ORNL-5388 - the Molten Salt Energy Technologies Web Site

I I,
i 7-1 7
ii
I
1
I
L
L
k
L
i
Three generic types of symbiotic reactor systems can be envisioned by considering
various combinations of thermal converters and fast breeders for the dispersed (D) and
energy center (S) reactors: thermal (D)/thermal (S), thermal (D)/fast(S), and fast(D)/fast(S).
In order for the generating capacity of a system to increase with time without an external
supply of fissile material, a net gain of fissile material (of some type) must occur.' Thus,
the growth potential of the thermal (D)/thermal(S) system is inherently negative; that is,
the installed nuclear capacity must decay as a function of,time since the overall conversion
ratio is less than 1. The thermal(D)/fast(S) system, however, does have the potential for
growth since the net fissile gain of the fast component can be used to offset fhe fissile
loss of the thermal reactors. However, a tradeoff between the support ratio [thermal(D)/
fast(S)] and the growth rate clearly exists for this system, since maximizing the support
ratio will mean that net fissile-consuming reactors will constitute the major fraction of
the system and the growth rate will be detrimentally affected.
provides a great deal more flexibility in terms of the allowable energy support ratio and
inherent growth rate.
The fast(D)/fast(S) system
To illustrate the tradeoff between the growth potential and the support ratio, the
"operating envelopes" shown in Fig. 7.2-1 have been generated using denatured PWR data
from Section 4.1 and LMFBR transmuter data from Section 4.5.1.
the locus of permissible symtiotic parameters (growth rate, support ratio) for the system
considered,' i.e., the permissible combinations of growth rate and support ratio for each
specific reactor combinations.
respectively, the classical (Pu/U)O, reference system with a U02 radial blanket, a (Pu/U)02
system with a Tho, radial blanket, and a (Pu/Th)02 system with a Tho2 radial blanket. At
each point along the curves connecting points A, B, and C, the transmuter is a combination
of the two reactors defined by the end points of each curve segment (see key in upper right-
hand corner).
muters in different proportions.
Each envelope represents
At points A, B, and C on the curves, the transmuter used .is
Points within the envelope correspond to combinations of the three trans-
The lower envelope in Fig. 7.2-la (repeated in Fig. 7.2-lb) illustrates the tradeoff
for the denatured PWRs and LMFBR transmuters, and the upper envelope depicts the fast/fast
analogue in which the denatured PWR is replaced by an ~12% denatured LRFPR. As indicated,
the fast(D)/fast(S) symbiotic system provides a higher growth rate for a given energy sup-
port ratio, and, moreover, the growth rate is always positive. The upper envelope in Fig.
7.2-lb represents the corresponding case using 20% denatured LMFBRs.
In all cases the fast reactor data utilized were taken from Section 4.5.1; that is,
homogeneous LMFBR cores were assumed. The use of a heterogeneous core for the transmuter
reactor would have the effect of displacing the curves in Fig. 7.2-1 upwards and to the
right. The employment of an advanced converter (a high conversion ratio thermal reactor)
would have a similar effect on the thermal/fast curve.