feat: add reactor control persistence and tests

src/reactor_sim/constants.py

@@ -0,0 +1,19 @@
+"""Physical constants and engineering limits for the simulation."""
+
+from __future__ import annotations
+
+SECONDS_PER_MINUTE = 60.0
+MEGAWATT = 1_000_000.0
+NEUTRON_LIFETIME = 0.1  # seconds, prompt neutron lifetime surrogate
+FUEL_ENERGY_DENSITY = 200.0 * MEGAWATT  # J/kg released as heat
+COOLANT_HEAT_CAPACITY = 4_200.0  # J/(kg*K) for water/steam
+COOLANT_DENSITY = 700.0  # kg/m^3 averaged between phases
+MAX_CORE_TEMPERATURE = 1_800.0  # K
+MAX_PRESSURE = 15.0  # MPa typical PWR primary loop limit
+CONTROL_ROD_SPEED = 0.05  # fraction insertion per second
+STEAM_TURBINE_EFFICIENCY = 0.34
+GENERATOR_EFFICIENCY = 0.96
+ENVIRONMENT_TEMPERATURE = 295.0  # K
+AMU_TO_KG = 1.660_539_066_60e-27
+MEV_TO_J = 1.602_176_634e-13
+ELECTRON_FISSION_CROSS_SECTION = 5e-23  # cm^2, tuned for simulation scale