Three-dimensional pin powers from LOFT Core-1 tip data

A stepwise process is outlined by which a three-dimensional (3-D) pin power distribution in the LOFT Core-1 (LOFT-I) can be constructed from a set of calculated X-Y power distributions and three different axial fission rate traces produced by the Traversing Incore Probe (TIP) system. At each of an arbitrary number of axial points in the core, three values are taken from the different TIP traces at the axial point. The relative power densities in three fuel pins adjacent to the respective TIP positions are then computed by multiplying the measured TIP output values by the respective pre-computed ratios of pin power to TIP output. An X-Y power distribution is chosen for the given axial point from one of two sets of calculated distributions (one for the rodded core and one for the unrodded) on the basis of minimizing the square of the error between the three pin powers inferred by the TIP measurements and the corresponding three pin powers of the calculated distributions. The chosen X-Y distribution is multiplied by a value proportional to the TIP inferred power for the fuel pin adjacent to the middle radial TIP position in order to establish the axial shape of the power distribution. The ratios needed to compute powers for the three pins adjacent to the TIP positions are computed for a number of LOFT-I core conditions and a table is presented that shows the error caused in one 3-D LOFT-I case by varying the degree of error in these ratios. The method, together with the present sets of X-Y distributions, was applied in fitting twenty different LOFT-I core power distributions. The differences between the constructed distributions and the calculated ones are tabulated for a center fuel pin, the pin with the maximum power and a pin at the core edge. The input and output of the fitting code are summarized