A calorimeter for graphite dose rate in advanced gas-cooled reactors

Abstract

A steady-state calorimeter of simple design, for determining absolutely the total energy deposition rate in a graphite sample inside the core of an advanced gas-cooled reactor, has been developed, and three have been tested in continuous operation in the Windscale reactor. The operating conditions are: temperature 300-400°C, pressure 270 lb in-2 and energy deposition (or dose) rate between 100 and 200 mw g-1 at full reactor power. The performance has been good and, despite subsequent heater failure, no significant change in the original calibration by electrical heating has been found. The average total dose to each of the instruments now exceeds 5 × 1011 rad (1 rad = 100 erg g-1). The major correction (about 10%), necessary to the measured graphite dose rate, arises from the effect of neutron captures by the stainless-steel can. The overall experimental accuracy (standard error) in the dose rate at full reactor power is estimated at ±3%. There could be an additional uncertainty in the deduced quantity, which is the CO2 (pore gas) dose rate, of up to about 4%, mainly because the fast neutron stopping powers of CO2 and graphite differ slightly. Reasonable agreement has been obtained between semi-empirical calculations of moderator dose rate, based on relative channel power outputs, and the actual measurements in the core.