Specifications and Economic Analysis of Nuclear District Heating

DHR400 is finalizing the preliminary design (some parameters might change with the optimization of DHR400 design) and seeking for construction license in early 2019. DHR400 has a target commercial operation date of 2021 for the first plant that is expected to be built in Xudapu, Liaoning, China. Due to the high reliability and inherent safety features, DHR400 can be located in the immediate vicinity of the targeted heating supply area.

Here is a 258 page IAEA review of SMR (Small Modular Reactors). THe DHR400 is described on pages 19-22.

Economic Analysis of DHR400 versus a Nuscale Combined Heat and Power System

There is an economic comparison of the DHR400 versus the Nuscale CHP reactor.

The DHR-400 is in demonstration phase and the NuScale is in licensing phase. DHR-400 is designed to produce district heating only. The first NuScale unit is designed for electricity generation, but the manufacturer has studied options to cogenerate electricity and heat.

Combined heat and power (CHP) plants generated 22% of the world’s fossil fuel electricity in 2016 (IEA 2018IEA. 2018. IEA world energy statistics and balances.) In total, CHP plants generated 16% of the global electricity in 2016.

The district heating and cooling model (DHC model) is a local operational model where investments can be modeled by running model several times according to different investment options (Kujanpää and Pursiheimo 2017Kujanpää, L., and E. Pursiheimo. 2017. Techno-economic evaluation of flexible CCS concepts in a CHP system.). The model describes a city-level energy system from the point of view of the district heating network operator.

NuScale CHP and DHR-400 (heat only) units as a part of a city-level district heating and cooling grid. In the studied reference system, NuScale CHP and DHR-400 look cost-efficient investments with internal rates of return from 7% to 20%. Modeled heat only reactor had better profitability than CHP reactor in almost all cases except when assuming higher than 50 €/MWh electricity prices. Large heat pumps with COP of 3.5 had similar economic performance than studied SMRs. Large heat pumps are less capital intensive than SMRs and easier to accept by public, but their potential is limited by the available heat sources.

The discounted payback period (DPP) of the SMR investments were from 5 to 18 years in the studied system. The average DPP was 8 years for two DHR-400 units and 9 years for four NuScale CHP units.

Investment cost had particularly large impact on the profitability of DHR-400 units and the studied range affected the DPP of two DHR-400 units by ±6 years. In the case of four NuScale CHP units, the investment cost uncertainty changed DPP by ±3 years. The importance of the investment cost increases with increasing number of units because additional units can operate fewer hours during the year.

DHR-400 units had better IRR than NuScale CHP in almost all cases, except with very high electricity prices. Increasing electricity price makes heat only investments less profitable because heat only units will have lower FLH as the model runs NGCC units more to get income from sold electricity. Apart from electricity price, natural gas and biomass prices were the most important system parameters when assessing the IRR of the SMR investments. Lower natural gas price increases the profitability of existing NGCC units, and lower biomass price increases the profitability of the biomass boilers.

Specifications of a DHR400

The reactor pool is a cylinder with an inside diameter of 10 meters and an overall height of 26 meters, containing the core structure, core shroud, four attenuation barrels, four inertial tanks, the residual heat removal system, the core supporting foundation and the seismic stabilizer brackets inside its 25 m depth of water. The pool is buried underground with an elevation of its bottom of -26 m. The pool is made of reinforced concrete with an inner layer of 5 mm stainless steel and an outer layer of 10 mm carbon steel. The thickness of the surrounding concrete layer is 1.0 m and the bottom plate is 2 m thick. The upper head includes a carbon steel truss and a stainless steel plate, connected to the concrete wall of the pool and provides support for the control rod driven mechanism and the control rod guide tubes. One meter below the upper head there is a gaseous space, which is connected to an engineered venting system to exhaust vapor and other gases. Above the reactor pool there is a 2 m thick movable reinforced concrete plate. The overall structure of the reactor pool provides great resistance to external events including airplanes. The large water inventory in the pool water provides large thermal inertia and a long response time, thus enhances the resistance to system transients and accidents. These features ensure that the core will not meltdown under any accident.

Inherent Safety Features
Instead of augmenting additional engineered safety systems the DHR400 emphasize on inherent safety features. The great heat capacity of the 1800 tons of water inside the reactor pool ensures that the reactor core will be kept submerged in all circumstances, thus no core meltdown could occur. It has negative temperature and void reactivity feedback, therefore the power increase can be effectively restrained. In the event of severe accident, the reactor can automatically shutdown by the inherent negative reactivity feedback, and the reactor core will be kept submerged for as long as 26 days even with no further intervention.

Containment System
There are four barriers precluding a radioactive release to the environment in DHR, including the fuel coating, the reactor pool, the earth around the pool and the reactor building on top of the pool. Due to the low operating temperature and atmospheric pressure on the top of the reactor pool, there are no high-pressure events and instead of a containment, a confinement building is sufficient for protection. The location of the reactor assembly below ground and submerged in 1800 tons of water makes DHR400 highly resistant to
external events including aircraft crashes. Additional protection is provided by the reactor building above the pool.

Primary Heat Exchanger
DHR400 uses 8 plate heat exchangers in its primary coolant system to transfer heat to the secondary loop. Plate heat exchanger is suitable for low temperature difference water to water heat exchange for its small resistance and high efficiency. The leak tightness of the plate heat exchanger is considered to be highly reliable. Even under the circumstances of leakage, the coolant leaks outwards to the pump room. This feature provides great advantages to radioactivity isolation.

Residual Heat Cooling System

The residual heat cooling system of DHR400 is consists of two parts, a 2.4 MW in-pool natural circulation cooling system and a 4 MW out-pool forced circulation cooling system. The temperature of the reactor pool water is kept below boiling point after shutdown and a temperature of 400 C can be achieved with the residual heat cooling system.

Safety Features
The DHR400 is designed with inherent safety features. These include a large volume of water in the reactor pool, two sets of reactor shutdown systems, pool water cooling system and a decay heat removal system. With these designs stable long-term core cooling under all conditions can be achieved.

SOURCES – IAEA, Energy Sources- A techno-economic assessment of NuScale and DHR-400 reactors in a district heating and cooling grid
Written By Brian Wang, Nextbigfuture.com

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