Critical vs. external neutron source driven systems

A number of factors dictate the choice of the fuel form. The ideal transmuter fuel should

combine the following features. - minimal generation of secondary actinides - neutronic features allowing optimal spectrum for transmutation while maintaining safe operation characteristics - high burnup capabilities - simple and economic handling and reprocessing - good thermal and mechanical properties that would promote reactor safety

A recent review of fertile free fuel candidates for transmutation of actinides in various

reactor types has been conducted at MIT [Long Y. et al., 2003].

Critical vs. external neutron source driven systems

The presence of significant quantities of MA in a fast spectrum core particularly in combination with fertile free matrix fuel leads to degradation of feedback coefficients and inferior transient behavior of the reactor. Some of the safety features of fast spectrum TRU transmutation systems can be significantly improved if they are designed as subcritical systems driven by external neutron source. The concept of Accelerator Transmutation of Waste (ATW) was originally conceived in Los Alamos Nation Laboratory [Bowman C.D. et al., 1992] and it is presently actively advocated by C. Rubia [Carminati F. et al., 1993]. In this concept, the neutrons that drive a sub-critical reactor are produced through spallation process. The beam of accelerated to high energy protons impacts typically heavy metal target and initiates a cascade of secondary lower energy neutrons and some protons. Some neutrons are produced via direct collisions with the nuclei of the target while the majority are produced as excited target nuclei get rid of their excess energy by “boiling off” additional neutrons. In such a process, up to 30 neutrons can be produced per one 1GeV incident proton.

Sub-critical accelerator driven systems (ADS) enable the design of reactor cores that would otherwise be very challenging to operate. In addition, the external neutron source allows the control neutronic reactivity by adjustment of proton beam current which eliminates the concerns over prompt critical power excursion accidents and need for burnable absorbers. However, it does introduce a concern about accelerator beam overpower. In addition, ADS systems are still