Page 164 - Dynamics and Control of Nuclear Reactors

12.15 Example of a PWR simulation 161

12.14 PWR safety systems
PWRs are operated so as to avoid accidents. But the probability of an accident is not
zero. Consequently, PWRs employ a “defense-in-depth” design approach and build
engineered safety systems into the plant.
The defense-in-depth approach is to incorporate multiple barriers to prevent
release of radionuclides into the environment. The first barrier is the UO 2 fuel pellet.
UO 2 contains most of the radionuclides. The migration of radionuclides into the gas
space inside the fuel pin increases as fuel temperature increases. But the high melting
point of the UO 2 (around 5000°F) reduces total release relative to the total release as
would occur upon melting.
The second barrier is the Zircaloy cladding (sheath). It contains the radionuclides
that have leaked into the gap between the sheath and the fuel pellets, unless high tem-
perature or high internal pressure causes failure of the cladding. It must be noted that
melting of Zircaloy (melting point of Zircaloy-4 is 3360°F) causes a new problem:
hydrogen production from the reaction between liquid zirconium and water. Hydro-
gen is a serious explosion hazard.
The third barrier is the primary system piping and vessels.
The fourth barrier is the containment building.
The engineered safety systems serve to limit temperature and pressure increases.
PWRs employ water injection systems to provide continued cooling of the fuel in
the event of a loss of primary coolant. Separate systems provide cooling water for
pipe breaks ranging from small leaks (with continuing full or partial primary pres-
sure) to complete loss of coolant (with complete primary system depressurization).
Containment cooling engineered systems limit containment pressure. They use a
water spray system or stored ice to reduce temperature and pressure in the
containment.
Electrical power is required to operate safety systems in the event of an accident
in Generation II and III reactors. Backup systems (batteries and Diesel generators)
provide power in the event of loss of off-site power. Failure of emergency power
systems during an accident is catastrophic in these systems. New designs avoid
the need for electrically-driven emergency cooling systems by employing emer-
gency cooling that is gravity-driven or provided by flow from a pressurized tank.

12.15 Example of a PWR simulation
Appendix J describes two lumped parameter models of a PWR plant [5]. It includes a
simple, linearized reactor core model and a complete nonlinear model for the pri-
mary and secondary components.
This chapter provides several simulation results for the linearized reactor core
model from Appendix J. The model includes neutronics and core heat transfer (no
representation of steam generators or BOP). Available input disturbances in a