The short answer to your question is that the calculator key for a square root symbol is [simbraille]$k>][/simbraille]
For information about transcribing calculator and computer keys, please see the “Nemeth 2007 update”, which is available from BANA’s website at this [url=http://brailleauthority.org/mathscience/math-science.html]link[/url], under the heading Nemeth Updates 2007 – information about transcribing calculator and computer keys is on pages N9-N11.
Thank you for your question.