Safety and Risk of Light Water Reactors - The Risks of Nuclear Energy Technology - page 125

5.6.8.2 Loss-of-Coolant Accident Due to Minor Coolant Leakages

Regarding the possible consequences the large 2F break is not necessarily the most
extreme or most severe accident involving leakages. Minor leakage accidents rather
follow a different sequence of events with similar consequences. In a small leak, the
primary coolant pressure will drop, and the filling level in the pressure vessel will
decrease. This initiates scram. When a primary pressure of 11 MPa (Kraftwerk
Union PWR) or 9.2 MPa (EPR) has been reached, the high-pressure safety pumps
feed borated water from the flooding tanks into the primary system. In case of
AP1000 high pressure borated water is fed from the core make-up tanks.
When the water supply in the flooding tanks has been depleted, pressure in the
primary cooling system must be reduced further. This is done by opening the main-
steam-blowdown valves (secondary system depressurization). This decreases pres-
sure and temperature on the secondary side. When the pressure on the primary side
drops below 1 MPa, the low-pressure emergency core cooling systems take over
further cooling. In case of EPR, AP1000 and US-APWR high capacity relief valves
are actuated to depressurize the primary coolant system to

< 1 MPa within a short

time period.
Depending on the size of the coolant leak, at least (in case of the KWU-PWR as
example)

- one or two out of the four high-pressure feed systems,
- one out of the two pressure accumulator feeds,
- one or two out of the four low-pressure emergency core cooling systems

must be available for feeding or for recirculation operation as a minimum require-
ment for accident control. In that case, the PWR in the long run can be transferred
into the safely coolable mode.
However, serious damage to the reactor core can develop when three or all four
systems of the emergency core cooling and residual heat removal systems or the
emergency power supply fail. In that case, severe core damage will arise and the
core will melt down.

References

1. Kessler G (2012) Sustainable and safe nuclear fission energy. Springer, Heidelberg
2. Smidt D (1979) Reaktorsicherheitstechnik, Sicherheitssysteme und St¨rfallanalyse f¨r
Leichtwasserreaktoren und Schnelle Br¨ ter. Springer, Berlin
3. Emend ¨ rfer D et al (1993) Theorie der Kernreaktoren, Band 2: Der instation ¨ re Reaktor. BI
Wissenschaftsverlag, Mannheim
4. Deutsche Risikostudie Kernkraftwerke Phase B (1990) Verlag T ¨ V Rheinland, K ¨ ln
5. Kersting E et al (1993) Safety analysis for boiling water reactors. A summary, GRS-98.
Gesellschaft f¨r Anlagen- und Reaktorsicherheit, Garching
6. Boland JF (1970) Nuclear reactor instrumentation (in-core). Gordon and Breach Science,
New York, NY