Elementary questions on biological shielding in nuclear submarines

Calvadosser

I'd be grateful to be pointed to some elementary information on the biological shielding in nuclear submarines. (Searching the web did not turn up what I was seeking).

I understand that the reactor is somewhere near the centre of the sub, with shielding bulkheads fore and aft of the reactor.

I'm curious about questions like:
- How do the crew pass from one side of the reactor to the other?
- Do they get a significant radiation dose if they stay in the vicinity of the reactor?
- Do the levels of shielding differ between former Soviet submarines and western subs?
- Is the weight of the shielding a significant fraction of the weight of the sub?

Thank you for any help.

NUCENG

Modern submarine design has the engineering spaces at after end or back of the sub. There are no berthing spaces in the engineering spaces so only the nuclear personnel have the need to be in that area on a frequent basis. All crewmembers wear radiation monitoring dosimetry. The reactor compartment has a shielded personnel access tunnel through the reactor compartment that passes through that area. There are valves in that area that need to be operated periodically but the tunnel is a no loitering area. There are tanks along the forward bulkhead providing additional shielding for the spaces forward.

At least in the older Soviet submarines the shielding was minimal to permit higher speeds and it was commonly reported that significant radiation exposures were common. Several classes of Soviet design reportedly used liquid metal reactors for high power density. If I am correct, there was only one US submarine built with a liquid metal plant and it was converted to a PWR later.

Astronuc

Seawolf (SSN-575) had a Na-cooled reactor designed and built by GE (1955), or their government entity.

http://en.wikipedia.org/wiki/S1G_reactor
http://en.wikipedia.org/wiki/S2G_reactor

zapperzero

The Alfa class submarines used lead-bismuth cooled fast neutron reactors. Very interesting design, but it went nowhere, because of maintenance problems. The Russians never perfected the external heaters that were supposed to keep the coolant fluid while the core was being fiddled with, so they have switched to lower-tech PWRs for their newer boats like Borey. The lead-bismuth alloy is also pretty corrosive, apparently.
The design was developed for its low weight to power ratio, but ironically it seems more suited for fixed installations.