4. A nuclear reactors fuel rods are immersed in water, which acts as a coolant and a
moderator. You notice a blue glow around the fuel rods. This is due to
a. Luminescence of radium produced during the decay of uranium
b. Superheating the water around the reactor vessel causing the release and
ignition of hydrogen
c. Ionisation of water resulting in electrical arcing with the walls of the reactor
vessel
d. Charged particles emitting radiation when they exceed the speed of light in
water

The correct answer is: d. Charged particles emitting radiation when they exceed the speed of light in water. This blue glow is called Cherenkov radiation, seen in nuclear reactors when fast charged particles (like electrons) move through water faster than light can travel in that medium.

Introduction

When looking into the core of a water-cooled nuclear reactor, a striking blue glow around the fuel rods is often visible. This is not fire, arcing, or burning hydrogen, but a well-understood physical phenomenon called Cherenkov radiation, produced when high‑energy charged particles move through water faster than light can travel in that water. Understanding this glow also helps clarify several misconceptions about nuclear reactors and radiation.

Explanation of the correct option (d)

d. Charged particles emitting radiation when they exceed the speed of light in water

• In water, light travels slower than in vacuum (about 75% of its vacuum speed), but high‑energy charged particles such as electrons from radioactive decay can travel through water at speeds greater than the local speed of light, though never faster than light in vacuum.

• When this happens, the particles emit Cherenkov radiation, analogous to a “light shockwave,” producing a characteristic bluish glow around reactor fuel rods immersed in water.

• The blue color comes from the spectrum of the emitted radiation, which is weighted towards shorter (bluer) wavelengths in the visible range.

Therefore, option (d) is correct.

Why the other options are wrong

Option (a)

a. Luminescence of radium produced during the decay of uranium

• While uranium decay chains may involve various daughter nuclides, reactor fuel is not producing significant radium that would glow visibly in this way.

• The glow seen in reactors is not from the chemical element radium; it is an electromagnetic effect in the water itself caused by fast charged particles.

• Also, the spectral and geometric characteristics of the observed blue glow match Cherenkov radiation, not luminescence from radium or other specific isotopes in the fuel.

Option (b)

b. Superheating the water around the reactor vessel causing the release and ignition of hydrogen

• If hydrogen were being produced and ignited in bulk near the fuel rods, there would be bubbles, violent boiling, and combustion, not a smooth, steady underwater blue glow.

• The core is submerged deep in water; burning hydrogen requires an oxygen-containing mixture and is not how normal reactor operation appears.

• Blue flame from hydrogen combustion is a gas-phase process in air, not a continuous glow in clear pool water.

Option (c)

c. Ionisation of water resulting in electrical arcing with the walls of the reactor vessel

• Electrical arcing in water would produce localized, unstable, sparking discharges, not the uniform, pool-wide, soft blue light associated with reactor cores.

• The reactor core and pool geometry, along with the shielding and electrical design, are not arranged to produce large-scale arcs between fuel and vessel walls during normal operation.

• Although ionisation of water does occur due to radiation, the visible effect of the blue glow is not an electrical arc; it is Cherenkov radiation emitted along the paths of fast particles in the water.

SEO-friendly recap for exam and article use

• The blue glow around nuclear reactor fuel rods immersed in water is a classic example of Cherenkov radiation.

• It occurs when high‑energy charged particles move faster than the speed of light in water, emitting a characteristic blue light.

• Explanations involving radium luminescence, hydrogen ignition, or electrical arcing do not match the physics or normal operating conditions of water‑moderated reactors.

Correct option: d. Charged particles emitting radiation when they exceed the speed of light in water.