diff --git a/src/reactor_sim/reactor.py b/src/reactor_sim/reactor.py
index f4f7cf1..ca1d6f1 100644
--- a/src/reactor_sim/reactor.py
+++ b/src/reactor_sim/reactor.py
@@ -371,14 +371,22 @@ class Reactor:
         state.time_elapsed += dt
 
         # Update inlet temperatures based on heat removed/added for next iteration.
+        env = constants.ENVIRONMENT_TEMPERATURE
         primary_cooling = temperature_rise(transferred, state.primary_loop.mass_flow_rate)
-        state.primary_loop.temperature_in = max(
-            constants.ENVIRONMENT_TEMPERATURE, state.primary_loop.temperature_out - primary_cooling
-        )
-        secondary_cooling = max(0.0, state.secondary_loop.temperature_out - constants.ENVIRONMENT_TEMPERATURE - 40.0)
-        state.secondary_loop.temperature_in = max(
-            constants.ENVIRONMENT_TEMPERATURE, state.secondary_loop.temperature_out - max(20.0, secondary_cooling)
-        )
+        if transferred <= 0.0 or state.secondary_loop.mass_flow_rate <= 1.0:
+            passive = 0.02 * max(0.0, state.primary_loop.temperature_out - env) * dt
+            primary_cooling = max(primary_cooling, passive)
+        state.primary_loop.temperature_in = max(env, state.primary_loop.temperature_out - primary_cooling)
+
+        if state.secondary_loop.mass_flow_rate <= 1.0:
+            target_temp = env
+            state.secondary_loop.temperature_out = self._ramp_value(
+                state.secondary_loop.temperature_out, target_temp, dt, self.secondary_pump.spool_time
+            )
+            state.secondary_loop.temperature_in = state.secondary_loop.temperature_out
+        else:
+            secondary_cooling = max(0.0, state.secondary_loop.temperature_out - env - 40.0)
+            state.secondary_loop.temperature_in = max(env, state.secondary_loop.temperature_out - max(20.0, secondary_cooling))
 
         LOGGER.info(
             (