I think it's important to compare with what you'd expect in the general population against what you'd expect in prisons. According to the Twelfth United Nations Survey of Crime Trends and Operations of Criminal Justice Systems the incidents of rape in the US is 28.6 per 1000 in 2008 (on a steady decline from 31.6 in 2003).
I came upon an article in reason.com reporting that the DOJ (U.S. Departement of Justice) has recently attempted to estimate the amount of sexual abuse in prisons in America. I've emailed the reporter asking for directions to the actual report, but the reason.com give these statistics that I found interesting:
The U.S. Department of Justice recently released its first-ever estimate of the number of inmates who are sexually abused in America each year. According to the department’s data, which are based on nationwide surveys of prison and jail inmates as well as young people in juvenile detention centers, at least 216,600 inmates were victimized in 2008 alone. Contrary to popular belief, most of the perpetrators were not other prisoners but staff members—corrections officials whose job it is to keep inmates safe. On average, each victim was abused between three and five times over the course of the year. The vast majority were too fearful of reprisals to seek help or file a formal complaint.
However you shouldn't assume that people who work in prisons are abusive, even going by accusation it's a low number of the total. A DOJ report from September 2009 reports that 4.7% of all staff members under the BOP (Federal Bureau of Prisons) has been accused of sexual assault, it doesn't however seem to break it down into categories containing the nature of the abuse.
On the bright side I did find out that congress has attempted to do something about prison rape back in 2003:
The Prison Rape Elimination Act of 2003 directs prison officials to make the prevention of sexual abuse in prisons a top management priority. The Prison Rape Elimination Act defines “prisons” broadly to include not only federal and state prisons and local jails, but also short-term lockups such as cellblocks and other holding facilities regardless of their size.
This is a placeholder until I can find the DOJ statistics.