Safety and Risk of Light Water Reactors - The Risks of Nuclear Energy Technology - page 122

Fig. 5.14 Development as
a function of time of coolant
pressure and coolant
volume in the reactor
pressure vessel after a large
leak in the cold leg [ 2 , 3 ]

5.6.8.1 Loss-of-Coolant Accident Due to 2F Break of the Main
Coolant Pipe

Large coolant leakages (2F break) do not necessarily require the scram system.
Although the scram signal is initiated when the lower limit of primary pressure is
underrun, void formation starting in the reactor core shuts down the reactor very
quickly automatically via the negative reactivity produced.
When the leak has opened, a pressure relief wave passes through the reactor
pressure vessel and piping system. At the leakage points, the critical outlet velocity
of a two-phase mixture (steam—water) is established. The pressure of the primary
cooling circuit drops to about 0.5 MPa within approx. 17 s. All primary coolant
(water) leaves the primary coolant system within approx. 13-15 s (Fig. 5.14 ). Film
boiling starts in the cooling channels of the fuel elements, and cladding tempera-
tures rise very sharply to 750-1,000 C (Fig. 5.15 ). Afterwards, automatic shut-
down of the reactor power to the level of decay heat power (afterheat level), and
simultaneous cooling by steam, cause a temporary decrease of temperature at the
fuel cladding. However, this temperature again rises slightly until the borated
cooling water of the pressure accumulators (Fig. 5.16 ) takes over core cooling
after some 50-90 s. These pressure accumulators start feeding borated water when
the primary pressure falls below 2.5 MPa (Phase 2, Fig. 5.16 ). The water level in the
reactor pressure vessel rises up between 20 s and 170 s (Fig. 5.14 ) to the upper core
edge again covering the core. It then fills the pressure vessel up to the lower inlet
and outlet nozzle edges. (Height of the rising center level in the pressure vessel
(Fig. 5.8 ) and the volume on the right scale of Fig. 5.14 are correlated).
This borated water cools the reactor core and keeps the reactor subcritical. After
further decrease of the primary pressure to

1.0 MPa the low pressure emergency
water injection system takes over (Phase 3, Fig. 5.16 ). When the reservoir of
borated water has been depleted, the water originating from the loss of coolant
and collecting in the reactor building sump is taken in, cooled in the residual heat
exchangers, and returned to the primary system (Phase 4, Fig. 5.16 ). In this way
cooling by borated water from the pressure accumulators is supported by the

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