Experimental study using an LWR-type critical assembly, NCA, has been conducted on the reactivity worth of the burnable poison (BP) that is employed in a small reactor, 4S, for achieving an extremely long core life. The experimental BP is composed of natural gadolinium oxide (Gd2O3) mixed in polyethylene. Three types of BPs are prepared: polyethylene without Gd, polyethylene with Gd/H ratio of 0.015at% and 0.943at%. Reactivity worth of each BP assembly is measured through change in critical water height. Power profile of fuel rods and flux profile are also measured. Analyses are conducted with a Monte-Carlo code, MCNP.

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