15.35. The low power density of gas-cooled necessitates very large cores for reactor plants having electrical generating ratings in the 600 MW or higher range. Prestressed concrete reactor vessels (PCRVs) to contain such systems are proven and safe, but are expensive. By dividing the desired capacity into smaller units, steel pressure vessels can be utilized. Also, the components and subsystems for such small modular units lend themselves to factory fabrication with resulting economies.

15.36. The plant capacity factor of the Fort St. Vrain plant was dis­appointing as a result of component problems, particularly with the water — lubricated circulator bearing seals. Although the modular HTGR (MHTGR) uses a proven magnetic bearing system for the circulator, which should be trouble-free, the Fort St. Vrain experience demonstrated the economic sensitivity of a large plant to excessive shutdowns. In a modular plant built up from small units, maintenance on individual unit components can be performed more easily than in a large plant. A multiunit plant also permits considerable fuel management flexibility.

15.37. A typical MHTGR plant consists of four identical modular re­actor units housed in separate adjacent reinforced concrete structures lo­cated below grade, but under a common roof. The reactor units, each producing 350 MW(th) of steam, are paired to feed two turbine generators in a separate energy conversion area to generate a total of 538 MW(el).