NRG en nucleair in het nieuws	Krantenartikelen

Zoeken in NRG krantenartikelen Acacia: A smaller solution Nuclear News Door: Dick Kovan 24 juli 2002   Aliki van Heek, of the Dutch company NRG, argued that although 100 -- 300-MWe HTRs may be relatively small for industrialized countries, they are still too large for many developing countries, and that smaller HTRs would more easily penetrate markets for cogeneration, desalination, hydrogen production, remote siting, etc. These will have their own requirements, which designers must take into account. For example, she noted, the cost of having onsite specialist maintenance staff would not be feasible in many of these applications. According to van Heek, NRG has designed a small HTR called Acacia (AdvanCed Atomic Cogenerator for Industrial Applications), aimed at these other markets. Acacia is a 60-MW thermal pebble bed reactor designed for heat and power cogeneration or distributed electricity generation. In the latter case, the plant could generate a maximum electrical output of 23 MWe using a combined cycle of gas and steam turbines. Economic performance is optimized by both simplification of the nuclear part and by the exclusive use of commercially proven systems in the energy conversion part. Key features of the design are the use of basic PBMR pebbles, a three year fuel cycle, and a secondary circuit using nitrogen as the heat carrier. NRG has adapted these technologies in very interesting ways. Unlike the typical pebble bed reactors in which the pebbles "flow" down through the core, Acacia's core is static, with no on-line refueling or shuffling done. And so there is no need for on-line fueling or defueling systems, which are one of the complicating features of pebble bed reactors. To refuel off-line only once every three years, however, other ways to control the reactor -- particularly reactivity -- are needed. The solution that NRG came up with was to control excess reactivity with boron carbide (B4C) burnable poison in the reflector, so that no new fue1 pebbles with burnable poison need to be developed. With this method in mind, NRG first looked at a basic cylindrical core, but found that with the burnable poison in a reflector surrounding the core, a suitable "flat" profile of reactivity was not possible over the planned three-year operating cycle. Instead, the designers opted for an annular core, with the poison in the inner reflector, which provides the necessary flat reactivity profile. As for using nitrogen gas turbines, Van Heek noted that direct-cycle systems require helium turbines to be developed and commercialized. For an indirect cycle,however, a nitrogen-driven turbine can be developed using conventional or existing components and systems. She explained that nitrogen acts much like air and that closed-cycle air gas turbines were developed in the past. She is confident that suitable nitrogen gas turbines can be developed for Acacia without significant R&D requirements or deployment risk. Dick Kovan July 2002

NRG, PO Box 25, NL-1755 ZG Petten, Netherlands, Tel +31-224564080, Fax +31-224563912 Informatie: info@nrg-nl.com Update 24 juli 2002