Nuclear Study on Plutonium Isotope Fission

Introduction

Recent research has provided significant updates on the fission properties of the plutonium isotope Pu-240. This study, conducted by researchers at the Los Alamos Neutron Science Centre (LANSCE) in the U.S., involved measuring the Prompt Fission Neutron Spectrum (PFNS) of Pu-240 when bombarded with neutrons of energy greater than 0.85 MeV. The findings revealed notable deviations from predicted values, which have implications for various fields, including reactor design, radiation shielding, and nuclear medicine.

Key Findings

1.     Prompt Fission Neutron Spectrum (PFNS):

o   PFNS refers to the spectrum of neutrons emitted immediately after a nucleus undergoes fission, before reaching a stable state.

o   This study was only the second attempt to measure the PFNS of Pu-240, with the first being for neutrons at 0.85 MeV. The new study used neutrons with higher energy levels.

2.     Deviations from Predicted Values:

o   The researchers observed significant differences between the measured PFNS and the values predicted by existing models.

o   This discrepancy suggests that current theoretical models need refinement to better predict the behavior of Pu-240 during fission.

3.     Higher-than-Expected Second-Chance Fission:

o   Second-chance fission occurs when a nucleus, initially non-fissionable, becomes fissionable after losing a neutron.

o   The study found a higher rate of second-chance fission than expected, along with indications of third-chance fission, though the latter was harder to directly observe in the data.

Significance of the Findings

1.     Implications for Reactor Design:

o   Accurate PFNS data are crucial for designing efficient and safe nuclear reactors. The deviations found in the study imply that reactor models need updates to incorporate the new findings, potentially leading to improved reactor performance and safety.

2.     Radiation Shielding and Nuclear Medicine:

o   Understanding the precise neutron emissions and energy spectra from Pu-240 fission aids in designing better radiation shielding and calculating accurate radiation doses in medical applications.

3.     Impact on Nuclear Data Libraries:

o   Nuclear data libraries like ENDF, JEFF, and JENDL rely on accurate fission data for various applications, including reactor operation, shielding design, and nuclear forensics.

o   The study's findings will necessitate updates to these libraries, improving their accuracy and reliability for scientific and industrial use.

Technical Details

1.     Experiment Setup:

o   The researchers used a tungsten disc struck by protons to produce neutrons of energy ranging from 0.01 to 800 MeV.

o   Neutrons moving at a specific angle were redirected to a chamber containing highly pure Pu-240.

o   Liquid scintillators surrounded the Pu-240 sample to track the output, measuring the energies of emitted neutrons and other fission products.

2.     Data Analysis:

o   The researchers carefully subtracted contributions from spontaneous fission, alpha particles, and other sources to isolate the PFNS data.

o   They reported their analysis for incident neutrons of energies between 1 and 20 MeV.

Pu-240 Profile

1.     Production and Characteristics:

o   Pu-239, produced when U-238 absorbs neutrons, can become Pu-240 if it captures another neutron.

o   Pu-240 undergoes spontaneous fission, emitting alpha particles, and is considered a contaminant in weapons-grade plutonium due to its properties.

2.     Applications and Limitations:

o   Pu-240’s presence in reactor-grade plutonium impacts its suitability for weapons. The isotope's spontaneous fission and alpha emission are undesirable in weapons-grade material, where its concentration is restricted to below 7%.

Conclusion

The new research on Pu-240’s fission properties provides critical updates that will influence various nuclear technologies. The deviations found in the PFNS from predicted models highlight the need for revised theoretical models and updated nuclear data libraries. This will enhance the accuracy and safety of reactor designs, improve radiation shielding, and aid in nuclear medicine applications. The study underscores the ongoing importance of experimental research in refining our understanding of nuclear processes and improving technological applications in the field.

MCQs

Question 1

What is the primary focus of the recent study on Pu-240?

A. Developing new nuclear weapons
B. Measuring the Prompt Fission Neutron Spectrum (PFNS) of induced fission in Pu-240
C. Finding new uses for plutonium in medicine
D. Discovering alternative energy sources

Answer: B. Measuring the Prompt Fission Neutron Spectrum (PFNS) of induced fission in Pu-240

Question 2

What energy level of neutrons was used in the recent study to bombard Pu-240?

A. Less than 0.85 MeV
B. Exactly 0.85 MeV
C. Greater than 0.85 MeV
D. Between 0.5 MeV and 0.75 MeV

Answer: C. Greater than 0.85 MeV

Question 3

Which facility conducted the recent study on the PFNS of Pu-240?

A. Indian Space Research Organisation (ISRO)
B. Los Alamos Neutron Science Centre (LANSCE)
C. CERN
D. NASA

Answer: B. Los Alamos Neutron Science Centre (LANSCE)

Question 4

What unexpected finding did researchers report in their study on Pu-240?

A. The absence of second-chance fission
B. Lower-than-expected fission rates
C. Higher-than-expected rate of second-chance fission
D. No deviations from predicted PFNS values

Answer: C. Higher-than-expected rate of second-chance fission

Question 5

Why is accurate PFNS data important for nuclear reactor design and other applications?

A. It helps in the production of new types of nuclear fuel
B. It ensures the safety and efficiency of nuclear reactors
C. It increases the explosive power of nuclear weapons
D. It reduces the cost of nuclear energy production

Answer: B. It ensures the safety and efficiency of nuclear reactors