Fig. 1 shows the energy transfers during one hour in a nuclear-powered desalination plant on an island. Fission in the reactor heats water in a sealed prima...
Question 1 Report
Fig. 1 shows the energy transfers during one hour in a nuclear-powered desalination plant on an island. Fission in the reactor heats water in a sealed primary circuit. A heat exchanger produces steam to turn a turbine and generator. The electrical energy operates pumps that remove salt from seawater. Cooling water carries thermal energy away to the sea.
(a) State the energy store of the uranium fuel before fission. [1]
(b) Calculate the efficiency of the plant when producing electrical energy. [2]
(c) Calculate the mean electrical power output in kW. [3]
(d) Describe how control rods are used to change the power of the reactor. [2]
(e) Explain why the cooling water is kept in a separate circuit from the water that passes through the reactor vessel. [2]
Answer Details
(a) Uranium fuel initially has a nuclear energy store. [1]
(b) Efficiency is useful output energy divided by total input energy.
\[\text{efficiency}=\frac{840\text{ MJ}}{2400\text{ MJ}}=0.35=35\%\]
The efficiency is \(35\%\). [2]
(c) Convert energy and time to SI units:
\[840\text{ MJ}=840\,000\,000\text{ J}\]
\[1\text{ h}=3600\text{ s}\]
\[P=\frac{E}{t}=\frac{840\,000\,000}{3600}=233\,333\text{ W}\approx233\text{ kW}\]
The mean electrical power is \(233\text{ kW}\), with \(230\) to \(233\text{ kW}\) accepted. [3]
(d) Inserting control rods absorbs more neutrons and reduces the number of fissions. [1] Withdrawing the rods absorbs fewer neutrons, so the fission rate and reactor power increase. [1]
(e) Water in the reactor circuit can become radioactive. [1] A separate cooling-water circuit prevents radioactive water from being released into the sea and contains the radioactivity. [1]
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