NDA (I) 2018 General Ability Test · Question 107 of 150

Correct — A, Potassium. Sodium and potassium together form the alloy known as NaK, whose eutectic composition is about 77 per cent potassium and 23 per cent sodium by mass and which stays liquid from roughly −12·6 °C all the way to 785 °C. That enormous liquid range is the whole point. Liquid metals conduct heat far better than water and can be run at high temperature without needing to be pressurised, which is why fast neutron reactors use them as coolants; but pure sodium freezes at about 98 °C, so a reactor cooled with it must be kept warm even when shut down. NaK is already liquid at room temperature, so a reactor that is frequently shut down and defuelled — as experimental ones are — can use it without trace heating. It cooled Britain's Dounreay Fast Reactor and the American SNAP-10A space reactor.

• (b)Calcium — Calcium is a group 2 metal with a melting point above 800 °C and it does not form a low-melting liquid alloy with sodium. Its industrial role is as a reducing agent and in cement and steel chemistry, not as a heat-transfer fluid.
• (c)Magnesium — Magnesium melts at about 650 °C. It does appear in reactor engineering, but as a cladding material — the magnesium alloy Magnox gave a generation of British gas-cooled reactors their name — not as a liquid coolant alloyed with sodium.
• (d)Strontium — Strontium is associated with reactors for an entirely different reason: strontium-90 is a notorious fission product that behaves like calcium in bone. It has no role as a coolant, and its melting point of about 777 °C rules it out anyway.

A reactor needs three distinct materials and confusing them is the commonest error in this topic. A moderator slows fast neutrons so that fission continues — heavy water, graphite or ordinary water. A coolant carries heat from the core to the steam generator — water, carbon dioxide, helium or a liquid metal. Control rods of cadmium or boron absorb neutrons and shut the reaction down. Liquid sodium and NaK are coolants only; they are poor moderators because sodium's nucleus is too heavy to take much energy from a neutron in a collision, which is precisely what a fast reactor wants.

The chemistry behind the answer is worth holding on to. Alloys frequently melt lower than either of the metals in them, because mixing two different-sized atoms disrupts the neat crystal lattice each would form alone; the lowest-melting mixture of all is called the eutectic. Sodium melts at 98 °C and potassium at 63 °C, yet the eutectic mixture of the two is a liquid below freezing point. India's own fast-reactor programme, the second stage of the three-stage nuclear plan, uses liquid sodium: the Fast Breeder Test Reactor at Kalpakkam has run on it since 1985 and the Prototype Fast Breeder Reactor at the same site is designed around sodium coolant. Candidates should note that the question is about the alloying partner, so the answer is potassium even though sodium is the metal the sentence starts from.

• NaK, the sodium-potassium alloy, has a eutectic of about 77 per cent potassium and 23 per cent sodium by mass and is liquid from about −12·6 °C to 785 °C.
• Liquid metals are chosen as coolants for fast reactors because they conduct heat well and need no pressurisation.
• A moderator slows neutrons, a coolant removes heat and control rods absorb neutrons — three separate jobs.
• Alloys often melt below either pure metal; the lowest-melting composition is called the eutectic.

• Naming heavy water when asked for a coolant — it is used as a moderator.
• Answering 'sodium' because the stem mentions it; the question asks for the alloying partner.
• Assuming a group 2 metal will behave like a group 1 one in an alloy.

NDA and UPSC both ask which material does which reactor job, usually with one true statement buried among plausible chemical claims.

• practice — not a real PYQ

  The main reason for using a liquid metal rather than water as the coolant in a fast breeder reactor is that the liquid metal

  • (a)slows down neutrons efficiently
  • (b)conducts heat well and needs no high pressure to stay liquid at high temperature
  • (c)absorbs neutrons and controls the reaction
  • (d)is cheaper than water

  Answer(b) conducts heat well and needs no high pressure to stay liquid at high temperature — and, unlike water, it does not slow the fast neutrons the reactor depends on.

• practice — not a real PYQ

  The eutectic alloy NaK used in some reactors is composed of

  • (a)sodium and calcium
  • (b)sodium and potassium
  • (c)sodium and krypton
  • (d)nickel and potassium

  Answer(b) sodium and potassium — roughly 77 per cent potassium to 23 per cent sodium by mass.