The presence of large amounts of plutonium in high exposure uranium will give rise to a large, positive, graphite reactivity, temperature coefficient. This will create control problems in normal operation and also in the event of loss of cooling. Some general suggestions are given for the solution of these problems. The magnitude of the coefficient is probably known within ± 25% at present and a program is planned to reduce this uncertainty. The uncertainty is of minor importance in the operating control problem but of considerable significance in the disaster control problem.