An ageing nuclear fleet

There are 436 operating commercial nuclear power generating units* (NPUs) worldwide at the end of 2021 producing approximately 10% of the electricity in the world. Apart from those early units commissioned in the 1960s that have ceased electricity production, a considerable number of these operating units were put into operation in the 1980s, and 285 or approximately 65% of the operating units have reached 30 years of age by 2021 upon first reactor criticality¹ at which point a reactor begins its working life and is followed soon by electricity generation (Figure 1). Given that these NPUs were typically provided with a design life of 40 years, it is time for their owner utilities to develop a plan to continue production, or to shut down and dismantle these ageing units at the end of their design lives.

Figure 1: Years in which nuclear power generating units achieve first reactor criticality

The case for life extension

Owing to its robust design and careful operations and maintenance, the condition of nuclear power plant is good generally. Therefore, it is often attractive to continue its operations from a simple economic perspective.

After operating for some years and approaching the end of its design life, an NPU will have completed the depreciation of the value of its initial fixed assets due to construction, and also have completed the payments of any commercial loans arising from the initial investment in the construction of the unit and thereby any associated financing charges. The remaining costs for operations beyond the design life will generally be the costs of the nuclear fuel, plant operations and maintenance, the costs or provisions for the subsequent management of used nuclear fuel and nuclear waste, and various taxes levied by the authorities. The costs of the dismantling of the NPU, or its decommissioning, will have already been provided for by the end of its design life though still subjected to continuous future review. It may be worth noting that continuous additions of fixed assets to the unit during its design life would be made to replace obsolete equipment and to introduce additional safety improvements at the plant, though the costs of these fixed assets, and therefore their depreciation, will only be a small fraction of the cost of construction.

Since an NPU is capital intensive, alleviating it from the depreciation of its initial fixed assets, financing charges and decommissioning provision is expected to significantly lower the cost of its electricity and, if external conditions remain largely unchanged, thereby offer a very competitive product for the electricity market or a considerable margin for profit to its owner utility.

However, a more comprehensive consideration is necessary before putting forward plant life extension, which is also known as “extended operation” or “long term operation” in the nuclear community. Factors such as the value of the nuclear power assets in the generation portfolio of the owner utility, market competitiveness in addition to simple plant economics, the future operational challenges of the plant and the alignment of nuclear power with national values may need to be taken into account.

The Nuclear Energy Agency (NEA) of the Organisation for Economic Co-operation and Development (OECD), for example, has put forward a set of assessment criteria in considering the life extension of nuclear power plants².

One criterion proposed by the NEA is “Production and asset portfolio”, which rates the value of the nuclear assets in the generation portfolio of the owner. It weighs the share of nuclear generation in the electricity generation mix, the ability of the nuclear power plants to “load follow” or the speed for responding to the change in demand of the electricity transmission grid and the presence of electrical connections to a neighbouring grid that may supply electricity for assistance or competition.

Another criterion is “Predictability of future electricity market”, which evaluates the certainty of the electricity market for nuclear power. It considers the predictability of future market prices of electricity, or any potential limitations or regulatory requirements that may restrict the operation of nuclear power plants.

The third criterion is about “Overnight cost of plant refurbishment” or the upfront cost of refurbishment completed overnight, which is measured against the overnight cost of a replacement plant. It also assesses the readiness of the plant owner to secure financing.

The fourth criterion is to assess the “Levelised cost of electricity generation after plant refurbishment” by evaluating the discounted cost of the electricity produced by nuclear power plant after refurbishment. The cost will be compared against the same cost of electricity produced in a replacement plant or imported from a neighbouring electricity transmission grid. In calculating the cost, a higher discount rate is generally adopted if the plant is considered a commercial investment, while a lower rate is generally justified if the plant is considered a social infrastructure.

The fifth criterion is the “Need for plant equipment upgrade or refurbishment”. It will be favourable if most of the plant equipment are up to date.

The sixth criterion is the “Impact of the refurbishment on the 10-year average of the plant capability factor.” It will be favourable if plant refurbishment for life extension has only a marginal effect on the capability of the plant to generate electricity in the long term.

The seventh criterion is “Risk and uncertainty” due to local or national politics, public opinions, financial support and regulatory views.

The final criterion is “National carbon policy and security of energy supply” which evaluates the effects of national policy or practices on reducing carbon emissions and of national view on the role of nuclear power for energy security.

Favourable scorings in the above eight criteria are expected to lead to positive government and plant owner opinions, and subsequently the decision to refurbish the NPU. The process is summarised in Figure 2.

Figure 2: Assessment process for plant life extension²

Review of plant equipment upgrade and refurbishment

Of the above considerations, the need for plant equipment upgrade or refurbishment is of particular engineering relevance. The process is conducted to demonstrate that the basis for regulatory licence for plant operation continues to be valid for the period of life extension of the plant. The International Atomic Energy Agency (IAEA) has put forward a proposal³ which begins with a scope setting and screening process to identify the systems and/or components (SCs) of the plant needed in the evaluation process. This process is followed by a review of existing plant programmes and practices for the applicability and improvement for life extension and of the effect of ageing on an SC so that it will continue to function properly. There is a further review that any analysis that demonstrates the integrity of a SC within a certain time period (or any “time limited ageing analysis” (TLAA) for the SC) will continue to be valid. The process to identify these SCs and their follow-up assessment and any subsequent management programme to control ageing effects (or “ageing management programme”) are illustrated in Figure 3.

Figure 3: Scoping and assessment of nuclear power plant for life extension³

Figure 4: Schematic layout of a pressurised water reactor nuclear power plant

The Pressurised Water Reactor technology which has been adopted by about 70% of the operating NPUs in the world and is shown in Figure 4 may serve as an example. A generating unit is comprised of a nuclear steam supply system built around a nuclear reactor housed in a pressurised reactor vessel. The system makes use of water under pressure as the working fluid and is enclosed in an air-tight containment building. Steam raised at the secondary side of the steam generator is delivered to a steam turbine which drives an electrical generator that produces electricity. Most of the structures, systems and components (SSCs) within the containment building, and indeed the building itself, are considered essential for nuclear safety and are therefore identified in the scoping analysis for evaluation for life extension. Most of the SSCs outside the containment building are excluded from the scoping analysis since equipment such as the steam turbine and electrical generator are already provided with their own maintenance programmes. SCs shortlisted in the scoping analysis, particularly the reactor vessel which is subject to nuclear irradiation that may lead to embrittlement in the material and thereby affect vessel integrity during certain plant operations, are evaluated for the effects of ageing such as fatigue, creep and corrosion on their integrity or review to be conducted on their TLAA (if available) to ascertain their adequacy for use during the period of life extension. Upgrading the NPU for life extension also provides a timely opportunity for upgrading or refurbishing other key plant items such as steam turbine, electrical generator, major pumps and motors, plant control systems, cables and sensors. This will increase power output and provide more flexibility and robustness in plant operations, and will also secure parts and equipment from alternative vendors in case the original equipment suppliers no longer offer related services.

International arena

The US, France and China are the top three countries in the world owning the highest number of operating NPUs. In the US, the majority of its 94 NPUs have received regulatory approval to extend their operating periods from 40 years to 60 years. Out of the 94 units, six have been licensed for another 20 years of operation. France expressed interest in extending the operating periods of her NPUs from 40 to 60 years in early 2022. While in China, Qinshan Unit 1, the first NPU in the mainland achieving first reactor criticality in 1991, was awarded a 20-year extension to its licence in 2021 for operating till 2041. Further extension of the operating periods of the other NPUs in China can therefore be expected.

However, it is prudent to note that regulatory approval may not assure the future of a nuclear power plant as issues may always emerge over time. For example, ten of the NPUs in the US ceased their operation despite the fact that regulatory approval had been received for extending their licences, owing largely to the availability of natural gas at relatively low prices brought about by the introduction of fracking technology for gas exploration during that time. The generally small generating capacity of these NPUs offering a lower level of economy of scale and the competitiveness of the electricity market in the US were also the reasons.

* It is noted that a nuclear power plant may have more than one NPU, as often found in other power plants. An NPU typically comprises a nuclear power reactor and a turbine generator.

Acknowledgements

The author wishes to express his gratitude to OECD and IAEA for their kind permission to reproduce Figures 2 and 3, which were taken from References 2 and 3 of which they are the respective copyright owners. The author also wishes to thank the support provided by Ms Jenny Lin in preparing this article.

About the author: Ir Richard Fung is a member of the HKIE. He has practised in the nuclear business for over 40 years serving nuclear power plant design and operations management.

References

1. Power Reactor Information System, International Atomic Energy Agency. [online]. Available at: <https://pris.iaea.org/pris/>. [Accessed on 2 September 2022].
2. The Economics of Long-term Operation of Nuclear Power Plants (2012). NEA number 7054, Nuclear Energy Agency, Organisation for Economic Co-operation and Development.
3. Safe Long Term Operation of Nuclear Power Plants (2008). Safety Reports Series Number 57, International Atomic Energy Agency.