Humans are still relatively backward, but they are making progress. Technologys traded to them by the Andalites , bought from the Hawjabran , and scavanged from a crashed Anati cruiser are being hybredized with Earth tech.
Earth has used nuclear power for around 60 years or so, and with access to alien technology their reactors have been greatly improved. They are still laughed at by races like the Andalites however. They use enriched Uranium-235.
The super shield is made by almost overloading the standard shield emmiters of Terran vessels. It makes the ship far more resistant to damage for a time, but can drain the powerplants of less advanced Terran vessels, resulting in the ship being quite vulnerible and they must be refueled at a station or shipyard.