Thyristor/SCR \$i^2t\$ and \$i^2\sqrt{t}\$ Rating
The \$i^2 t\$  is associated with the devices energy capability.

The maximum non-repetitive on-state surge current is generally quoted for one 10 millisecond sinusoidal. Such non-recurrent ratings are usually specified to allow fuse and circuit breaker short-circuit
protection. The I2t rating for a 10ms period is another parameter used for fuse protection, where I is non-repetitive rms current. 

However... This is only part of the story. When employed in a crowbar circuit, especially with a local DClink capacitor that will equally be rapidly discharged via the SCR, the energy capability of the SCR is closer to \$i^2 \sqrt t\$ 

The mathematical derivation of \$i^2 \sqrt t\$  can be seen in this SCR application notes.

https://edisciplinas.usp.br/pluginfile.php/2587503/mod_resource/content/2/Onsemi_Thyristor_Theory_and_Design_Considerations_Handbook_HBD855-D.pdf