The main aims of the core design are subject to several The main aims of the core design are subject to several
important constraints.
important constraints.
The first important constraint is that the core temperatures The first important constraint is that the core temperatures remain below the melting points of materials used in the remain below the melting points of materials used in the reactor core. This is particular important for the nuclear reactor core. This is particular important for the nuclear
fuel and the nuclear fuel rods cladding.
fuel and the nuclear fuel rods cladding.
There are also limits on heat transfer are between the fuel There are also limits on heat transfer are between the fuel elements and coolant, since if this heat transfer rate elements and coolant, since if this heat transfer rate becomes too large, critical heat flux may be approached becomes too large, critical heat flux may be approached leading to boiling transition. This, in turn, will result in a leading to boiling transition. This, in turn, will result in a
THERMAL-HYDRAULIC IN NUCLEAR REACTOR
The coolant pressure drop across the core must be kept low to The coolant pressure drop across the core must be kept low to minimize pumping requirements as well as hydraulic loads minimize pumping requirements as well as hydraulic loads
(vibrations) to core components.
(vibrations) to core components.
Above mentioned constraints must be analyzed over the core Above mentioned constraints must be analyzed over the core live, for all the reactor core components, since as the power live, for all the reactor core components, since as the power distribution in the reactor changes due to fuel burn-up or distribution in the reactor changes due to fuel burn-up or core management, the temperature distribution will core management, the temperature distribution will
similarly change.
similarly change.
Furthermore, since the cross sections governing the neutron Furthermore, since the cross sections governing the neutron physics of the reactor core are strongly temperature and physics of the reactor core are strongly temperature and density dependent, there will be a strong coupling between density dependent, there will be a strong coupling between thermal-hydraulic and neutron behaviour of the reactor thermal-hydraulic and neutron behaviour of the reactor
core.
core.
21
THERMAL-HYDRAULIC IN NUCLEAR REACTOR
Thermal hydraulic considerations are important Thermal hydraulic considerations are important when selecting overall plant charac-teristics.
when selecting overall plant charac-teristics.
Primary system temperature and pressure are Primary system temperature and pressure are key characteristics related to both the coolant key characteristics related to both the coolant selection and plant thermal performance. This selection and plant thermal performance. This thermal performance is dictated by the bounds of thermal performance is dictated by the bounds of the maximum allowable primary coolant outlet the maximum allowable primary coolant outlet temperature and the minimum achievable temperature and the minimum achievable condenser coolant inlet temperature. Because condenser coolant inlet temperature. Because this at-mospheric heat sink temperature is this at-mospheric heat sink temperature is relatively fixed, improved thermodynamic per- relatively fixed, improved thermodynamic per- formance requires increased reactor coolant formance requires increased reactor coolant outlet temperatures. Figure 2-1 illustrates the outlet temperatures. Figure 2-1 illustrates the relation among reactor plant temperatures for a relation among reactor plant temperatures for a 2222
THERMAL-HYDRAULIC IN NUCLEAR REACTOR
Bounds on the achievable primary outlet temperature depend Bounds on the achievable primary outlet temperature depend on the coolant type. For liquid metals, in contrast to water, on the coolant type. For liquid metals, in contrast to water, the saturated vapor pressure for a given temperature is the saturated vapor pressure for a given temperature is low, i.e., less than atmospheric pressure at outlet low, i.e., less than atmospheric pressure at outlet
temperatures of interest of 500° to 550°C.
temperatures of interest of 500° to 550°C.
For water-cooled reactors, on the other hand, high primary For water-cooled reactors, on the other hand, high primary outlet temperatures require correspondingly high system outlet temperatures require correspondingly high system pressures (7 to 15 MPa), which increases the stored energy pressures (7 to 15 MPa), which increases the stored energy in the primary coolant and requires increased structural in the primary coolant and requires increased structural piping and component wall thicknesses. Single-phase gas piping and component wall thicknesses. Single-phase gas coolants offer the potential for high outlet temperatures coolants offer the potential for high outlet temperatures
without such inherently coupled high pressures.
without such inherently coupled high pressures.
23
THERMAL-HYDRAULIC IN NUCLEAR REACTOR
For these reactors the system pressure is dictated by the For these reactors the system pressure is dictated by the desired core heat transfer capabilities, as gas properties desired core heat transfer capabilities, as gas properties that enter these heat transfer correlations are strongly that enter these heat transfer correlations are strongly dependent on pressure. The resulting pressures are dependent on pressure. The resulting pressures are moderate, i.e., 4 to 5 MPa, whereas achievable outlet moderate, i.e., 4 to 5 MPa, whereas achievable outlet temperatures are high, i.e., 635° to 750°C. The nu-merical temperatures are high, i.e., 635° to 750°C. The nu-merical value of the plant thermal efficiency depends on the value of the plant thermal efficiency depends on the maximum temperature in the secondary or power- maximum temperature in the secondary or power- generation system. This temperature is lower than the generation system. This temperature is lower than the reactor coolant outlet temperature owing to the reactor coolant outlet temperature owing to the temperature difference needed to trans-fer heat between temperature difference needed to trans-fer heat between the primary and secondary systems in the steam generator the primary and secondary systems in the steam generator
or inter-mediate heat exchanger.
or inter-mediate heat exchanger.
THERMAL-HYDRAULIC IN NUCLEAR REACTOR
In the boiling water reactor a direct cycle is employed. The In the boiling water reactor a direct cycle is employed. The reactor outlet temperature is therefore identical (neglecting reactor outlet temperature is therefore identical (neglecting losses) to the inlet tem-perature to the turbine. This outlet losses) to the inlet tem-perature to the turbine. This outlet temperature is also limited to the saturation con-dition, as temperature is also limited to the saturation con-dition, as BWRs do not operate under superheat conditions. In a BWRs do not operate under superheat conditions. In a typical BWR, how-ever, the average outlet enthalpy typical BWR, how-ever, the average outlet enthalpy achieved corresponds to an average quality of 15%. The achieved corresponds to an average quality of 15%. The PWR and BWR reactors achieve approximately equal PWR and BWR reactors achieve approximately equal thermal efficiencies, as the turbine steam conditions are thermal efficiencies, as the turbine steam conditions are comparable even though the primary system pressure and comparable even though the primary system pressure and temperature conditions significantly differ (Table 2-1). Note temperature conditions significantly differ (Table 2-1). Note that because of detailed differences in thermodynamic that because of detailed differences in thermodynamic
cycles the example PWR plant achieves a slightly higher cycles the example PWR plant achieves a slightly higher
25
THERMAL-HYDRAULIC IN NUCLEAR REACTOR
Relations among reactor plant temperatures for typical PWR.
Relations among reactor plant temperatures for typical PWR.
THERMAL-HYDRAULIC IN NUCLEAR REACTOR
Other plant characteristics are strongly coupled with thermal Other plant characteristics are strongly coupled with thermal hydraulic consid-erations. Some notable examples are as hydraulic consid-erations. Some notable examples are as
follows.
follows.
Primary coolant temperature Primary coolant temperature
- Corrosion behavior, though strongly dependent on water - Corrosion behavior, though strongly dependent on water
chemistry control, is also temperature-dependent.
chemistry control, is also temperature-dependent.
- The reactor vessel resistance to brittle fracture degrades - The reactor vessel resistance to brittle fracture degrades with accumulated neu-tron fluence. Vessel behavior under with accumulated neu-tron fluence. Vessel behavior under low-temperature, high-pressure transients from operating low-temperature, high-pressure transients from operating conditions is carefully evaluated to ensure that the vessel conditions is carefully evaluated to ensure that the vessel has retained the required material toughness over its has retained the required material toughness over its
lifetime.
lifetime.
27
THERMAL-HYDRAULIC IN NUCLEAR REACTOR
Primary system inventory Primary system inventory
- The time response during accidents and less severe - The time response during accidents and less severe transients strongly depends on coolant inventories. The transients strongly depends on coolant inventories. The reactor vessel inventory above the core is important to reactor vessel inventory above the core is important to behavior in primary system rupture accidents. For a PWR, behavior in primary system rupture accidents. For a PWR, in particular, the pressurizer and steam generator in particular, the pressurizer and steam generator inventories dictate transient response for a large class of inventories dictate transient response for a large class of
situations.
situations.
- During steady-state operation the inlet plenum serves as - During steady-state operation the inlet plenum serves as a mixing chamber to homogenize coolant flow into the a mixing chamber to homogenize coolant flow into the reactor. The upper plenum serves a similar function in reactor. The upper plenum serves a similar function in multiple-loop plants with regard to the intermediate heat multiple-loop plants with regard to the intermediate heat exchang-er/steam generator while at the same time exchang-er/steam generator while at the same time protecting reactor vessel nozzles from thermal shock in protecting reactor vessel nozzles from thermal shock in 2828
THERMAL-HYDRAULIC IN NUCLEAR REACTOR
System arrangement System arrangement
- The arrangement of reactor core and intermediate heat - The arrangement of reactor core and intermediate heat exchanger/steam gen-erator thermal centers is crucial to exchanger/steam gen-erator thermal centers is crucial to
the plant’s capability to remove heat by natural circulation.
the plant’s capability to remove heat by natural circulation.
- Orientation of pump shafts and heat exchanger tubes - Orientation of pump shafts and heat exchanger tubes coupled with support designs and impingement velocities coupled with support designs and impingement velocities is important relative to prevention of trou-blesome is important relative to prevention of trou-blesome
vibration problems.
vibration problems.
29
THERMAL-HYDRAULIC IN NUCLEAR REACTOR