Question

The spent fuel elements from a fission reactor are much more intensely radioactive than the original fuel elements. (a) What does this tell you about the products of the fission process in relationship to the belt of stability, Figure 21.2? (b) Given that only two or three neutrons are released per fission event and knowing that the nucleus undergoing fission has a neutron- to-proton ratio characteristic of a heavy nucleus, what sorts of decay would you expect to be dominant among the fission products?

Short answer

(a) The increased radioactivity of spent fuel elements indicates that the fission products have a higher neutron-to-proton ratio and lie far from the belt of stability, making them unstable and radioactive. (b) Due to the high neutron-to-proton ratio of heavy nuclei undergoing fission, beta decay is expected to be the dominant radioactive decay process among the fission products, as it converts excess neutrons into protons and moves the nucleus towards the belt of stability.

Step-by-step solution

(a) Radioactivity compared to the belt of stability

The belt of stability is a graphical representation of stable atomic nuclei. It shows the relationship between the number of protons (Z) and the number of neutrons (N) for stable isotopes. Nuclei on the belt are stable and non-radioactive, while nuclei far from the belt are unstable and radioactive. The increased radioactivity of the spent fuel elements indicates that the products of the fission process are not located on the belt of stability. These fission products instead lie far from the belt due to their unstable nature. The fission process has created new isotopes with a higher neutron-to-proton ratio, which means that they are much more radioactive than the parent nucleus.

(b) Dominant decay types

A fission event typically releases two or three neutrons. The nucleus undergoing fission has a neutron-to-proton ratio which is characteristic of heavy nuclei. Heavy nuclei usually have more neutrons than protons in order to stabilize the repulsive electric forces between protons. Since heavy nuclei have a higher neutron-to-proton ratio, their decay process typically involves converting excess neutrons into protons to reduce this ratio and become more stable. Beta decay is a common radioactive decay process for heavy nuclei, where a neutron is converted into a proton by the emission of an electron (called a beta particle) along with an anti-neutrino. This decay type decreases the neutron-to-proton ratio, thus moving the nucleus towards the belt of stability. In conclusion, based on the high neutron-to-proton ratio of the heavy nuclei undergoing fission and the limited number of neutrons released per fission event, one can expect beta decay to be the dominant radioactive decay process among the fission products.

Key concepts

Belt of Stability

The concept of the 'belt of stability' is critical in understanding the nature of atomic nuclei and their radioactivity. Essentially, the belt of stability is a region on a graph where stable atoms live; more specifically, it represents the ratio of neutrons to protons (N/Z ratio) that isotopes must have to be considered stable and non-radioactive. Atoms inside this so-called belt do not undergo radioactive decay naturally, as they have found the perfect balance in their internal structure.

When we consider fission products, which are the elements generated as a result of nuclear fission, they often are located outside this belt. This is because the intense process of splitting a heavy nucleus into two or more smaller nuclei often results in new isotopes with excessive neutrons, rendering them unstable and radioactive. They must undergo decay processes to reach stability. Consequently, they emit radioactivity as they transition closer to or reach the belt of stability. Our understanding of where these elements fall relative to the belt of stability tells us much about their radioactive nature and decay pathways. It also reinforces why spent fuel elements from fission reactors are incredibly radioactive compared to the original fuel due to their distance from the belt.

Fission Products

Fission products are the nuclei created when a large atomic nucleus, like that of Uranium-235 or Plutonium-239, is split into smaller parts. These products are a blend of highly radioactive isotopes since they are freshly forged in the extreme environment of a nuclear reactor.

Quite intriguingly, these products are rarely ever stable. They possess an imbalance of neutrons and protons, a direct consequence of their origin from a heavy nucleus with a greater number of neutrons for stability. The excess of neutrons makes these fission products prime candidates for radioactive decay as they seek to achieve a more stable state by adjusting their internal N/Z ratio.

This quest for stability can lead to the emission of various types of radiation, depending on how far from the belt of stability each isotope lies. With spent fuel elements being significantly more radioactive than the original fuel, we are provided with a treasure trove of information on how these products decay and what types of particles they emit in the process.

Beta Decay

Among the various ways that an unstable atom can achieve stability is through a process known as 'beta decay.' This form of radioactive decay is crucial for nuclei with a neutron excess as it helps them to even out their neutron-to-proton ratio.

In beta decay, a neutron within the atomic nucleus is transformed into a proton. This transformation releases a beta particle (which is essentially an electron) and an antineutrino, both ejected from the nucleus. This conversion increases the atomic number of the element by one but leaves the atomic mass unchanged, effectively moving it one spot to the right on the Periodic Table of Elements.

Given the neutron-rich nature of fission products, beta decay plays a dominant role in their journey towards stability. By emitting beta radiation, they can reduce their neutron count and 'climb' their way back onto the belt of stability. This process is reflective of the intensive radioactive nature of spent nuclear reactor fuel, which is laden with such neutron-rich fission products undergoing beta decay.