Regional Basic Professional Training Course in Korea

Regional Basic Professional Training Course (BPTC) on Nuclear Safety
end, the temperature at the centre of the pellet is restricted to 2590o C. This limit is
translated, in the design of the system, by a limit on the linear power density
(590W/cm = maximum excess power).
Non‐boiling crisis in any point of the core. Boiling crisis (or DNB) would lead to
clad overheating and therefore to a possible loss of integrity of the first barrier.
This limit is translated in the safety system design by a limit on the boiling crisis
ratio (or DNBR) equal to 1.30 with the correlation W3 used for the studies of 900
MWe and equal to 1.17 with the correlation WRB‐1 used for 1300 MWe reactors.
The boiling crisis ratio is defined by:
DNBR =
critical heat flux
local thermal flux
The criteria previously mentioned obviously take into account all the technological
uncertainties (notably linked to fuel manufacture), or the calculation. It is imperative that
these criteria be respected in accidents having some probability of occurring (category 2).
For accidents that are not very likely to arise, these criteria are exceeded, but the safety
system guarantees limitation of the doses released.
2) The integrity of the second barrier (main primary system) is guaranteed by:
Not exceeding the design pressure (172.3 bars) and demonstrating fatigue
resistance in category 2.
A study of stresses sustained by the material in categories 3 and 4 and comparison
with criteria linked to maximum acceptable deformations. Furthermore, the
maximum pressure must not exceed 120% of the design pressure in category 3.
3) The integrity demonstration of the third barrier is ensured by the study of containment
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