diff --git a/FEATURES.md b/FEATURES.md
index 0c50e25..fb452f0 100644
--- a/FEATURES.md
+++ b/FEATURES.md
@@ -3,7 +3,7 @@
 - **Core physics**: point-kinetics with delayed neutrons, temperature feedback, fuel burnup penalty, xenon/iodine buildup with decay and burn-out, and rod-bank worth curves.
 - **Rod control**: three rod banks with weighted worth; auto controller chases 3 GW setpoint; manual mode with staged bank motion and SCRAM; state persists across runs.
 - **Coolant & hydraulics**: primary/secondary pumps with head/flow curves, power draw scaling, wear tracking; pressure floors tied to saturation; auxiliary power model with generator auto-start.
-- **Heat transfer**: steam-generator UA·ΔT_lm model; core and secondary thermal solvers with passive cool-down when flow is low.
+- **Heat transfer**: steam-generator UA·ΔT_lm model with a pinch cap to keep the primary outlet hotter than the secondary, coolant heating uses total fission power with fuel heating decoupled from exchanger draw, and the secondary thermal solver includes passive cool-down when flow is low.
 - **Steam cycle**: three turbines with spool dynamics, load dispatch to consumer, steam quality gating for output, generator states with batteries/spool.
 - **Protections & failures**: health monitor degrading components under stress, automatic SCRAM on core or heat-sink loss, relief valves per loop, maintenance actions to restore integrity.
 - **Persistence & ops**: snapshots auto-save/load to `artifacts/last_state.json`; dashboard with live metrics, protections/warnings, heat-exchanger telemetry, component health, and control shortcuts.
diff --git a/src/reactor_sim/reactor.py b/src/reactor_sim/reactor.py
index 216ec36..fb6c51c 100644
--- a/src/reactor_sim/reactor.py
+++ b/src/reactor_sim/reactor.py
@@ -358,8 +358,7 @@ class Reactor:
             transferred = 0.0
         else:
             transferred = heat_transfer(state.primary_loop, state.secondary_loop, total_power)
-        net_power = total_power - transferred
-        self.thermal.step_core(state.core, state.primary_loop, net_power, dt)
+        self.thermal.step_core(state.core, state.primary_loop, total_power, dt)
         self.thermal.step_secondary(state.secondary_loop, transferred)
 
         self._step_turbine_bank(state, transferred, dt)
diff --git a/src/reactor_sim/thermal.py b/src/reactor_sim/thermal.py
index 7eb8b89..867fa13 100644
--- a/src/reactor_sim/thermal.py
+++ b/src/reactor_sim/thermal.py
@@ -15,7 +15,7 @@ LOGGER = logging.getLogger(__name__)
 
 def heat_transfer(primary: CoolantLoopState, secondary: CoolantLoopState, core_power_mw: float) -> float:
     """Return MW transferred to the secondary loop."""
-    if secondary.mass_flow_rate <= 0.0:
+    if primary.mass_flow_rate <= 0.0 or secondary.mass_flow_rate <= 0.0:
         return 0.0
     delta_t1 = max(1e-3, primary.temperature_out - secondary.temperature_in)
     delta_t2 = max(1e-3, primary.temperature_in - secondary.temperature_out)
@@ -26,7 +26,20 @@ def heat_transfer(primary: CoolantLoopState, secondary: CoolantLoopState, core_p
     else:
         lmtd = (delta_t1 - delta_t2) / math.log(delta_t1 / delta_t2)
     ua = constants.STEAM_GENERATOR_UA_MW_PER_K
-    transferred = max(0.0, min(core_power_mw, ua * lmtd))
+    ua_limited = ua * lmtd
+
+    # Prevent the heat exchanger from over-transferring and inverting the outlet temperatures.
+    primary_capacity = primary.mass_flow_rate * constants.COOLANT_HEAT_CAPACITY
+    secondary_capacity = secondary.mass_flow_rate * constants.COOLANT_HEAT_CAPACITY
+    approach = 2.0  # K minimum approach between loop outlets
+    pinch_limited = 0.0
+    if primary_capacity > 0.0 and secondary_capacity > 0.0:
+        temp_gap = primary.temperature_out - secondary.temperature_in - approach
+        if temp_gap > 0.0:
+            pinch_watts = temp_gap / ((1.0 / primary_capacity) + (1.0 / secondary_capacity))
+            pinch_limited = max(0.0, pinch_watts / constants.MEGAWATT)
+
+    transferred = max(0.0, min(core_power_mw, ua_limited, pinch_limited))
     LOGGER.debug("Heat transfer %.2f MW with LMTD=%.1fK (ΔT1=%.1f ΔT2=%.1f)", transferred, lmtd, delta_t1, delta_t2)
     return transferred
 
@@ -51,11 +64,20 @@ def saturation_pressure(temp_k: float) -> float:
 class ThermalSolver:
     primary_volume_m3: float = 300.0
 
-    def step_core(self, core: CoreState, primary: CoolantLoopState, power_mw: float, dt: float) -> None:
+    def step_core(
+        self,
+        core: CoreState,
+        primary: CoolantLoopState,
+        power_mw: float,
+        dt: float,
+        residual_power_mw: float | None = None,
+    ) -> None:
+        if residual_power_mw is None:
+            residual_power_mw = power_mw
         temp_rise = temperature_rise(power_mw, primary.mass_flow_rate)
         primary.temperature_out = primary.temperature_in + temp_rise
         # Fuel heats from any power not immediately convected away, and cools toward the primary outlet.
-        heating = 0.005 * max(0.0, power_mw - temp_rise) * dt
+        heating = 0.005 * max(0.0, residual_power_mw) * dt
         cooling = 0.025 * max(0.0, core.fuel_temperature - primary.temperature_out) * dt
         core.fuel_temperature += heating - cooling
         # Keep fuel temperature bounded and never below the coolant outlet temperature.