Civil Engineering Reference
In-Depth Information
Fig. 2.10
Criticality or
effective multiplication
factor, k
eff
, as a function of
the volume ratio V
H2O
/
V
UO2
in a lattice cell for two
examples (3.2 % U-235
enriched U-235/U-238 fuel
and 3.0 % Pu-239/Pu-241
enriched plutonium/
uranium dioxide fuel [
37
])
Fuel rod diameter: 10.75 mm
Fuel rod diameter: 9.75 mm
For the present line of PWRs, the sum total of the individual contributions to
changes in various energy ranges finally leads to a negative coefficient of the
moderator temperature which, however, also depends on the concentration of
boric acid dissolved in the coolant and the burnup condition of the reactor core.
Figure
2.10
shows the criticality or effective multiplication factor, k
eff
,asa
function of the volume ratio of the moderator (Light Water Reactor) to fuel in a
lattice cell (Fig.
2.5
) in a LWR core [
37
]. This volume ratio of moderator to fuel
varies if the moderator density or the distance, p, between the fuel rods with
diameter, d, of the lattice cell changes. Left of the maximum of the curves in
Fig.
2.10
the lattice cell is called undermoderated with lower pitch to fuel rod
diameter, p/d, ratio. It is called overmoderated in the range right of the maximum of
the curves (higher p/d ratio) [
23
,
36
,
37
].
The three curves of Fig.
2.10
are valid for fuel with 3.2 % U-235 enriched
uranium (U-235/U-238) dioxide fuel and for 3.0 % (Pu-239/Pu-241) enriched
so-called mixed plutonium/uranium dioxide fuel as well as for two fuel rod
diameters in case of the plutonium/uranium fuel.
LWRs are always designed with an undermoderated lattice cell in the fuel
element (left side of the k
eff
curves in Fig.
2.10
). In this case a temperature increase
followed by a decrease of the moderator (H
2
O) density and of the effective water
volume V
H2O
as well as of the ratio V
H2O
/V
UO2
results in a decrease of the
criticality or effective multiplication factor k
eff
(shift to the left). For LWRs the
moderator/coolant-temperature coefficient is:
