Lessons Learned from the Commissioning Process of the 3rd Mochovce NPP Unit in Slovakia

Abstract

The paper is focused on broader considerations regarding the commissioning process of the 3rd Unit of nuclear power plant VVER-440 type in Mochovce (Slovakia). The new nuclear plant built in Europe is getting much more slowly than expected, declared or scheduled. Besides the nuclear power plant in Olkiluoto (Finland) and also Flamanville (France), the 3rd Mochovce Unit has finally been in full operation since 6 November 2024. Nevertheless, the more than 30 years of construction process, which was intermittently stopped and frozen, make this success story exceptional. Lessons learned from commissioning are every time specific for different countries but commissioning of nuclear power plant without presence of general designer, respecting all safety requirements and taking full responsibility for this process is unique. Still, in general, the actual Slovak experiences and knowledge could help optimise new buildings in Europe, including dreams about small modular reactor deployment or the building of other clean and sustainable use of advanced nuclear facilities in the future.

1. Introduction

According to the data published in the IAEA PRIS database [1], until March 2025, 416 nuclear reactors were in commercial operation (additional 23 suspended from operation), and 62 were under construction worldwide. Unfortunately, after finishing of the last European nuclear power plant (NPP) in Flamanville-3 Unit in France (first grid connection on 21 December 2024), there are only some announcements about possible new builds in the European Union. All these announced projects, including small modular reactors, are still far from real construction or commissioning and long-time gap in commissioning of new NPPs is expected. Preservation of adequate knowledge and skills for the future seems to be extremely important.

The actual situation in the energy supply requires a serious reconsideration of the energy policy and a new vision and impulses for the sustainable and long-term use of nuclear power as part of the future, focusing on decarbonised and stable energy sources. In Europe, this reconsideration is critical, considering broken contracts for cheap oil, gas, wood or coal deliveries from Russia. Prices of all these conventional fossil energy resources dramatically increased. For the last two decades, the leitmotiv has been “green”, as clearly shown by the label “Green Deal”, strongly promoted by the European Commission and endorsed by the Member States and the European Parliament. The very recent communication campaign “You are Europe”, univocally associating fundamental European values (i.e., human rights, liberty, peace, diversity, unity, stability) with “green” intermittent renewable sources of electricity, confirms it. However, only a “green” energy policy is not sustainable. Focusing on the environment alone is insufficient, and a more balanced approach is necessary. The economics and security of energy supplies fully integrated into societal sustainability must be respected. The measures proposed thus far, at EU and national levels, to reduce the effects of the energy crisis are helpful in the short-term but not sufficient in the longer-term perspective: taxing the excessive profits of energy companies (the inframarginal generators as called by the Commission), creating additional debt by distributing finance, pleading for reducing consumption by lowering indoor temperatures during the winter, may help but not solve the problem [2,3,4]. We urgently need efficient and radical structural long-term reforms. Without these, dramatic economic and social consequences can be incurred [5].

There is no doubt that nuclear energy has been a primary source of low-carbon electricity and has played a significant role in avoiding greenhouse gas emissions. The question from the past, “renewables or nuclear?” has changed to the answer “renewables and nuclear”. Although the support of renewables seems to be accelerated to the maximum, its electricity production is not stable and sustainable. The combined use of stable nuclear gives both sides synergy. Nuclear energy also has the potential to contribute further to the decarbonisation of non-electric energy sectors like industry and transportation by deploying novel technologies.

The vision for sustainable use of nuclear energy should be comprehensive. Therefore, at least the following items need to be considered:

(A)

Improvement and build of a power plant in the frame of the Generation III+ projects [2,3,4,5,6].

(B)

Existing plant lifetime extension up to about 60 or 80 years [7,8,9,10].

(C)

Small and micro modular reactors are involved in the wide commercial use [11].

(D)

Reconsider, simplify, refresh, and accelerate 1–2 most promised projects in the frame of Generation IV.

(E)

Accelerate activities towards commercial use of thermonuclear fusion (ITER, DEMO).

By general considerations about risks and uncertainties of nuclear power [12,13,14,15], it is essential to mention that:

(1)

Very high upfront capital costs and a long realization period in some cases over about 15 years, as well as prolongation of the lifetime of the existing nuclear fleet, can play a crucial role in the realistic consideration of total costs.

(2)

Several factors influence the environment in which new nuclear projects (including deep geological repositories) are developed (public acceptance, regulatory risks, fuel supply or geopolitical risks), resulting in cost and construction time overruns.

(3)

By the end of the year 2024, 276 reactors worldwide were operating for longer than 30 years. It represents more than 66.5% of power reactors. From a long-term perspective regarding the projected design lifetime, the next 160 reactors [1] will probably be shut down in 2030 if appropriate measures towards long-term operation (LTO) are not taken. This fact causes plenty of uncertainties. Additionally, only 65 (15%) new units have been put into operation during the last decade, and only 31 (7%) units in the decade before. The United States prolonged the operational license of 73 units for up to 60 years. There is a similar trend in other countries, where the operational lifetime approaches 40 years; these countries try to extend the lifetime individually by the decision from periodic safety reviews (PSR).

(4)

It is assumed that the decommissioning costs are about 15% of the overnight construction costs, compared to less than 5% at all other energy production technologies. Due to the prolonged lifetime of NPPs, this portion gets relatively smaller in the discounted calculations of electricity generation costs, and the utility (government) obtains additional time for the upgrade of decommissioning plans and financial resources collection.

(5)

The nuclear industry is the only one obliged to collect money for the complete decommissioning of nuclear facilities during their operation lifetime.

2. NPP Mochovce in Slovak Power Generation

The peaceful use of nuclear energy has a long tradition in Slovakia. The build of the first Slovak nuclear power plant, NPP A-1, was started in 1957 [16]. Currently, 5 VVER-440 units (all V-213 types) operate in Slovakia [17]. The owner, as well as the operational licence holder, is the company Slovenské elektrárne, a.s. (SE, a.s). In 2023, these units generated a total of 18 343 GWh. Nuclear power is a solid base for a low-carbon economy. With 61.3% of electricity produced from nuclear, Slovakia is in second place (behind France with 64.8%) in the world [1].

The construction of the NPP Mochovce started in 1981; the decision had already been made in the 70s. The political and economic changes resulted in the suspension of the construction in the early 90s. In 1996, a “Mochovce NPP Nuclear Safety Improvement Programme” was developed in the framework of the completion projects for Units 1 and 2 in accordance to existing legislative requirements as well as best international practices and recommendations [18,19,20,21]. These units have been in operation since 1999 and 2000, respectively. Completion of Units 3 and 4 was postponed. After the privatisation of 66% of the utility Slovenské elektrárne, a.s. by Italian company ENEL in 2006, the decision to commission both units in 53 months was taken in 2008. Although the new owner was motivated for commissioning, the reality was far from expectations. Some milestones in the completion of NPP Mochovce units are shown in

Table 1. Milestones of Mochovce first 3 units completion. All VVER-440, V-213 type, owner Slovenské elektrárne, a.s.

Type of Licence	Milestones	Unit 1	Unit 2	Unit 3
	Designed net capacity Net capacity Gross capacity	408 MWe 467 MWe 500 MWe	408 MWe 469 MWe 500 MWe	440 MWe 440 MWe * 471 MWe *
	Construction start date	13.10.1983	13.10.1983	27.1.1987
Permit for commissioning and early use of the building as well as permit for management of nuclear material spent fuel and radioactive waste	Physical start-up (loading of the first fuel assembly into the reactor)	27.4.1998	4.10.1999	9.9.2022
	First criticality date	9.6.1998	1.12.1999	22.10.2022
	First grid connection	4.7.1998	20.12.1999	31.1.2023
	144 h test run (as a part of the energy start-up stage)	7.–13.10.1998	13.–19.3.2000	8.–14.10.2023
Permit for commissioning and early use of the building as well as permit for management of nuclear material spent fuel and radioactive waste	Trial operation	29.10.1998	11.4.2000	6.11.2024
	Permanent operation	29.1.1999	11.7.2000

* The designed net capacity was kept due to a valid construction licence. It can be expected that the gross capacity will increase later.

.

The detailed study of dates shown in Table 1 shows that the timespan between critical experiments and issuing the licence for permanent operation increased significantly. It was connected to the substantial increase in legislative requirements and the precise evaluation of all tests and experiments performed during the physical and power commissioning of the unit 3 from all the participating institutions, including the Slovak Nuclear Regulatory Authority. The general view on the NPP Mochovce with 4 VVER-440 units is presented in Figure 1. The simplified thermal scheme of the Mochovce unit 3 is shown in Figure 2.

3. Specialty of Mochovce Unit 3 Critical Experiment

This section provides a concise and precise description of the experimental results, their interpretation, as well as the experimental conclusions that can be drawn. The initial approach to criticality was a procedure undertaken with a great deal of respect towards nuclear safety. Firstly, the available reactivity was near its maximum at the critical experiment because there was no fuel burnup. Practically, no fission products were present. This excess positive reactivity was compensated for by chemical shim—boric acid with a large thermal neutron absorption cross-section. However, this boric acid absorber was removable from primary circuit coolant; hence, the possibility of a more significant positive reactivity insertion existed. Secondly, standard nuclear instrumentation as ex-core ionization chambers can be at first criticality off the scale at their low end. Therefore, the reactor safety and control system may not be able to control automatically the reactor itself. Thirdly, even if the standard start-up instruments are wired into the reactor shutdown systems, their response becomes increasingly longer as the neutron flux density levels decrease. Lastly, the value of the critical parameter, boric acid concentration (at the given control rod position and moderator temperature), was not precisely known. Design estimates were only available.

During the first approach to criticality, boric acid was gradually removed from the moderator until criticality was reached. In this case, the neutron leakage was nearly constant, and the effective multiplication coefficient k_eff was increased by raising the thermal utilization factor f until the k_eff value became equal to 1 (reactivity is 0). The criticality experiment used the well-known Inverse Count Rate Ratio or 1/N method. The neutron flux density during the subcritical multiplication was monitored via low-range ex-core detectors (standard and non-standard), where the count rate gets infinitely large as the core approaches k_eff equal to 1. Notably, the count rate of low (source) range detectors was proportional to the neutron population in the core. Therefore, instead of plotting count rate N directly, its inverse value 1/N was plotted on a graph of 1/N versus boric acid concentration. As criticality was reached, 1/N approaches zero.

At the NPP Mochovce Unit 3, the boron dilution process was performed in two main stages during reaching the first criticality. The first one was the as called “big boron dilution”, in which pure condensate was injected into the primary circuit at a volumetric flow rate of 20 m³/h. In this stage, boric acid concentration was continuously decreasing from its starting value of 15.7 g/kg (moderator temperature of 200.7 °C and the initial subcriticality of −38.7$) up to the beginning of the so-called—starting interval—defined by reactivity range of −2.862 ÷ 0$. That was reached at a boric acid concentration of 8.26 g/kg. At this moment, the second stage, the so-called “small boron dilution”, starts with a volumetric flow rate of 8 m³/h (later reduced to 4 m³/h) until the critical condition is approached. The criticality was finally reached at these parameters:

-

Boric acid concentration of 7.4 g/kg,

-

Stabilized reactor power of 6.8 × 10⁻³% P_nom,

-

The position of the sixth group of control assemblies of 157.1 cm,

-

The temperature of the primary circuit of 201.25 °C,

-

The pressure in the primary circuit of 11.88 MPa.

It is essential to mention that each nuclear unit is unique, and the final level of parameters can be slightly different from designed values due to many factors that cannot be fully predicted before. The shape of criticality experiment and reaching the first criticality of Mochovce unit 3 is presented in Figure 3. The figure points out the start-up neutron detector count rate dependence (logarithmic scale) on time across the first criticality experiment. In the range of the “big boron dilution” high statistical fluctuation of the detector response is observed. The first local maximum indicates reaching the first critical condition followed by continuous increase of power and its stabilization at the minimum controlled power level (6.8 × 10⁻³% of nominal thermal reactor power).

4. Operational Characteristics of Mochovce Unit 3

Despite many difficulties, as well as postponement, Mochovce Unit 3 was successfully commissioned at parameters shown in Figure 4 and Figure 5. It is necessary to note that the basic design was relatively old and robust, outcoming from the 70s (the first two units of VVER-440, type V-213 were put in operation in 1980 and 1981 as the NPP Rivne 1&2 in the former Soviet Union); therefore, its design improvement was limited by initial conditions. Another negative factor was the missing of the general designer’s (OKB Gidropress) presence in the locality during the last years of commissioning. As a positive factor, experiences from the commissioning of Mochovce Units 1 and 2 can be mentioned, although over 20-year time gap. Having in mind all these factors, robustness and experiences from many operating years of VVER-440, V-213 type, the readiness for continuous and safe operation is well.

As it can be seen in top right corner, inlet and outlet temperatures are slightly different for each from 6 loops due to small differences of pipelines design and placement (almost normal for PWRs). The thermal power achieved of about 1355 MWt, which correspond to 98.9% of projected nominal power level. The power level was slightly limited by the real operational capacity of the steam separator. Its complex improvement is not trivial and is planned for next outage.

5. Lessons Learned from NPP Mochovce 3rd Unit Commissioning

Only several papers about NPP commission experiences were published in the last decade. A general comprehensive review was published in [22], where the summary of the lessons learned from commission-related events was provided. This summary was done by the centralised office of the European Clearinghouse on Operational Experience Feedback, Institute of Energy in Petten. Some partial and specific knowledge connected to NPP commissioning can be found also in [23,24,25], but almost all papers refer world-wide issues dominantly from last century and do not reflect actual specific problems with NPP commissioning in the Europe. Although there exist many scientific papers oriented towards physical phenomena (embrittlement, corrosion, swelling, etc.) with impact on the NPP commissioning or its next effective and long-term operation [26,27], the actual and exact lessons learned from NPP technological commissioning in the Europe was not reported in detail. It can be note that only 3 units were put in operation in frame of western and central Europe in the last 20 years and papers of this type are rare. Therefore, these lessons learned from complex commissioning process of Mochovce NPP can be considered as unique.

Based on the authors’ personal experiences from Mochovce 3rd Unit (MO3) commissioning as well as from evaluation of various tests and experiments performed during physical as well as power commissioning (PhysC and PowC), it was possible to summarise obtained knowledge into concentrated form for the use by commissioning of next units. Comprised knowledge can be included in the following bullets:

-

It is necessary to use up-to-date and revised documentation incorporating all changes and corrections during commissioning.

-

When signalling unreliable instrumentation data during complex tests, conservative decision-making principles must be followed.

-

During PhyC and PowC tests, the main unit parameter controllers should be in automatic mode unless the test requires a different mode.

-

Do not allow makeshift solutions that tend to fail and increase the risk of failure (e.g., makeshift generator wake-up solutions).

-

An approved, clear, and precise procedure must be used for the simulation and blocking of signals, as well as protection and control, including equipment resetting.

-

Clear and unambiguous definition of competencies and responsibilities in examinations and tests.

-

Ensure effective communication between all those who prepare, implement and evaluate each test.

-

Anticipate that hidden bugs on devices are discovered during PowC tests, and the given test may not be successful the first time. Pay attention to accompanying fault signalling.

-

Continuously monitor unit parameters and periodically evaluate deviations from expected parameters.

-

Carry out a thorough inspection and diagnosis of current connections on electrical equipment during their progressive loading, which can prevent faults and fire due to insufficiently tightened connections and contacts of electrical wiring.

-

Organise regular briefings, preferably at T-48h or T-24h, to ensure accurate test preparation, especially if the test is being taken for the first time.

-

Communicate openly, promptly, proactively and efficiently with the Regulatory Authority.

Additionally, it is necessary to mention some weak points that should be addressed in the commissioning of Mochovce Unit 4 (MO4).

(A)

Poor public reception of the NPP commissioning (media, third sector, “green organisations”, etc.)

This point could also move for the better in the context of energy shortages. However, there is a need to work actively here and to publish information about the contribution of NPPs not only to the profits of SE, a.s., but mainly what the public benefits from it today.

Similarly, for triggering day-to-day information, it can’t just stay in the press department. There must be a technically proficient spokesperson. He/she should answer the phone seriously; otherwise, the managing director may be completely overwhelmed and have no space and operational management for the launch. Even so, he/she has many activities as the main communicator towards the Nuclear Regulatory Authority and the Ministry of Economy and Foreign Affairs.

(B)

Mindset changes

There were noticed some preparation reserves. For example, in statements such as: “… when it happens, we will deal with it, but it seems unlikely to me that it happens…” The correct approach should be: “… we have analysed and evaluated the problem with low probability. However, if one does occur, we will follow the tested plan.”

(C)

Human Factor

It is mainly related to the “mental fatigue originating in permanent readiness” but as well as the looming problems of ageing of key personnel in the prospect of the start-up of MO4 commissioning in 2–3 years. A long-term vision must be offered to both the launchers and the subcontractors [28,29,30,31].

As lessons learned from the MO3 commissioning, next bullets summarized steps for proposed tasks towards smooth commissioning of new units (applicable also for the MO4, but also for built of new nuclear facilities):

• Update service contracts and provide all-round engineering support, especially for equipment other than the MO12.
• Identify operational experiences with critical facilities from the other VVER units for other than the MO12.
• Set up a proper workflow and precisely define the tasks in the preparation, execution and evaluation of tests.
• Record the current state of technology to support effective planning and decision-making.
• Summarise and archive experiences and best practices for the start-up of the MO4.
• Considering the age structure and human potential to maintain critical knowledge into the next period.
• Complete the risk management system, focusing on risks arising from installing new components and equipment in the case of which operational experience or engineering support is unavailable.

From the point of view of the start-up of the MO4, the following recommendations are the most important:

PEOPLE:

• Given the imminent loss of key start-up personnel and subcontractors with the prospect of the MO 4 commissioning in 2–3 years (turnover, retirements, or other projects), it is necessary to retain critical knowledge (including specific nuclear knowledge).

PROCESSES:

• Before the actual commissioning of the MO4, review in detail the workflow (sequencing of the individual steps) of the preparation, execution and evaluation of the individual tests. During the review, focus on the setup of roles (= who is responsible for …) and the throughput of the whole process for:

  -

  Assessment of the current state of the relevant technological systems for test preparation and execution, including other related technology (auxiliary systems, etc.). The status assessment should also include an assessment of any deficiencies identified in previous operations, which may be long-term.

  -

  Assess and ensure readiness for planned tests (technology, personnel, documentation, collection and evaluation of necessary data).

  -

  Gathering the information needed for the morning briefing should include an assessment of the progress and results of the tests carried out, an assessment of any failure to meet the specified criteria and a decision on the way forward.

  -

  The procedure for recording unresolved non-conformities from previous tests and the process for considering them in decision-making and further action (this may include longer-term non-conformities).

  -

  Direct involvement of the Design Authority in evaluating the test results and participation of its representatives in the morning briefing.

Note: care must be taken to ensure a clear personal responsibility for a particular action—excluding collective responsibility.

○

Consistently record non-conformances/recommendations from evaluations of individual stages of block start-up with tracking of how they were/are resolved.

○

Ensure a clear definition of responsibilities between the construction and NPP departments for the commissioning of the MO4.

○

Consistent (independent) control of the protocols submitted to the regulatory authority.

○

Use updated and revised documentation incorporating all changes and corrections during the commissioning of MO3.

TECHNOLOGY:

Knowledge transfer in relation to solutions of technical problems identified during the MO3 commissioning towards commissioning of next units, e.g.,:

• This issue of neutron-physical characteristics during physical triggering of the unit. Clarification of discrepancies between the reconstructed reactor core power distribution by the standard in-core monitoring system and the computationally predicted reactor core power distribution. This is performed during the evaluation of power field deformations induced by incorrectly positioning of control assembly at different power levels.
• Safety system response analyses are especially useful for low power levels.
• Monitoring of reactivity balance effects during triggering.
• Correctness and speed of processing of ex-core detector responses. Evaluation of reactor period and reactivity during approaching the critical state. Period measurements during verification of the dynamic properties of the neutron flux measurement system.
• Problems related to separators and lower efficiency of the secondary circuit (ensure transfer of technical solutions from the MO3).
• Ensure the transfer of experience in solving problems with the accuracy of pressure measurements in the primary circuit.

Finally, the actual message for each NPP is that the basic precondition for safe and long-term operation is the precise mapping of starting NPP operation conditions during its built and commissioning process.

Having in mind the lessons learned from the whole process of the MO3 commissioning timetable of Mochovce 4 commissioning was improved (see Figure 6 and Figure 7). It is important to note, that although all procedures for commissioning process were carefully prepared and deeply consulted with regulatory authority, each deviation from the process was deeply analysed and procedures must be repeated to the moment, till the authority inspectors will be fully satisfied.

6. Conclusions

Based on experiences with the commissioning of the NPP MO3, we summarised the most relevant issues that impacted the long duration of physical and power commissioning of this unit. A collection of lessons learned from the commissioning of the MO3 can be applied to the MO4 and the commissioning of new nuclear facilities in Slovakia or elsewhere. However, after the Olkiluoto3, the Flamanville3 and the MO3 successful commissioning, several years without any new NPP units in Europe can be expected. This knowledge and experience can be unique for a long time also due to taking responsibility of the former general designer (OKB Gidropress), whose activities and involvement in the locality were minimised during the last years of commissioning.

Actual industrial needs in Europe as well as deficiency of economically acceptable energy cause that existing NPP lifetime extension increases up to about 60 or 80 years. It is important to note that the log-term and safe operation start before commissioning and the whole process lasting several decades should be controlled properly. Large database of surveillance specimens from design materials is needed and proper surveillance specimen programs should be started in time.

We would be glad to contribute at least a little to avoid possible mistakes or nonoptimal processes that could negatively impact the time and budget of new nuclear builds in Europe.