Civil Engineering Reference
In-Depth Information
nuclear plant as far as possible or, should defects occur nevertheless, their conse-
quences must be limited reliably.
The radioactive inventory of the core of a 1 GW(e) reactor with high burnup fuel
is approx. 10
21
Bq. It arises mainly from the fission products present in the fuel
elements during reactor operation. Most of the fission products are contained in the
fuel elements (fuel matrix and fuel cladding). This does not apply to some fission
product gases which are accumulated in the fission gas plenum of the fuel rods.
The fuel elements can only be destroyed by overtemperatures, e.g. melting of the
fuel or rupture of the fuel rod cladding due to overpressure. At the beginning of the
accident this causes fission product gases to be released, e.g. tritium, carbon-14,
argon, krypton, xenon, and then also highly volatile fission products, such as I-131,
Cs-137, Sr-90 etc.
Overtemperatures arise from an imbalance between the heat production and the
heat removal in the reactor core during reactor operation.
However, imbalances between the heat produced and the heat removed can arise
also when the reactor is shut down, as the radioactive substances generate heat by
radioactive decay. This decay heat power (afterheat) (Sect.
2.9
) is roughly 6 % of
the nominal reactor power shortly after shutdown of the reactor, and 1 % after
approx. 6 h, 0.3 % after 1 week, 0.1 % after 3 months and 0.04 % after 1 year and
0.006 % after 3 years for a burnup of 50 MWd/kg. This decay heat power is slightly
dependent on burnup and increases somewhat with higher burnup for cooling time
periods up to about 100 years [
1
-
3
,
37
-
38
].
Against the background of these considerations, the goals of protection listed
below are required which should be ensured in a nuclear reactor under all
conditions.
5.2 Goals of Protection for Nuclear Reactors and Fuel
Cycle Facilities
In case of a disturbance during power operation, controlling, limiting and safety
shut down systems intervene as foreseen in the plant design and reduce the power
level or shut down the nuclear power plant. However, even after shutdown of the
nuclear chain reaction, the reactor core needs to be cooled because of the decay heat
(afterheat) produced. Based on these reactor physics characteristics arise the fol-
lowing basic engineered safeguards requirements (goals of protection) which must
be fulfilled at all times:
- Safe shutdown of the nuclear power plant: It must be possible to shut the reactor
core down safely at any time and hold it in this shut down condition.
- Core cooling: The reactor must be cooled sufficiently at all times during
operation and after shutdown
