A recent US study on Prompt Fission Neutron Spectrum (PFNS) is significant for updating the design of India’s second stage of its nuclear power program.
Prompt Fission Neutron Spectrum (PFNS)
Prompt Fission Neutron Spectrum (PFNS) refers to neutrons emitted immediately after a Pu-240 nucleus captures a neutron but before it stabilises.
- Past Studies: Until now, only one study has examined PFNS for Pu-240-induced fission at 0.85 mega-electron-volt (MeV). Recently, U.S. researchers conducted a second study using neutrons with energy levels higher than 0.85 MeV.
- Latest Findings: The study revealed significant discrepancies between predicted and measured PFNS, providing valuable insights for reactor designers and nuclear medicine practitioners.
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- Isotopes
About: Isotopes are atoms of the same element that have the same number of protons (i.e., atomic number, “Z”) but a different number of neutrons, meaning that their mass number, “A”, varies.
- Take hydrogen, for example. It has three naturally occurring isotopes–1H, 2H, and 3H.
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Experimental Findings
- Research: Scientists at Los Alamos Neutron Science Center (LANSCE) conducted experiments by exposing a pure Pu-240 sample to neutrons ranging in energy from 0.01 to 800 MeV.
- Detection : The setup employed liquid scintillators to detect emitted particles, utilizing a small Pu-240 sample to minimize alpha particle emissions.
- Focus of Measurement: The researchers focused on measuring the energies of neutrons and other fission products, particularly emphasizing neutron-induced fission data.
About Plutonium-240 and its Fission
- Capturing of Neutrons: When a Pu-239 nucleus captures a neutron, it can either undergo fission or transform into Pu-240. Pu-240 is commonly found in nuclear reactors and nuclear weapon test fallout.
- Behaviour of Pu-240: When Pu-240 captures a neutron, it usually becomes Pu-241. If Pu-240 undergoes fission, the energy of its fission products is uncertain.
- Current models rely on complex calculations to estimate this energy output.
- Plutonium Production: Plutonium is produced from Uranium-238 in nuclear reactors. Plutonium-239 is a weapon-grade fissile material used in nuclear weapons. Both Pu-239 and Pu-240 are by-products of nuclear reactor operations and nuclear bomb detonations.
Fission and Fusion:
- Nuclear Fission: Nuclear fission occurs when a neutron collides with a large atom, causing it to become excited and split into two smaller atoms, known as fission products. This process triggers a chain reaction as additional neutrons are released. Moreover, the splitting of an atom releases a substantial amount of energy.
- Uranium and Plutonium are preferred as fission fuels in nuclear reactors due to their ease of initiation and control. In these reactors, the energy released from fission heats water into steam.
- The steam then drives a turbine, generating carbon-free electricity.
- Fusion: Nuclear fusion occurs when two atoms collide to create a heavier atom, such as when two hydrogen atoms fuse to form helium. This process mirrors the sun’s energy production, generating vast amounts of energy—significantly more than fission.
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Relevance of PFNS Study to India’s Prototype Fast Breeder Reactor
- PFBR Use: The PFBR utilises plutonium from CANDU reactor spent fuel, which contains Pu-240. Reprocessed spent fuel from the PFBR will also contain Pu-240.
- Importance: Understanding Pu-240 behaviour is crucial for enhancing reactor efficiency and safety.
Characteristics of Pu-240
- Formation of Pu-239: Pu-239 is formed when U-238 absorbs neutrons in a reactor. As Pu-239 captures more neutrons, it transforms into Pu-240, accumulating gradually over time.
- Instant Fission: Pu-240 spontaneously undergoes fission, emitting alpha particles. It is classified as a contaminant in weapons-grade plutonium, where its concentration is typically kept below 7%.
- Reactor-Grade: Plutonium containing more than 19% Pu-240 is categorized as reactor-grade.
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Three Stage Nuclear Programme of India
Stage I: Pressurized Heavy Water Reactors (PHWRs)
- Pressurized heavy water reactors (PHWRs) are used in the first phase of India’s three-stage nuclear power development.
- These reactors create plutonium-239 as a byproduct in addition to power.
Stage II: Fast Breeder Reactor (FBR)
- Fast breeder reactors (FBRs) are used in the second phase of India’s three-stage nuclear power development.
Composition and Type of Fuel:
- Type of Fuel: FBRs use a mixed oxide (MOX) fuel composed of plutonium-239 recovered from spent fuel from the first stage and natural uranium.
- Fission Process: In order to produce energy in FBRs, plutonium-239 undergoes fission.
- Breeding Fuel: FBRs are able to “breed” more fuel than they consume because uranium-238 in the mixed oxide fuel transmutes to more plutonium-239.
Stage III: Thorium Based Reactors
- In the third stage of India’s three-phase nuclear power programme, self-sustaining reactors powered by uranium-233 and thorium-232 will be deployed.
Features of Reactors
- Refueling: Reactors classified as thermal breeder reactors are able to be refuelled with naturally occurring thorium following the initial fuel charge.
- Fuel Composition: The main fuel used in the reactor is thorium-232, which is converted to uranium-233 to provide energy.
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June 24, 2024
