POWER PLANT DESIGN | RUTA


V DHNP with the RUTA reactor The RUTA-70 reactor plant was developed in the 1990s by IPPE and NIKIET. It is a pool-type water-cooled reactor plant with forced circulation of the coolant at a rated thermal power of 70MW. It can also operate with natural circulation of the coolant at a thermal power below 30% of the nominal rate (see Table 1). The RUTA reactor’s simple design means the construction and operating costs are low. But its design is inherently


safe, with: ● Low pressure in the primary circuit (atmospheric pressure above the water surface in the reactor pool),


● Large heat storage of water in the pool due to its significant volume,


● Low power density in the reactor core, ● Heat removal from the core in the natural circulation


mode at an energy release below 30% of the nominal power, and as residual energy release during cooling down of a shutdown reactor,


● Three-circuit scheme of heat transfer to consumers, maintaining the lowest pressure in the primary circuit.


The reactor core and its reflector are in the lower part of the pool, while the main part of the reactor plant equipment, including heat exchangers between the primary and secondary circuits are in dry boxes outside the reactor pool. Forced circulation of the primary coolant is provided by two circulation pumps — one in each of the loops. The reactor core, 1.5m in diameter and height, comprises


91 hexagonal fuel assemblies (FAs) with 120 fuel rods in each similar in cross section, to the VVER-440 reactor fuel assembly. Two fuel options are being considered: uranium dioxide (as in VVER-400) or cermet fuel (60% UO2


In district heat configuration RUTA has three circuits.


There is a natural circulation in the primary circuit with an option of boosting the reactor by using circulation pumps. Heat transfer from the primary circuit to the second and


from the second to the third is carried out through sealed heat exchange surfaces. The second circuit, consisting of two autonomous loops, is used for normal and emergency cooling of the reactor. The third circuit is a consumer’s circuit, the district heating network.


Features of using the RUTA reactor plant in district heating systems A specific feature of the RUTA pool reactor is atmospheric pressure in the air space above the pool. The absence of overpressure in the reactor vessel results in a relatively low heat potential generated by the reactor. With a reasonable depth of the pool (up to about 20m)


and acceptable values of the heat exchange surfaces of the primary and network heat exchangers, the temperature of the district heating water supplied from the RUTA reactor unit can reach about 90°C. In the RUTA-70, with natural circulation of water in the pool, the temperature of the feed water is 90°C and the return water temperature is 60°C. Considering the low temperatures of the district heating water, the following approach was adopted to configure heat supply systems with RUTA reactors. The water in the network circuit is heated in the


and 40%


aluminium alloy). In the latter, the cermet fuel matrix offers a supplementary safety barrier (in addition to the fuel rod cladding) and low operating temperature. At the beginning of the fuel cycle, the excessive reactivity


margin in the core is partially compensated for by a burnable absorber (gadolinium) distributed in the core in such a way as to ensure a uniform distribution of energy release in it. The rest of the excessive reactivity margin is compensated for by the control rods.


intermediate (network) heat exchangers of the reactor unit to 90°C and then (if necessary) the water is supplied to the peak boilers, where it is brought to the required temperature. In this case, the reactor, operating at a stationary (maximum) power level, covers the major part of the annual heat load schedule. The peak part of the load schedule is covered by additional heating of the district heating water in the peak boilers. For district heating networks with standard temperature control schedules, the optimal value of the installed nuclear plant capacity is 30–50% of the maximum load of the heating system. When implementing the proposed scheme, the RUTA


reactors will supply 70-80% of the total annual heat consumption, and peak heat sources will supply 20-30%. One of the possible ways to increase the temperature of


Table 1: Basic characteristics of the RUTA-70 reactor plant Reactor rated power (Nnom


), MW


Primary coolant circulation: - up to 30% Nnom - from 30 to 100% Nnom


Core cooling


Core dimensions (diameter/height), m Fuel


Enrichment in 235 U, % Uranium loading, kg Number of fuel assemblies Number of fuel rods in one fuel assembly


Fuel rod lattice spacing in the fuel assembly, mm Water volume in the reactor pool, m3


Water temperature at the inlet/outlet of the core, °C 40 | October 2021 | www.neimagazine.com


the water in the district heating system by 5-10°C is to use forced circulation in the primary circuit by using pumps which create additional useful pressure in the reactor primary circuit.


70


Natural Forced


Double-circuit 1.42/1.53 UO2


or cermet


(0.6 UO2 3.0 (UO2


4,165 91


120 12.2 250


75/101


+ 0.4 Al alloy) ), 4.2 (cermet)


With a low power rate and low load parameters of the district heating network, DHNP can fully satisfy the heat demand, eliminating the use of fossil fuels — as required by district heating network conditions in Finland. Boosting the reactor by the circulation pumps does not


contradict the concept of increased safety and reliability of RUTA reactors, since the natural circulation provides a high level of safety and reliability in a wide range of powers from the minimum level to about 70% of the nominal power. Full power is realised by creating additional backpressure in the primary circuit by circulation pumps. There is automatic switching between the circulation modes in the primary circuit (natural/forced) when the pumps are started. The temperature of the district heating water for the


variant optimised in this way will be 95/70°C, which corresponds to the temperature adopted in the typical control schedule for low-temperature heating systems.


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