Add delayed kinetics and steam-drum balance

tests/test_neutronics.py

@@ -43,3 +43,15 @@ def test_xenon_penalty_caps():
     assert dynamics.xenon_penalty(state) == 0.05
     state.xenon_inventory = 0.2
     assert dynamics.xenon_penalty(state) == pytest.approx(0.01)
+
+
+def test_delayed_precursors_follow_rod_banks():
+    dynamics = NeutronDynamics()
+    state = _core_state(power=600.0, flux=5e6)
+    dynamics.step(state, control_fraction=0.2, dt=1.0, rod_banks=[0.2, 0.2, 0.2])
+    initial_sum = sum(state.delayed_precursors)
+    assert len(state.delayed_precursors) == 3
+    assert initial_sum > 0.0
+
+    dynamics.step(state, control_fraction=0.95, dt=2.0, rod_banks=[0.95, 0.95, 0.95])
+    assert sum(state.delayed_precursors) < initial_sum