UPDATE 22 (2011-6-10): ISIS has published new satellite imagery of the Yongbyon nuclear facility.  The imagery shows a number of new buildings and renovations.  I have archived the information, along with other recent satellite imagery of Yongbyon, below:

These construction and renovation activities took place after the April 2009 departure of IAEA monitors and U.S. experts charged with disabling three plutonium production facilities at the Yongbyon site.  According to North Korea, after their departure, it began constructing the gas centrifuge plant at the Fuel Fabrication Facility.  These other construction activities could be related to reactor fuel fabrication, uranium enrichment, or uranium conversion, including uranium tetrafluoride or uranium hexafluoride production.  Whatever the exact purpose, these activities show that more is going on at Yongbyon than commonly believed.  North Korea should provide information about these new buildings.

Above pictures (39.801395°, 125.754409°, Google Maps): Commercial satellite images showing this original pilot fuel fabrication building on August 10, 2009, February 24, 2010 and June 24, 2010.  A new roof can be seen in the June 24, 2010 image.  The entire building may have undergone a renovation during this time period.

Above Picture (39.770379°, 125.750889°, Google Maps): Comparison of satellite imagery showing new roof on building adjacent to the gas centrifuge plant.  The new roof could indicate that North Korea renovated the entire building.

Above Picture (39.768070°, 125.754132°, Google Maps): Comparison of satellite images of the southeast corner of the Fuel Fabrication Facility. Original structures were deconstructed beginning almost nine years ago.  The skeletons of the buildings remained in place for many years until construction of new buildings began after April of 2009.

Above Picture (39.768160°, 125.752472°, Google Maps): Comparison of satellite images over time showing the construction of a new building after April 2009.

Above Picture (39.769873°, 125.746629°, Google Maps): Comparison of commercial satellite images showing the construction of a new roof on a building on the western edge of the Fuel Fabrication Facility.  The new roof could indicate that North Korea renovated the entire building.

UPDATE 21 (2011-4-27): The Institute for Science and International Security released new photos of the Yongbyon LWR facility construction site:

According to ISIS:

Left image: Commercial satellite image from March 8, 2011 of the new construction near the site of the destroyed cooling tower for the 5 megawatt reactor at the Yongbyon nuclear center.  North Korea has stated this is a construction site for a new light water reactor.  The cylinder seen in the image measures approximately 21 meters across.  There is also a new clearing with a crane and possibly construction materials in an area near the 5 MW reactor.  The cylinder may appear to be oval-shaped, however, this is an optical illusion caused largely by the shadows cast and partly by the angle of the image.  The diameter of the cylinder remains relatively constant around the circle—accounting for the angle.

Right image: A comparison of an area adjacent to the new construction as seen in a November 3, 2010 and the March 8, 2011 image.  This area is mostly empty in the earlier image.  In the March 2011 image, there appears to be some site clearing, a crane, and what could be construction materials.  It is unclear if this area is or will serve as construction support or staging, of if a structure will be built there.

UPDATE 20 (2011-4-8): The Choson Ilbo offers satellite imagery of progress at the Yongbyon LWR construction site:

(New image on the right dated 2011-3-24)

According to the article:

A satellite photograph of North Korea’s Yongbyon nuclear complex taken on March 24 shows a dome-shaped structure and construction taking place right next door that were not visible in a surveillance photo taken in November last year. The dome-shaped structure measuring around 30 m in diameter is apparently around 40 m tall.