BR-5 is a Soviet 5 MWt reactor built in 1959 as an irradiation facility. It is fueled with Pu02 sintered in 0.2-in. diameter stainless steel tubes, 19 to an assembly. There are 88 such assemblies, with the fuel tubes held at the bottom but free to move at the top. The core is sodium cooled with upward flow (see Fig. 4.41) (45).

The system has a prompt positive contribution to the power coefficient but also has a delayed and overwhelming negative contribution.

At one time about $ 2 worth of positive reactivity was present in the core in the form of cover gas bubbles. There was some concern over safety at

the time, but the problem of entrainment was cleared. There have been no other incidents. Since then some fuel cladding failures have occurred, al­though these are due to excessive burn-up rather than any accident event (46a). Despite a number of fuel cladding failures there has never been any indication of failure propagation.