The concept of public interest is not defined anywhere in federal or provincial legislation. Court rulings have clarified that the term “public interest” is always contextualized in the legislation in question.
In the example you gave, the minister would have to be assured that there is an analysis of the costs and benefits, that risks are identified and mitigated, and consultations have been undertaken with the other policy makers and stakeholders, such as indigenous peoples.
There are also requirements for transparency. The intent behind the regulatory sandbox must be published in regulatory plans. When a sandbox is launched, it must be posted on the website. Lastly, the President of the Treasury Board must prepare an annual report listing all regulatory sandboxes.
That would also apply to the example of a nuclear reactor.