Unless shellfish are delivered to the consumer immediately upon completion of shucking, refrigeration facilities shall be provided capable of cooling the shucked shellfish to a temperature below 50 degrees Fahrenheit within two hours after the shellfish are shucked, and keeping them at this temperature until delivered to the consumer. If shucked shellfish are frozen, they shall be kept in a frozen condition until delivered to the consumer. The refrigerator or icebox shall have an impervious lining. The floor shall be graded to drain quickly. An accurate thermometer shall be kept in the refrigerator or icebox. Refrigerators shall be kept clean and sanitary at all times.