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The Safest and Most Reliable Generation IV Reactor: Nuclear Technology Experts'​ Opinion

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Mark Gino Aliperio's picture
Student Graduate KEPCO International Nuclear Graduate School

Nuclear Power Plant Engineer. In my study at KEPCO International Nuclear Graduate School in which I specialized in Project Management in Nuclear Power Plant (NPP) Construction, my team and I...

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  • Jun 18, 2020

by Mark Gino Aliperio

Generation IV nuclear power plants are deemed to be an important source of baseload power for long-term goals. Currently, there are several designs but few will be deployed in the near future due to strategic, economic, and even political reasons. Such factors pose a challenge in selecting an ideal Generation IV technology to commit time and financial resources to. This study presents a systems approach to decision making for an appropriate Generation IV nuclear power plant program. A methodology commonly used in Systems Engineering projects, the Analytic Hierarchy Process, was applied to evaluate the survey results from expert opinions. As input to the decision analysis methodology, a set of selection criteria was developed and evaluated by nuclear power technology experts. Safety and Reliability emerges as the most important goal area, followed by Waste Management and Economics in terms of priority. This article presents which of the six (6) Generation IV reactors is the most competitive in the goal area of Safety and Reliability.

A Review on the six (6) Generation IV Nuclear Systems

1. Very-High-Temperature Reactor (VHTR)

  • VHTR is a thermal reactor cooled by helium gas and moderated by graphite (solid, can be recycled). The core outlet temperature (COT) of over 900°C and aiming for 1000°C enables the production of hydrogen for other co-generative industrial applications. VHTR is a small modular reactor (SMR) that has potential for high burn-up, complete passive safety, low operation and maintenance (O&M) cost, and modular construction that could partially compensate the loss of economies of scale. Although its basic technology has been established in former high-temperature gas reactors with hundreds of operation hours, the main research needs for VHTR are fuel, materials, and hydrogen production.

2. Molten Salt Reactor (MSR)

  • MSR is the only Gen IV reactor that utilizes liquid fuel (uranium is dissolved in the fluoride salt coolant) which circulates through the graphite core channels (also acts as moderator). This novel feature provides the foundation for an enhanced safety profile based on low pressure operation, eliminating the need for solid fuel fabrication and handling criticalities. Compared with solid-fueled reactors, MSR systems offer far more complete and efficient fuel consumption having lower fissile inventories and large negative reactivity feedback. Such characteristics may enable MSRs to have competitive economics, but needs more research and development (R&D) works in fuel treatment, materials, and reliability.

3. Super-Critical-Water Cooled Reactor (SCWCR)

  • SCWCR, considered to an evolution of actual boiling water reactor (BWR), is a high-temperature and very high-pressure water-cooled reactor which operated above the thermodynamic critical point of water, giving a higher net electrical efficiency (10% higher than BWR). The super-critical water directly drives the turbine without the need for any secondary steam system (e.g. steam generator, dryer, recirculating system, etc.), thus improving economics because of plant simplification resulting in potential cost reductions of 30% compared with present pressurized-water reactors (PWR). With operational experience and passive safety features similar to those of BWRs, R&D is still needed on materials and thermal-hydraulics.

4. Gas-Cooled Fast Reactor (GFR)

  • Like other helium-cooled reactors, GFR will be a high-temperature and a fast-spectrum reactor that employs similar technology with VHTR, which is suitable for electricity generation and thermochemical hydrogen production for industrial applications. With a high COT of 850°C, it enables an elevated efficiency for helium Brayton cycle. GFR is the only Gen IV design with no operating antecedent and experience, Moreover, the main R&D needs for this reactor system are fuels, thermal-hydraulics, and material as core internals are exposed to high temperatures and elevated irradiation.

5. Sodium-Cooled Fast Reactor (SFR)

  • SFR is a fast reactor that uses liquid sodium as coolant allowing high power density with low coolant volume operated at low pressure. Having a high specific heat, sodium is a good coolant and is less corrosive than lead, but it chemically reacts with water and air, so a sealed coolant system is required. SFR builds on 390 reactor-years of operational experience and remains the forefront as the main technology of interest in GIF. Three variants are proposed: a 50-150 MWe modular-type; a 300-1500 MWe intermediate-to-large size pool-type; and a 600-1500 MWe large size loop-type reactor. While SFR is the most researched type of fast reactor, R&D is focused on safety in loss-of-coolant scenarios, fuels and its handling, and advanced recycle options.

6. Lead-Cooled Fast Reactor (LFR)

  • LFR is a flexible fast neutron reactor that can be fueled by depleted uranium or thorium matrices. It is cooled by liquid lead or lead-bismuth eutectic (LBE) which has a high boiling point, does not react to water and air, and has an excellent neutron and thermos-fluid-dynamic properties. Compared to LBE, pure lead is more abundant, less expensive, and less corrosive at high temperature. A wide range of unit size envisaged from a battery type producing 20-180 MWe, to modular-types producing 300-400 MWe, and to large size plants of 1400 MWe. COT of 550°C is achievable but 800°C is aimed to enable hydrogen production. The main research needs for LFR are fuels and advanced materials.

Safety and Reliability Goal Area

The goal area of safety and reliability is the most important priority in selecting a nuclear energy system. It ensures normal operation and prevents off-normal situations to deteriorate into severe accidents. The operating temperature and pressure of a nuclear system is also weighed as high-temperature systems may require a large cooling mechanism and high-pressure systems pose explosion risks during accident scenarios. In addition, core damage goal ensures effective mitigation and decay heat removal during accident scenarios. History of operation is added as a criteria, which is originally not in GIF’s metric. The viability and success of a new nuclear program cannot rely only upon on its technology but also requires proven experience from pilot project prototypes and research demonstration. Furthermore, plant protection goals would guarantee that future nuclear plants are designed to withstand external events, and highly secure it from internal threats through passive safety systems, that would eventually increase the public confidence on the safety of nuclear infrastructures.

The figure above presents the roll up of goals and criteria for this goal area. Under the goal of SR1 Operational Safety and Reliability there are four criteria to be considered, SR1-1 radiation exposure (routine), SR1-2 radiation exposure (accident), SR1-3 reliability, and SR1-4 operating temperature and pressure. Under the goal of SR2 History of Operation there are four criteria to be considered, SR2-1 operational experience, SR2-2 pilot projects, SR2-3 feasibility, and SR2-4 research and development. Under the goal of SR3 Core Damage there are two criteria to be considered, SR3-1 decay heat removal features and SR3-2 susceptibility to loss-of-coolant accidents. Under the goal of SR4 Plant Protection there is one criteria, SR4-1 passive safety features.

Technology Experts' Opinion

To quantify the metric of the selection process, opinions of nuclear power technology experts were obtained through a survey. The goals were compared in pairs. Each pairing was evaluated by the respondents based on importance in a 1 to 5 scale, where 1 implies equal importance for both, otherwise, one is more important than the other. goals. The figure below shows the sample survey form as well as the affiliations of experts who participated and demonstrates a wide range of companies and institutions involved in the different sectors of nuclear power technology such as research, academics, plant design and engineering, operation, utility, maintenance, and fuel design and fabrication.

Analytical Hierarchy Process and Decision Analysis

To determine the importance factor to be used in decision-making process, the answers from the survey for each pairing are analyzed. The values of judgement of 1 to 5 scale of the survey is transmuted to a 1 to 9 scale for decision analysis. The interpretation of such values for a pair a comparing criteria j and k, are described in the figure below. The Analytical Hierarchy Process (AHP) is employed in this project for decision analysis and is implemented through Super Decisions, a decision support software.

Evaluation of Safety and Reliability Goal Area

Safety and Reliability appears to be the most important goal area having a score of 0.726. This goal area contains the three highest-scoring goals of Operational Safety & Reliability (SR1), Core Damage (SR3), and Plant Protection (SR4). History of Operation (SR2) has the 5th relative importance for all goals. The table below presents the driving factors of this goal area for the six Gen IV reactor designs.

According to the experts surveyed, the goal of Operational Safety and Reliability (SR1) has the highest level of priority, having a score of 0.259. This goal is described by the criteria of radiation exposure (routine and accident), reliability, and operating temperature and pressure. Reactors operating at higher temperatures may have improved thermodynamic efficiency, but have drawbacks in terms of cooling mechanism during off-normal situations. It also requires additional or large heat exchangers. Furthermore, LFR, MSR, and SFR operate at low pressure, thus possessing significant safety advantage. Reactor designs that operates at high pressure possess explosion risks especially during accident scenarios.

VHTR design is aimed to prevent any accidents that may occur at the facility resulting in significant releases of radioactivity. It also eliminates fuel heating above the temperature limit, guaranteeing its integrity. SFR technology guarantees a safety level at least equivalent to today’s European Pressurized Reactor (EPR). While liquid metals are highly effective coolants, SFR core internals are sensitive to blockages and power disturbances. In addition, SFR safety is highly influenced by the reactive nature of sodium. Although SCWR avoids problems relating to phase change of water, the design doesn’t present any intrinsic advantage in terms of safety. For example, the use of supercritical water in a nuclear reactor raises many concerns. While SCWR is the only Gen IV system that uses water as coolant, it is still uncertain if it will become significantly safer than Gen III reactor systems.

GFR appears less effective in mitigating off-normal scenarios compared to the other Gen IV systems because its coolant is not capable of retaining radioactive materials. As safety demonstrations rely almost exclusively on the reliability and protection performance, GFR requires much more development before it can meet the safety levels required for Gen IV reactors. Unlike SFR, by using a non-reactive coolant, LFR designs shows similar drawbacks associated with using highly corrosive and erosive metals (Lead or LBE) that are chemically toxic. In addition to that, the LFR temperature range is also limited due to lead’s high solidification point. This makes LFR highly reliant on operating procedures which is not a desirable characteristic of a Gen IV reactor system. Lastly, as the only reactor that utilizes liquid fuel, MSR has several advantages such as the absence of solid fuel handling and having a large negativity reactivity feedback which are unique safety characteristics not found in solid-fueled reactors. However, safety systems depend mainly on the reliability and performance of a fuel salt drainage system in case of accidents. Furthermore, the use of salt has drawbacks such as being corrosive and having a relatively high crystallization temperature.

Core Damage (SR3) is the second most important goal with a score of 0.230. This goal is evaluated by two criteria of decay heat removal features (SR3-1) and susceptibility to loss-of-coolant accidents (SR3-2). To provide a passive means of removing decay heat, natural convection in the core and primary circuit is essential. Design reviews suggest that natural convection-based cooling of the core and primary circuit is feasible SFR and LFR, but such features still have to be verified. In the case of SCWR, cooling through natural convection may be achieved with the aid of condenser systems isolation. For MSR and VHTR, coolant circulation is considered to be undesirable, but systems can be used to cool the primary circuit from the outside, i.e. through conduction and radiation. However, with GFR, decay heat removal required forced convection to be maintained in the core.

Plant Protection (SR4) is the 3rd most important goal with a relative score of 0.179. This goal is evaluated by the criteria of passive safety features which ensures that Gen IV designs will deliver acceptable safety performance in case of accident conditions. Although no cooling system connected to the primary circuit, VHTR has a safety cooling system located outside the reactor vessel. VHTR and SFR are supplemented with passive systems triggered by temperature changes. Due to the risk of structural collapse, SFR and LFR requires a high level of reliability which makes the safety system architecture complicated. Also, the risk of reaction between sodium and water, further makes the overall safety architecture of SFR more complex. On the other hand, SCWR may use similar established experience of BWR’s safety systems such as turbo pumps and passive steam condensing system. Lastly, GFR still features a complex safety system that needs further research and development.

As mentioned, History of Operation (SR2) is introduced as one of the goals as the success of a new nuclear program can’t rely only upon its technology but also requires proven experience from pilot projects and research demonstration. Four of the Gen IV systems (VHTR, MSR, SFR, LFR) have significant operating experience already in most respects of their design, which provides a good basis for further needed research and development, and is likely to mean that they can be in commercial operation as soon as anticipated. Since SCWR builds both on much BWR experience and that from hundreds of fossil-fired power plants operated with supercritical water, GFR remains to be the only Gen IV system with no operating antecedent, so a prototype is not yet expected in the upcoming years. Also, GFR specifications are highly ambitious that raises a number of technological dilemma.


For Safety and Reliability Goal Area, core outlet temperature, pressure, decay heat removal features, and operational experience are considered as main drivers. Reactor designs operating at lower temperature and pressure possess significant safety advantage. Moreover, using natural convection for decay heat removal is seen to be more advantageous in the event of accident scenarios. History of operation, as a new goal introduced into the metric provides a good basis for experience from pilot projects. It is found that SFR satisfies this goal area operating at a relatively low temperature (500-550°C) and pressure. Furthermore, it is built on approximately 400 reactor-years of experience. SFR also employs natural convection that provides a passive means of removing decay heat.

In general, the results of this project are partially conclusive as it based on experts’ opinion as well as literature review. It is important to note that there are still a number of factors and criteria that may affect the entire selection and evaluation procedure. For example, political factors are not reflected in this project but it can significantly influence future judgement, and may overshadow other critical criteria. Nonetheless, the outcomes of this project can be used as reference for decision making. However, the level of priorities or importance is flexible depending on an implementing agency or a country’s specific need for future generation of nuclear systems.


This article is a part of a work published in the Journal of the Korea Management Engineers Society, Vol. 24, No. 2, 2019. This work was supported by the 2019 Research Fund of the KEPCO International Graduate School, Republic of Korea.

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Thank Mark Gino for the Post!
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Michael Keller's picture
Michael Keller on Jun 22, 2020

Appears all the experts are from Korea. Not so sure that leads to a broad enough base to reach a reasonable consensus. A mix of experts from across the globe would be better.

On a more specific level, all of the SFR's built have been utter commercial failures. List includes France, Great Britain, Japan. Costs have been in the billions of dollars with capacity factors dismally poor. Unclear how the survey group reached a conclusion that the SFR approach is competitive. History points in exactly the opposite direction.

Another specific observation, high pressures in and of itself does not lead to explosions. The measure is what happens if the high pressure rapidly devolves or ends up in the wrong place. For instance, if the reactor depressurizes, does the fuel melt because the core cannot be cooled. If a high pressure steam generator experiences tube leaks, is a low-pressure reactor damaged by the coolant being ejected from the core.

The general idea of comparing the technologies has merit, but should be grounded in using more quantitative data, as opposed to subjective opinions. Such an approach does require more effort, but provides a much better contrast. The necessary information is available, although the evaluation criteria would need some adjustments.

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