Quantitative techniques have been successfully employed in verification of information and communication systems. However, the use of such techniques are still rare in the area of security. In this paper, we present a toolkit that implements transient analysis on a key update method for wireless sensor networks. The analysis aims to find out the probability of a network key being compromised at a specific time point, which result in fluctuations over time for a specific key update method called Leave-based key update. For such a problem, the use of current tools is limited in many ways such as rapidly constructing a compact formal model, computing the time point where the risk is maximum, or terminating the transient analysis after the fluctuations disappear and system stabilizes. Our toolkit, LBTool, is not only resolving the above-mentioned issues, but also demonstrating how to construct models in an analytical way and how to speed up the analysis by eliminating redundant computations. The toolkit can be generalized to other key update methods by replacing the analytical model construction.