feat: link turbine output to transferred heat

src/reactor_sim/thermal.py

@@ -5,6 +5,8 @@ from __future__ import annotations
 from dataclasses import dataclass
 import logging
 
+import math
+
 from . import constants
 from .state import CoolantLoopState, CoreState
 
@@ -14,8 +16,9 @@ LOGGER = logging.getLogger(__name__)
 def heat_transfer(primary: CoolantLoopState, secondary: CoolantLoopState, core_power_mw: float) -> float:
     """Return MW transferred to the secondary loop."""
     delta_t = max(0.0, primary.temperature_out - secondary.temperature_in)
-    conductance = 0.05  # steam generator effectiveness
-    transferred = min(core_power_mw, conductance * delta_t)
+    conductance = 0.15  # steam generator effectiveness
+    efficiency = 1.0 - math.exp(-conductance * delta_t)
+    transferred = min(core_power_mw, core_power_mw * efficiency)
     LOGGER.debug("Heat transfer %.2f MW with ΔT=%.1fK", transferred, delta_t)
     return transferred