Modern Physics Questions and Answers – Page 2

The correct answer is Option B — 1960 and Natural Uranium. CIRUS (Canada India Reactor Utility Services) was established in 1960 at BARC Mumbai with Canadian assistance. It used natural uranium as fuel and heavy water (D2O) as moderator. CIRUS had a thermal power of 40 MW and played a crucial role in India's nuclear research including production of plutonium used in India's first nuclear test in 1974. Option A is incorrect: 1956 and enriched uranium describes Apsara reactor which was India's first reactor not CIRUS. CIRUS used natural uranium not enriched uranium. Option C is incorrect: 1972 and plutonium oxide describes Purnima-1 reactor which was India's third major reactor. It was India's first fast neutron reactor. Option D is incorrect: 1985 and natural uranium describes Dhruva reactor which is India's largest research reactor. Dhruva also uses heavy water as moderator but was established 25 years after CIRUS.

The correct answer is Option B — 1956. Apsara was India's first nuclear research reactor established in August 1956 at BARC (Bhabha Atomic Research Centre) Trombay Mumbai. It was a swimming pool type reactor using enriched uranium fuel and ordinary water as moderator. It was built with British assistance. Prime Minister Jawaharlal Nehru named it Apsara. Option A is incorrect: 1947 is the year of India's independence. There was no nuclear reactor in India at that time. The nuclear programme was just beginning under Homi Bhabha. Option C is incorrect: 1972 is the year Purnima-1 reactor was established at BARC which used plutonium oxide as fuel. It was India's first fast neutron reactor not the first nuclear reactor. Option D is incorrect: 1985 is the year Dhruva reactor was established at BARC. It is India's largest research reactor with 100 MW thermal power.

The correct answer is Option C — Less than the total mass. In nuclear fusion, the product nucleus has slightly less mass than the sum of masses of the individual nuclei. This mass difference called mass defect is converted into energy according to E = mc². This is why nuclear reactions release enormous energy. Option A is incorrect: If mass were equal, no energy would be released because E = mc² with zero mass defect gives zero energy. The energy released in fusion proves mass is lost. Option B is incorrect: Mass cannot increase in fusion. If mass increased that would mean the reaction absorbs energy which contradicts observed energy release. Option D is incorrect: Double the total mass is physically impossible as it would require creation of matter from nothing violating conservation of mass-energy. Even a tiny mass defect produces huge energy because c squared is enormously large (9 x 10^16 m^2/s^2).

The correct answer is Option B — Another name for nuclear reactor. The term Atomic Pile was the original name given to the first nuclear reactor, Chicago Pile-1, built by Enrico Fermi in 1942. It was called a pile because the reactor was literally constructed by piling up layers of uranium and graphite blocks alternately. Today the term nuclear reactor is more commonly used. Option A is incorrect: Radioactive waste refers to byproducts of nuclear reactions and is stored in special containers — it has nothing to do with an atomic pile which is a functional device for energy production. Option C is incorrect: Storage tanks for nuclear fuel are separate facilities. Atomic pile is an active device where controlled fission takes place, not just storage. Option D is incorrect: Devices to measure radiation include Geiger-Muller counters, dosimeters, and scintillation counters. An atomic pile produces energy through nuclear fission and does not measure radiation. The first atomic pile used 50 tonnes of natural uranium, 385 tonnes of graphite blocks as moderator, and cadmium control rods.

The correct answer is Option C — Heavy water and Graphite. In nuclear reactors, fast neutrons released during fission must be slowed down to thermal speeds to efficiently cause further fission in U-235. A moderator is used for this purpose. Heavy water (D₂O) and graphite (carbon) are the most commonly used moderators. They slow neutrons without absorbing them significantly. Option A is incorrect: Cadmium is used as a control rod material, not a moderator. Cadmium absorbs neutrons very efficiently, and control rods made of cadmium are inserted into the reactor to reduce the number of neutrons and control the reaction rate. Option B is incorrect: Lead is used as radiation shielding material in nuclear facilities because it is dense and absorbs gamma rays. It is not used as a moderator because it does not effectively slow neutrons. Option D is incorrect: Uranium is the fuel in the reactor, not the moderator. U-235 undergoes fission when it absorbs a slow neutron; it does not slow down neutrons. Regular water (H₂O) can also act as a moderator in light water reactors.

The correct answer is Option C — Enrico Fermi. The first nuclear reactor, named Chicago Pile-1 (CP-1), was built under the direction of Italian-American physicist Enrico Fermi at the University of Chicago. On December 2, 1942, the first self-sustained controlled nuclear chain reaction was achieved here. Option A is incorrect: Albert Einstein is famous for E=mc² and the photoelectric effect, for which he won the Nobel Prize in 1921. While his letter to President Roosevelt in 1939 urged nuclear research, he did not directly lead or build any nuclear reactor. Robert Oppenheimer was the scientific director of the Manhattan Project and is known as the father of the atomic bomb, not the first nuclear reactor. Option D is incorrect: Ernest Rutherford discovered the nucleus of the atom and the proton. He died in 1937, before the first nuclear reactor was built in 1942.

In an uncontrolled chain reaction (such as in an atom bomb), when U-235 absorbs a neutron and undergoes fission, it releases approximately 3 neutrons. Each of these 3 neutrons causes another fission releasing 3 more neutrons each, so after 2nd generation: 9 neutrons, 3rd: 27, and so on. Option A is incorrect: The sequence 1, 2, 4, 8 represents doubling (ratio 2) which would occur if each fission released only 2 neutrons. While some reactions release 2 neutrons, the standard value for U-235 is approximately 2.5, rounded to 3 for chain reaction calculations. Option C is incorrect: The ratio 5 (1, 5, 25, 125) is incorrect as U-235 fission does not release 5 neutrons per event. Option D is incorrect: Ratio 4 (1, 4, 16, 64) is also not the correct neutron multiplication for U-235 fission.

The correct answer is Option B — Nuclear Reactor. In a controlled chain reaction, each fission of U-235 releases 2-3 neutrons and enormous energy. This energy is harnessed in nuclear reactors to produce electricity. The chain reaction is kept controlled (k=1) using control rods made of cadmium or boron. Option A is incorrect: Nuclear weapons use uncontrolled chain reactions, not controlled ones. The energy in weapons is released explosively. Option C is incorrect: Chemical factories use chemical reactions, not nuclear chain reactions. The energy scale of nuclear reactions is millions of times greater than chemical reactions. Option D is incorrect: Solar panels convert sunlight (photovoltaic effect) into electricity. They have nothing to do with nuclear chain reactions. Nuclear reactors work on the principle of controlled fission where the energy heats water to produce steam, which drives turbines to generate electricity.

The Geiger-Muller counter is the most widely known instrument for detecting radioactive radiation. Option B is correct: The Geiger-Muller (GM) counter, invented by Hans Geiger and Walther Müller in 1928, detects ionizing radiation including alpha, beta, and gamma particles. It works on the principle of gas ionization: radiation entering the tube ionizes the gas inside, producing an electrical pulse that is counted. It is used in nuclear physics labs, hospitals, and for monitoring radiation in nuclear power plants. Option A is incorrect: A cyclotron is a particle accelerator, not a radiation detector. Option C is incorrect: A spectrometer is used to analyze the spectrum of light or radiation; it separates radiation by wavelength or energy, but it is not primarily used for counting radioactive particles. Option D is incorrect: An oscilloscope is an electronic instrument that displays electrical waveforms on a screen. It is used in electronics for measuring voltage signals, not for detecting radioactive radiation.

Nuclear fusion is the process that powers the Sun and other stars. Option B is correct: Nuclear fusion is the process in which two light atomic nuclei, typically hydrogen isotopes (deuterium and tritium), combine under extremely high temperature and pressure to form a heavier nucleus (helium), releasing enormous energy. Fusion releases much more energy per unit mass than fission. Option A is incorrect: Splitting of a heavy nucleus describes nuclear fission, which is the opposite process. Fission is used in nuclear power plants and atomic bombs. Option C is incorrect: Emission of neutrons from a nucleus can occur during fission or in some radioactive decays (neutron emission), but it does not define nuclear fusion. Option D is incorrect: Decay of a radioactive nucleus is radioactive decay, which involves alpha, beta, or gamma emission from an unstable nucleus. It is a different nuclear process from fusion. The hydrogen bomb uses uncontrolled nuclear fusion triggered by a fission bomb.