Probabilistic Analyses and Risk Studies - The Risks of Nuclear Energy Technology - page 145

6.9.2 Safety Improvements Implemented in Reactor Plants
After the Risk Studies

As mentioned in the previous sections, rather simplified and pessimistic assump-
tions were made in the German Risk Study Phase A and the US Reactor Safety
Study [ 1 , 2 ] about the sequence of accident events in which the molten core
penetrates the bottom of the reactor pressure vessel leading to subsequent accident
conditions which can lead to a relatively early loss of containment integrity. This
leads to an overestimation of the accident consequences and risks. More recent
theoretical studies and preliminary experimental results indicate that (Chap. 10 )

- the assumed large steam explosions leading to early containment failure can be
considered to be impossible
- the assumed early containment failure after core melt through under high
primary coolant pressure can be counteracted by primary coolant depressuriza-
tion or strengthening of the anchorage of the reactor pressure vessel
- the penetration of the molten core into the concrete does not necessarily lead to
contacts with the sump water. However, if it contacts sump water, a loss of
containment integrity would occur only after time periods of approximately 5-
12 days. Water spray systems can lower the internal pressure in the containment.
Exventing filters introduced after the publication of the reactor risk studies
(WASH-1400 and German Risk Study Phase B) can avoid overpressurization
of the outer containment. However, the danger of a hydrogen detonation still
remains. Hydrogen recombiners can decrease the amount of hydrogen released.
The containment can be proven to even withstand to large severe hydrogen
detonations. For reasons of aerosol physics [ 11 ], a considerable percentage of
radioactive aerosols will have settled within the containment under fog or
rainlike conditions and shortlived radioisotopes will have decayed away by the
time containment failure was assumed to occur (Fig. 6.3 ). The release of
radioactivity into the environment then decreases by several orders of

magnitude.

These results were confirmed during the decades after the reactor risk studies
appeared by large scale experiments (Chap. 10 ) .

References

1. (1975) Reactor safety study: an assessment of accidents risks in US Commercial Nuclear
Power Plants. In: Rasmussen NC (ed). US Nuclear Regulatory Commission, WASH-1400
(NUREG-75/014), Washington
2. Deutsche Risikostudie Kernkraftwerke Phase A (1980) Gesellschaft f¨r Reaktorsicherheit
(GRS). Verlag T ¨ V Rheinland
3. Kessler G (2012) Sustainable and safe nuclear fission energy. Springer, Heidelberg
4. Lewis EE (1977) Nuclear power reactor safety. Wiley, New York, NY