Nuclear Technology: Fusion, radiation and the nuclear order

Pairs 1, 2 and 3 are correct. The becquerel (Bq) is the SI unit of activity: one decay per second. The gray (Gy) is absorbed dose: one joule of radiation energy absorbed per kilogram. The sievert (Sv) weights the absorbed dose by the type of radiation and the sensitivity of tissues, so it measures biological harm; public dose limits (1 mSv per year) are set in sieverts. Pair 4 is wrong: the curie (Ci) is an older unit of activity (3.7 × 10¹⁰ decays per second, the activity of one gram of radium-226), not of dose.

All three statements are correct. In food irradiation the food passes near a sealed gamma source (or an electron or X-ray beam) and never touches radioactive material; the energy used is too low to make atomic nuclei radioactive, so the food does not become radioactive. Cobalt-60 is the usual gamma source, supplied in India by BRIT. Low doses inhibit sprouting (potatoes, onions), medium doses kill insects and delay ripening, and higher doses destroy pathogens in spices. BARC's KRUSHAK plant at Lasalgaon (Nashik) irradiates onions, and FSSAI regulations allow irradiation of listed foods with labelling.

The answer is (b). BARC uses mutation breeding: seeds or plant parts are exposed to gamma rays (or other radiation) to induce random heritable mutations, and breeders then select and cross the useful mutants for traits such as yield, earliness, seed size or disease resistance. The 'Trombay' groundnut (TG), mustard, pulse and rice varieties have been released for cultivation this way. It is not genetic modification: no foreign gene is inserted, so these varieties are not regulated as GM crops. Option (d) describes food irradiation, which preserves the produce and creates no new variety.

Only statement 1 is correct. The usual PET tracer is FDG (fluorodeoxyglucose labelled with fluorine-18, half-life about 110 minutes), which collects in cells that use a lot of glucose, such as many cancers, active brain areas and inflamed tissue. Statement 2 is wrong: a positron is the antiparticle of the electron and travels only about a millimetre in tissue before meeting an electron; the two annihilate into a pair of 511 keV gamma photons flying in opposite directions, and it is these photons that the ring of detectors records. Short-lived isotopes such as F-18 are made in medical cyclotrons.

All four are natural sources. According to UNSCEAR, radon (a radioactive gas from the decay of uranium in soil and rocks) is the largest single source of natural exposure, typically about half. Cosmic rays add more at high altitude and during flights. Our bodies contain radioactive potassium-40 (in all potassium) and carbon-14 (formed in the atmosphere and taken up through food), which irradiate us from within. Terrestrial gamma rays from soil and building materials are the other component; coastal Kerala's monazite sands give some of the highest natural background levels in the world.

Statements 1 and 2 are correct. The first discharge began on 24 August 2023. ALPS filters out most radioactive elements, but tritium is part of the water molecule itself (as HTO) and cannot be filtered out, so TEPCO dilutes the water to below 1,500 becquerels per litre, about one-fortieth of Japan's regulatory standard, before releasing it through an undersea tunnel. Statement 3 is the reverse: the IAEA's comprehensive report of 4 July 2023 found the plan consistent with international safety standards, with negligible radiological impact on people and the environment; China and some neighbours still objected.

All three statements are correct. Chernobyl unit 4 (Ukraine, then USSR) was an RBMK, a graphite-moderated, light-water-cooled reactor with a design flaw that made power rise when coolant turned to steam; a test on 26 April 1986 led to a power surge, explosions and a graphite fire. At Fukushima Daiichi (Japan, 11 March 2011) the operating reactors shut down when the earthquake struck, but the tsunami flooded the emergency diesel generators; without cooling, decay heat melted three cores. The INES has seven levels, and both are the only accidents rated level 7 ('major accident').

Statements 2 and 3 are correct. Fusion is not a chain reaction: only a few grams of fuel are in the vessel at any time, and any disturbance lets the plasma cool and the reaction stop within seconds, so a Chernobyl-type runaway is not possible. Each D-T reaction gives a helium nucleus (3.5 MeV) and a 14.1 MeV neutron, about 80% of the 17.6 MeV total; the neutrons carry heat to the blanket and breed tritium. Statement 1 is wrong: fusion produces no long-lived high-level waste such as fission products and actinides; neutrons do activate structural materials, but ITER expects most of that waste to decay within about 100 years.