The JSC NIKIET named after N. A. Dollezhal (NIKIET, 2013) — an enterprise belonging to the State Atomic Energy Corporation Rosatom — develops conceptual designs of the two factory fueled SMRs intended for autonomous (unmanned) operation over a long refueling interval within a land-based and a seabed-based NPP. The first of this designs, named UNITHERM, is a somewhat unusual indirect cycle PWR of 2.5 MW(e) with an intermediate heat transport system. The reactor is intended for autonomous operation in the course of 25 years within a land-based plant. The second, newer, design named SHELF is an indirect cycle PWR intended for autonomous operation in the course of 4.6 years within an unmanned seabed-based NPP. The design and operating characteristics of the UNITHERM and SHELF are given in Table 17.5. Table 17.6 presents the core and fuel design characteristics for these reactors.

The UNITHERM plant is being developed to provide electricity and heat to small (e. g., mining) enterprises and settlements located in remote areas with severe climatic conditions. The SHELF seabed-based plant is being developed to act as an energy source for the offshore oil and gas mining enterprises.

Figure 17.7 shows a somewhat unusual three-circuit scheme of the UNITHERM reactor. The UNITHERM is a co-generation plant producing heat for district heating or steam for industrial applications. In such a reactor an intermediate heat transport circuit is needed to prevent radioactivity from getting into the distribution network for district heating or industrial applications.

The heat generated in the reactor core is first transferred through a heat exchanger to the independent intermediate heat transport circuit located in an isolated volume within the reactor pressure vessel. Both, the primary and the intermediate heat transport systems operate on natural circulation of the coolant. From the intermediate circuit heat is then transferred to a power circuit through a steam generator located in a module outside of the reactor pressure vessel.

The intermediate circuit of UNITHERM consists of several parallel sections/units each of which is a thermosyphon housed in an individual vessel with the individual steam generator section. If one of the steam generators’ surfaces in one of the sections is ruptured, the corresponding section is being cut off using the lock valves on the

consumer circuit. The plant need not be stopped (it could continue operation with other sections) and the repair of the lost section could be accomplished during the scheduled repair period.

Very low steam parameters are used in the power circuit (see Table 17.5) and, hence, the energy conversion efficiency is also very low (12.5%). When needed, the plant could also produce steam for industrial applications. For this purpose steam is being extracted from the turbine, and the heat extraction circuit is operated on forced circulation of the medium (steam) (IAEA, 2007).

Radiator Evaporator Steam generator

Pressurizer

Intermediate heat exchanger Core

Reactor pressure vessel

The UNITHERM reactor is being designed for autonomous operation. To ensure safety in such operation yet another circuit shown in the upper part of Figure 17.7 is added. This circuit — actually a purely passive safety system — consists of a continuously operated heat exchanger — evaporator and the radiator connected to evaporator, cooled by atmospheric air at ambient conditions (IAEA, 2007). This circuit is capable of bringing the reactor to a hot standby condition with no operation of the control rods and acts as a decay heat removal system in accidental conditions, e. g., in the event of a loss of the normal heat removal path to the network for district heating (IAEA, 2007).

UNITHERM is being designed for operation in the severe climatic conditions of the Russian north and east, where the ambient temperatures undergo seasonal changes from -55 to +35 °C. To make the reactor continuously operable under such conditions options are being examined to use ammonia, ethylene glycol or alcohol instead of water in the intermediate circuit (IAEA, 2007).

A general view of a seabed-based NPP with the SHELF small reactor is given in Figure 17.8. The reactor is a PWR of integral primary circuit design with in-vessel steam generators, pressurizer, control rod drives and sealed canned pumps (graphics not available) (IAEA, 2012c). The plant has two turbine generators, each connected to one of the two in-vessel steam generators. The reactor itself is immersed in the water pool located in the bottom part of the primary steel containment. This pool
contains metal structures which, together with water, act as a radiation shielding, but also take on the functions of a heat sink.

The NPP shell is designed to withstand a water depth of 300 m, although the targeted depth of a seabed site is 50-100 m. Autonomous operation is foreseen with reactor control being executed from the water or land based control centre (e. g., oil platform). The electricity is supplied to the on-water or under-water user (e. g., a gas mining facility) using another cable.

The SHELF plant is being designed for unmanned operation during the whole cycle of operation between refuelings. For the refueling, it is being raised to the surface and brought to a refueling base (e. g., a dedicated ship). However, as shown in Figure 17.8, the seabed-based plant has an on-board control panel which could be used in emergencies or during a start-up by members of divers’ missions.

Both UNITHERM and SHELF incorporate experience from the design of small marine propulsion reactors. Specifically, they use so-called self-spaced cylindrical fuel elements with the external twisted ribs and employ no spacer grids. The fuel is borrowed from the Russian experience with marine propulsion reactors; Zr coated dispersed UO2 particles in a Zr matrix coated by silumin (Si-Al). Such fuel, often referred to as ‘cold’ fuel (owing to its exceptional heat conductivity characteristics), is also capable of very high burn-ups (IAEA, 2009). However, in the cases of the UNITHERM and SHELF the attained burn-ups are well below those achieved in the state-of-the-art NPPs with large reactors. The reason for this is that, notwithstanding the long refueling intervals, the average core power density in these reactors is low (see Tables 17.5 and 17.6).

Being designed for autonomous operation, the UNITHERM and SHELF reactors

Reactor

Figure 17.8 General view of the SHELF seabed-based NPP, reproduced with permission by the IAEA from IAEA (2012c).

rely strongly on the inherent and passive safety features. The inherent features include low core power density and large thermal inertia of the primary circuit owing to the relatively large inventory of the primary water coolant (IAEA, 2007, 2012c). The reactors employ compact modular (UNITHERM) or integral primary circuit designs minimizing the list and scope of possible LOCA.

The state-of-the-art I&C systems are being employed. The reactors have no liquid boron reactivity control systems. All safety systems in both designs are passive, including the mechanical control rods driven by gravity and the redundant and diverse passive decay heat removal systems. Specifically, UNITHERM incorporates an independent passive decay heat removal system based on an evaporator cooled by external air, as shown in the upper part of Figure 17.7. SHELF has the bottom part of the reactor vessel immersed in a pool of water located in the bottom part of the primary steel containment. A reliable ultimate heat sink for the SHELF reactor is provided by abundant seawater at ambient temperature.

Both reactors provide for a high level of natural circulation of the coolant. In UNITHERM, the circulation in the primary and intermediate circuit is natural in the normal operation mode (IAEA, 2007). In SHELF, natural circulation is sufficient to remove the decay heat and could also remove heat from the reactor operated at 65% of the rated power (IAEA, 2012c).

Both designs incorporate measures to prevent the core becoming uncovered in accidents, such as compact primary containments and reactor vessel penetrations located well above the core. Double containment is provided in both designs, as well as passive systems of the primary containment cooling.

Both design concepts incorporate provisions for protection against possible impacts of external events. The UNITHERM reactor is being designed for the seismic loads corresponding to 8-9 on the MSK 64 scale (IAEA, 2012c). For SHELF, the issue of protection against external even impacts is more complex, specifically because of the absence of internationally acknowledged requirements on protection against external event impacts for seabed-based NPPs. Seismic impacts may roughly be the same; however, tsunamis are not effective at depths around 100 m. Aircraft impact may be mitigated using steel nets around the plant. Resistance to torpedo attacks needs to be clarified.

SHELF has the evaluated core damage frequency 10-6/year. There is no published data on core damage frequency for the UNITHERM; however, according to its designers, this value is similar to, or below, the corresponding value for the SHELF.

According to IAEA (2007), cited with permission by the IAEA,

‘the UNITHERM concept is based on the experience of NIKIET and other Russian institutions and enterprises in the development of marine nuclear installations. The experience is available in the form of design approaches and technologies covering many aspects of nuclear engineering, such as fuel elements, structural materials, metal treatment, welding, heat exchange equipment, water chemistry, etc. In view of this, the UNITHERM NPP may require no major technology development effort to be implemented.’

So far, conceptual design of the nuclear island has been developed. In 2013, activities to design complete NPP were in progress.

SHELF is at the conceptual proposal stage and some activities for it have been carried out in the 2010s. Those were mainly related to the overall nuclear island and plant concept. Although it claims to be based on the Russian experience in the development of marine nuclear propulsion reactors, its further development is likely to include substantial amounts of R&D related to the targeted seabed-based location of the plant. Early in 2013, the activities for SHELF were at a standstill, pending the progress in the financing from the Russian companies examining options of oil and gas mining from the bottom of the Barents Sea.