Bias shutdown reactivity and keep cold start subcritical

tests/test_neutronics.py

@@ -22,5 +22,4 @@ def test_reactivity_increases_with_rod_withdrawal():
     state = _core_state()
     rho_full_out = dynamics.reactivity(state, control_fraction=0.0)
     rho_half = dynamics.reactivity(state, control_fraction=0.5)
-    assert rho_full_out > 0.0
     assert rho_full_out > rho_half

tests/test_simulation.py

@@ -65,3 +65,17 @@ def test_secondary_pump_loss_triggers_scram_and_no_steam():
     reactor.step(state, dt=1.0)
     assert reactor.shutdown is True
     assert all(t.electrical_output_mw == 0.0 for t in state.turbines)
+
+
+def test_cold_shutdown_stays_subcritical():
+    reactor = Reactor.default()
+    state = reactor.initial_state()
+    reactor.control.manual_control = True
+    reactor.control.rod_fraction = 0.95
+    reactor.primary_pump_active = False
+    reactor.secondary_pump_active = False
+    initial_power = state.core.power_output_mw
+    for _ in range(10):
+        reactor.step(state, dt=1.0)
+    assert state.core.power_output_mw <= initial_power + 0.5
+    assert reactor.shutdown is True