 INES 1 |
Reactor scram followed by the actuation of safety valve in the reactor primary circuit |
At 11:15 (Moscow time) during research reactor operation at the preset power level Nt-200 MWt (Ne-48 MW) reactor protection on the "turbine trip" signal actuated due to failure in the turbine protection system circuit. The reactor was brought to subcriticality and simultaneously, according to design algorithm, the second scram system actuated to inject boric acid solution from the Safety Injection Tank (SIT) by compressed nitrogen action. During the transient reactor pressure dropped from 5.0 MPa to 4.8 MPa and then started to build up. Personnel attempts to stabilize the pressure by activating engineered features for dumping excess steam from the reactor and its subsequent condensation proved to be unsuccessful. 5.5 min. after reactor condensation scram at reactor pressure of 5.63 MPa according to the design one of the five safety valves opened and reactor pressure started to drop 1 min. later when the pressure decreased to 4.5 MPa the safety valve closed automatically. No breach of safe operation limits and conditions occurred. During the time that the safety valve remained open 1.1 t of steam with a total activity of about 90 mCi (with a less than 20 mCi share of radionuclides with a half-life period of more than 24 hours) was released into the atmosphere. As a result contamination of a limited area of the Institute's site occurred (where there are no buildings with staff permanent habitation, e.g. the cooling towers area) with a maximum dose-rate of 3 mcR/s (about 11 mR/h) at the initial moment of the event. Radiological situation in the reactor facility rooms did not change and no radiation effects on personnel were observed. No contamination and radiation effects on the public outside the institute's territory were found.
Justification of rating:
1. Off site impact criterion. the event is not rated under this criterion since during the actuation of the safety valve no activity releases to the environment outside the Institute's territory occurred.
2. On site impact criterion. The event is not rated under this criterion since the on site impact was below the significance/relevance/threshold for level 2.
During the time that the safety valve was open about 1.1 t of the steam with a total activity of about 90 mCi was released into the environment. This resulted in the contamination of a limited area of the Institute's site with a maximum dose-rate of 3 mcR/s (about 11 mR/h) at the initial moment of the event. Radiological situation in the reactor facility rooms remained unchanged, and no radiation effects on personnel occurred. (When performing survey of the contaminated area during about 2 hours three radiation monitoring technicians received exposure corresponding to the reference level of the daily exposure for this category of personnel. i.e. their individual exposure dose did not exceed 22 mrem).
3. Degradation of defence in depth criterion. Under this criterion the event is rated according to Sections III-2 and III-3.4 of INES User's Manual for reactor facilities (taking into account the power of this research reactor and the maximum potential on-site and off-site consequences of the accident).
- Expected initiator occurred - reactor scram (see section III-A.2.1). Safety function availability - full. Consequently, according to Table II (A-1) the basic level is 0. Taking into account an additional factor of Safety culture deficiencies (violation of operating procedure by personnel) the basic level is increased by 1.
- Contamination spread on the limited area of the Institute's territory which led to exceeding the prescribed limit per this zone (210 mcR/h) as a result of violation of operating procedure by personnel is rated as level 1.
The final rating of the event is level 1 - anomaly.
Event date: Wed, 31-01-1996
