ORNL-5388 - the Molten Salt Energy Technologies Web Site

c 4-23
L
i a
L
L
4.2. SPECTRAL-SHIFT-CONTROLLED REACTORS
N. L. Shapiro
Combustion Engineering, Inc.
The Spectral -Shift-Control led Reactor (SSCR) is an advanced thermal converter
reactor that is based on PWR technology and offers improved resource utilization, partic-
ularly on the denatured fuel cycle.
is designed to minimize the number of reactions in control materials throughout the plant
life, utilizing to the extent possible captures of excess neutrons in fertile material as
a method of reactivity control.
material serves to reduce fuel makeup requirements.
The SSCR differs from the conventional PWR in that it
The resulting increase in the production of fissile
In the conventional PWR, long-term reactivity control is achieved by varying the
concentration of soluble boron in the coolant to capture the excess neutrons generated
throughout plant life. The soluble boron concentration is relatively high at beginning
of cycle, about 700 to 1500 ppm, and is gradually reduced during the operating cycle by the
introduction of pure water to compensate for the depletion of fissile inventory and the
buildup of fission products.
The SSCR consists basically of the standard PWR with the conventional soluble boron
reactivity control system replaced with spectral-shift control. Spectral-shift control is
achieved by the addition of heavy water to the reactor coolant, in a manner analogous to
the use of soluble boron in the conventional PWR. Since heavy water is a poorer moderator
of neutrons than light water, the introduction of heavy water shifts the neutron spectrum
in the reactor to higher energies and results in the preferential absorption of neutrons
in fertile materials. In contrast to the conventional PWR, where absorption in control
absorbers is unproductive, the absorption of excess neutrons in fertile material breeds
additional fissile material, increasing the conversion ratio of the system and decreasing
the annual makeup requirements. At beginning of cycle, a high (approximately 50-70 mole X)
D20 concentration is employed in order to increase the absorption of neutrons in fertile
material sufficiently to control excess reactivity. Over the cycle, the spectrum i s
thermalized by decreasing the D20/H20 ratio in the coolant to compensate for fissile
material depletion and fission-product buildup, until at end of cycle essentially pure
light water (approximately 2 mole % D20) is present in the coolant.
The basic changes required to implement spectral-shift control in a conventional
PWR are illustrated in a simplified and schematic form in Fig. 4.2-1. In the conventional
PWR, pure water is added and borated water is removed during the cycle to compensate for
the depletion of fissile material and buildup of fission-product poisons. The borated
water removed from the reactor is processed by the boron concentrator which separates the
discharged coolant into two streams, one containing pure unborated water and the second