Add HX diagnostics and pump curves; retune coolant demand

src/reactor_sim/constants.py

@@ -20,6 +20,8 @@ AMU_TO_KG = 1.660_539_066_60e-27
 MEV_TO_J = 1.602_176_634e-13
 ELECTRON_FISSION_CROSS_SECTION = 5e-16  # cm^2, tuned for simulation scale
 PUMP_SPOOL_TIME = 5.0  # seconds to reach commanded flow
+PRIMARY_PUMP_SHUTOFF_HEAD_MPA = 8.0  # approximate shutoff head for primary pumps
+SECONDARY_PUMP_SHUTOFF_HEAD_MPA = 7.0
 TURBINE_SPOOL_TIME = 12.0  # seconds to reach steady output
 GENERATOR_SPOOL_TIME = 10.0  # seconds to reach full output
 # Auxiliary power assumptions
@@ -31,6 +33,7 @@ PRIMARY_OUTLET_TARGET_K = 580.0
 SECONDARY_OUTLET_TARGET_K = 520.0
 PRIMARY_NOMINAL_PRESSURE = 7.0  # MPa typical RBMK channel header pressure
 SECONDARY_NOMINAL_PRESSURE = 7.0  # MPa steam drum/steam line pressure surrogate
+STEAM_GENERATOR_UA_MW_PER_K = 30.0  # overall UA for steam generator (MW/K)
 # Threshold inventories (event counts) for flagging common poisons in diagnostics.
 KEY_POISON_THRESHOLDS = {
     "Xe": 1e20,  # xenon