Nuclear batteries are 1) heavier, 2) more expensive, 3) politically complicated.

Atomic batteries aren't really controversial or politically sensitive and have been used to great effect by many nations involved in space travel.

There's some problem with Tritium expose if the craft exploded on launch but those haven't really stopped it usage.

It's been developed for spacecraft use by Russia, China and the US, that's it. It IS politically sensitive in Europe.

The issue is getting the plutonium.

nasa almost used up the last bits of our reserves of it for the last mars lander.

Thankfully it appears that the united states department of energy is re-visiting generating plutonium-238 for NASA:

http://en.wikipedia.org/wiki/Plutonium-238#United_States_sup...

Yet another reason to build thorium reactors since the "waste product" is PU-238.

4) Rare and are designed to last ages, whereas this thing will burn up far earlier, wasting the rare ingredients.

That was asked during the early hours of the power "uh oh" phase, and someone involved replied that Europe has no ability to manufacture RTGs for political reasons.

I can't link the source but it was a tweet.

I've seen it a few places that they mentioned in the press conference that (mostly for political reasons) there has been almost no development of RTGs in Europe at all so it simply weren't an option.

> what would be the reason they didn't put a nuclear battery on board

Not every launch makes it to space.

I also heard from reddit that they didn't want to have radiation contaminate the area where Philea landed, apparently it's a bit hard to keep all of it inside of the lander.