Want some more RAM - you got it. Want it to be radiation tolerant and non-volatile - we have got what you need. Arduino Mega Ferroelectric RAM shield. It uses one Ramtron SOIC-28 package non-volatile memory - like FM16W08-SG or FM18W08-SG. It is a parallel memory that is mapped right on your ram. And non-volatile means that you don't have to use that EEPROM any more, you can save data right in your RAM.
Board has room for one memory and one latch, needed for the bus. Besides that there is also a little AND gate to make protocols of AVR and FRAM work with each other - there were some pin differences. The schematic is tested and working but the board layout isn't.
And just a little extra something - there is a parallel FRAM on the bottom of the board. It hooks up to your SPI and works like a serial flash chip. You can use Ramtron FM25640B-G and other pin compatible devices.
Info about how to use an external RAM with Arduino Mega.