e-learning view The Manual here
Radiological barriers and controls




  • 50% of fuel pins failed from overheating
  • 0.5% of total activity was released into coolant
  • Immediate evacuation out to 2 km
  • Doses less than 1 mSv
  • Release of 20 TBq; 10% 131I, 5% 137Cs


An accident at a nuclear power station, involving overheating of the fuel, led to failure of about half of the fuel pins and a subsequent release of radioactive material. Failure of about half the fuel pins, without significant fuel melting, would release about 0.5% of the total core inventory. Local police, in consultation with the licensee and the regulatory authority, took the immediate decision to evacuate people within a 2 km radius of the facility and as a result, no one received doses above 1 mSv. Assessment of the release by experts at the facility suggested that the total activity of about 20 TBq comprised 10% 131I, 5% 137Cs and the remaining was made up of noble gases.

INES procedure (click the highlighted flowchart items to follow the procedure)

Figure 4, page 145 of the Manual

Figure 5, page 146 of the Manual

Calculate radiological equivalence

Here's the calculation:

10% of the total release of 20 TBq was 131I = 2 TBq 131I
5% of the total release of 20 TBq was 137Cs = 1 TBq 137Cs
The rest was noble gases and adds nothing to the calculation

(40 x 1(137Cs)) + (1 x 2(131I)) = 42 TBq 131I

Level 4 defines that the release should be:
“of the order of tens to hundreds of TBq 131I”

The lower boundary for Level 4 is about 50 TBq 131I. (footnote 2, page 17 of the Manual)

The rating based on activity release is very close to Level 4

Note that "Immediate evacuation out to 2 km" is not relevant for rating. Doses were less than 1 mSv so Level 0 is based on individual dose.

Return to flowchart

Figure 6


0.5% of the total activity was released into coolant

To rate an event at Level 4 based on the criteria related to impact on radiological barriers and controls at facilities requires:
Release from the fuel assemblies > 0.1% of the core inventory of a power reactor

The overall rating for the evacuation near a nuclear reactor is Level 4, based on fuel damage.