Calculation of the self-shielding factors for the upper reflector in MNSRs

The self-shielding factors for the upper reflector of the Miniature Neutron Source Reactors (MNSRs) are evaluated in this paper. Unlike nuclear fuels the values of these factors for the reflector in MNSRs are greater than unity. Different modes are used to fill the upper reflector in the shim tray, namely: the In-Out Radial Cumulative (IORC) mode, the In-Out Radial Singular (IORS) mode, the Out-In radial Cumulative (OIRC) mode, the Out-In Radial Singular (OIRS) mode, the Axial Singular Mode (ASM), and the Axial Cumulative Mode (ACM). The volumetric reactivity of the upper reflector in MNSRs is best utilized in average in the OIRC filling mode (0.003877mk/cm3), while it reaches a maximum with the ACM of 0.01607 mk/cm3. The presence of the upper reflector does not compromise the safety of MNSRs.

► Fuels tested:(Pu,Am,Zr,Y)O2;(Am,Zr,Y)O2;Am2Zr2O7 + MgO;(Pu,Am)O2 + Mo;(Am,Zr,Y)O2 + Mo.
► The HELIOS irradiation has been concluded, as planned, successfully.
► The objectives of the experiment, temperature and burn-up, have been fulfilled.
► The irradiation of such kind of nuclear fuel didn’t show any particular problems.