  A new method for the measurement of sample reactivity worth in a fast neutron
reactor named the inverse kinetics method is proposed in the paper. The sample
reactivity worth could be obtained by measuring the reactivity step change in
the process of sample fetching and placing in the delayed critical reactor.
Compared with the traditional period method, the advantage is that the accuracy
of reactor reactivity control will not exert any influence on the uncertainty
of reactivity worth measurement. The inverse kinetics method has been used to
measure the reactivity worth of {\phi}20mmx9mm Au, V and Be samples at the
center of the upper surface of a highly enriched uranium fast neutron reactor,
and the results are 5.17, 4.40 and 5.90 respectively, which are consistent with
those obtained with the period method. The standard uncertainty of measurement
of the results is about 0.03, which achieves an obvious improvement compared
with that (~0.08) of the period method.
