Calculator Inputs
Formula Used
Decay heat power is the deposited energy per second.
P = A × E × f
- P = decay heat power (W = J/s)
- A = activity (Bq = decays/s)
- E = average energy released per decay (J/decay)
- f = deposition fraction (0–1)
If decay adjustment is enabled: A = A0 × 2−t/T½, where T½ is half-life and t is elapsed time.
How to Use This Calculator
- Enter the activity and choose its unit.
- Enter average recoverable energy per decay and unit.
- Set deposition fraction to represent escaping energy.
- Optional: enable decay adjustment for older activity values.
- Click Calculate to view power above the form.
Example Data Table
| Activity | Energy/decay | Deposition fraction | Decay heat power |
|---|---|---|---|
| 1 Ci | 0.5 MeV | 1.0 | 2.96×10-3 W |
| 100 MBq | 1.0 MeV | 0.8 | 1.28×10-5 W |
| 5 GBq | 3.0 MeV | 0.6 | 1.44×10-3 W |
Understanding Decay Heat Power
Decay heat is the thermal power released as unstable nuclei transform into more stable products. Each decay deposits energy into surrounding material through charged particles, recoil, and absorbed photons. Engineers use decay heat estimates to size shielding, coolant flow, and thermal storage for radioactive sources and waste packages.
How Activity Relates to Heat Output
Activity (A) measures the number of decays per second. Because power is energy per unit time, multiplying decays per second by energy per decay produces watts. For example, 1 Bq means one decay per second. Converting from curies (Ci) to becquerels (Bq) standardizes the calculation.
Choosing a Realistic Energy per Decay
Use an average energy value that represents energy released per decay across all emitted radiation. If a radionuclide emits multiple particles or photons, compute a weighted average using emission probabilities. Many reference tables list decay energies in MeV; this calculator converts eV, keV, or MeV into joules.
Deposition Fraction and Escaping Radiation
Not all decay energy becomes local heat. Neutrinos typically escape, and high-energy gamma rays may exit small sources. The deposition fraction (f) models this by scaling the deposited energy from 0 to 1. Thick shielding and dense materials usually increase deposition, raising predicted heat near the source.
Optional Time Decay Adjustment
If your activity value was measured earlier, the current activity can be lower due to radioactive decay. When you enable time adjustment, the calculator applies A = A0 × 2−t/T½. This is helpful for inventory planning, long-term storage, and tracking heat output as a function of time.
Interpreting Results in Watts
The output is decay heat power in watts (J/s). Small laboratory sources often produce milliwatts or microwatts, while large inventories can produce substantial heat requiring active cooling. Use the reported current activity line to confirm whether the tool treated your input as initial or current.
Accuracy, Uncertainty, and Data Quality
Uncertainty typically comes from activity measurement error, incomplete decay energy data, and deposition assumptions. If geometry is complex, deposition can vary with distance and shielding thickness. For best practice, run scenarios with low and high deposition fractions and compare results against conservative safety margins.
Typical Applications and Best Practices
Common uses include estimating heat loads in sealed sources, hot-cell tooling, radiotherapy waste handling, reactor shutdown inventories, and shipping cask thermal design. Document your chosen energy per decay and deposition fraction alongside the results. Export CSV for records and PDF for reporting and review.
FAQs
1) What is decay heat power?
Decay heat power is the thermal energy released per second from radioactive transformations. It equals activity (decays per second) multiplied by deposited energy per decay, giving watts.
2) Why does this calculator ask for deposition fraction?
Some radiation escapes without heating the material, especially neutrinos and penetrating gamma rays. Deposition fraction lets you model how much decay energy actually becomes local heat.
3) Which activity unit should I use?
Use whatever you have measured. The tool converts common units to Bq internally. Curie-based values are supported alongside metric Bq prefixes for convenience.
4) How do I pick energy per decay?
Use an average decay energy from a reliable reference, or compute a weighted average from emissions and probabilities. If some energy escapes, reduce deposition fraction rather than lowering the decay energy itself.
5) When should I enable the time decay option?
Enable it when your activity value represents an earlier time and you want the present heat output. Provide half-life and elapsed time to adjust activity using exponential decay.
6) Can the result be used for cooling design?
It is a good first estimate. For design, include safety factors and consider geometry, shielding, and heat transfer limits. Complex systems may require detailed transport and thermal simulations.
7) Why might my calculated watts seem very small?
Most sources have modest decay energies and activities. Many common lab sources produce milliwatts or less. Verify unit selections, energy input, and whether deposition fraction should be closer to one for your setup.