 INES 1 |
Slightly increased radioactivity release outside the reactor while preparing for removal of leaky fuel assembly |
On 25.07.97 MIR- M1 pool type pressure tube research reactor (mainly used for materials testing purposes) was shut down because of increased activity indications of the Burst Can Detection System (BCDS) caused by depressurization of the operating fuel assembly (FA).
While performing operations to reduce pressure in the primary circuit on 26.07.97 fission products release into the reactor pool occurred that subsequently led to increased discharge of radioactivity from the Institute's vent stack. No contamination outside the Institute's site was detected, and no radiation effects on the public occurred. Personnel overexposure was not observed.
Justification:
1. Off-site impact criterion:
The event is not rated by this criterion since the amount of unauthroized release of radioactivity into the environment outside the Institute's territory was below the lower threshold.
2. "On-site impact"criterion.
The event is not rated by this criterion since the amount of activity release on site (within the Institute's territory) was below the significance threshold for lebel 2.
3. "Degradation" of defence-in-depth.
Under this criterion the event is rated according to Sections IV-5.1.3, IV-5.1.7, and IV-3.3 of the "INES user's Manual" for non-reactor installations.
Degradation of the first safety barrier occurred, i.e. fuel rod damage. In the course of mitigating the consequences of this event the safety function of "confining the radioactive material" was not appropriately assured, and fission products release into the primary circuit occurred followed by the spread of contamination on a limited part of the Institute's territory which did not lead to exceeding the regulatory prescribed limit for this area (240 mcR/h).
The availability of the remaining safety functions was full.
Taking into account the additional factor of "safety culture deficiencies" (violation of operating procedure by personnel occurred resulting in the fission products release into the primary circuit) the event is rated as level 1.
Event date: Sat, 26-07-1997
