An apparatus and method to detect combustion conditions using ion signals for use in a feedback control of a reciprocation engine is presented. The fuel assembly itself contains mixed oxides of a low enrichment concentration uranium oxide containing 3 to 8 wt % in the average enrichment concentration of the fuel assembly, or mixed oxidecontaining not less than 2 wt %, but less than 6 wt % in the average enrichment concentration of fissile plutonium of. A servo system steers the catheter tip in the direction of the tissue to be removed.