Expose heat exchanger diagnostics and retune conductance

src/reactor_sim/dashboard.py

@@ -443,6 +443,7 @@ class ReactorDashboard:
         )
         right_y = self._draw_section(right_win, right_y, "Generators", self._generator_lines(state))
         right_y = self._draw_section(right_win, right_y, "Power Stats", self._power_lines(state))
+        right_y = self._draw_section(right_win, right_y, "Heat Exchanger", self._heat_exchanger_lines(state))
         right_y = self._draw_section(right_win, right_y, "Protections / Warnings", self._protection_lines(state))
         right_y = self._draw_section(right_win, right_y, "Maintenance", self._maintenance_lines())
         self._draw_health_bars(right_win, right_y)
@@ -603,6 +604,14 @@ class ReactorDashboard:
         ]
         return lines
 
+    def _heat_exchanger_lines(self, state: PlantState) -> list[tuple[str, str]]:
+        delta_t = getattr(state, "primary_to_secondary_delta_t", 0.0)
+        eff = getattr(state, "heat_exchanger_efficiency", 0.0)
+        return [
+            ("ΔT (pri-sec)", f"{delta_t:6.1f} K"),
+            ("HX Eff", f"{eff*100:6.1f}%"),
+        ]
+
     def _protection_lines(self, state: PlantState) -> list[tuple[str, str]]:
         lines: list[tuple[str, str]] = []
         lines.append(("SCRAM", "ACTIVE" if self.reactor.shutdown else "CLEAR"))

src/reactor_sim/reactor.py

@@ -351,6 +351,8 @@ class Reactor:
             transferred = 0.0
         else:
             transferred = heat_transfer(state.primary_loop, state.secondary_loop, total_power)
+        state.primary_to_secondary_delta_t = max(0.0, state.primary_loop.temperature_out - state.secondary_loop.temperature_in)
+        state.heat_exchanger_efficiency = 0.0 if total_power <= 0 else min(1.0, max(0.0, transferred / total_power))
         self.thermal.step_secondary(state.secondary_loop, transferred)
 
         self._step_turbine_bank(state, transferred, dt)

src/reactor_sim/state.py

@@ -75,6 +75,8 @@ class PlantState:
     secondary_pumps: list[PumpState] = field(default_factory=list)
     generators: list[GeneratorState] = field(default_factory=list)
     aux_draws: dict[str, float] = field(default_factory=dict)
+    heat_exchanger_efficiency: float = 0.0
+    primary_to_secondary_delta_t: float = 0.0
     time_elapsed: float = field(default=0.0)
 
     def snapshot(self) -> dict[str, float]:
@@ -115,6 +117,8 @@ class PlantState:
         generators_blob = data.get("generators", [])
         generators = [GeneratorState(**g) for g in generators_blob]
         aux_draws = data.get("aux_draws", {})
+        hx_eff = data.get("heat_exchanger_efficiency", 0.0)
+        delta_t = data.get("primary_to_secondary_delta_t", 0.0)
         return cls(
             core=CoreState(**core_blob, fission_product_inventory=inventory, emitted_particles=particles),
             primary_loop=CoolantLoopState(**data["primary_loop"]),
@@ -124,5 +128,7 @@ class PlantState:
             secondary_pumps=[PumpState(**p) for p in sec_pumps_blob],
             generators=generators,
             aux_draws=aux_draws,
+            heat_exchanger_efficiency=hx_eff,
+            primary_to_secondary_delta_t=delta_t,
             time_elapsed=data.get("time_elapsed", 0.0),
         )

src/reactor_sim/thermal.py

@@ -18,8 +18,8 @@ def heat_transfer(primary: CoolantLoopState, secondary: CoolantLoopState, core_p
     if secondary.mass_flow_rate <= 0.0:
         return 0.0
     delta_t = max(0.0, primary.temperature_out - secondary.temperature_in)
-    # Require a larger temperature difference to push full power across the steam generator.
-    conductance = 0.02
+    # Require a modest temperature difference to push full power across the steam generator.
+    conductance = 0.05
     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)