thought I would get the answer from the horses mouth.....just finished work at the Seafarers Centre and asked the ( Latvian) Captain of the " Norasia Bellatrix", which is in port at the mo.
Now the name might not ring a bell , but she is the repaired and rebuilt ex " Hanjin Pennslyvania", which suffered massive explosions and fire on 11/11/2002 whilst westbound off Sri Lanka.
( see e.g. http://www.nationalfireworks.org/penn-ship.htm ).
The container holds are all seperated by a watertight bulkhead, of course, but have an access point at the fore or aft end & ladders down to the double bottoms. There is insufficient space to even open a container's doors.
To quote the Captain: " If anything happens to a container in a hold there is absolutlely nothing we can do about it,there is no space to do anything."
Obviously all the container holds and hatch lids have closable vents to allow air circulation. Because of the closely packed container stacks, once a fire or smoulder starts, all you can hope for is to starve it of Oxygen,operate the CO2 system & apply cooling water outside, if safe !
BUT , if an explosion starts the ball rolling and breaches the hold or hatch then heaven help you !
From my own experience I only worked on one container ship, the " Dart Americana" and the hold system was the same then as now. If anything,the containers are even more closelyy packed in now, due to accountants and economy.
Several of the older style container ships carried refrigerated units below deck and there was sufficent space to access the refrigeration machinery for service, these units had air cooled condenser coils for normal above deck use but below deck were connected to a ships reticulation water cooling system.
One major problem that plagued us was if the ship had a black-out (as often happened) not all the units would re-start then it was down below checking each one and restarting those that had stopped, and on a large container ship in those days could take several hours.