HTR-PM — Exergy Analysis: Design vs Operating Data
The HTR-PM (High Temperature Reactor — Pebble-bed Module) is a helium-cooled reactor using TRISO pebble fuel. Two reactor modules supply steam to a single turbine. The Shindao Bay plant (Shandong province, China) is the first industrial-scale deployment of this concept, commissioned in 2021.
This page presents two exergy balances for the same reactor: the nominal design model (turbines at design efficiency) and a Shindao Bay model calibrated on operating data published by Dong et al. (2025). The comparison illustrates how an exergy balance can diagnose the source of performance gaps in a real plant.
The Shindao Bay Operating Data
Very little information is publicly available about the HTR-PM secondary circuit. The Shindao Bay model presented here was constructed from data visible in Figure 5 of Dong et al. (2025), which shows the Human-Machine Interface (HMI) of the plant’s coordinated control system. This operational dashboard displays helium, steam and feedwater temperatures, core power, electrical output, efficiency, power level, and mass flow rates.
Note: this model has not been validated by the original authors. Despite several requests, it has proven impossible to obtain direct information on the secondary circuit. The extraction pressures and flow rates were assumed; a polytropic efficiency of 70 % was applied to all turbine stages — the value that alone reproduces the announced electrical output of 64 MWe.
The comparison between announced design values and HMI readings is striking:
| Parameter | Design | Shindao Bay (HMI) | Relative difference |
|---|---|---|---|
| T helium (°C) | 750 | 563.7 | −24.8 % |
| Core power (MWth) | 250 | 197.5 | −21.0 % |
| Electric power (MWe) | 105 | 64 | −39.1 % |
| T steam (°C) | 566 | 519.9 | −8.1 % |
| Efficiency (%) | 40 | 34.1 | −14.7 % |
| Power level (%) | 100 | 79.5 | −20.5 % |
Source: Dong, Z., Zhang, Z., Dong, Y. et al. Testing the feasibility of multi-modular design in an HTR-PM nuclear plant. Nat Commun 16, 2778 (2025). https://doi.org/10.1038/s41467-025-58194-7
The discrepancies are significant. The plant was operating at approximately 80 % of rated power, which partially explains the reduced performance — but the electrical output shortfall (−39 %) is disproportionate relative to the thermal power reduction (−21 %), pointing to turbine efficiency as the primary driver.
Cycle Summary
| Nominal design | Shindao Bay | |
|---|---|---|
| Thermal power (MWth) | 190.8 | 198.9 |
| Electric power (MWe) | ~79 | ~68 |
| η energy (%) | 41.3 % | 34.2 % |
| η Xh (%) | 60.9 % | 50.4 % |
The steam generator receives 72.7 kg/s of feedwater at 125 bar and 159.8 °C (Shindao Bay conditions) and delivers steam at 110.8 bar and 519.9 °C. The feedwater train comprises three LP heaters (LPH1–3) and one HP heater (HPH4).
Exergy Balance Comparison
| Component | Nominal design | Shindao Bay | Change |
|---|---|---|---|
| Core (Tk = 624 °C) | 43.5 % | 35.8 % | −7.7 pts |
| Steam generator He→steam | 20.6 % | 19.1 % | −1.5 pts |
| Turbines T1–T6 | 18.9 % | 30.5 % | +11.6 pts |
| Condenser (Tk = 15 °C) | 10.4 % | 8.8 % | −1.6 pts |
| FWH (HPH4 + LPH1–3) | 5.3 % | 4.6 % | −0.7 pts |
What the Balance Reveals
The exergy balance localises the performance gap with precision: the turbines alone account for the entire difference, increasing from 18.9 % to 30.5 % of total irreversibilities (+11.6 pts). The source (core + SG) and condenser contributions are only modestly affected.
This is exactly the diagnostic function of an exergy balance. A purely energetic analysis would show that η energy falls from 41.3 % to 34.2 % — a 7.1-point drop — but would not identify where the losses originate. The exergy balance points directly to the turbines.
The individual turbine efficiencies in the Shindao Bay model tell the same story:
| Stage | Nominal η | Shindao Bay η |
|---|---|---|
| Turbine 1 | 0.931 | 0.839 |
| Turbine 2 | 0.914 | 0.812 |
| Turbine 3 | 0.894 | 0.779 |
| Turbine 4 | 0.878 | 0.744 |
| Turbine 5 | 0.852 | 0.717 |
| Turbine 6 | 0.835 | 0.691 |
The degradation is progressive from stage 1 to stage 6 — the LP stages are proportionally the most affected. This pattern is consistent with operation at partial load (~80 %), where the steam conditions at each stage inlet deviate increasingly from the design point as expansion proceeds.
Whether the low turbine efficiencies reflect the partial-load operating point, teething issues specific to a first-of-a-kind prototype, or both, cannot be determined from the available data alone. The exergy balance does not answer this question — but it identifies the turbines as the only component worth investigating.
Note on the HTR-PM Steam Generator
The HTR-PM steam generator is a helical-coil once-through design integrated within the reactor vessel. It is modelled here as a single unit receiving helium at the design (nominal) or measured (Shindao Bay) temperature and delivering superheated steam. The SG contribution to total irreversibilities (19–21 %) reflects the gas→steam heat exchange penalty discussed in the AGR Hartlepool analysis.
Unlike gas-cooled reactors with separate SG sections (economiser, evaporator, superheater, reheater), the helical-coil SG cannot be decomposed into sub-components in the Thermoptim model without additional geometric data. The SG irreversibility is therefore reported as a single value.
References
- Dong, Z., Zhang, Z., Dong, Y. et al. Testing the feasibility of multi-modular design in an HTR-PM nuclear plant. Nat Commun 16, 2778 (2025). https://doi.org/10.1038/s41467-025-58194-7
- Chen, F., Han, Z. Steady-state thermal fluids analysis for the HTR-PM equilibrium core. International Journal of Advanced Nuclear Reactor Design and Technology 3, 11–17 (2021). https://doi.org/10.1016/j.jandt.2021.04.001
Thermoptim models: HTR_PM_2023_trad_OK.prj (nominal) and HTR_PM_2023_68MW.prj (Shindao Bay). Exergy balances processed with ExerBalanceHX.