Abstract
The degree to which a given physical subdivision of a critical reactor acts as a neutron source for other lobes is not analytically characterized for general application. ATR and ATRC both operate at power, and individual power-producing lobes rely on each other as neutron sources in order to maintain constant power, which in general requires either exactly critical multiplication within a reactor or an external neutron source. This work shows that lobe powers in ATR or ATRC can be related with subcritical multiplication theory, demonstrating that the lobes are not individually critical. This explanation is significant for ATR due to the desire to irradiate a large variety of experiments simultaneously, each having its impact on the core neutron population. This work also establishes the lack of relationship between experiment reactivity worth and cross-sections calculated from fundamental nuclear data. For any physical subdivision of any other critical reactor, it is likewise true that the subdivision undergoes only subcritical multiplication.
•The Advanced Test Reactor (ATR) is conceptualized with fuel divided among 5 lobes.•The Advanced Test Reactor Critical facility (ATRC) is conceptualized similarly.•An individual lobe relies on adjacent lobes as an external neutron source.•No lobe is individually critical; each exhibits only subcritical multiplication.•This fact is true of subdivisions of any other critical reactor.•No relationship has been found to predict reactivity worth from cross-sections.