Is a superconductor 'perfect'? i.e. does current flow forever without decaying? In a (say) circular superconducting loop which has a current initially induced in it, and without any further external influences, and at a temperature above 0 K and below the transition temperature, will the current flow forever (e.g. years ?).
Are there imperfections in practical superconductors (e.g. YBCO) that actually cause the superconductivity to not be exactly zero? Are there other loss mechanisms?
(Edit) to clarify the mechanisms I am wondering about:

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*The carriers (pairs) are circulating around the loop (do they all go in the same direction (at the same speed), or do some (N+X) go clockwise and others (X) go anti clockwise to have a net of just $N $?). Since the carriers are localized, they will have a small effect on the generated magnetic field and flux. Won't these temporal variations in $B$ cause electromagnetic radiation and therefore a loss of energy?


*Notwithstanding @Stanislav's comment, at $T > 0 K$, there is a distribution of thermal energy levels in all the particles in the system; couldn't some of the 'high' energy particles destroy the superconductivity briefly locally? And would this cause a loss of energy?


*Even a metallic superconductor has grain boundaries; do these cause no degradation in the superconductivity?


*A ceramic superconductor can be quite non-uniform at the molecular scale. Does this make superconducting not 'perfect'?
I am asking these because while I understand superconductors conduct extremely well and usefully for practical applications, a superconductor with $R=0$ is quite a precise value of $R$ and perhaps there are actually loss mechanisms that might show up and be detectable over long time scales (years?).
 A: A supercurrent flows forever, since electron pairs in a superconductor form the Bose-Einstein-Condensate (BEC). In the BEC every boson has a minimum and quantized kinetic energy and, thus, cannot transfer their energy to other particles by arbitrarily small portions. So the bosons flow as long as there is no external energy exceeding the quant of the boson energy.
Answer to point 2 (couldn't some of the 'high' energy particles destroy the superconductivity briefly locally ?). External 'high' energy particles can destroy the pairs briefly locally. In equilibrium, kinetic energies of internal particles are stationary (like energies in molecules and atoms), that is the high kinetic energies are linked to zero total momentum of every particle. Therefore observable excitations are only a few kT, which cannot destroy pairs below Tc. Note, the thermal distributions at low Tc are rather quantized than smooth, so thermal excitations are rather a few kT than much larger values.
A: The basic idea is that for a superconductor the state of least energy can have a non-zero current. If this is already a state of least energy then there is no further lower-lying state for it to decay to. Loss mechanisms (in the sense of energy loss) therefore do not matter. So if the material is not subject to some external shock such as something falling on it or heating it up then yes the current can flow forever without the need for any energy input.
In practice such a current will not last forever because of other considerations: the material might be heated up because it was left in an ordinary room and the refrigeration failed owing to a power cut, or maybe it gradually underwent chemical reactions or a change owing to natural radioactivity, etc.
A: You say "temperature above 0K", however any superconductor losses the superconductivity properties above a certain temperature. If the material is a superconductor and you induce a current initially, then the current will flow for a transient time. Then the temperature may change the material because of the random generation of rogue waves. Then if the system is large enough to neglect this molecular change, then the current is unaffected, if is not large enough, then the current may decrease.
A: Sometimes it helps to think unconventionally. Let's assume that the electric current not only generates a magnetic field, but also aligns the magnetic dipoles of the subatomic particles with this macroscopic field. We keep the electric current small and thus do not disturb the cryogenic cooling. After a certain time, the magnetic dipoles of many subatomic particles are aligned in such a way that a self-holding (like a permanent magnet) occurs. Now we can switch off the circuit.
What can happen? If the temperature increases, the thermal movements of the subatomic particles and (Currie temperature) predominate and the magnetic dipoles move chaotically, the common magnetic field collapses and a Lenz current destroys the coil.
What else can happen? An external magnetic field brings the magnetic field of the ultracold coil out of its self-hold, a Lenz current occurs and destroys the coil.
What else argues for the self-holding of aligned magnetic dipoles rather than a lossless orbiting stream of Cooper electrons? In the production of superconducting wires, its difficult-to-roll material is embedded in copper and then rolled into micrometre-fine wires. In the process, it cannot be ruled out at all that the superconducting material forms continuous conductors at all. Nor is this important. What is important is to wind materials with unpaired electrons, which tend easily to self-hold the induced magnetic field, into magnetic coils and to minimise the thermal movements of the unpaired electrons.
