Hi Anny,
I'm an ALARA Engineer from a long ways back so I'm not as current on recent issues as many of the other folks on this board. I did get to work on Fort St. Vrain in its last few years of operation and actually worked for one of the original design engineers when I started out working for the Navy so I might be able to get you started. The Hanford DOE guys and maybe some of the other HP types that got a chance to work on Peach Bottom Unit 1 can give you some other pointers.
From an ALARA standpoint, an HTGR is a dream design. I believe the total plant dose at Fort St. Vrain was around 300 mR/yr. At Palisades, that was a typical worker exposure back in the 1980s. There were no real contamination issues and from an HP standpoint, it looked like you'd get pretty bored working there.
The plant really never ran very well. My former manager told me that it had a lot to do with physicists at General Atomic making firm price commitments for deliveries of systems that had never been designed or built before. That said, the design was really interesting and if you could find a copy of the FSAR for Fort St. Vrain, it would give you a good leg up on radiological design considerations.
Issues that I remember from those days included water intrusion into the primary coolant system, condensation of water in the reactor shutdown system, and instrumentation issues with measuring really low dewpoints at really high temperatures. The condensation problem was because the design used a system where a LOCA caused a rupture disk that held back a hopper full of borated balls to break dumping the balls into the core. The disk had a small orifice to allow pressure equalization during normal reactor startup and shutdown but also allowed and water vapor in the primary system leak through and condense on the borated balls thus gluing them together.
Water intrusion into the primary system was the worst case scenario. Reactivity control was severely jeopordized with the additional moderator and the steam would react with the graphite directly leading to all sorts to badness. The secondary loop was water/steam so the ideal design would be to keep primary pressure higher than secondary to prevent intrusion. I remember that Fort St. Vrain had some extraordinary steam generators but don't remember much more.
The plant used a cryogenic primary coolant treatment system if I remember correctly although it seems like you could have used a straight activated charcoal system. I remember they also had an RO treatment system but don't remember the particulars. I do seem to remember that almost all the plant dose was related to maintenance and repair work on their primary coolant circulators as I believe those were the source of several water intrusions but that's about all the help I can be. When I was taking graduate classes at Penn State in NucE, there was a lot of thinking around a HTGR design that used magnetohydrodynamics to generate electricity using an all gas design but there were significant technical issues to pulling it off. Maybe you folks can do it now.
Bill
