Forget the nonsense about the inability of over a billion Chinese to deal with the a few hundreds of thousands or even a million refugees. It is a mere excuse. The real reason behind the Chinese support for North Korea? China needs Kim argues Dingli Shen in Le Monde Diplomatique:

China’s current interests are to develop its economy and reunify the nation. For the ultimate goal of reunification, China’s main task must be to oppose any move towards Taiwanese independence. North Korea puts in check the tens of thousands of US troops in South Korea, helping to reduce US military pressure over the peninsula’s reunification. To some extent North Korea helps China to share the threat of US military force in the Asia-Pacific region. As an ally, North Korea also helps China defend the main gate of its northeast regions and contributes to China’s national security. It is impossible for China to abandon North Korea or to adopt comprehensive sanctions against the country just because of nuclear testing.

So what should China do? Trust the US to stop Japan from going nuclear and agree to limited sanctions.

Now that North Korea has announced its weapons test, China needs to take part in the sanctions (import/export restrictions on nuclear technology) that the international community will surely impose on North Korea. But China should block economic sanctions; it can only afford to choose between two evils: insufficient sanctions that would show lack of responsibility or harsh sanctions that would force North Korea to extremes or bring about a regime change.