Thermal-Hydraulics and Safety Analysis
This domain tests heat transfer, two-phase flow, critical heat flux, accident progression and the acceptance criteria used in safety analysis.
Core rules
- Fuel heat flux q″ is related to linear heat rate q′ by q″ = q′/(πD), where D is the fuel-rod diameter. Heat transfer regimes move from single-phase convection to subcooled boiling, nucleate boiling, transition boiling and film boiling.
- Critical heat flux is the heat flux at which boiling crisis occurs. Departure from nucleate boiling ratio is DNBR = CHF/q″. A higher DNBR is safer; PWR limits are typically around 1.3.
- A loss-of-coolant accident progresses through blowdown, refill, reflood and long-term cooling. The key acceptance criteria include peak cladding temperature below 1204°C, local cladding oxidation below 17%, hydrogen generation below 1% of the theoretical maximum, coolable geometry and long-term cooling.
- Decay heat is about 6.5% of full power at shutdown, roughly 1.5% at one hour and about 0.5% at one day.
- Natural circulation can remove heat after loss of forced flow if a heat sink and flow path exist.
Worked example
A PWR fuel rod has a local CHF of 1.20 MW/m² and an actual heat flux of 0.80 MW/m². DNBR = 1.20/0.80 = 1.50. If the safety limit is 1.30, the rod has margin. Now suppose power is raised by 20%: q″ becomes 0.96 MW/m², so DNBR = 1.25, below the limit. The reasoning is that DNBR falls inversely with heat flux if CHF is unchanged. In a transient, CHF also changes with pressure, flow and quality, so the margin must be evaluated along the transient, not only at steady state.
Traps
Do not compare DNBR to 1.0; the design limit is above 1. Do not use average heat flux when the hot rod controls. Do not ignore the difference between blowdown and reflood cooling. Do not apply the 1204°C criterion to normal operation; it is an accident acceptance limit. Do not forget that decay heat continues after the reactor trips.
Key points
- DNBR = CHF/q″; higher is safer.
- PWR DNBR limits are typically about 1.3.
- LOCA phases are blowdown, refill, reflood and long-term cooling.
- PCT limit is 1204°C in the standard LOCA criteria.
- Decay heat is about 6.5% at shutdown and about 1.5% at one hour.
- Natural circulation requires a flow path and heat sink.