Abstract
This study originated from a specific problem that arose in conjunction with the IRIS (International Reactor Innovative and Secure) project (Carelli et al., 2004; Carelli, 2009). IRIS adopts an integrated primary system reactor (IPSR) configuration with all the primary loop components of a classical Pressurized Water Reactor (PWR) contained inside the vessel (Fig. 1). Among the reactor core internals are the steam generator (SG) units (Cinotti et al., 2002) with the primary fluid flowing outside the tube bundles and subjecting them to significant external pressure. In this situation buckling affects the tube collapse modality and codes become extremely conservative, to the point that up to five years ago design procedures based on the ASME Boiler & Pressure Vessel code (Section III) required an external diameter to thickness ratio (D/t) less than 8.5, leading to an increased thermal resistance in the heat exchange process between primary and secondary fluids, with detrimental consequences on the dimensioning of the heat transfer surface. A reduction in the tube thickness would allow the reduction of the overall heat transfer surface needed to exchange the same amount of power, with consequent saving on tube lengths and/or number of tubes. On the other hand, if the design of the steam generator units is not modified, an increase in the exchanged thermal power and a consequent up rating of the reactor can be obtained. Besides IRIS, other recent proposals for next generation power plants based on PWR technology consider an IPSR design (Ingersoll, 2009; Karahan, 2010; Ninokata, 2006). Such integrated design is particularly suitable for small sized units, i.e., reactors with a power less than 300 MWe following the IAEA’s definition (IAEA, 2007). A significant number of small sized PWR IPSRs is currently under development (e.g., RITM-200, ABV, CAREM, SMART, MRX, NHR-200, Westinghouse SMR, mPower, NuScale, see http://www.world-nuclear.org/info/inf33.html). Packing all the PWR primary components into the reactor pressure vessel (RPV) (Fig. 2) offers several advantages (Ingersoll, 2009): (i) all large coolant pipes are eliminated (only small feed water and steam outlet pipes penetrate the vessel wall); (ii) the total inventory of primary coolant is much larger than for an external loop PWR (this feature increases the heat capacity and thermal inertia of the system and hence yields a much slower response to core heat-up transients); (iii) typically the heat exchangers are placed above the core creating a relatively tall system that facilitates more effective natural circulation of the primary coolant in the case of a coolant pump failure; (iv) the vessel accommodates a relatively large pressurizer volume that provides better control 14
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