Safety and Risk of Light Water Reactors - The Risks of Nuclear Energy Technology - page 105

Fig. 5.3 Primary coolant
circuit system of a PWR
(fourfold redundant primary
cooling system) [ 2 ]

SG = Steam generator

PR = Pressurizer

MCL = Main primary coolant pipe

MCP = Main primary coolant pumps

RPV = Reactor pressure vessel

any condition. In a loss-of-coolant accident (LOCA), the maximum cladding
temperature of the fuel rod should not exceed 1,200 C[ 1 , 2 , 13 ].

5.6.2 Neutron Physics Design of LWRs

Preparatory critical experiments must be run to confirm the calculated enrichment
by fissile U-235 in the fuel, the number of fuel elements, and criticality, k eff .This
implies the slight supercriticality necessary at the beginning, which must be com-
pensated by burnable poisons (boron, gadolinium), boric acid, and poisons in partly
inserted control rods. This initial positive excess in k eff compensated by burnable
poisons is consumed during reactor operation mainly by the negative contribution
arising from the buildup of fission products and also by the depletion of fissile
material. This is followed by tests confirming the negative contribution of control/
shutdown rods (see also Chap. 2 ) .
The aggregate negative contribution of all shutdown rods to k eff must compen-
sate the positive contribution arising between full load (including a positive con-
tribution assumed in accident analysis) and zero power. At zero power, a relatively
low coolant temperature must be assumed.
These are the most important engineered safeguards design parameters of
LWRs:
The negative Doppler fuel temperature coefficient and the negative coolant
temperature coefficient.
For a 1.3 GWe PWR, these are a