"Exposure of personnel to radiation has been held well below permissible limits: the maximum exposure of any individual in any quarter has been <0.5 rem." This is within the ALARA standards.
http://moltensalt.org/references/static/downloads/pdf/NAT_MS...
RE: no real fluid-to-fluid heat exchanger.
MSRE had salt-to-salt and salt-to-air heat exchangers made from hastelloy-N. https://www.flickr.com/photos/oakridgelab/31537153147/
RE: or power conversion cycle, which is where lots of maintenance troubles arise from.
That's the part which is not highly radioactive. Fluid fuel reactors have simplest fuelling mechanism of any reactors. The fuelling of a fluid-fuel reactor is even simpler than fuel injectors of diesel engine. Where the need for "maintanence" arise in this simple machine??
Only moving parts are the pump impellers and the control rods. ORNL had some difficulty designing the bearing, seals etc. for the salt pump. But, even that is solved today with concentrating-solar salt pumps.
Ah, the good old days! 25-50% of the radioiodine from MSRE is completely unaccounted for. No one knows where it went. This is wholly unacceptable from a modern regulatory approach. It will have to be found.
http://moltensalt.org/references/static/downloads/pdf/ORNL-4...
> MSRE had salt-to-salt and salt-to-air heat exchangers made from hastelloy-N
Fair enough. It's generally in the salt-to-water heat exchanger where major maintenance concerns take place in a power reactor. Check out steam generator problems in PWRs and SFRs to see what I mean. You're right that the radiation will be low there if they use an intermediate salt. But the baseline maintenance problems come in at the steam generator in most plants. So MSRE avoided any problems there but any power MSR will have them like any other reactor. Good old BWRs get around this via direct cycle.
> Where the need for "maintanence" arise in this simple machine??
Pumps. Heat Exchangers. Valves. Flanges. Welds. Vessels. Graphite. Reflectors. Fission product processing equipment. Instruments/sensors. Control mechanisms. There are very complex systems with lots of things working together in a very tough environment (high radiation, high temperature). Maintenance is a major challenge.
<0.5 rem per quarter (or 5 mSV) is in compliance with today's dose limit. ORNL was a competent American national lab with high standards even back in the 1960s. Another hollow argument from today's incompetent nuclear industry.
"Exposure of personnel to radiation has been held well below permissible limits: the maximum exposure of any individual in any quarter has been <0.5 rem." http://moltensalt.org/references/static/downloads/pdf/NAT_MS...
"The maximum annual dose allowed for radiation workers is 20 mSv/yr" https://www.world-nuclear.org/information-library/safety-and...
135Xe also can be removed (halflife ~9.2 hours). It is famously a very powerful neutron poison with important effects on reactor operation; those advocating thermal breeders count on it being removed before it soaks up neutrons. But this leads to an increase in production of its decay product, 135Cs, which has a halflife of 2.3 million years.
A lot has changed since MSRE ran, and MSRE was a fairly simple experiment to prove a concept. It didn't have all the systems necessary to make practical energy, and so it didn't have the maintenance issues you'd expect. Also, it only ran for like 5 years, vs the 60-80 we're hoping for these days from these facilities (inherently requires lots of maintenance).
Have you ever done work planning for ALARA at a light water reactor? Those who have raise eyebrows really high when academic reactor designers start saying how easy the maintenance will be. Admiral Rickover's quotes from 1953 just keep on coming back to haunt us [1]. He didn't curse us to death, but he sure warned us that we need to be working really hard on new/effective solutions to these kinds of problems. Most advanced nuclear advocates skip over these points.
RE: Design it carefully: This takes practical experience. The number of people today designing new reactors who have this kind of experience is very close to zero. We will have to re-learn.
[1] https://en.wikiquote.org/wiki/Hyman_G._Rickover#Paper_Reacto...