How to deal with career breaks in CV I am about to finish my first postdoc (2 yrs) and the positions and grants that I am applying to frequently require me to specify and explain any career breaks. I feel somewhat uneasy about this since I was out of work for a year and a half after finishing my PhD, due to the fact that I could not find a postdoc position.

During this time I was not able to do any science as I was not affiliated with an institution. I also submitted nearly 90 applications, almost all of which were tailor made. This required a lot of time and effort. 

The 1.5 yr gap is the only stain on my otherwise good CV and academic track record. Any suggestions on how to approach this issue would be much appreciated.
I wouldn't bother to explain a break this short on your CV directly. I would instead just have a "canned" response ready when people ask you about it that puts a spin on it to look as good as possible. Something like:

"I wanted to make sure that I stayed in the field that I wanted to be in, and proper opportunities can be hard to come by, and short-lived. I applied for over 90 post-doc positions over that time, but decided that I wanted to make sure that I was open if the right opportunity came along, so I didn't take other work. Instead, I lent extra support to my partner as they started a new position, and made sure that the applications that I put out were top-notch."

Based on your question and comment, none of that is likely dishonest, it's just a nice and clean way to describe what happened in your life during that time. They really just want to make sure you didn't get bored in your field, but seems like you stayed engaged and dedicated to your field, so I wouldn't see it as a problem.