The JSC AKME Engineering (AKME Engineering, 2013), a joint venture of the State Atomic Energy Corporation Rosatom and the private JSC EvroSibEnergo, has
developed the design of the SVBR-100 reactor. The SVBR-100 is a small modular reactor of 101.5 MW(e) per module cooled by lead-bismuth eutectics. It is based on the experience of the propulsion reactors of the Russian alpha-class submarines which successfully operated in 1970s-1980s and gained operational experience of 80 reactor-years. The Russian Federation is the only country in the world with positive experience of reactors cooled by lead-bismuth eutectics. However, this experience so far relates to non-civilian application reactors.

The Russian program for marine propulsion reactors has resolved the two major issues relevant for lead-bismuth eutectics coolant, namely those of corrosion/erosion — free operation of structural materials in the coolant flow at temperatures below ~500 °C and of 210Po trapping and removal, based on data from p. 126 of Current status, technical feasibility and economics of small nuclear reactors (NEA-OECD, 2011) and on data from IAEA (2007). Moreover, a reported accident with spillage of the primary coolant has demonstrated that extra-irradiation doses to personnel could be effectively avoided during spillages and during the subsequent repair works at a factory (IAEA, 2012b).

The design and operating characteristics of the SVBR-100 are summarized in Table 17.3. Table 17.4 gives the core and fuel design characteristics.

Unlike its marine propulsion prototypes in which beryllium moderators were used in the core, the SVBR-100 is a reactor with fast neutron spectrum. Generically, this means that a substantial amount of R&D would be required for qualification and licensing of the new reactor.

The SVBR-100 is being designed as a modular reactor for single-module or multi-module nuclear power plants with optional co-generation capacity. Being a fast reactor, it could be flexible in fuel cycle options, i. e., easily adjustable to operation with different types of fuel — uranium, uranium-plutonium, uranium-transuraniums and uranium-thorium-based oxides or nitrides — in a once-through or closed nuclear fuel cycle, effectively matching the fuel cycle options of the day (IAEA, 2007). When operated with advanced ‘dense’ types of fuel (e. g., nitrides) in a closed nuclear fuel cycle, SVBR-100 could ensure preservation of its fissile inventory through an infinite number of recycles. In this case the reprocessing becomes essentially reduced to just removal of fission products and addition of some fertile material, e. g., depleted uranium (Kuznetsov and Sekimoto, 1995).

The SVBR-100 could be used for base load electricity generation within single or multi-module plants of different capacity. A load following option is, in principle, possible and could be considered for future design modifications. Several conceptual studies of 4- and 16- module SVBR-100 based plants have been developed in the Russian Federation (IAEA, 2007). Some of these designs consider a partly underground location of the reactor modules which, together with other features of the SVBR-100, makes these reactors the closest match to the philosophy of SMRs being pursued in the USA.

Figure 17.5 presents the schematics of the equipment layout in the SVBR-100 primary circuit. Different from sodium-cooled fast reactors, the SVBR-100 plant has no intermediate heat transport system. This is because lead-bismuth eutectics do not react exothermically with water or air. The reactor is pool type with primary

circulation provided by the pumps with externally located drives (see Figure 17.5). There is a very small excess pressure in the primary circuit (inert gas with an excess pressure of 0.1 MPa fills in the space above the coolant-free level). The circulation scheme is optimized to prevent gas or steam bubbles from passing through the reactor core in accidents.

Two reactor vessels are provided, the main vessel and the guard vessel. The guard vessel is immersed in a tank of water at atmospheric pressure with bubbling devices to facilitate decay heat removal in normal shutdown and in accidents. The secondary circuit operates on saturated steam, so that steam separators are being included in the design.

The relatively high melting temperature of the lead-bismuth eutectics (125 °C)

requires heating the reactor internals and the coolant before initial supply of the latter to the reactor vessel. It also may require heating of the shutdown reactor if the spent fuel composition in the reactor does not ensure enough of decay heat to keep the primary coolant liquid. Systems to accomplish such operations have been developed and applied in the Russian marine propulsion reactor program. Moreover, a procedure for safe freezing/unfreezing of the reactor coolant based on a particular time-temperature curve has been developed and tested on a land-based facility. No further details of these technologies are available. The overall layout of the reactor module appears very compact, as shown in Figure 17.6.

The inherent and passive safety features of the SVBR-100 include (IAEA, 2007, 2012a):

• low-pressure primary coolant system, contributing to the prevention of LOCA (Level 1 of the defense in depth);

• very high boiling temperature of Pb-Bi eutectics (1670 °C at atmospheric pressure), double reactor vessel, relatively high temperature of Pb-Bi eutectics freezing (125 °C at atmospheric pressure) and location of the reactor module in a water tank, practically excluding LOCA

and limiting potential radioactivity release in accidents with core melt (Level 1 and Level 4 of the defense in depth);

• chemical inertness of Pb-Bi in air and water, preventing fires and explosions (Level 1 of the defense in depth);

• a pool type design of the reactor with high heat capacity of the primary circuit, ensuring high thermal inertia in transients (Level 2 of the defense in depth);

• negative optimum reactivity feedbacks, including very small reactivity margin for fuel burn-up achieved with ‘dense’ types of fuel at the expense of high conversion or a very small breeding ratio (1.05) in the reactor core, preventing reactivity-induced accidents (Level 1 of the defense in depth);

• a high level of natural circulation of the primary coolant sufficient to remove the decay heat from the core (Level 3 of the defense in depth);

• a primary coolant flow path organized to prevent the possibility of steam or air bubbles from getting into the reactor core, to avoid prompt criticality events owing to positive void worth (Level 1 of the defense in depth).

In addition to the above-mentioned, the SVBR-100 incorporates shutdown systems based on mechanical control rods inserted by gravity and by the force of springs and two diverse passive decay heat removal systems. A steam generator leak localizing system is also included in the design to prevent the ingress of pressurized steam from the secondary into the primary circuit owing to a steam generator tube rupture

(Level 3 of the defense in depth). Reinforced concrete containment is provided to prevent hypothetical radioactive releases beyond the plant boundary.

The seismic design of the SVBR-100 incorporates features to ensure safe reactor shutdown at 0.5 g PGA. Specifically, the reactor is immersed in a water tank which acts as a seismic-resistant structure.

The predicted core damage frequency of 10-8/year (IAEA, 2012c) is primarily because Pb-Bi coolant is chemically inert with water and air, allows for primary circuit operation at near-atmospheric pressure and has a proven capability to self-cure cracks (freezing temperature 125 °C). Moreover, it has a very high boiling temperature (1670 °C), requires no intermediate heat transport system in plant design and ensures a high level of natural circulation in loss-of-flow accidents. Altogether, the above — mentioned features make it possible to develop a simple and robust small reactor design. Additionally, the integral layout of the primary circuit includes a free level of the coolant with inert gas volume above it. The primary flow path is organized in a way that gas or steam bubbles potentially coming from steam generator are released to the gas volume above the free level before the coolant is directed to the reactor core.

As of early 2013, the R&D phase for the SVBR-100 has been completed and design activities were in progress to prepare for the construction of prototype reactor. The focus of the R&D was to adapt the naval lead-bismuth coolant reactor technology to a commercial power reactor (IAEA, 2007). Current activities are focused on the design of fuel, reactor core and primary circuit components, with fuel element design being a priority. As a possible future evolution of the SVBR-100 technology the conceptual proposal of a smaller, perhaps, barge-mounted reactor of 10 MW(e), named SVBR-10, has been developed (IAEA, 2010).