Final answer:
No valve is allowed between the safety valve and the boiler or on the discharge pipe to the atmosphere at any time, due to safety regulations. The correct answer is D.
Step-by-step explanation:
The question posed concerns the placement of valves in relation to safety valves and the boiler in a steam generation system. Namely, it addresses the prohibition of placing any valve between the safety valve and the boiler, as well as on the discharge pipe between the safety valve and the atmosphere. This safety principle is part of boiler safety regulations to ensure that the steam pressure within the boiler can be relieved in case of excess pressure, without being obstructed by additional valves which could compromise the system's integrity and safety.
The correct answer to the question is d) at any time. This means no valve of any description is allowed to be placed between the safety valve and the boiler, nor on the discharge pipe between the safety valve and the atmosphere, under any circumstances. This is a critical safety measure to prevent overpressure accidents and ensure that the steam and heated water systems, especially in nuclear plants like the one described, where water within the reactor is radioactive and separate from the nonradioactive water in the condenser and steam generator, operate safely.
In nuclear reactors, maintaining proper coolant levels is essential to prevent a loss of coolant accident. If the reactor's water level drops and exposes the fuel, the chain reaction is halted, but decay heat from fission products can still generate significant heat. Thus, the system relies on auxiliary cooling water and pumps as additional safety features. It is critical that these systems are designed without any impediments, such as unnecessary valves, that could jeopardize their function during emergency scenarios.