Times Online -- Pyongyang may have A-bomb in 30 days
Good article, all the reasons for restarting the reactor have to do with producing atomic weapons which could take place between 30 days and a year.
The UN has never been able to verify how much plutonium the country produced before the reactor was shut down in 1994. Some analysts have speculated that it might have kept enough for two atomic bombs.
In this case North Korea might be restarting the nuclear reactor to produce a “polonium trigger” needed to ignite a nuclear explosion. This would enable it to produce a functioning nuclear weapon within just 30 days.
North Korea’s unstable Stalinist regime has also spent recent years developing a long range missile that would enable it to send bombs as far as Japan. “North Korea has a nuclear weapons development programme on an industrial scale, and the means for delivering them.
Most analysts have assumed they have two bombs - this recent activity may mean they don't have triggers for them or they want more or this is a bargaining chip.