A neutronics investigation simulating fast reactor environments in the thermal-spectrum advanced test reactor

Abstract

Developing advanced fuel and materials for fast-spectrum nuclear reactors is an immense challenge with tremendous potential in maximizing the value of advanced nuclear reactors. Historical developments and modern materials show promise for overcoming these challenges, but the lack of fast neutron test capabilities in the United States is a significant obstacle for meaningful progress. Modern modeling techniques were utilized to investigate a proposed method for fast neutron irradiations in an existing thermal-spectrum reactor, the Advanced Test Reactor (ATR). This method builds upon past ideas, where fast flux is increased by surrounding the specimens with fissionable “booster fuel” but diverges from historical approaches by using an already developed fuel element design used in the Belgian Reactor 2 (BR2) as the booster fuel while leveraging modern 3-D modeling and simulation techniques. Design evaluations and neutronics predictions were performed to evaluate the performance of a BR2 fuel element irradiated in an ATR flux trap with test pins in the central channel of the BR2 fuel element. These efforts have yielded promising results.Adding a BR2 fuel element in the northeast (NE) flux trap of ATR was predicted to result in a 150% increase in the incident fast neutron flux with a fast (>0.1 MeV) to thermal (<0.625 eV) neutron flux ratio ranging from approximately 50 to 150, dependent on the material used for thermal neutron filtering and volume of moderator within the central channel of the BR2 element.The predicted annual fast neutron fluence (>0.1 MeV) ranges from 7.9 × 1021 to 9.1 × 1021 n/cm2. Given the relatively large fast to thermal neutron flux ratio, the calculated radial power profiles within 4.3 mm outer diameter fueled specimens irradiated within the BR2 booster fuel element are fairly representative of fast neutron reactors. These predicted radial power profiles are not flat, but they are more prototypic than those seen in advanced fuels tests (Capriotti et al., 2017) which began in ATR in 2003. Another distinct advantage of this experiment design is that full-scale test pins can be irradiated to supplement the ongoing series of reduced scale advanced fuels tests (Beausoleil et al., 2020). The proposed experiment design irradiated within a BR2 fuel element in a flux trap of ATR offers an improved alternative to the current testing of advanced reactor fuels in ATR. Selected results from this design evaluation are documented herein and substantiate this conclusion.