An island council is choosing a replacement for an ageing diesel power station. Fig. 3 compares two possible designs. Both designs use moving steam to rotat...
Question 1 Report
An island council is choosing a replacement for an ageing diesel power station. Fig. 3 compares two possible designs. Both designs use moving steam to rotate a turbine, then a generator produces an electric current for homes. The council also needs to consider waste, fuel deliveries and the effect of each option on the atmosphere. In the fission design, a small fuel assembly remains in the reactor for several years. In the diesel design, fuel ships must arrive every week.
A proposed fission station would use 18 kg of fuel each year. This fuel contains 1.4 × 1015 J of usable energy. The island needs 7.0 × 1014 J of electrical energy each year.
(a) Describe two features of electricity generation that are common to both designs in Fig. 3. [2]
(b) Which design releases carbon dioxide directly while generating electricity? Give one reason why this can affect Earth’s climate. [2]
(c) Explain why radioactive waste from the fission station needs careful storage after it has been removed from the reactor. [3]
(d) Calculate the minimum overall efficiency needed for the proposed fission station to meet the island’s annual electrical-energy demand. [3]
(e) Give two factors, other than efficiency, that the council should consider before selecting the fission design. [2]
Answer Details
(a) Both designs heat water to make steam, and the steam turns a turbine. In each case the turbine drives a generator to produce an electric current. Any two of these common features gain credit. [2]
(b) The diesel station releases carbon dioxide directly while generating electricity because it burns diesel fuel. Carbon dioxide is a greenhouse gas: it absorbs outgoing infrared radiation, increasing the greenhouse effect and contributing to global warming. [2]
(c) Radioactive waste emits ionising radiation. This can damage living cells and may cause mutations or cancer. The waste must therefore be shielded and stored securely for a long time, until its activity has fallen sufficiently. [3]
(d) \[\text{efficiency}=\frac{\text{useful electrical energy output}}{\text{energy input}}=\frac{7.0\times10^{14}\text{ J}}{1.4\times10^{15}\text{ J}}=0.50\]
The minimum required overall efficiency is 0.50, or 50%. [3]
(e) Two relevant factors are the cost of construction or decommissioning and arrangements for safe disposal or storage of radioactive waste. Other acceptable factors include accident risk, trained workers, security of fuel supply and public acceptance. [2]
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