A Solution to the Problem of Incorrect Measurement of the Core Inlet Temperature in Pool Type Research Reactors with Downward-Flow Cooling

The thermal power of research reactors is usually measured through measuring the temperature gradient over the core. In pool type reactors, with downward-flow cooling, the inlet temperature is measured with a precise thermometer located above the core. Due to the high radiation field in this area, the thermometer must be located at a reasonable distance from the core upper surface, and this may lead to an incorrect reading of the core inlet temperature, which will eventually cause an incorrect evaluation of the core thermal power.

The discrepancy between the measured inlet temperature and the true inlet temperature is caused mainly at transient condition (e.g. during reactor start up), due to the fact that the water, entering the pool, is usually much cooler than the bulk pool temperature. This stream of cooler inlet water makes a short cut to the core inlet, while the thermometer measures the higher bulk temperature.

In order to solve this problem, a designated device was designed at IRR-1. This device is positioned on the grid plate, in the reflector zone, similar to a standard fuel element. It consists of an inner pipe which sucks water from the upper surface of the core, to a point, two meters above the core level, and then down to the water plenum under the core. The thermometer is located at the uppermost part of this pipe, where the radiation level does not damage it. The flow through the device is driven by the pressure gradient in the core.

Flow simulation, using computation fluid dynamics (CFD) code, shows that the water temperature does not change (less than 0.1°C) while it flows from the core inlet up to the measurement point. The actual performance of this device has been tested by operating the reactor at zero power with only the primary cooling loop in operation, and then, after reaching a steady state, the secondary cooling loop was started, thus causing a transient condition. This test demonstrated that the maximum deviation of the measured temperature from the actual inlet temperature was 0.7 0C (in comparison to 2.20C using the former method).