Withdrawal of all Control Rods with Scram and a Plant Shutdown

0 200 400 600 800 1000 1200

0 20 40 60 80

Time (s)

Temperature (°C)

3-shaft - FLOWNEX PB Inlet 3-shaft - FLOWNEX PB Outlet

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

0 20 40 60 80

Time (s)

Reactivity (%)

Fig. 5.11: Reactor temperatures and reactivity after withdrawal of all control rods without scram, three shaft configuration simulated using Flownex core model.

The increase in the core inlet temperature leads to a negative reactivity, and therefore the re-actor power is reduced. Less heat is transferred to the helium, and this large effect causes the reactor outlet temperature to decrease, as it can be seen in Fig. 5.11. During the same period of time, the bypass valve is maintained at an opened position. Further description about the bypass valve is given in load rejection transient case. The loss of forced circulation in the cir-cuit is stopped (Fig. 5.12). Similar effects to those discussed in a load rejection case appear here as well, but increased, since the system pressure ratio reduces to unity for pressure equalisation. The cooling of the core is done by means of radiation and convection mecha-nisms via the reactor cavity cooling system.

0 20 40 60 80 100 120 140

0 20 40 60 80

Time (s)

Mass Flow Rate (kg/s) 0

1000 2000 3000 4000 5000 6000 7000 8000

0 20 40 60 80

Time (s)

Pressure (kPa)

3-shaft - FLOWNEX LPC Inlet 3-shaft - FLOWNEX Manifold

Fig. 5.12: Reactor mass flow and system pressure after opening of the bypass valve, three shaft configuration simulated using Flownex core model.

120% has been reached. The first scram signal, which is a shutdown criterion for the reactor protection system, is activated at a neutron flux which is equal or greater than 120% of the nominal value. At t=25 s, the criteria for shut down has been reached, and all control rods are inserted. This causes an immediate decrease in reactor power. Hence, the system cannot main-tain a full power operation and a decoupling from the grid and a shut down of the power tur-bine is requested. Another option for a sub-critical reactor core in this case is maintaining a hot stand-by of the reactor, whereby a stable operation of the power turbine is maintained using the bypass valve. This is necessary in order to prevent the turbine from coasting down and to prevent further severe consequences to the components. The current simulation has been performed using both three shaft and single shaft system configurations. It is shown in Fig. 5.13 that the shut down causes the reactor power to decrease. Both core models show a good agreement qualitatively. Yet, the results achieved by the WKIND core model indicate that the core reaches a slightly higher power level than the power level reached by Flownex core model. These small differences between the models are a result of the reactivity coeffi-cients in Flownex, which were not completely consistent with the coefficoeffi-cients implemented in the WKIND core model.

0 50 100 150 200 250 300 350

0 10 20 30 40

Time (s) Reactor Thermal Power (MWth)

FLOWNEX - 1 shaft WKIND - 1 shaft FLOWNEX - 3 shaft WKIND - 3 shaft

Fig. 5.13: Reactor power after a control rods withdrawal with scram and load rejection, single and three shaft configurations, WKIND and Flownex core models.

The interactions between the PCU and the pebble bed core are great, and hence, due to the opening of the bypass valve, the mass flow decreases as the compressors power decreases.

Fig. 5.14 indicates that the reactor inlet temperature increases rapidly because of the opening of the bypass valve. As the power turbine which uses as the main heat sink for the reactor thermal power is bypassed, the hot helium returns via the recuperator into the system and into the inlet of the core. This result is also shown later for a load rejection case. Furthermore, the

models demonstrated the same trends in the dynamic characteristics of other system compo-nents parameters, such as the recuperator temperatures, the system pressures etc. [21].

400 500 600 700 800 900 1000

0 10 20 30 40

Time (s)

Temperature (°C)

FLOWNEX - 3 shaft, PB Inlet FLOWNEX - 3 shaft, PB Outlet WKIND - 3 shaft, PB Inlet WKIND - 3 shaft, PB Outlet

Fig. 5.14: Reactor temperatures after a control rods withdrawal with scram and load rejection, three shaft configuration, WKIND and Flownex core models.

It has been observed in both core models, that the large thermal capacity of the reactor core allows relatively fast load changes in the system, without requiring fast response from the core. Thus, the energy stored in the core can be decreased or increased, with minimal core temperature changes. It must be mentioned that similar results were achieved using the single shaft system configuration coupled to each of the core models. Furthermore, it is possible to observe that small differences exist between WKIND and Flownex core models. These can be contributed to the reactivity coefficients applied in the point kinetics model used in Flownex, which differ than those used in the space dependent model in WKIND. In addition, WKIND uses a certain dependency of the control rods position implemented in its 1D neutronics ap-proximation, which is somewhat different than the reactivity curve used in Flownex. How-ever, the discrepancies observed between the fuel temperature and the temperature of the sur-rounding matrix are not strong, due to the relative slow decrease in reactor power. The strong changes in the core inlet temperature are observed in the results calculated by both core mod-els and they correspond to the changes in the reactor power. As the agreement between the core models is deemed acceptable, the following transient simulations will only employ the Flownex core model, and will mostly treat the differences between the single and the three shaft configurations. Furthermore, the effect that the design of the plant has on the major sys-tem components during normal and upset conditions shall be investigated.