Read and complete the text using the phrases below.

NUCLEAR FUSION

[1] All elements around us, including those in our bodies, are the product of nuclear fusion. Fusion powers stars like the Sun, within which all the elements heavier than hydrogen are cooked up. We really are made of stardust. If we can harness the stars’ power on Earth, fusion [1 __].

[2] Nuclear fusion is the merging together of light atomic nuclei to form heavier ones. When pressed together hard enough, hydrogen nuclei can merge to produce helium, giving off energy – a great deal of energy – in the process. Gradually, by building up heavier and heavier nuclei through a series of fusion reactions, all the elements that we see around us can be created from scratch.

[3] Fusing together even the lightest nuclei, such as hydrogen, is tremendously difficult. Enormous temperatures and pressures are needed, so [2 __], like the Sun and other stars. For two nuclei to merge, the forces that hold each one together must be overcome. Nuclei are made up of protons and neutrons locked together by the strong nuclear force. The strong force is dominant at the tiny scale of the nucleus, and is much weaker outside the nucleus. Because protons are positively charged, their electrical charges repel one another, so pushing each other apart slightly as well. But the strong force glue is more powerful so the nucleus holds together.

[4] Because the strong nuclear force acts over such a short precise range, its combined strength is greater for small nuclei than for large ones. For a weighty nucleus, such as uranium, with 238 nucleons, the mutual attraction will not be as strong between nucleons on opposite sides of the nucleus. The electric repulsive force, on the other hand, [3 __] and so becomes stronger for large nuclei because it can span the whole nucleus. It is also boosted by the greater numbers of positive charges they contain. The net effect of this balance is that the energy needed to bind the nucleus together, averaged per nucleon, increases with atomic weight up to the elements nickel and iron, which are very stable, then drops off again for larger nuclei. So fission of large nuclei happens relatively easily as they can be disrupted by a minor knock.

[5] For fusion, the energy barrier to overcome is least for hydrogen isotopes that contain just a single proton. Hydrogen comes in three types: ‘normal’ hydrogen atoms contain one proton surrounded by a single electron; deuterium, or heavy hydrogen, has one proton, one electron and also one neutron; tritium has two neutrons added, so it is even heavier. The simplest fusion reaction therefore is [4 __] a lone neutron. Although it is the simplest, scorching temperatures of 800 million kelvins are needed to ignite even this reaction.

[6] On Earth, physicists are trying to replicate these extreme conditions in fusion reactors to generate power. However, they are decades [5 __]. Even advanced fusion machines take in more energy than they give out, by orders of magnitude. Fusion power is the holy grail of energy production. (A) Very few atoms are needed to produce huge amounts of energy, there is little waste and certainly nothing as nasty as ultraheavy elements that come out of fission reactors. (B) Fusion power doesn’t produce greenhouse gases either, promising a self-contained, reliable source of energy assuming its fuel, hydrogen and deuterium, can be manufactured. (C)

a. Off from achieving this in practice;

b. Is still felt at larger separations;

c. Could even be the key to unlimited clean energy;

d. The combination of hydrogen and deuterium to form tritium plus;

e. Fusion only happens naturally in extreme places.

Look at the three lettered spaces in the text (A), (B), (C) that indicate where the following sentence can be added to the passage. Where would the sentence best fit?

Compared with fission technology, fusion reactions are relatively clean and, should they work, efficient.

4. Read the text again and find the words that mean the same as the following phrases.

a. used to describe actions that affect two or more things equally;

b. to succeed in dealing with or controlling something;

c. a range of levels or numbers used for measuring something;

d. closely connected with something;

e. made something increase, or become better;

f. made it difficult for something to continue in the normal way;

g. to become fewer or less;

h. to control and use the force or strength of something to produce power or achieve something;

i. to start or make something to burn;

j. to combine or make two or more things to form a single thing.