Interesting idea. 2km deep could work. I'm not sure how cooling or maintenance would work.

Depth below surface | Typical temperature (°C) | Indicative cost to drill 1.2 m diameter hole

500 m | 15–25 | $5–10 million

1 km | 25–40 | $10–20 million

2 km | 50–70 | $25–45 million

3 km | 75–100 | $50–80 million

4 km | 100–130 | $90–140 million

5 km | 130–160 | $150–250 million

maybe not the article I read, but similar:

https://spectrum.ieee.org/underground-nuclear-reactor-deep-f...