Which one of the following elements is essential as a structural material in nuclear reactors because of its low neutron absorption cross section?

Introduction / Context:
This question belongs to applied chemistry and nuclear technology. It asks which element is widely used in the construction of nuclear reactor components because of its special nuclear properties, particularly its interaction with neutrons.

Given Data / Assumptions:

• The elements listed are cobalt, nickel, zirconium, tungsten and chromium.
• We are looking for an element used as a structural material in nuclear reactors.
• An important requirement for such materials is low neutron absorption, especially for fuel cladding and other components close to the reactor core.
• We assume basic knowledge of common reactor materials.

Concept / Approach:
Inside a nuclear reactor, neutrons must be carefully controlled to sustain the chain reaction. Structural materials that surround the fuel rods should not absorb too many neutrons; otherwise, they would act as neutron poisons and reduce reactor efficiency. Zirconium and its alloys, especially zircaloy, are famous for their very low neutron absorption cross section and good corrosion resistance in high temperature water, making them ideal for fuel cladding. Other listed metals are used in industry but are not as suitable for this specific nuclear function.

Step-by-Step Solution:
1) Cobalt and nickel are transition metals used in superalloys and magnet applications, but they have higher neutron absorption and are not standard for fuel cladding.2) Tungsten has a very high melting point and is used in filaments and electrodes but significantly absorbs neutrons and is dense.3) Chromium is commonly used in stainless steel for corrosion resistance but again is not the primary choice for low neutron absorbing cladding.4) Zirconium has excellent corrosion resistance in water and steam at high temperatures and, most importantly, a low neutron absorption cross section.5) Therefore, zirconium and its alloys are widely used as structural components, especially as cladding for fuel rods in many types of nuclear reactors.

Verification / Alternative check:
Standard descriptions of pressurised water reactors and boiling water reactors list zircaloy, a zirconium based alloy, as the common cladding material for uranium dioxide fuel pellets. This fact is directly linked to zirconium's nuclear properties. In contrast, cobalt, nickel, tungsten and chromium are rarely mentioned as primary reactor structural materials because of their less favourable neutron interaction characteristics and other material constraints.

Why Other Options Are Wrong:
Cobalt: Important in high strength alloys and magnetic materials, but its neutron absorption characteristics are not suitable for widespread use as reactor cladding.
Nickel: Used in stainless steels and alloys but not preferred as a main low absorption cladding material.
Tungsten: Though extremely refractory, it has high neutron absorption and is not commonly used as structural material inside the reactor core.
Chromium: Primarily used in steel for corrosion resistance; it does not match zirconium in low neutron absorption performance for reactor applications.

Common Pitfalls:
Students sometimes choose metals like tungsten simply because they know it has a high melting point and seems robust. However, in nuclear reactors, neutron absorption characteristics are as important as mechanical strength. Another mistake is to focus on general industrial use rather than the specialised requirements of reactor design. Remember that zirconium's key advantage is its combination of corrosion resistance and low neutron capture, which is exactly what this question is testing.

Final Answer:
The element widely used as a structural material in nuclear reactors is zirconium.