Email This Author Comment On Article About The Author More Articles By This Author

Nuclear power has demonstrated excellent technical and economic performance in many countries but, like any advancing technology, it continues to chase improvements. The industry's accumulated experience is now being used to develop advanced nuclear power plant designs.

With the majority of Generation I reactors developed in the 1950s and '60s now decommissioned, Generation II reactors represent the bulk of fleets in operation globally today. Generation III (and 3+) are advanced reactors under construction in several countries but have yet to enter operation, while Generation IV designs are still on the drawing board and will not become operational before 2030 at the earliest.

Enhanced safety and improved economic competitiveness are common goals for these advanced designs. The lessons learned from the nuclear accidents of Chernobyl and Three Mile Island (US) have been applied since the first stages of Generation III plant design, with the major safety objective of reducing the likelihood of accidents, as well as mitigating their consequences in the event that they occur.

In the past, basic design and licensing requirements were developed on a country-specific basis and, even within the same country; different utilities defined their own requirements. Starting in the 1980s and following the energy market liberalisation that opened national borders to different utilities, a tendency towards overall standardisation has developed among both vendors and utilities so that designs will be suitable for deployment in different countries.

In this context the Utility Requirements Document (URD), developed by the Electric Power Research Institute (EPRI) in the US and the European Utility Requirements (EUR) document developed by the major European utilities provide a common framework and guidance for development of next-generation nuclear power plants. They enable standardised plant designs that can be offered in different countries without any major design changes.

Renewed focus on safety

For the first time, the requirements have directly addressed not only the design basis accidents, but also the severe accidents -- i.e. certain unlikely event sequences beyond design basis accidents and involving significant core damage. Design features both to improve prevention of severe accidents involving core damage and to mitigate their consequences are being incorporated in the design of advanced plants. Examples of such preventative measures are:

• Larger water inventories (large pressurizers, large steam generators), lower power densities, negative reactivity coefficients to increase margins and reaction periods for operators, thereby reducing system challenges.

• Redundant and diverse safety systems with proven high reliability and improved physical separation between systems. Each of the main safety systems is subdivided into identical sub-systems (four in the EPR plant), with each one capable of carrying out the entire safety function on its own and housed in its own building with electrical supply and support systems. This seeks to overcome the risk of simultaneous failure of all the safety systems because of internal (flooding, fire, etc.) or external (earthquake) events.

• Passive cooling and condensing systems. Passive systems rely only on natural forces (gravity, natural circulation, evaporation) to achieve their safety function without the use of active machinery (pumps, heat exchanger, etc.). Such systems automatically establish and maintain safe shutdown conditions in the plant without operator action following design-basis events, including an extended loss of both on-site and off-site AC power sources. This minimises the potential contribution of a station blackout to core melt sequences.

• Improved man-machine interface to reduce the operator burden during an accident.

Addressing severe accidents phenomena

Mitigation of severe accidents phenomena is also considered in new plant designs:

• High-pressure melt ejection and direct containment heating. Early containment failure from high-pressure melt ejection is prevented by incorporating the means to reliably depressurise the primary system prior to vessel melt-through. Direct containment heating is minimised by arrangements to collect and confine the molten core debris.

• Hydrogen combustion. Large containment volume to minimise hydrogen concentration and the installation of igniters and/or autocatalytic recombiners to reduce the likelihood of hydrogen explosion.

• Core-concrete interactions. There are different strategies to assure corium cool ability and prevent core-concrete interaction. One concept provides a core catcher to collect the molten core outside the reactor pressure vessel and provide cooling of the molten material. Another is based on an in-vessel retention concept (retaining the molten core debris in the reactor vessel via water cooling of the external surface of the reactor vessel) to avoid core-concrete interaction.

The containment is the last barrier to the prevention of large releases of radioactive material into the environment. This means that the outer containment must remain intact and leak tight in case of a severe meltdown accident. Acceptable technologies for the primary containment are metallic, reinforced concrete with a liner, or pre-stressed concrete with or without a liner. Containment system design enhancements include higher design pressure and low-leakage factors, as well as measures to protect the containment, which include a reactor cavity flooding system, hydrogen control systems and the means to spread and cool a molten core.

Containment systems for Generation III plants are characterised by a secondary containment that collects possible leakage from primary containment. In addition, the primary containment is protected from external events (including gas cloud explosion, aircraft crash, etc.) by a shield building.

Following the Fukushima nuclear accident, it will be necessary to critically reappraise the causes and the weak points identified by the sequence of events leading to the accident in order to properly check that the safety features implemented in the Generation III plants are adequate and sufficient to prevent similar accidents and their consequences in future plants.

Italian Nuclear Industry is ready to compete

Italy was a pioneer of civil nuclear power and by the mid-1960s had the third most advanced nuclear power generation programme in the world behind the US and the UK. After the Chernobyl accident in 1986 however, the Italian people voted in favour of a referendum, which initially called for the restriction and suspension of the nuclear programme but ultimately stopped all activity in the nuclear sector. However, even in the absence of a domestic market, the Italian Nuclear industry has kept its design and manufacturing capabilities, and since the 1990s the core of the Italian nuclear industry has been involved in projects outside Italy.

After the recent events in Fukushima and the decisions taken by the Italian government -- namely the recent law dealing with the licensing of nuclear plants in Italy being repealed -- the future of nuclear power in Italy is once again under discussion.

As the Italian nuclear industry has retained its manufacturing and design capabilities, it is ready to compete in today's nuclear energy market, which for the next few years will be found, once more, beyond Italy's national borders. In China, Ansaldo Nucleare -- in a joint venture with Mangiarotti Nuclear -- is charged with design activities related to the steel containment vessel (CV) for the Sanmen 1 plant and also to support manufacturing at the Haiyang SNPEMC workshop and construction at Sanmen site. Moreover, the Joint Venture has designed and manufactured the Passive Residual Heat Removal Heat Exchanger, installed in the first AP1000 plant.

Meanwhile, in the US, excavation work has already commenced at two sites, V Summer in South Carolina and Vogtle in Georgia, where procurement activities for both have involved Italian companies. In particular, IBF has been awarded the supply of RCL piping, while Mangiarotti has been awarded the supply of accumulators, the core make-up tank, the passive residual heat removal heat exchanger and the pressurizer.

Today, more than 30 Italian companies are currently involved in the construction of Generation III plants, three of which are detailed in the following:

AP1000 Plant

The Westinghouse AP1000 is a 2-loop PWR with a net electrical power of 1117 MWe. It has evolved from the smaller AP600, and was the first Generation III+ reactor design certified by the US Nuclear Regulatory Commission (NRC), in 2005. Simplification was a major design objective in order to enhance the construction, operation, maintenance and safety of the plant. The AP1000 plant uses passive safety systems to further enhance plant safety and to satisfy utilities requirements

EPR Plant

Areva NP (formerly Framatome ANP) has developed a large (4590 MWt -- typically 1750 MWe gross and 1630 MWe net) European Pressurised Water Reactor, which was confirmed in the middle of 1995 as the new standard design for France and received French design approval in 2004. It is a 4-loop design derived from the German Konvoi design with features from the French N4. The main safety systems are organized into four sub-systems or `trains'. Each is capable of providing 100 per cent safety functions alone. Each train is installed in one of the four emergency buildings, separated by the reactor building. Simultaneous failure of the trains is thereby avoided.

ESBWR Plant

GE-Hitachi Nuclear Energy's ESBWR (Economic Simplified BWR) is a Generation III+ technology that utilises passive safety features and natural circulation principles and is essentially an evolution of a previous design, the 670 MWe SBWR. The emergency core cooling system has eliminated the need for pumps by using passive and stored energy. One of the ESBWR passive safety systems, the isolation condenser, was designed by Ansaldo, which also manufactured the first prototype.