The High Temperature engineering Test Reactor (HTTR) attained its initial criticality in November 1999 using a ring core geometry. This core was formed by 19 fuel columns, as shown in Fig. 9-1. Should there be an inadvertent loss of coolant pressure, this ring core geometry has the advantage of being able to leak the decay heat radially. The maximum fuel temperature can therefore be lowered so that release of fission products is prevented. Therefore, active safety engineering devices are not required to achieve a passive safety reactor. It is intended to perform critical experiments on this ring core geometry, which have not been previously carried out.
Fundamental neutronic characteristics have been observed. As mentioned, the core reached critical with the 19 fuel columns, which is somewhat beyond the 16±1 fuel columns, expected by calculation. The reason why the calculated value significantly underestimated the experimental one was investigated. Several problems were found which are specific to the ring core geometry. One of them is the use of a detailed model against the complex heterogeneous structure of the fuel columns (Fig. 9-2). The other is the sophisticated evaluation of the coupling effects between the fuel columns facing each other over the central region made of the graphite columns. In the early stage of planning critical experiments, it has been recognized that some experimental methods regarding the core reactivity have to be improved. One is a revised method for the determination of the excess reactivity. In this method, the value of the excess reactivity is given by the cumulative sum of the reactivity increments for each step of fuel column additions is not actual but virtual cores. Using this method, the mutual interaction effect between the control rods can be precisely taken into account. As for the shut down margin, the delayed integral counting method is proposed for the rod drop experiment. Precise measurement of the control rod worths (effectiveness of control rod insertion on reactivity) can be done even for a long insertion time such as 10 seconds (Fig. 9-3). These efforts have made it possible to understand the whole neutronic properties of the HTTR in ring core geometry. |
