In financial theory the "risk free rate" is usually proxied as the yield on government bonds. Since the US government is able to print its own currency, it will not ever need to default on its own debt which is denominated in US dollars.
Short of world going dark, you will get your principal and interest back.
Riskless? Not quite. If the total dollars you get back buy less than they used to in terms of goods and services you have lost wealth in real terms.
Investing firm Newfound Research has a mantra:
Risk cannot be destroyed, but only transformed.
This is clear when you consider the trade-off you must always face —the possibility of "failing slow" or "failing fast".
When you are young, you risk "failing slow":
The primary risk of investors with growth mandates (e.g. investors early in their lifecycle) is “failing slow,” which is the failure to grow their capital sufficiently to outpace inflation or meet future liabilities. In this case, our aim should be to diversify as much as possible without overly de-risking the portfolio.