theme() in both the filter and the score.
1. Ask for the Tilt

The filter keeps companies with meaningful nuclear exposure. The score ranks them by that exposure, and the Tilt holds the top 25. With the weight expression blank, each position is proportional to its exposure, and no position can exceed 10%.
2. Refine the rules
The first version held some companies worth a few hundred million dollars. Ask for a size floor:

market_cap >= 1000 means at least $1 billion, since market_cap is in millions of US dollars.
Tilt records the change in the Tilt’s History, alongside the original rules.
To make the same change by hand, open Selection rules, edit the filter expression, and select Apply changes. The Theme importance control in that section sets how much each theme counts in the score. Tilt drafts the new score expression for you to review before it applies.
3. Review the result
The Methodology tab shows a backtest against a benchmark and every rule by section:

Variations
How theme() scores companies explains where exposure scores come from.