since 2023, nuclear energy has been officially classified as a ‘green’ technology under the EU taxonomy, criteria and rules for environmentally sustainable economic activities [1,2]. However, despite its low-carbon profile, the long-term operation and decommissioning of NPPs raises complex challenges related not only to technical reliability and economic feasibility but also to environmental safety, social impact, and regulatory compliance. These concerns contribute to hesitancy and distrust among both domestic and international investors regarding continued support for nuclear energy development.

In Ukraine, nuclear power has historically covered up to 60% of its energy demand within its design lifespan. Although the energy balance is shifting towards different sources [3,4,5,6], nuclear power remains a crucial component of the energy supply for both the industrial and civilian sectors [7,8].

However, since 2010, the 30-year design lifespan of Ukrainian NPPs has expanded, raising questions about their continued operation. Thus, a decision has been made to extend the lifetime of 1000 MW power units by 10 and 20 years for WWER-440 (440 MW) power units, but these lifetimes are either approaching their end or have already been exhausted. The decision regarding the continued operation of nuclear power units is based on expert assessments of their current conditions and predictions of future changes in technical status and safety implications. According to specialists in state nuclear regulation, operating power units are functioning safely beyond their design lifespan, maintaining an acceptable level of risk, and complying with safety standards for reactor facilities, relevant scientific and technical documentation, and international practises [9,10,11].

At the same time, sustainable decision-making regarding decommissioning must integrate a broader perspective. This includes minimising environmental impacts such as radioactive waste volumes and emissions, as well as addressing social factors like workforce transition and the burden on future generations. These dimensions are central to a sustainability-oriented approach and have been incorporated into the comparative assessment of decommissioning options presented in this study.

International experience shows that extending the service life of nuclear power plants to 50 years is common practice. After the Fukushima accident in 2011, Japan imposed a lifetime limit of 40 years on reactors, and in 2023, the government allowed reactors to continue operating for more than 60 years, subject to strict safety inspections every 10 years after 30 years of operation. As of 2024, the average age of Japan’s nuclear fleet is 33.5 years, with most reactors being between 31 and 40 years old [12,13,14]. Germany has decided to phase out its nuclear reactors, completing the process in April 2023, which can be used as a basis for reassigning the lifetime and phasing out nuclear power units. In many countries, nuclear reactors are reaching or exceeding the 40-year lifetime (23 countries out of 32 operating nuclear reactors) [15,16,17].

The decommissioning of nuclear power plants is a complex and lengthy process that requires significant technical, financial, and organisational resources. Despite the fact that 204 nuclear reactors have already been shut down, the full decommissioning procedure (deconstruction and site remediation) has been completed for only eleven of them, with an electrical capacity exceeding 100 MW. At the same time, about 200 more reactors around the world are expected to reach the end of their service life over the next two decades. In this context, a comparative cross-country study conducted in [18], which analyses existing models of commercial nuclear power plant decommissioning in France, Germany, Sweden, Switzerland, the United Kingdom, and the United States, is of particular importance and serves as a basis for the development of an effective policy in the field of nuclear decommissioning at the international level. However, effective management decisions require thorough and scientifically grounded safety analysis [19,20,21,22]. In addition to safety considerations, the economic feasibility of continued operation must also be evaluated [23]. A crucial factor is the cost–benefit analysis, which includes assessing fixed operating expenses, modernisation costs, and revenues from electricity sales while ensuring the safety of long-term operations. The economic feasibility of the long-term operation of nuclear power units depends on timely modernisation, efficient cost management, innovative approaches to operation, and legislative support. This ensures stable energy supply, minimisation of environmental risks, and economic sustainability in the future.

Therefore, to make important management decisions, it is necessary to conduct an economic feasibility study, taking into account the analysis of the technical condition of equipment, safety and reliability assessment, costs of implementing innovative technologies, equipment modernisation, operating costs, and projected benefits from long-term operation of NPPs based on the relevant regulatory framework. Thus, Section 2 will present an “Analysis of approaches to the justification of NPP lifetime extension”, followed by Section 3 which will discuss the “Development of the concept of economic justification for the sustainable operation of NPPs”, ending thus with Section 4 “Results and Discussions”, and Section 5 regarding the “Conclusions”.

2. Analysis of Approaches to Justification of NPP Lifetime Extension

Safety issues are of primary concern when extending the operation of NPP units beyond their design life, and many scientific studies have focused on addressing this challenge. To ensure safety, nuclear power plants employ strategies motivated by the inherent risks associated with nuclear facilities and lessons learned from past incidents, such as the Chornobyl and Fukushima accidents. Fundamental safety concepts involve monitoring, controlling, and proactive measures to prevent potential internal and external hazards [24]. A key approach is to ensure deep containment (DiD) by implementing multiple safety barriers to prevent releasing ionising radiation and radioactive substances into the environment. In this regard, the authors of [25,26,27] suggested technical and organisational measures to protect these barriers and maintain their effectiveness.

These measures are integrated into three levels of protection to provide the necessary safety standards. The authors in [28,29] suggest considering and implementing higher levels of protection, including measures for severe accident management (level 4) and off-site emergency response (level 5). This approach involves using deterministic and probabilistic safety assessment methods to verify and confirm the effectiveness of safety measures, ensuring their reliability even in severe accident scenarios [25,27]. Probabilistic Safety Assessment (PSA) is also employed to evaluate the likelihood and potential impact of severe accidents, facilitating the development of Severe Accident Management Guidelines (SAMGs) to mitigate their consequences. SAMGs play a critical role in managing and controlling severe accidents by monitoring their progression and evaluating the effectiveness of containment systems, thereby minimising radioactive releases [26,28].

Modern nuclear power plants are equipped with advanced safety features designed to prevent, contain, and mitigate radioactive releases in the event of accidents on a beyond-design basis. These features include passive safety systems without active control or human intervention. These are crucial for minimising the consequences of severe accidents, particularly those triggered by extreme natural events [29,30,31,32]. Following the Fukushima accident, significant changes were made to improve the ability of NPPs to cope with beyond-design-basis accidents worldwide.

Thus, international, sectoral, and local regulations were revised; the design, systems, and components (SSC) of nuclear power plants for managing extreme situations and their inspection and control procedures were improved; and changes were made to the off-site accident management and emergency response procedures that go beyond the design basis. These measures encompass residual heat removal, reactivity control, and the prevention of fuel assembly damage. The corresponding systems, structures, and components (SSCs) are engineered to withstand hazards far beyond the design basis, including extreme earthquakes and flooding events [33,34,35,36,37]. Additionally, new regulations and standards have been introduced to specifically address severe accident scenarios. For example, the US regulation 10CFR 50.155 and the new Japanese safety standards focus on mitigating the consequences of beyond design basis accidents and ensuring reliable safety measures [36,37].

The risk-based safety margin approach assesses and manages changes in the safety margin of a plant over time, ensuring that the safety margin is maintained even with plant age and undergoes modifications [38,39,40,41]. This approach evaluates safety by utilising data from technical diagnostics and predictions of future conditions of power equipment. This allows for the effective management of degradation processes and timely interventions, such as modernisation, equipment replacement, or adjustments to load conditions [39,42]. Assessing the technical condition and forecasting of power equipment is based on analysing the degradation (damage) and failures, considering factors such as stress–strain states, erosion and corrosion wear, load cycles, and other relevant parameters. This approach makes it possible to assess the current state, compare it with regulatory and design documentation, analyse the reliability margin, and predict the further sustainable operation of the power system.

NPPs thus ensure safety beyond design through a combination of advanced safety features, rigorous safety assessments, enhanced regulatory standards, and systematic qualification of critical components. These measures enhance the resilience of nuclear power plants to severe accidents and extreme events, thereby safeguarding the environment and public health. However, beyond ensuring safety, it is also essential to assess the economic feasibility of extending the operation of the unit beyond its design life. This evaluation must consider the substantial safety requirements, operational risks to personnel and the environment, potential social discontent, and the associated economic costs of maintaining safety standards [43].

Economic assessment is usually based on operational efficiency beyond the design timeframe and the associated costs of introducing new technologies, modernisation costs, the energy efficiency of facilities, etc. [44]. This suggests the optimisation of modes, using more efficient devices and service systems, and combining different energy sources [45,46,47,48,49,50]. An assessment of the economic efficiency and payback of nuclear power use within the design timeframe in correlation with the thermal performance and environmental impact (zero carbon footprint) shows that its use is economically feasible [51]. At the same time, it is important to take into account, in addition to active costs during the transition to a new stage of operation of energy equipment, the costs of staff training, adaptation of work processes to new conditions, and long-term financial consequences of changes in production and maintenance processes. The assessment should consider both the initial investment and potential benefits, such as lower energy costs, enhanced productivity, and improved product or service quality [52,53,54,55].

However, as the literature increasingly emphasises, a comprehensive economic evaluation of nuclear energy systems must also include long-term cost–benefit analyses related to the decommissioning stage. Recent studies explore full lifecycle costs, including waste management, site remediation, and environmental recovery, along with opportunities for value recovery through recycling and reuse strategies [56,57,58]. These works suggest that sustainable decommissioning not only minimises environmental risks but can also improve the overall economic viability of nuclear energy when properly integrated into planning and policy frameworks. In addition, sustainability-oriented models incorporate social acceptance, long-term safety, and resource circularity into economic decision-making, expanding the analytical scope beyond traditional cost-efficiency parameters [59,60].

At the same time, power plant assessment is becoming increasingly complex as more and more criteria are involved in the analysis and input data are rapidly changing in a dynamic technological and economic environment. Instead of a separate analysis of the individual characteristics of power plants, a multi-criteria assessment based on hierarchically structured criteria, cluster analysis, systematisation, and classification using the faceted method is becoming increasingly relevant. This approach allows for a comprehensive assessment of nuclear power units, taking into account technological, economic, and environmental aspects of sustainable development. For example, study [61] proposes a comparative analysis of different types of power plants using nine final criteria structured in accordance with the Analytical Hierarchical Process (AHP) methodology. The method of pairwise comparison, as a method of expert evaluation, made it possible to take into account the weighting coefficients of the criteria and reflects their significance. In the scenario based on subjective weighting of the criteria, the main focus is on sustainability indicators, which enabled the final criteria in the overall assessment of each type of power plant, as well as the individual results of power plants for each of the criteria, to be influenced. Due to the complexity of energy systems, multicriteria evaluation and multi-criteria decision-making (MCDM) methods are typically employed, often incorporating fuzzy set theory to develop a comprehensive ranking of models while accounting for the inherent uncertainty of such projects [62,63,64]. At the same time, the risks associated with innovation should be considered, such as possible problems with technological failures and the need for additional maintenance or maintenance costs.

3. Methods and Materials

3.1. Development of the Concept of Economic Justification for the Sustainable Operation of NPPs

In order to conduct an economic feasibility study and plan decommissioning activities, it is necessary to develop a concept that considers key aspects of the feasibility of these activities, the possibility of extended service life, and safety for personnel and the environment. For this purpose, the main aspects that significantly impact the economic assessment and criteria to be analysed when making management decisions were identified (Figure 1).

The main goal of this concept (Figure 1) is to build a model of energy enterprise management that includes long-term planning of the entire range of measures to determine the feasibility of decommissioning or extending the service life of all operating nuclear facilities and radioactive waste management facilities located at the industrial site of a nuclear power plant, taking into account the following conditions:

• Safety priority: strict adherence to all safety standards and regulations, as well as minimising negative impacts on personnel, the population, and the environment from nuclear installations and radioactive waste management facilities during the final stage of their life cycle, along with the waste generated at this stage;
• Priority of justification of capital investments: ensuring efficient use of existing facilities and existing infrastructure at the industrial site to solve the tasks of decommissioning or prolongation of the operation period;
• Priority of stability in the future: ensuring the possibility of operating the existing facilities and creating new facilities at the NPP site to replace the decommissioned facilities;
• Priority of protection of future generations: minimisation of negative economic, social, environmental, and other consequences beyond the planned period from decisions made and decommissioning activities implemented at the NPP site during the planned period.

The concept development requires a set of statistical data. However, according to the “Energy Strategy of Ukraine for the period up to 2050” [65], an additional 15 years of operation beyond the original 30-year design period was preliminarily established. By the time the power unit is finally shut down, it is anticipated that owing to maintenance, repairs, modernisation, and reconstruction of its principal and auxiliary systems during its operational life, those systems necessary for decommissioning and final shutdown will still have sufficient residual life to carry out these tasks.

3.2. Predictive Assessment of Activity of Main Equipment and Structural Elements of Power Units by the Time of Their Final Shutdown

Information about the presence of radioactivity and the radiation characteristics of systems and components to be decommissioned is crucial for the initial planning of maintenance and dismantling activities, assessment of radioactive waste (RAW) streams, selection of waste management technologies, and other related factors. The characterisation of contamination in energy facilities, essential for planning decommissioning activities, includes the following aspects:

• Assessment of contamination of systems and elements resulting from their direct activation during reactor plant operation;
• Assessment of contamination of systems and elements not related to their direct activation;
• Assessment of dose fields.

To assess contamination resulting from direct activation, computer modelling methods (also known as calculation methods) have been developed and are widely used. However, for contaminations not associated with direct activation, as well as for dose fields, purely computational methods are less developed and provide significantly less reliable results. The most reliable information is provided by calculation-experimental methods based on direct measurement data.

Radioactive contamination of equipment and structural elements resulting from their direct activation is volumetric in nature. Materials located inside and around the reactor core are subject to activation, primarily through neutron radiation capture. The levels of activation are influenced by several factors, including the chemical composition of the material, the neutron spectrum, the duration and schedule of irradiation during operation, and the time elapsed after the operation ends.

The most activated materials are the structural elements of the active zone of the reactor facility. The less activated part is the biological shielding made of concrete and reinforced concrete, which is irradiated with a weaker neutron flux and contains materials with smaller activation cross sections. According to predictions [66,67,68,69,70,71], solid radioactive waste classified as high level waste will be predominantly formed by reactor vessels and their internal and external constituents, and the total weight of solid radioactive waste, related to high-level radioactive waste and resulting from direct activation, is estimated as a thousand tonnes/unit and is consistent with the order of magnitude estimate for WWER-440-type reactor units at Lovisa NPP (Finland) [72,73,74]. All available estimates demonstrate that the activated parts will remain highly active for a long time (hundreds of years).

A thorough calculation of the volume and degree of material contamination due to direct activation should be conducted during the development of decommissioning programme (project) stages for each reactor plant. This calculation should be based on an analysis of design documentation, operational history, and data from a comprehensive engineering and radiation survey.

Radioactive contamination of equipment and structural elements not related to their direct activation is of a surface nature [75,76]. The primary source of such contamination is the direct contact of components and materials with the primary circuit coolant. Contamination of the primary circuit water with activated corrosion products occurs as a result of contact with the reactor vessel, which is made of austenitic stainless steel, fuel assemblies composed of zirconium alloy, and other in-core elements [77,78]. Non-ideal sealing of fuel elements leads to the release of fission products into the coolant, which also contribute to the total contamination of elements and materials in direct contact with the primary coolant.

Comparison of data collected at operating WWER-type reactor plants shows that after 2–3 years of ageing, the contamination levels of the main parts of the primary circuit (pipelines, main circulation pump, etc.) lie within the range of 104–105 Bq/cm². The main contribution to the surface activity is given by radionuclides ⁵⁴Mn, ⁶⁰Co, ¹¹⁰mAg, and ¹³⁷Cs. The contribution of ⁶⁰Co to the total activity ranges from 20 to 80%. The radionuclide distribution and contamination levels in elements with a high thermal gradient (heat exchangers, steam generators) may differ markedly from the rest of the primary circuit. Contamination levels in auxiliary systems such as drains, valves, or filters also require special attention.

For WWER reactor plants, the most complete information on the levels of radioactive contamination of equipment and structural elements not related to their direct activation was obtained during a comprehensive engineering and radiation survey at the Greifswald NPP (Germany) [79,80]. The information covered all systems and elements of the NPP, including the secondary circuit process systems and concrete structures. A complete radiochemical analysis for all isotopes of interest required considerable effort, time, and money. The data obtained were used for nuclide distribution modelling based on measurements of a limited number of key nuclides (⁶⁰Co and ¹³⁷Cs). At NPPs, measurements of a limited number of key nuclides (⁶⁰Co and ¹³⁷Cs) should be performed during the development of programmes (projects) of individual stages of decommissioning for each reactor unit based on the data of a comprehensive engineering and radiation survey and the results of subsequent radiation surveys.

Extrapolation for the decommissioning period of data on dose fields measured during the operation of reactor facilities (during scheduled maintenance, comprehensive engineering and radiation survey, etc.) is a rather difficult task and requires complete identification of radionuclides and determination of their contribution to the formation of these fields. A forecast of dose fields for the decommissioning period can only be made based on the results of contamination assessments of systems and components.

For WWER-type reactor units, the most complete information on dose fields at decommissioning was obtained during radiation surveys at the Greifswald NPP (Germany) [81,82]. Predominantly, the dose fields from the main components of the first circuit after 2–3 years of exposure turned out to be relatively low (less than 0.1 mSv/h), although some hot spots with levels up to 10 mSv/hour were detected.

3.3. Methodology for Estimating the Amount of Radioactive Waste Generated During Decommissioning

The methodology used in this concept is similar to that adopted in [81,82,83,84,85,86], based on the following assumptions, taking into account the practical experience of the Gracefwald NPP (Germany) decommissioning with WWER-440-type power units:

• Annual volumes of radioactive waste generated during decommissioning at the stages of final closure, mothballing and dismantling are predicted to be equal to the annual volumes of operational waste for a power unit of this type;
• Annual volumes of radioactive waste generated during decommissioning at the stage of ageing are an order of magnitude less than annual volumes of operational radioactive waste for the power unit of this type;
• The volume of high-level solid radioactive waste generated during the dismantling phase is determined by the reactor, including all internal and external components, as well as the internal biological shielding layer;
• The volume of intermediate-level solid radioactive waste generated during the dismantling phase is equal to the volume of high-level solid radioactive waste produced at the same stage;
• The volume of high-level radioactive solid waste generated during the final closure and mothballing phases is significantly lower than the volume produced during the dismantling phase;
• The volume of low-level radioactive solid waste generated during the dismantling phase is much greater than the volume of intermediate-level radioactive solid waste produced at the same stage;
• No medium- or high-level radioactive solid waste is generated during the holding phase;
• During the final closure phase, the volume of low-level radioactive solid waste is equal to the volume generated during the dismantling phase;
• In the preservation phase, the volume of low-level radioactive solid waste is significantly smaller than that produced during the dismantling phase;
• In the ageing phase, the volume of low-level radioactive solid waste is two orders of magnitude smaller than that produced during the dismantling phase;
• The volume of intermediate-level solid radioactive waste generated during the final closure and mothballing phases is the same as the volume generated during the dismantling phase;
• In the case of the immediate dismantling option, where there is no mothballing phase, the corresponding volumes of all types of solid radioactive waste are generated additionally during the dismantling phase.

Average annual volumes of operational radionuclides (cube residue) for WWER-1000-type power units of operating NPPs of Ukraine for the period 1996–2006 make 142.7 cubic metres/unit.

When estimating the volumes of medium- and high-level solid radioactive waste from decommissioning, it was assumed that the specific densities of compacted products are for 2000 kg/m³ for concrete and 5000 kg/m³ for metal.

The estimated volumes of unprocessed radioactive waste generated at various stages of the two considered decommissioning options for a power unit with a WWER-1000 (B-320)-type reactor are shown in Figure 2 and Figure 3.

The estimated total volumes of reprocessed RAW from the decommissioning of NPP power units under the considered decommissioning options are listed in

Table 1. Estimated total volumes of reprocessed RAW from decommissioning of NPP power units (cu.m.).

No Powerunit	Deferred Dismantling *				Immediate Dismantling
	Solt Melt and Sorbents	Low-Level Waste	Intermediate-Level Waste	High-Level Waste	Solt Melt and Sorbents	Low-Level Waste	Intermediate-Level Waste	High-Level Waste
1	967	1130	312	167	851	1126	312	167
2	967	1130	312	167	851	1126	312	167

* for the variant with the duration of the ageing phase of 30 years.

. The costs of RAW processing generated during decommissioning are included in the implementation cost of the decommissioning measures.

4. Results and Discussion

Selection of the Optimal Option for Decommissioning of NPP Power Units

Based on the above information, the following possible options were considered when selecting the optimal option for decommissioning of NPP power units:

(Option 1) immediate dismantling.

(Option 2) delayed dismantling with sub-options considering different durations of the endurance stage:

(Option 2.1) 20 years;

(Option 2.2) 30 years;

(Option 2.3) 40 years.

In line with the requirements of [73], the selection of the optimal decommissioning option should be based on the cost–benefit principle, which involves a balanced evaluation of the following key factors:

(Factor A) safety requirements established by the current legislation;

(Factor B) the results of assessments of possible hazardous effects on personnel, the public, and the environment;

(Factor C) the radiation state of the installation and its predicted change in time;

(Factor D) the physical state of the facility and its foreseeable change over time;

(Factor E) aspects of radioactive waste management up to disposal;

(Factor F) the possibility of reusing components of the facility or recycling constituent materials;

(Factor G) the possibility of releasing materials for unrestricted use;

(Factor H) plans for the future utilisation of the area occupied by the facility;

(Factor I) availability of equipment and technology for decommissioning;

(Factor J) availability of personnel, and the possibility of utilising the knowledge and experience of operating personnel;

(Factor K) availability of appropriate financial support;

(Factor L) domestic and foreign experience of decommissioning;

(Factor M) social aspects.

Similarly to the approach outlined in [74], the selection of the optimal decommissioning option was conducted using a multifactor analysis method to balance the aforementioned factors. This methodology allows for the combined analysis of both quantitative and qualitative criteria (indicators), supported by objective baseline data available at the time of the analysis and the results of forecast calculations. In the absence of such data, the criteria are assessed based on expert evaluation.

The main prerequisite for the comparison was the consistency of the initial and final states of the facility for both decommissioning options of the power unit. This included the initial and final conditions of the facility, the cost of equipment and materials needed for similar tasks, labour costs for performing the work, and radiation protection measures for personnel, the population, and the environment during the execution of similar tasks.

In defining the comparison criteria, the primary goals were to ensure the comprehensiveness of the analysis by considering all key factors and the accuracy of the analysis by accounting for possible interrelationships (correlations) among the criteria. To achieve maximum coverage, all initially analysed criteria (indicators) were categorised into four relatively independent groups:

• Financial and economic indicators;
• Organisational and technical indicators;
• Indicators of negative impact on personnel, population, and the environment;
• Socio-psychological indicators, excluding those from the first three groups.

The weighting of these criterion groups was based on expert evaluation. A panel of specialists, including economists, technical experts, and professionals in environmental and social impact assessment, was engaged to assess the relative importance of each group in the context of power unit decommissioning. According to their consensus, the first three groups—financial/economic, technical/organisational, and environmental/health impacts —were assigned a weight of 3 points each, reflecting their direct and substantial influence on project feasibility, safety, and regulatory compliance. The socio-psychological group, while important, was weighted at 1 point due to its more indirect impact on operational outcomes.

To further substantiate this approach, a sensitivity analysis was conducted [87]. The results confirmed that moderate variations (±20%) in the assigned weights did not significantly alter the prioritisation of decommissioning options, thus supporting the robustness of the weighting scheme.

The initial list of analysed criteria (indicators) and their relationship to the main factors is presented in

Table 2. Initial list of analysed criteria (indicators) for comparison of options for decommissioning of NPP power units.

No.	Criterion Name	Related Factors	Commentary	Weight Factor
Financial and economic indicators				3.00
1	Total costs of decommissioning activities	A, B, C, D, E, F, G, H, I, K, L, M		0.75
2	Costs of implementation of individual stages of decommissioning	A, B, C, D, E, F, G, H, I, K, L, M	Excluded as highly correlated with criterion 1	0
3	Total RAW disposal costs from decommissioning	A, B, C, E, F, G, I, K, L, M		0.75
4	Possibility to optimise the decommissioning cost schedule	D, E, F, I, K, M		0.75
5	Time to peak decommissioning costs	C, D, E, I, K, M	Excluded as highly correlated with criteria 4 and 15	0
6	Recyclability of elements and materials	B, C, D, E, F, G, H, L		0.75
Organisational and technical indicators				3.00
7	Ensuring compliance with safety requirements	A, B, C, D, E, F, G, H, I, J, K, L, M	Excluded as a prerequisite for any scenario	0
8	Labour costs for decommissioning and individual stages of decommissioning	C, D, E, F, G, H, I, J, L	Excluded as highly correlated with criterion 1	0
9	Possibility to optimise the schedule of labour costs for decommissioning	B, C, D, E, H, I, J, K, L		0.60
10	Timeline for achieving the ultimate goal of decommissioning	A, B, C, D, E, G, H, I, L		0.60
11	Duration of the individual phases of decommissioning	C, D, E, G, H, I, L	Excluded as highly correlated with the previous criterion	0
12	Staffing of the works	A, E, H, J, M	Excluded as a prerequisite for any scenario	0
13	Information support of works	A, C, D, E, I, J, L, M		0.60
14	Perspective on the application of improved technologies	A, C, D, E, F, G, I, K, L, M		0.60
15	Time margin until peak receipts of radioactive waste from decommissioning for disposal	A, E, I, K, M		0.60
Indicators of negative impact on personnel, population, and the environment				3.00
16	Relative magnitude of the collective dose to personnel during decommissioning	A, B, C, E, I, L, M		1.50
17	Absolute value of the collective dose to personnel during decommissioning	A, B, C, E, I, L, M	Excluded as highly correlated with the previous criterion	0
18	Relative and absolute values of individual and collective doses to the population during withdrawal from operation	A, B, C, E, I, L, M	Excluded as negligible under conditions of further NPP operation	0
19	Relative and absolute values of emissions and discharges into the environment from the decommissioning of operations	A, B, C, D, E, I, L, M	Excluded as negligible under conditions of further NPP operation	0
20	Total volume of recycled RAW from decommissioning of operation	A, B, C, D, E, F, G, H, I, L, M		1.50
21	Volumes of RAW generated at individual stages of decommissioning	A, B, C, D, E, F, G, H, I, L, M	Excluded as highly correlated with the previous criterion	0
Socio-psychological indicators				1.00
22	Public opinion preferences	A, E, H, J, K, L, M		0.50
23	Burden on subsequent generations	A, B, C, D, E, G, K, L, M		0.50

. As shown in the table, each analysed criterion is associated with at least five main factors, and each factor corresponds to at least three criteria.

To ensure the accuracy and reliability of the multivariate analysis, criteria that exhibited strong correlations (correlation coefficient > 0.85) were excluded to avoid redundancy and potential distortion of the results. The correlation analysis was performed using a combination of expert judgement and correlation matrices, where applicable. In addition, criteria that were identical across all decommissioning scenarios or represented prerequisites rather than differentiating factors were also excluded. Detailed justifications for the exclusion of specific criteria are provided in Table 2.

Following the selection process, the remaining criteria were grouped and assigned equal weighting factors within their respective categories based on expert evaluation of their relative significance. Among the selected indicators, 46% were quantitatively assessed (criteria Nos. 1, 3, 10, 15, 16, 20 in Table 2), while 54% were evaluated qualitatively (criteria Nos. 4, 6, 9, 13, 14, 22, 23).

In the comparison, the two decommissioning options were categorised for each criterion (indicator) according to their level of acceptability (less preferable—1 point; more preferable—2 points). For the delayed dismantling option, quantitative and qualitative indicators were averaged across the three considered sub-options. For the criteria subject to quantitative assessment, a difference of over 10% between the options was considered significant. For criteria subject to qualitative assessment, differences in principle (e.g., ‘positive’, ‘neutral’, ‘negative’) were regarded as significant. If no such differences were present, both options for the criterion were categorised as equal in terms of acceptability (1.5 points).

The assessed level of acceptability (scores) for each criterion was considered, with a weighting factor corresponding to that criterion (Table 2). The comparison results are presented in

Table 3. Comparison of options for decommissioning of NPP power units.

No.	Criterion Name	(Option 1) Immediate Dismantling	(Option 2) Delayed Dismantling
Financial and economic indicators		4.51	4.51
1	Total costs of decommissioning activities	1.50	0.75
2	Total RAW disposal costs from decommissioning	1.13	1.13
3	Possibility to optimise the decommissioning cost schedule	0.75	1.50
4	Recyclability of elements and materials	1.13	1.13
Organisational and technical indicators		3.90	5.10
5	Possibility to optimise the schedule of labour costs for decommissioning	0.60	1.20
6	Timeline for achieving the ultimate goal of decommissioning	1.20	0.60
7	Information support of works	0.90	0.90
8	Perspective on the application of improved technologies	0.60	1.20
9	Time margin until peak receipts of radioactive waste from decommissioning for disposal	0.60	1.20
Indicators of negative impact on personnel, population, and the environment		3.75	5.25
10	Relative magnitude of the collective dose to personnel during decommissioning	1.50	3.00
11	Total volume of recycled RAW from decommissioning of operation	2.25	2.25
Socio-psychological indicators		2.00	1.00
12	Public opinion preferences	1.00	0.50
13	Burden on subsequent generations	1.00	0.50
Total scores		14.16	15.86

. As indicated in the table, the option of delayed dismantling of NPP power units proves to be more advantageous according to the evaluation methodology used, with a total point difference exceeding 10%. Based on this outcome, the delayed dismantling option was selected as the foundation for developing a decommissioning strategy for NPP power units.

In the comparison, the two sub-options under consideration (options 2.1, 2.2, 2.3) were categorised for each criterion according to their level of acceptability (least preferred—1 point; medium preferred—1.5 points; most preferred—2 points). The comparison results are shown in

Table 4. Comparison of sub-options during decommissioning of NPP power units (delayed dismantling).

No.	Criterion Name	Delayed Dismantling
		(Option 2.1) 20 Years	(Option 2.2) 30 Years	(Option 2.3) 40 Years
Financial and economic indicators		4.89	4.52	4.14
1	Total costs of decommissioning activities	1.50	1.13	0.75
2	Total RAW disposal costs from decommissioning	1.13	1.13	1.13
3	Possibility to optimise the decommissioning cost schedule	1.13	1.13	1.13
4	Recyclability of elements and materials	1.13	1.13	1.13
Organisational and technical indicators		4.20	4.50	5.10
5	Possibility to optimise the schedule of labour costs for decommissioning	0.90	0.90	0.90
6	Timeline for achieving the ultimate goal of decommissioning	1.20	0.90	0.60
7	Information support of works	0.90	0.90	0.90
8	Perspective on the application of improved technologies	0.60	0.90	1.20
9	Time margin until peak receipts of radioactive waste from decommissioning for disposal	0.60	0.90	1.20
Indicators of negative impact on personnel, population, and the environment		3.75	4.50	4.50
10	Relative magnitude of the collective dose to personnel during decommissioning	1.50	2.25	2.25
11	Total volume of recycled RAW from decommissioning of operation	2.25	2.25	2.25
Socio-psychological indicators		1.75	1.50	1.25
12	Public opinion preferences	0.75	0.75	0.75
13	Burden on subsequent generations	1.00	0.75	0.50
Total scores		14.59	15.02	14.99

. As seen from the table, the total points indicate that the difference between the sub-options is less than 10%, which, according to the methodology applied, confirms their near equivalence. Based on this result, the intermediate sub-option (option 2.2) was used as a base case for further analysis, and the optimisation of decommissioning of power units Nos. 1–2 was performed within the limits of differences allowed by the considered sub-options.

To verify the reliability of the proposed model, a sensitivity analysis was conducted to assess the impact of changes in key parameters (weights and assumptions) on the final results. Each group of criteria (financial and economic, organisational and technical, environmental, social, and psychological) was varied within ±20% of the initial values, while keeping their total number unchanged, and the change in the ranking of alternatives and total scores under these variations was analysed. The results showed that the model demonstrates stability to moderate changes in weighting coefficients; the deferred dismantling option remained preferred for all such variations, with a difference in scores ranging from +8% to +14%, and the ranking of alternatives remained unchanged in 85% of cases, with a deviation of total scores of no more than 10% from the initial values, which indicates sufficient stability of the model and confirms the reliability of the results obtained. The weights of certain criteria with a high level of influence (e.g., Criterion 16—Collective dose to personnel; Criterion 20—Volume of radioactive waste processed; Criterion 1—Total costs) were changed by ±25%. The recalculated total scores showed a minimal impact on the final rating; in no case did the preference change from deferred to immediate dismantling. The maximum change in the total score difference was ±0.5 points, which was not enough to change the decision as it remained above the 10% threshold. Moderate changes in the weighting coefficients do not lead to a change in the preferred option, both in the overall group and by individual criteria, which indicates sufficient stability of the model and confirms the reliability of the results.

Optimisation of decommissioning of NPP power units with delayed dismantling was performed based on the following conditions:

• Minimisation of gaps in the schedule of implementation of each of the ‘active’ stages of decommissioning (decommissioning, final closure, conservation, and dismantling stages) consolidated for all units;
• Achievement of maximum uniformity across all blocks of the schedule of labour costs for decommissioning;
• Achievement of maximum uniformity of the aggregate schedule of RAW generation and processing from decommissioning across all units at the above mentioned ‘active’ stages.

Comparison of block-by-block decommissioning schedules, dynamics of labour costs, and annual volumes of processed RAW from decommissioning of NPP power units for the basic and optimised variants of decommissioning with delayed dismantling is shown in Figure 4, Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9. As shown in the figures, the continuity of the decommissioning schedule for each of the ‘active’ stages, summarised across all units, is maintained in both options. However, the optimised option offers significant advantages in terms of the uniformity of labour costs and the workload of RAW processing facilities during the dismantling stage. This contributes to better resource planning, reduced peak loads on personnel and infrastructure, and improved operational safety.

Based on these comparative results, the optimised variant is accepted as the basis for developing a long-term strategy for the decommissioning of NPP power units, as it ensures a more balanced, resource-efficient, and safer approach to dismantling.

Figure 4 and Figure 5 illustrate the sequencing of power unit shutdowns and dismantling operations. In the optimised variant (Figure 5), the distribution of work is better balanced across the timeline, avoiding overlaps of high-intensity dismantling stages. This allows for more effective allocation of resources and reduces the likelihood of bottlenecks.

Figure 6 and Figure 7 present the dynamics of labour costs over the decommissioning period. The basic variant (Figure 6) shows sharp fluctuations in labour demand, which can result in inefficient staffing, higher training costs, and workforce stress. In contrast, the optimised scenario (Figure 7) demonstrates a more even distribution of labour costs, which supports smoother workforce transition and improved cost control throughout the project.

Figure 8 and Figure 9 show the annual volumes of processed radioactive waste. The optimised approach (Figure 9) results in a more stable and manageable RAW processing load, which reduces the need for temporary storage expansion and improves the safety and sustainability of RAW handling procedures.

The optimal variant of NPP power unit decommissioning is defined as the delayed dismantling variant with the duration of the endurance stage of 30 years. Generalised characteristics of this optimal option are given in

Table 5. Characteristics of the optimal option for NPP unit decommissioning.

Stage Name	Start Date	Duration, Years	Labour Costs, Person–Years
Termination of operation	2032	4	1745.5
Final closure	2036	5	1714
Preservation	2041	4	1441
Maturation	2045	30	1279
Dismantling	2075	9	1753.5
Total		52	7933

. Refined assessment of RAW volumes from NPP unit decommissioning is presented in

Table 6. Refined total volumes of processed RAW from decommissioning for optimal variants of NPP power units (cu.m.).

Unit No.	Salt Fusion and Sorbents	Low-Active Solid Radioactive Waste	Medium-Active Solid Radioactive Waste	Highly Active Solid Radioactive Waste
1	967.3	1130.3	312.6	167.4
2	940.2	1130.3	312.6	167.4

.

The goal of decommissioning activities is to ensure their optimal organisation within the unified production and organisational structure of NPPs. This allows for efficient planning and execution of work during the decommissioning phase, while ensuring full compliance with safety regulations, rules, and standards.

To achieve this goal, the process of organising and implementing decommissioning activities involves the following tasks:

• Optimising the organisational structure by clearly defining the functions of existing NPP departments based on their functional purpose and assigning responsibilities among involved third-party organisations;
• Establishing necessary (including temporary) organisational structures to effectively carry out the tasks at hand;
• Implementing flexible planning of activities, including resource allocation, based on regulatory requirements, necessity, and priority of specific tasks;
• Defining requirements for the content and procedures of task execution;
• Preparation, approval, and implementation of necessary documents;
• Training, retraining, and certification of necessary personnel, as well as maintaining their qualifications;
• Control over the execution of works;
• Obtaining permits (licences) to carry out activities in accordance with the legislation;
• Provision of resources of the required quality, at the required time, and in the required quantities.

5. Conclusions

The present study proposes a model for ensuring the sustainable operation of NPP power units and its economic justification when planning NPP power unit decommissioning. Based on the proposed model, a step-by-step consideration of economic indicators to ensure sustainable operation was outlined for concept development. The concept includes both immediate and deferred dismantling options during NPP decommissioning, with a comprehensive comparison made using a multifactor analysis methodology based on expert assessments. The results of this analysis indicate that the deferred dismantling option is the preferred choice for decommissioning NPP power units.

The deferred dismantling option enables optimisation of the decommissioning process by adjusting the holding stage duration for an individual power unit within 20–40 years. The multifactorial analysis takes into account the costs associated with radioactive waste accumulation, technical indicators, labour costs, and socio-psychological factors. It is crucial to highlight that any upgrades to equipment during decommissioning should prompt a review of the concept, ensuring that these factors are incorporated into recalculations for the decommissioning plan.

From a sustainability perspective, the study integrates key principles of environmental and social sustainability. Environmentally, the approach promotes more effective planning and reduction in radioactive waste volumes through better optimisation of decommissioning schedules and extended decay periods before final dismantling. The possibility of recycling materials is also included in the analysis, contributing to a reduced environmental footprint. Socially, the deferred dismantling approach allows for a smoother workforce transition and reduces the burden on subsequent generations by distributing risks and responsibilities over a longer timeframe, thus supporting intergenerational equity.

While the reuse of components and materials from decommissioned facilities for purposes beyond the decommissioning process is not considered significant, these materials are planned to be treated as waste and disposed of by the end of the dismantling phase. However, should any components or systems be repurposed for operational NPP units, such decisions will be carefully justified within the decommissioning-stage programme framework.

Throughout the decommissioning phase, all activities will be carried out within the scope of the unit’s operating licence, adhering to the organisational and technical safety standards used during its operation. The deterministic consequences and risks are aligned with the safety assessment provided in the latest updated safety analysis report. For each subsequent decommissioning phase, safety assessments will be integrated into the design and implementation projects. Prior to the commencement of each phase, relevant safety analysis reports will be developed in full compliance with the applicable safety regulations, rules, and standards, and submitted for review by the State Nuclear and Radiation Safety Regulatory Authority as part of the design documentation.

This study also provides a foundation for further research into extending the model’s applicability to other energy facilities, including fossil fuel and renewable energy plants. Future work could explore the role of automation and technological advancements, such as AI and robotics, in improving the decommissioning process. By integrating such innovations, the model could enhance efficiency, reduce human exposure to hazardous environments, and ensure more sustainable energy transitions.