Nuclear clear to go as improved design lowers public safety fears

Paul Boughton

The resurgence of interest in nuclear power generation is being boosted by fewer public fears about safety issues, by the industry developing improved reactor designs and control systems and by an impressive safety record in the industry.

Typical of the new order is the Economic Simplified Boiling Water Reactor (ESBWR) designed by the nuclear division of GE Energy. The company has recently submitted the Design Certification application for its new reactor design to the US Nuclear Regulatory Commission in a 7500-page application.

GE Energy says the 1500MW ESBWR is a third generation reactor design because of its design simplicity and passive safety features. It depends on fewer active mechanical systems, with their associated pumps and valves, and relies on more reliable passive systems that utilise natural forces, including natural circulation and gravity.

It evolved from GE’s 1350MW Advanced Boiling Water Reactor (ABWR), which the NRC certified for US construction in 1997. The ABWR is a design that has already been proven with more than 18 reactor years of operating data from plants completed in Japan in the
mid-to-late 1990s.

The ESBWR is considered to be an evolutionary design because while it incorporates much of the ABWR’s key and proven design features, including its advanced digital monitoring and controls technology and advanced construction techniques, it also incorporates new technology advances.

Safe emphasis

Framatome also emphasises safety with its SWR1000, an advanced boiling water reactor. Again, the new safety concept of this medium-capacity model emphasises passive safety features, based on gravity and natural convection.

By increasing the volume of water in the reactor pressure vessel, the reactor core remains well covered with water during depressurisation. This lengthens the time period until the start of core heatup, providing a considerable safety margin before the coolant needs topping up. There is also a low susceptibility to human error, as the passive systems are not accessible to personnel during plant operation.

Emergency condensers serve to remove heat from the reactor if there is a drop in reactor pressure vessel water level. The tubes of the emergency condensers are submerged in the core flooding pools and are filled with water when the water level in the RPV is normal. If reactor water level should drop, the water drains from the tubes. Steam from the reactor then enters the tubes and condenses, the resulting condensate flowing by gravity down into the RPV. The emergency condensers come into action automatically without any need for electric power or switching operations.

The Westinghouse AP600 is a 600MW PWR that is based on previous PWR designs but which features innovative passive safety features that permit a greatly simplified reactor design. This has led to fewer plant components, increasing reliability and reducing construction costs. Now, the company is promoting the next version, the 1000MW AP1000, which is less expensive on a per kilowatt output basis.

Simplification of plant systems, combined with increased plant operating margins, reduces the actions required by the operator. The AP1000 has 50percent fewer valves, 83percent less piping, 87percent less control cable, 35percent fewer pumps and 50percent less seismic building volume than a similarly sized conventional plant. These reductions in equipment and bulk quantities lead to major savings in plant costs and construction schedules.

The AP1000 plant configuration comprises two Delta-125 steam generators, each connected to the reactor pressure vessel by a single hot leg and two cold legs. There are four reactor coolant pumps that provide circulation of the reactor coolant for heat removal. A pressuriser is connected to one of the cold leg piping to maintain subcooling in the reactor coolant system.

Like the AP600, the AP1000 utilises modularisation technique for construction, which allows many construction activities to proceed in parallel. This technique reduces plant construction time down to 36 months from first concrete to fuel loading.

First EPR

In Finland, Framatome is building its first European Pressurised Water Reactor (EPR) and says the plant is a milestone in the recent development of nuclear energy. It has a capacity of about 1600MW and is scheduled to start commercial operation in 2009. The design features innovations to prevent core meltdown and mitigate any potential consequences. It also offers exceptional resistance to external hazards such as aircraft crashes and earthquakes.

In a PWR, the pressurised water in the primary system is used as a moderator to slow down the neutrons, allowing a nuclear reaction to occur in the core, and to transfer the heat generated during the reaction to the steam generators.

The EPR has four steam generators, or heat exchangers, one for each of the four heat removal loops that comprise the primary system. On their primary side, they receive heat from the nuclear reactor, and on their secondary side, they deliver heat to the non-nuclear part of the facility. That secondary heat produces steam to power the turbine generator.
At the smaller end of the sale, the Pebble Bed Reactor also claims a dramatically higher level of safety and efficiency. Instead of water, it uses pyrolytic graphite as the neutron moderator, and an inert or semi-inert gas such as helium, nitrogen or carbon dioxide as the coolant, at very high temperature, to drive a turbine directly. This eliminates the complex steam management system from the design and increases the transfer efficiency.

Pebbles are held in a bin and an inert gas, helium, nitrogen or carbon dioxide, circulates through the spaces between the fuel pebbles to carry heat away from the reactor. Ideally, the heated gas is run directly through a turbine. However, if the gas from the primary coolant can be made radioactive by the neutrons in the reactor, it may instead be brought to a heat exchanger, where it heats another gas or steam.

The primary advantage of a pebble bed reactor is that it can be designed to be inherently safe. As the reactor gets hotter, the rate of neutron capture by the U-238 increases, reducing the number of neutrons available to cause fission. This places a natural limit on the power produced by the reactor.

The reactor vessel is designed so that, without mechanical aids, it loses more heat than the reactor can generate in this idle state. The design adapts well to safety features. In particular, most of the fuel containment resides in the pebbles, and the pebbles are designed so that a containment failure releases at most a 0.5mm sphere of radioactive material.

Higher efficiency

The reactor is cooled by an inert, fireproof gas, so it cannot undergo a steam explosion as a light-water reactor can. A pebble-bed reactor can have all of its supporting machinery fail, and the reactor will not crack, melt, explode or spew hazardous wastes. It simply goes up to a designed idling temperature, and stays there. In that state, the reactor vessel radiates heat but the vessel and fuel spheres remain intact and undamaged. The machinery can be repaired or the fuel can be removed.

A large advantage of the pebble bed reactor over a conventional light-water reactor is that it operates at higher temperatures. This enables a turbine to output more mechanical energy from the same amount of thermal energy so the power system uses less fuel per kilowatt-hour.

The technology has been under development in South Africa since 1993. The PBMR project entails the building of a demonstration reactor project near Cape Town and a pilot fuel plant near Pretoria. The current schedule is to start construction in 2007 and for the demonstration plant to be completed by 2010. The first commercial modules are planned for 2013.

PBMR (Pty) Ltd has recently signed a memorandum of understanding with the Chinese developers of pebble bed technology, Chinergy Co, whose pebble bed concept is based on a 10MW research reactor that was started up in Beijing in December 2000. The 10MW prototype, the HTR-10, is a conventional helium-cooled, helium-turbine design.

The first 200MW production plant is planned for 2007. There are firm plans for thirty such plants by 2020 generating 6GW. By 2050, China plans to generate as much as 300GW. If PBMRs are successful, there may be a substantial number of reactors deployed.
These developments could herald a new future for nuclear power if the trend follows the pattern of so many new ideas over the last fifty years.

The original concept was hailed as the world's salvation for its power requirements but the industry promptly shot itself in the foot with its inability to counter the fear of radiation leaks, followed by various accidents that did not encourage public confidence.

Perhaps the public were still stunned by the bombs dropped on Japan only a few years before while materials technology and control techniques were not up to the forces that they had to restrain. Today, lessons have been learned, technical advances have been made and there should be no reasonable bar to a global nuclear future. 

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