Modern Physics Questions and Answers

India's three-stage nuclear power program was conceived by Dr. Homi Jehangir Bhabha and aims to eventually utilize India's vast thorium reserves to generate electricity. Stage-1 uses Pressurized Heavy Water Reactors (PHWRs) fueled by natural uranium. These reactors produce plutonium-239 as a byproduct from U-238. Stage-2 uses Fast Breeder Reactors (FBRs) fueled by plutonium-239 produced in Stage-1. These reactors breed more fuel than they consume and also convert thorium-232 into uranium-233. Stage-3 uses reactors fueled by uranium-233 produced from thorium in Stage-2. This allows India to use its enormous thorium reserves (world's largest at ~25%) to generate electricity for centuries. Option A is incorrect: The program is entirely for civilian nuclear power not weapons development. Option C is incorrect: India's program is not for reactor export but for domestic energy security using indigenous resources. Option D is incorrect: The three stages refer to fuel cycles not weapons testing. India has conducted nuclear tests separately.

The correct answer is Option B — 2 to 3 neutrons with an average of 2.5. When U-235 absorbs a slow neutron and undergoes fission it splits into two smaller nuclei (fission fragments) and releases approximately 2 to 3 fast neutrons along with enormous energy (about 200 MeV) and gamma radiation. The average is 2.5 neutrons per fission. These released neutrons when slowed by a moderator can go on to cause further fissions creating a chain reaction. Option A is incorrect: If only 1 neutron were released the chain reaction would barely sustain itself (k=1 at best). In practice due to neutron losses to moderators coolants and structural materials at least 2 neutrons must be released for practical reactors. Option C is incorrect: U-235 does not release 5 neutrons per fission event. The actual value is approximately 2 to 3. Option D is incorrect: Ten neutrons per fission is far from the actual value of 2.5. The 2.5 average means enough neutrons for chain reaction while still being controllable.

The correct answer is Option A — USA and Trinity Test 1945. The United States conducted the world's first nuclear weapon test codenamed Trinity Test on July 16, 1945, at the Trinity site in the Jornada del Muerto desert of New Mexico. The device tested was a plutonium implosion bomb called the Gadget. The test was part of the Manhattan Project directed by J. Just three weeks later atom bombs were dropped on Hiroshima and Nagasaki. Option B is incorrect: The Soviet Union conducted its first test called Joe-1 (RDS-1) on August 29, 1949. This made USSR the second nuclear state. It was in response to the USA nuclear monopoly. Option C is incorrect: The UK conducted its first test called Hurricane on October 3, 1952, at Monte Bello Islands Australia. This made UK the third nuclear state. Option D is incorrect: China conducted its first nuclear test called 596 (also called 059 by Chinese) on October 16, 1964, in the Lop Nur test site in Xinjiang. This made China the fifth nuclear state after USA USSR UK and France.

For a nuclear reactor to sustain a chain reaction two conditions are essential. First each fission of U-235 must release more than one neutron on average (U-235 releases approximately 2.5 neutrons per fission). This ensures that even if some neutrons are lost or absorbed there are enough remaining to trigger further fissions. Second the fast neutrons produced in fission must be slowed down to thermal energies by a moderator (heavy water, graphite, or ordinary water). Slow thermal neutrons have a much higher probability of being captured by U-235 nuclei and causing fission compared to fast neutrons. Option A is incorrect: High temperature and high pressure are requirements for nuclear fusion (in the Sun and hydrogen bombs) not for fission chain reactions in reactors. Fission reactors operate at moderate temperatures. Option C is incorrect: Heavy water is a moderator and lead is a radiation shield. While these may be present they are not the two defining conditions for self-sustained chain reaction. Option D is incorrect: 90% enrichment is for nuclear weapons not reactors. Reactors use only 3-5% enriched uranium.

The fundamental difference between a nuclear reactor and an atom bomb lies in whether the chain reaction is controlled or uncontrolled. In a nuclear reactor the chain reaction is carefully controlled using control rods (cadmium or boron) to maintain multiplication factor k equal to 1. Each fission triggers exactly one more fission producing steady continuous energy used for electricity generation. In an atom bomb there are no control rods or moderators. The multiplication factor k is much greater than 1. Option A is incorrect: Both atom bombs and nuclear reactors primarily use fission not fusion. The hydrogen bomb uses fusion triggered by a fission bomb. Option C is incorrect: Both reactors and bombs can use U-235. What differs is the enrichment level and whether the reaction is controlled. Option D is incorrect: Physical size is irrelevant to the fundamental difference. Critical mass determines bomb feasibility not reactor vs bomb distinction.

The correct answer is Option B — Cold water and Liquid Sodium. In nuclear reactors a coolant is used to transfer heat produced by nuclear fission away from the reactor core. The most common coolants are ordinary water (H2O) or heavy water (D2O) in water-cooled reactors and liquid sodium in fast breeder reactors. The coolant absorbs heat and carries it to heat exchangers where steam is produced to drive turbines. Option A is incorrect: Graphite is used as a moderator to slow down neutrons not as a coolant. Cadmium is used in control rods to absorb neutrons. Neither absorbs and transfers heat efficiently as a coolant. Option C is incorrect: Boron is used in control rods. Lead is used as radiation shielding. Neither is used as a primary coolant for heat removal in reactors. Option D is incorrect: Helium and argon are noble gases. While helium is used as coolant in some high-temperature gas-cooled reactors, argon is not used as nuclear reactor coolant. The primary coolants for most reactors are water and liquid sodium.

The correct answer is Option C — Calcium. Calcium is a common alkaline earth metal (atomic number 20) that has no fissile or fertile properties. It cannot sustain a nuclear chain reaction and is therefore NOT used as nuclear fuel. Calcium is used in biological systems (bones, teeth) and in metallurgy. Option A is incorrect: Uranium is the most widely used nuclear fuel. U-235 is fissile (can sustain chain reaction with slow neutrons) and U-238 is fertile (can be converted to Pu-239). Natural uranium contains 99.3% U-238 and 0.7% U-235. Option B is incorrect: Plutonium-239 is a fissile material produced in reactors from U-238. It is used as fuel in fast breeder reactors. India's PFBR at Kalpakkam will use Pu-239 as fuel. Option D is incorrect: Not all are fuels because Calcium cannot be used as nuclear fuel. Nuclear fuels include fissile (U-235, Pu-239, U-233) and fertile (Th-232, U-238) materials.

The correct answer is Option C — Cadmium. Cadmium is NOT used as nuclear reactor fuel. Instead it is used to make control rods in nuclear reactors. Cadmium has an extremely high neutron absorption cross-section meaning it captures neutrons very efficiently. Control rods made of cadmium or boron are inserted into the reactor to absorb excess neutrons and control the rate of chain reaction. Option A is incorrect as a distractor: Deuterium (heavy hydrogen) is used in fusion reactors as fuel and heavy water (D2O) containing deuterium is used as moderator in fission reactors. Option B is incorrect as a distractor: Uranium (specifically U-235) is the primary fuel in most nuclear fission reactors worldwide. Natural uranium has 0.7% U-235. Option D is incorrect as a distractor: Plutonium-239 is produced in reactors from U-238 and is used as fuel in fast breeder reactors like India's PFBR at Kalpakkam. Boron is also used for control rods along with cadmium.

The correct answer is Option D — Kamini at IGCAR Kalpakkam. Kamini (Kalpakkam Mini Reactor) was established in 1996 at Indira Gandhi Centre for Atomic Research (IGCAR) Kalpakkam Tamil Nadu. It is the only reactor in the world that uses Uranium-233 as fuel. Uranium-233 is produced from Thorium-232 in other reactors. Kamini uses light water as moderator. Option A is incorrect: Apsara is located at BARC Mumbai and uses enriched uranium not uranium-233. Option B is incorrect: Dhruva is at BARC Mumbai and uses natural uranium with heavy water moderator. Dhruva is India's largest research reactor at 100 MW thermal power. Option C is incorrect: CIRUS was at BARC Mumbai and used natural uranium with heavy water moderator. CIRUS was shut down in 2010. Kamini proves India's capability to use thorium-uranium-233 fuel cycle which is significant given India's vast thorium reserves.

The correct answer is Option C — Purnima-1 (1972). Purnima-1 (Plutonium Reactor for Neutron Investigation in Multiplying Assemblies) was established in 1972 at BARC Mumbai. It was the first fast neutron research reactor in India and used plutonium oxide as fuel and light water as moderator. It achieved criticality on May 18, 1972. Option A is incorrect: Apsara (1956) used enriched uranium as fuel and light water as moderator. It was India's first reactor not Purnima-1. Option B is incorrect: CIRUS (1960) used natural uranium as fuel and heavy water as moderator. It was Canada-assisted reactor. Option D is incorrect: Dhruva (1985) uses natural uranium as fuel and heavy water as moderator. It is India's largest indigenous research reactor with 100 MW thermal power. Purnima-1 was shut down and upgraded to Purnima-2 in 1984 which used Uranium-233 as fuel.