⚖️ Exergy Analysis of Nuclear Reactor Thermodynamic Cycles
This section presents the exergy analysis of the thermodynamic cycles of the nuclear power plants studied in the book, complementing the energy-based analysis of Chapter 8.
Exergy balances quantify the quality of energy transformations — not just their quantity — and reveal precisely where irreversibilities occur and how large they are in each component of the cycle. This makes them a powerful tool for cycle optimisation and for comparing reactor designs on a thermodynamically rigorous basis.
Exergy is denoted Xh and exergy efficiency η Xh throughout this section.
🔧 Tools Used
The exergy balances presented here were generated using two complementary tools:
Thermoptim — a professional thermodynamic simulation environment. Its exergy structure feature automates the generation of exergy balances directly from the thermodynamic model, eliminating the risk of manual errors. Unlike an energy balance, an exergy balance is not conservative — irreversibilities accumulate without a simple closure check — making automated calculation essential for complex multi-component cycles.
Nuscle (Nuclear Secondary Circuit Lite Emulator) — an open-source tool developed specifically to model Water-Cooled Reactor (WCR) thermodynamic cycles at accessible complexity. Nuscle includes a direct gateway to Thermoptim, allowing exergy balances to be generated in a few steps for any of the WCR cycles it covers: EPR Flamanville, VVER-1000, RBMK-1000, CANDU Pickering, NuScale US600, NuScale US460, ABWR, and VVER-70.
Important note on model accuracy: Nuscle models have structurally lower exergy efficiencies than detailed models of the same reactor, because their feedwater heating circuit is simplified. The gap varies by reactor type. For the EPR: detailed model gives η Xh = 73.4 %, Nuscle gives 65.6 % — a gap of 7.8 points. This systematic difference must be kept in mind when comparing Nuscle and detailed results.
🌡️ Source Temperature Convention
The exergy balance of a nuclear cycle depends critically on the choice of source temperature Tk — the temperature at which heat is considered to be “supplied” to the cycle. The following conventions are used throughout this section:
| Reactor type | Heat exchange type | ΔT above coolant | Example |
|---|---|---|---|
| WCR (PWR, BWR, CANDU, RBMK) | Liquid → boiling | +35 °C | EPR: Tprimary 300 °C → Tk = 335 °C |
| Superphénix (SFR) | Liquid Na → steam | +50 °C | Tmax Na 550 °C → Tk = 600 °C |
| Canadian SCWR | Supercritical water → steam | +50 °C | Toutlet 625 °C → Tk = 670 °C |
| AGR Hartlepool | Gas CO₂ → steam | +60 °C | Tmax CO₂ 639 °C → Tk = 699 °C |
| HTR-PM | Gas He → steam | +60 °C | Tmax He → Tk = 624 °C |
The reference environment is T₀ = 15 °C. The condenser cold source temperature varies by design:
- Standard (river/sea water): Tk = 15 °C
- Air-cooled condenser (ABWR, NuScale US460): Tk = 35 °C (NRC design basis)
- NuScale US600 detailed model: Tk = 31 °C (model-specific)
Because Tk varies between reactor types, direct comparison of η Xh values across different reactor families must be done with care.
📊 Summary of Results
Detailed models:
| Reactor | Tk (°C) | Tc (°C) | η Xh | η energy | Source % | Cond % | Turbines % | FWH % |
|---|---|---|---|---|---|---|---|---|
| EPR Flamanville 3 | 334 | 15 | 73.4 % | 38.5 % | 30.0 % | 22.7 % | 32.8 % | 1.5 % |
| ABWR | 330 | 35* | 72.4 % | 37.8 % | 31.2 % | 18.6 % | 33.5 % | 7.9 % |
| CANDU Pickering | 292 | 15 | 68.4 % | 33.5 % | 24.1 % | 25.7 % | 34.1 % | 3.2 % |
| NuScale US600 | 339 | 31* | 67.6 % | 35.8 % | 56.6 % | 12.8 % | 25.9 % | 4.6 % |
| VVER-1000 | 313 | 15 | 65.9 % | 33.5 % | 19.7 % | 17.8 % | 46.8 % | 9.3 % |
| RBMK-1000 | 324 | 15 | 65.2 % | 33.7 % | 30.1 % | 17.0 % | 38.2 % | 7.1 % |
| Superphénix | 600 | 15 | 64.7 % | 43.3 % | 25.6 % | 16.0 % | 16.0 % | 3.2 % |
| AGR Hartlepool | 699 | 15 | 61.8 % | 43.5 % | 37.3 % | 9.7 % | 18.1 % | 3.5 % |
| HTR-PM | 624 | 15 | 60.9 % | 41.3 % | 43.5 %† | 10.4 % | 18.9 % | 5.3 % |
| NuScale US460 (77 MWe) | 318 | 28* | 59.8 % | 30.7 % | 44.0 % | 12.0 % | 38.9 % | 2.8 % |
| VVER-70 | 283 | 15 | 57.5 % | 27.7 % | 39.5 % | 19.2 % | 36.6 % | 1.6 %‡ |
Nuscle models (WCR — η structurally lower than detailed models):
| Reactor | Tk (°C) | Tc (°C) | η Xh | η energy | Source % | Cond % | Turbines % | FWH % |
|---|---|---|---|---|---|---|---|---|
| RBMK-1000 | 324 | 15 | 67.2 % | 34.8 % | 28.6 % | 17.5 % | 37.0 % | 8.2 % |
| ABWR | 330 | 35* | 66.8 % | 34.9 % | 21.5 % | 19.8 % | 37.8 % | 11.5 % |
| EPR Flamanville | 334 | 15 | 65.6 % | 34.5 % | 23.9 % | 18.1 % | 37.8 % | 14.3 % |
| NuScale US600 | 339 | 15 | 64.6 % | 34.2 % | 51.9 % | 18.0 % | 21.5 % | 7.7 % |
| CANDU Pickering | 292 | 15 | 63.6 % | 31.2 % | 24.1 % | 20.5 % | 43.1 % | 6.6 % |
| VVER-1000 | 313 | 15 | 56.2 % | 28.6 % | 22.1 % | 13.5 % | 32.8 % | 34.8 % |
| NuScale US460 | 339 | 15 | 52.0 % | 27.5 % | 40.9 % | 12.3 % | 34.5 % | 4.2 % |
* Air-cooled condenser design basis (NRC SDAA) or model-specific condensation temperature. † HTR-PM source % = reactor core only (43.5 %). The steam generator He→steam (20.6 %) is modelled as an internal exchanger without Tk annotation; core + SG = 64.1 %. ‡ VVER-70 FWH %: estimated — feedwater temperature Talim not documented in available sources; see Nuscle Models page.
📚 Pages in This Section
| Page | Content | Status |
|---|---|---|
| Fundamental Concepts | Energy vs exergy, irreversibilities, productive units, exergy structures | ✅ Available |
| Using Exergy Balances | How to read a balance, optimisation guidance, source temperature effects | ✅ Available |
| Detailed Models — Component Analysis | Component-by-component analysis for all detailed models | ✅ Available |
| Nuscle Models — WCR Comparison | Cross-reactor comparison using Nuscle; pedagogical use | 🔜 Coming soon |
| Cogeneration and Exergy | The thermodynamic case for nuclear cogeneration | 🔜 Coming soon |
The exergy analysis in this section is based on models and results first presented on LinkedIn as part of the book’s companion publication series.