CAREM – An Innovative, Small PWR

I want to tell you about a development project in Argentina that has received little attention in the nuclear industry press. Its existence demonstrates the continuing international interest in nuclear technology, even at a time when many industry observers declared that the atomic age had come and gone.

CAREM ( http://www.invap.net/nuclear/carem/carem_index-e.html) is classified as a very small nuclear power plant, with an electrical output of 27 MWe. The plant is a pressurized light water reactor with some innovative features that attempt to make the system suitable for developing countries. CAREM’s designers consciously chose to develop a small power source with most of the characteristics of established nuclear plants.

CAREM is being developed jointly by INVAP, a company based in Patagonia, Argentina and CNEA (Comision Nacional de Energia Atomica) the National Atomic Energy Agency of Argentina. The team has a long history that includes successful development of nuclear research reactors, uranium enrichment, nuclear fuel manufacturing, radioisotope production, nuclear medical devices and space technology. According to company literature, the CAREM project has been in progress for a little over ten years and has included system design work, component testing and computer code development and validation.

The CAREM heat source is a smaller version of the cores used in most water cooled reactors. The basic element is a pellet of uranium dioxide about the size of the tip of a pinkie finger. These pellets are loaded end to end into Zircaloy-4 tubes to protect them from corrosion in the hot water environment. About 108 of the thin tubes are assembled together with spacer grids to form hexagonal bundles that provide them with mechanical strength. There are 18 guide tubes in each bundle that can accommodate control rods. The 61 fuel bundles are about half the length of those used in most existing reactors, requiring less costly handling and storage equipment.

Outside the core there are some unusual features. The CAREM primary system is fully contained inside the pressure vessel, with the stated goal of eliminating the large diameter piping used in most water cooled reactors. The idea is that this will reduce the possibility of a large loss of coolant accident. There are 12 small steam generators located on the inner circumference of the vessel. The bottom of the steam generators is slightly higher than the core itself. The ring of steam generators is outside the outer diameter of the core to allow full refueling access from the top of the pressure vessel.

Directly above the core is a large volume of heated water that flows into the steam generators. The primary coolant enters the top of each steam generator and exits from the bottom. The primary water flows on the outside of the tubes containing the secondary water. Below the steam generators, there is a large volume of water surrounding the core that protects the pressure vessel from neutron damage. The large water volume inside the pressure vessel allows operators plenty of time to respond to a loss of coolant accident.

The elevated location of the steam generators and the low resistance flow path provides natural coolant circulation based solely on the density variations in the coolant as it gets hotter in the core and colder in the steam generators. There are no coolant pumps.

There is also no pressurizer in the system, instead, the pressure vessel is tall enough to contain a steam bubble at the top of the vessel, keeping the coolant at saturation conditions for an operating pressure of 12.25 Mpa and a coolant temperature of 326 degrees C. There is no boron in the coolant during normal operation, resulting in a strong negative temperature coefficient of reactivity that allows smooth reactor response to power transients.

The control rod drive mechanisms are hydraulic devices fully enclosed inside the pressure vessel in order to minimize penetrations. They are designed so that a loss of power to the pressurizing pump causes the rods to drop into the core. Six of the 25 rod assemblies have mechanisms designed for rapid insertion. They are normally fully withdrawn during plant operation. The other 19 rod assemblies are designed for fine control of reactivity; they will also drop into the core when power is lost to the pressurizing pump, but they respond more slowly because of tighter tolerances in the hydraulic pistons.

The secondary plant is a conventional steam plant with a single turbine and several stages of feed water pre-heat provided by extracting saturated steam at several points in the turbine. The filtration and polishing system purifies the feed water. In combination with the unique steam generator design, this eliminates the need for routine steam generator blowdowns.

CAREM has a suite of safety systems that are similar to those of conventional light water reactors. The systems keep the core from melting under anticipated accident conditions and keep any fission products that might be released by a melted core inside the containment building. The systems include redundant shutdown with safety control rods and boron injection, three safety relief valves for overpressure protection, a primary and secondary containment, emergency condenser for primary depressurization and residual heat removal, and high and low pressure primary water injection systems which operate without electrical power.

Though somewhat simpler than first generation nuclear power plants, CAREM will have difficulty competing economically in most power markets. The large pressure vessel will be a major cost component; it is approximately the same size and thickness as the ones used for 600-1000 MWe reactors. In a power system that produces only 27 MWe, the pressure vessel and its surrounding containment structure will demand a large investment per kilowatt of capacity.

The designers seem to recognize this fact. In their literature, they describe CAREM first as a research and development product that competes in the international R&D market and secondly as a National Product that serves as a bridge between research reactors and full scale commercial power plants. For an investment on the order of $100 Million USD ($4000 USD per kilowatt capacity), CAREM allows a nation with limited resources to develop the skills needed to license and operate a large, pressurized water reactor plant and build many of its components. It is only after describing these uses that the designers mention that the smallish plant could compete in isolated electricity or cogeneration markets where fossil fuel supplies are limited and expensive.