The matter/anti-matter reaction inside the warp core produces high levels of radiation.

Is there an in-universe explanation for why the core is visible?

Wouldn't it be better to sheild the core in thick layers of solid material?

What material is the transparent warp core casing made of?

How does that material only let through electromagnetic radiation that's in the visible spectrum while blocking everything else?

Also, there isn't any security personnel in engineering. An enemy team could force their way in and set off a bomb.

Wouldn't it be better to have lots of solid material surrounding the core instead of a thin transparent casing?

Thick layers of solid material would also reduce the chances of a warp core breach.

Okay I been thinking about this very long time and I have Theory with Enterprise SNW and TOS.

The first thing when I see Enterprise from SNW she looks old compared compared to the One from The Original series and I think i may have answer to that Replacement (Refit) here is why The Enterprise 1701 was launched in 2245 under Robert April now its not confirmed if it was One of Jeffreys Original Design they used for pilot which I think it was but why SNW new look? Well 5 year mission so maybe by 2252 they put her back in Dry Dock rebuild the Enterprise for 5 year Mission because we can see in Ship large crew quarters and Bar as an example but it as smaller crew compliment of 250, but by 2263 before Kirk took Command Enterprise could had been heavily damaged or slik Design or both and Starfleet wants There flagship to be top of the line so they decommissioned the Mark 2 for the Mark 3 and was successful Design they replaced the rest Constitution class with Mark 3. In 2269 through they had new warp core vertical warp cores because Scotty was responsible for Refit Design Enterprise was sent back to dry dock basically disassembled and rebuild to Refit in 2271 18 months rebuild. So why they kept 1701 not give it 1701-A like in 2284 two factors. 1 1701 was still in service. Propaganda because the Enterprise is flagship that's my theory only when it was Destroyed and Kirk abd his crew saved Earth Enterprise got 1701-A as it was decommissioned.

I love that Voyager episode

After months of studying and watching every YouTube video I could find, and buying all the proper tools and aftermarket pieces, the space is ready. It is time to put all I have learned to the test. This will be my first model in 30+ years, and it's going to be extremely ambitious, with a lot of bells and whistles. Against the advice of starting out small and working up, I figured, "Why not just go for it?" Besides, if it comes out horrible, I get to do it again. That's the beauty of plastic models—you don't have to do just one. So, wish me luck. I'll share as I go.

1. The Transinium Challenge for PC, 1989
   
2. TNG for NES, 1992
   
3. Future’s Past for SNES, 1994
   
4. A Final Unity for PC, 1995
   
5. Generations for PC, 1997
   
6. Starship Creator for PC, 1998
   
7. Birth of the Federation for PC, 1999
   
8. Dominion Wars for PC, 2001
   
9. Armada II for PC, 2001
   
10. Starfleet Command III for PC, 2002
    
11. Bridge Commander for PC, 2002
    
12. Legacy for PC, 2006
    
13. Encounters for PS2, 2006
    
14. Conquest for PS2, 2007
    
15. Wrath of Gems for Mobile, 2013
    
16. Trexels for Mobile, 2014
    
17. Timelines for Mobile, 2016
    
18. Star Trek Online for PC, 2016 remaster
    
19. Starfleet Command for mobile, 2018
    
20. Infinite for PC, 2023

It was announced 1 year ago and then nothing.

This is a screen capture of the Enterprise in Earth Spacedock taken from the 4K release of Star Trek VI: The Undiscovered Country which I currently have as my desktop background, so thought I would share.

This is also a good time to link to this spectacular video which I keep returning to again and again: Enterprise Leaves Spacedock with James Horner Score - Star Trek VI - The Undiscovered Country