This information I obtained from northkoreanrefugees.com and although it doesnt give an exact answer, it helps a little.
Q: How many North Korean refugees are there in China? A: Because of the Chinese government's tight control, it is impossible to conduct surveys to determine the numbers with any accuracy, but based on past figures provided by NGOs working in the field, it is estimated that there are approximately 100,000 North Koreans in China as of 2007. Some of these refugees are in shelters; others are living and working in hiding in autonomous ethnic-Korean areas. Female refugees frequently become the "brides" of local men-but in reality these marriages are little more than human-trafficking arrangements. Q: Why does China forcibly repatriate North Korean refugees? A: China continues to violate the International Convention Relating to the Status of Refugees by refusing to recognize North Korean defectors as refugees. China and North Korea have a secret agreement with regard to the forcible repatriation of North Korean refugees. China is probably considering its own position in northeast Asia and the consequences if North Korea were to collapse. China fears that a mass influx of North Korean refugees could precipitate a collapse of the North Korean system. Both of those events would have an adverse effect on Chinese economic growth. It is likely also that China fears what the effect would be on the two million ethnic Koreans living in the border region, as well as on other, smaller, ethnic groups.