A nuclear brick is a small RTG or nuclear reactor built to the form-factor of a brick and controlled by a wireless network. They are to power brick furnaces and heaters for smelting, casting, glass melting and curing, heating of habitats and greenhouses, heating of chemicals, and other thermal processes. A plutonium-powered RTG could last several years but has a much lower specific power than an americium-242-powered nuclear reactor. A 1.6 kg americium core can produce 70 kW of thermal power but this core must be replaced every 80 days of operation. Americium-242 exists on Earth as a byproduct of normal nuclear reactors.  The enrichment process required to create significant amounts of americium would be very expensive, however, and there are less expensive ways of producing heat. Americium costs about 1 500 000 $ per kg.
A martian settlement, as soon as it starts producing significant amounts of food and propellant. will be a thermally rich environment. Energy production from nuclear reactors produces between 50 and 75% of it's output as heat. So these bricks might not be much use, except perhaps for isolated vehicles or instruments.
- Genuth, Iddo, 2006, Americium Power Source, TFOT The Future of Things.