15.77. Although it has been possible to eliminate most of the active safety subsystems in the passive plants, the entire plant remains a very sophisticated complex system. Therefore, plant reliability and its effect on the plant availability factor is an issue. There are differences among the concepts described regarding the amount of design innovation involved. Proponents of the AP600 and the SBWR claim that the technology is sufficiently proven so that a “demonstration plant” is not required. On the other hand, there appears to be agreement that a “lead plant” to gain experience before other plants are constructed would be desirable. Since the MHTGR and the ALMR are more innovative, but are modular, the road to commercialization would clearly start with a “demonstration” mod­ule. Also, other designs are being proposed. For example, a hybrid system uses active systems to cope with non-LOCA events, with passive features available as a backup for serious accidents. This approach is claimed to result in a higher plant availability than in an all-passive design, since shutdowns would be shorter [12].

15.78. As a result of experience with the Fort St. Vrain plant, the MHTGR is claimed to be in an advanced state of development. However, as a result of various problems, the operation of the plant was uneconom­ical. Therefore, the operability of a modular plant with some advanced features still requires some demonstrations to assure utilities that their investment risk will be reasonable. As mentioned in §15.62, there is ample time for an ALMR demonstration, both to permit further development and provide a firmer basis for cost estimation.