Nuclear Technology: Nuclear basics and reactors

The answer is alpha particles. An alpha particle is a helium nucleus (two protons and two neutrons): it is heavy and doubly charged, so it loses its energy quickly by ionising the atoms it passes and is stopped by paper or the outer dead layer of skin. Beta particles (electrons or positrons) need a few millimetres of aluminium, and gamma rays (high-energy photons) need thick lead or concrete. The flip side is that alpha emitters are most dangerous when swallowed or inhaled, which is why polonium-210 and radon are internal hazards.

Both statements are correct. The Sun's energy comes from thermonuclear fusion: in its core (about 15 million kelvin) hydrogen nuclei are fused, step by step, into helium, and the small loss of mass appears as energy. Nuclei are positively charged, so they repel each other (the Coulomb barrier); only at very high temperatures do they move fast enough to come within the very short range of the strong nuclear force and fuse. That is why fusion on Earth needs plasma at well over 100 million degrees in devices such as tokamaks.

Statements 2 and 3 are correct. Radioactive decay is a property of the nucleus and is not changed by ordinary heat, pressure or chemical combination, so statement 1 is wrong: the half-life is fixed for an isotope. Each half-life halves the remaining nuclei: 1/2, 1/4, then 1/8 after three half-lives. Carbon-14 (half-life about 5,730 years) is useful only up to roughly 50,000 years, after which too little remains; billion-year-old rocks are dated with long-lived isotopes such as uranium-238 or potassium-40.

Three are fissile. Uranium-235 (the only fissile nuclide found in useful quantity in nature), plutonium-239 and uranium-233 split readily when they absorb slow neutrons. Thorium-232 is fertile, not fissile: on absorbing a neutron it turns, through thorium-233 and protactinium-233, into fissile uranium-233. In the same way uranium-238 is fertile and breeds plutonium-239. This fertile-to-fissile conversion is the logic of India's three-stage programme.

Statements 1 and 3 are correct. Formulated by Homi J. Bhabha, the programme links India's modest uranium and large thorium resources. Stage 1: PHWRs burn natural uranium and convert part of the uranium-238 into plutonium-239. Stage 2: fast breeder reactors (PFBR at Kalpakkam) use plutonium-based mixed oxide fuel in the core; a blanket of uranium-238 breeds more plutonium than is consumed, and thorium blankets can later breed uranium-233. Statement 2 is wrong because thorium in stage 2 sits in the blanket to be converted, not in the core as the main fuel. Stage 3: reactors based on the thorium–uranium- 233 cycle.

Statements 1 and 2 are correct. The PFBR is a 500 MWe sodium-cooled, pool-type fast breeder designed by IGCAR and built by BHAVINI, a DAE public sector company set up for fast reactors. The Prime Minister witnessed the start of core loading on 4 March 2024, and the reactor attained first criticality on 6 April 2026 at 8:25 PM after AERB clearance (DAE and PIB, 7 April 2026). Statement 3 is wrong: a fast reactor has no moderator at all, because it needs fast neutrons; the PFBR uses uranium-plutonium mixed oxide fuel and liquid sodium as coolant.

Statements 1 and 2 are correct. NCERT's binding-energy curve rises steeply for light nuclei, peaks at about 8.8 MeV per nucleon near iron-56, and falls slowly for heavy nuclei such as uranium. Moving towards the peak from either side gives more tightly bound products, so both fission of heavy nuclei and fusion of light nuclei release energy. Statement 3 is the reverse of the truth: the products are slightly lighter, and this mass defect appears as energy according to E = mc².

Statements 1 and 2 are correct. Gamma rays and X-rays are both high-energy photons, but they are named by origin: gamma rays come from an excited nucleus, while X-ray tubes produce X-rays when fast electrons are decelerated or when inner-shell electrons change orbits. In beta-minus decay a neutron changes into a proton, an electron and an antineutrino, so Z rises by one (carbon-14 becomes nitrogen-14). This decay is driven by the weak nuclear force. Statement 3 is wrong: a free neutron is unstable and decays in about fifteen minutes on average (half-life about ten minutes).