In the common practice of propulsion reactor core designing the main fuel burn-up reactivity margin is compensated with the gadolinium self-shielded burnable absorbers (SBA). The well-developed profiling of gadolinium SBAs (through variation of gadolinium density and height in the SBA, the SBA number in FAs of various profiling zones) provides the consistent burn-up rate of fuel and poison in the core, with the minimum positive overshoot of reactivity; it also ensures flattening and sufficient stability of power distribution within the core volume in the course of the reactor run. Implementation of gadolinium SBAs made it possible to increase the burn-up reactivity margin, to significantly increase absorber burn-up and minimize interference of CPS control rods to compensate reactivity during the reactor cycle. Reduction of the reactivity margin that is compensated by the CPS control rods resulted in reducing non-uniformity of power distribution and burn-up within the core volume and improving the conditions of fulfilling nuclear safety requirements. The use of gadolinium SBAs in combination with boron-containing carriers is very promising for the cores with increased energy margin in terms of attenuation of the effect of shortening the reactor cycle duration by means of even isotopes of gadolinium, and increase in the core uranium intensity (reduction in the number of displaced fuel elements) From the point of view of neutronics, the important positive feature of boron in contrast to gadolinium consists in the fact that in the course of burning of absorbing isotope 10B, 4Не and 7Li are generated; and in contrast to even gadolinium isotopes, they actually do not absorb neutrons. The concept of optimization of boron and gadolinium combination consists in replacing part of gadolinium SBAs with boron-containing carriers. The task is to select the parameters with the aim to minimize the core reactivity “imbalance” in the course of the reactor run, with the ensured 10В burn-up. The calculation studies performed demonstrate that in the core of the propulsion reactors with high energy margin it is reasonable to replace part of the gadolinium SBAs with boron-containing carriers, thus allowing the reactor cycle to be extended by 2-3% due to reduction in the number of even isotopes of gadolinium. The highest economic effect comes out with the use of borated rods without cladding. In this case, along with the extension of the reactor cycle, the reduction of the core cost is provided due to cutting down the cost of the absorber.

Key words
Propulsion reactor, core, self-shielded burnable absorbers (SBA), boron absorber, uranium intensity, incomplete burn-up of isotopes, energy content, “imbalance" minimization

Article Text (PDF)

UDC 621.039.51

Problems of Atomic Science and Technology. Series: Nuclear and Reactor Constants", issue 3:11, 2014