4.4.5 PWR wet steam plant
The reheater is not part of the boiler. This function is carried out by live steam from the reactor and bled-steam from the HP turbine in vessels mounted adjacent to the turbine. There is therefore no requirement for separate HP and LP by-passes to permit independent operation of the reactor and steam generators.
A single by-pass circuit to dump HP stop valve inlet steam to the condenser is employed. Warm-through of the reheater is accomplished as part of the turbine start-up. Rapid starting and utilisation for two-shifting is not the role of PWR plant.
The reasons for the supply of by-pass systems on PWR plant are therefore:
- (a) To enable the nuclear plant to accept transients without tripping or actuating safety valves.
- (b) To remove stored energy and residual heat following a turbine and reactor trip to bring the plant to equilibrium.
- (c) To permit a manually controlled cooldown of the plant to the point where the residual heat removal system can be put into operation.
- (d) Following a turbine trip or shutdown, to maintain the reactor and steam generators in a hot-standby condition, ready for turbine restarting.
A diagram of a typical by-pass system configuration is shown in Fig 1.80. The main by-pass is through six parallel sets of control valves into the three condenser sections. Valves 1, 3 and 5 are modulating valves used for all three functions but sized to be capable of performing the cooldown duty (c) on their own. Valves 2, 4 and 6 are capable of being switched open or closed and are only used for duties (a), (b) and (d). An additional dump to atmosphere is also provided for duty (d), since infrequent discharges to atmosphere are permissible for short durations. This reduces the duty on the condenser, permitting considerable economies to be made. Thus, in the event of a turbine-generator trip, valves 1, 3 and 5 are opened first, followed by valves 2, 4 and 6 and finally, if necessary, the atmospheric dump valve. For one reactor, the total capacity is arranged to be approximately 70% of the full MCR steam flow with about 10% being accommodated by the atmospheric dump valve. In association with further relief valve capacity in the reactor system, this permits a trip of the total turbine-generator capacity. A reactor trip only requires a by-pass capacity of 40% and is accomplished without discharge to atmosphere.
Both condenser and atmospheric sets of valves are required to open fast, particularly for duties (b) and (d). The opening time of approximately three seconds permits containment of the steam pressure rise within the settings of the atmospheric valves for the reactor trip case. The valve closing rates are designed to protect the turbine and condenser in the event of a system failure. A closure time of 5 seconds is provided to protect against high condenser/turbine exhaust pressures in the event of a condenser cooling water failure. The effects on other plant are similar to those discussed for superheat plant and are taken into account in the design.
The provision of sufficient by-pass capacity to avoid tripping the reactor in the event of a turbine-generator trip justifies the by-pass on economic grounds. If the reactor were allowed to trip for a turbine problem lasting for perhaps only 30 minutes, there would be a severe availability penalty.
The problems of implementing a by-pass system are addressed in Chapter 2. These matters are properly dealt with at the design stage and result in systems which provide economic benefits by achieving better co-ordination between the turbine and boiler systems.