11.2 By-pass systems for nuclear plant
A by-pass system is important in a nuclear plant for the following reasons:
- Sudden large load rejection can be accepted without incurring a reactor trip or actuation of the steam generator safety valves. (The reactor control system will accommodate either a sudden load rejection of 10% of full-load or a gradual rejection of 5% per minute. Load rejection beyond these figures results in the need for system by-pass.)
- Stored energy and residual heat can be removed following a turbine or reactor trip. This will bring the plant to no-load conditions without the actuation of the steam generator safety valves.
- The plant can be maintained at hot standby conditions.
- Manually-controlled cooldown of the plant can be achieved to the point when the 'residual heat removal system' can be operated.
Figure 2.85 shows the arrangement of the by-pass system for a nuclear plant.
The by-pass consists of a steam line that begins at the main steam manifold and has three pairs of lines branching off it to the three condenser shells via the isolation/control/isolation valve arrangement and the dump tubes. For each pair of lines, one line is designated 'Bank 1' and is used in a modulating mode so that the required conditions can be regulated. The other line is designated 'Bank 2' and is either open or closed.
There is another bank on a separate line from the manifold (not shown in Fig 2.85) which exhausts directly to atmosphere. This 'Bank 3' is used when there is a large loss in load or a reactor trip. Once steam temperature has reduced sufficiently (usually after a few minutes), the 'Bank 3' valves are closed.
The by-pass pressure control valves may be operated either pneumatically or hydraulically; but a pneumatically-controlled system will be discussed here.
When load is rejected, a load rejection controller opens all the by-pass valves (Banks 1, 2 and 3) sequentially in about three seconds. The controller receives a signal obtained from the difference between the actual and the programmed reactor-coolant temperature. The controller subsequently closes the valves when load is regained.
The plant trip controller opens a proportion of the pressure control valve when the reactor trips and subsequently reduces the opening to suit the reactor decay heat generation. The controller receives a signal obtained from the difference between the actual primary average temperature and the no-load primary temperature.
The steam header controller operates the first valve bank of pressure control valves in two different modes of operation, depending upon whether the plant is required to be on hot standby or cooldown. In the first mode, the 'Bank 1' valves are modulated to maintain a constant steam header pressure. This enables the plant to be automatically maintained at the hot shutdown condition. It is also used to maintain steady conditions while the turbine-generator is being synchronised. In the second mode, the operator controls the signal to the valve directly. This permits the operator to open the 'Bank 1' valves progressively to cool the plant down.
Each of the spraywater control valves opens to deliver a preset minimum flow whenever its associated steam by-pass control valve is not closed. Thereafter the spraywater flow is arranged to be directly proportional to the steam pressure measured in the dump tube, after the control valve downstream isolating valve and before the first orifice plate. As the flow through the dump tube is critical, the spraywater flow is proportional to the steam flow.
In the event of a loss in electrical power or air supply, the pressure control valves will fail-closed. The spraywater control valves, however, remain fixed in their last held position and control is regained by using remote manual operation. The pressure control valves will also fail closed when the condenser pressure is too high. The by-pass system carries alarms to warn of low spraywater pressure and high dump tube steam temperature.