Fast Withdrawal of All Control Rods

It can clearly be seen, that simultaneously with the withdrawal, a very strong increase occurs in the reactor thermal power. This increase is almost seven folds greater than the reactor ini-tial power. The sharp increase is followed by a rapid power decrease. This is due to the effect of the prompt increase in fuel temperature on the position of the control rods, which is calcu-lated by the heterogeneous fuel temperature module in WKIND. Fig. 5.7 shows the discretisa-tion in the core and the change in the axial temperature profile during the event.

Fig. 5.7: Reactor axial fuel temperature as a function of time after withdrawal of all control rods without scram.

The immediate increase in reactor thermal power is evident. The change in the axial power distribution can be observed for each point in the matrix. The increase in power, caused by the strong insertion of reactivity into the reactor, plays a significant role for the change in the re-actor fuel temperature.

For a short time, the temperature of the coated particles greatly differs from the moderator temperature (see Fig. 5.8). The reactivity equivalent of the withdrawal of all control rods from a stationary position to an end position is about 2$². This reactivity is compensated by an in-crease of 130 K in the fuel temperature. After about 20 s also the average moderator tempera-ture reaches a corresponding higher level, so that the isothermal temperatempera-ture coefficient af-fects the global reactivity.

2 Dollar ($) is a term used in nuclear chain reaction kinetics to define the increase in reactivity between critical and prompt critical.

Fig. 5.8: Temperature difference between moderator and fuel temperature in the axial centre of the core after withdrawal of all control rods without scram.

This emphasises the fact that for a short relaxation time, the difference between the fuel and the moderator temperature reduces the power of the core remarkably. The heterogeneous par-ticle model implemented in WKIND is more realistic compared with Flownex. Flownex ho-mogeneous model predicts an unacceptable power increase by a factor of about 140, since the fuel temperature is strongly coupled to the slowly increasing moderator temperature. Fig. 5.9 shows a plot of fuel temperature, coating temperatures, average moderator temperature and surface temperature of the spheres in the axial centre of the core.

1000 1050 1100 1150 1200 1250 1300

0 10 20 30 40 50 60 70 80 Time (s)

Temperature (K)

Fuel temperature Average moderator temperature Inner coating temperature Outer coating temperature Surface temperature

Fig. 5.9: Fuel temperature, coating temperatures, average moderator temperature and surface temperature in the axial centre of the core after withdrawal of all control rods without scram.

As the control rods are removed, the addition of external reactivity increases the fission power, which causes an increase in the fuel temperature. As for the fuel, which is represented by the UO2 layer in the kernel; the fuel temperature increases much above the average tem-perature of the matrix, before the kernel reaches a homogeneous temtem-perature. It can further be

seen, that by the time the peak has been created, the fuel temperature increases in more than 84 K, compared with the moderator temperature. This phenomenon takes place in the short time during which over power and over temperature occur in the particle. The strong reactiv-ity feedback will then cause the total reactivreactiv-ity to drop to a negative value, and hence the re-actor power will decrease. Due to the large heat capacity of the graphite, no large change oc-curs in the reactor outlet temperature and in the average fuel temperature despite of the in-crease in reactor power, as shown in Fig. 5.10. The following figures show the simulation results done with Flownex core model, assuming the same boundary conditions as in the case calculated with WKIND core model.

1.00E+00 1.00E+01 1.00E+02 1.00E+03 1.00E+04 1.00E+05

0 20 40 60 80

Time (s) Thermal Power (MWth)

0 200 400 600 800 1000 1200 1400

0 20 40 60 80

Time (s) Fuel Average Temperature (°C)

Fig. 5.10: Reactor thermal power and fuel temperature after withdrawal of all control rods without scram, three shaft configuration simulated using Flowenx core model.

As emphasised earlier, the power reaches unacceptably high levels. As a consequence, a com-plete plant shut down is initiated. The reactor power drops immediately, whereas the fuel temperature continues to rise. Under such conditions, the fuel particles are destroyed, and a release of fission products will occur. This example also strengthens the need for using a more detailed model than the Flownex core model, which over-predicts the reactor power level and under-predicts the average fuel temperature. In addition, it must be mentioned that a depres-surisation accident is not assumed in this case. In such a severe incident the decay heat should be removed by means of natural convection. Under such extreme conditions, the system must be completely isolated from the core, and hence it is no longer of importance to treat the PCU as an integral part of the simulation. The evolution of the core temperature after a depressuri-sation event and the natural convection heat removal mechanism which takes place in the core could be analysed with the aid of THERMIX [70].

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.