DN detection during SLSF tests

During two tests in the Sodium Loop Safety Facility (W2 and P4), two systems were operated to detect delayed neutrons from exposed fuel. One monitored directly the sodium in the in-pile loop with a transit delay from the flux region of approx.2 seconds; the other conducted a sample stream of sodium to external detectors with a transit delay which could be varied from approx.10 to approx. 40 seconds. Detectors of a wide range of sensitivity were operated in pulse and current mode; DN signals varying from <1 mm/sup 2/ recoil to many grams of molten fuel could be recorded reliably without saturation. During the W2 and P4 tests a continuous DN record was made. Massive fuel failure signals were observed during reactor transients in both tests, including events interpreted as fuel melting and exposure of large areas. The steady signal from the blockage in P4 was studied as a function of reactor power and sodium temperature.