In the case of a small breach in fuel rod cladding, only a small fraction of the fission products like Cs nuclides can be released into the primary loop during the stable operation state of the reactor, making it difficult to detect. A quick reduction of power can be conducted to increase the activities of Cs nuclides in primary loop and the burnup of the target failure rod can be calculated by the Cs activity ratio measured after the power reduction transient. Here we simulate the changes of gas pressure and temperature in the gap between the fuel pellet and the clad-ding and the water behavior near breach during the power transient. The results show that both the gap gas pressure and temperature exhibit a drop peak after power reduction. If there is a breach in the cladding, the coolant water in the primary loop will enter the cladding and then carry away some soluble nuclides like 134Cs and 137Cs, resulting in the activity increase of fission products in the coolant. This study gives a brief explain of power transient process for soluble fission products release, providing reference data for the safe operation of the reactor.