Backgrounders

Friday, May 03, 2002

MARITIME FORCES PACIFIC NUCLEAR PROPELLED VESSEL ACCIDENTS

General

The base maintains a well-equipped and trained Nuclear Emergency Response Team to ensure the safety of nuclear-propelled vessel visits to Esquimalt Harbour. There are two conceivable types of accidents for which the NERT is trained: the contained or the uncontained.

As the terms imply, the contained accident is one in which any radioactive material that might escape from the reactor or the primary containment system is fully contained within the hull of the vessel at all times. Even more unlikely, but conceivable nevertheless, is the uncontained accident in which radioactive material is released into the environment.

Protective Barriers

Nuclear-propelled vessels operated by allied navies have an exemplary safety record. That record is the direct result of conservative, safe design that errs on the side of caution; exceptional crew selection and training; rigourous application of nuclear regulatory requirements and safety standards; and, proven operating practice. As a result, in over 40 years of operation and 112 million miles steamed, there has never been a measurable release of radiation from an allied naval vessel.

A key factor in the design of these vessels is the multiple barriers that are in place to contain the radioactive material and prevent any release to the environment. Radioactive material is securely contained in fuel plates, which in turn are retained within an extremely robust steel nuclear reactor pressure vessel. In addition, there are two other protective barriers that are in place, referred to as the primary and secondary containment respectively.

The primary containment is a pressure-tight compartment located within the vessel that contains all the nuclear components of the propulsion plant. Any leakage from the reactor would be contained within this compartment.

The secondary containment system is the hull of the submarine, which is extremely strong and is designed to withstand the pressures of operating at great depth. Therefore, any release from the primary containment area would be contained within the hull of the boat and would be prevented from escaping to the environment.

Figure 1 – NPV Containment Systems

Figure 2. - The Contained Accident: In this instance, while there has been damage to the reactor, all radioactive material is fully contained within the primary containment.

As a result of this accident, an intense radiation field will be generated in the vicinity of the submarine. This ‘hull shine’ caused by the emission of gamma radiation will be very similar in nature to an X-ray. While a hazard in the immediate vicinity of the sub, it will not be dangerous to personnel in excess of 250 meters. In addition, like an X-ray, there will be no lasting effects to the environment once the vessel has been moved.

Figure 3. This shows the danger area extending from an NPV that has suffered a contained accident.

In the event of a contained accident, the actions taken by the NERT will be to evacuate all personnel from within the 250 m danger radius and monitor the NPV for any further radiation risks. There will be no risk to the local communities; however, they will be fully engaged and apprised of the situation through the Provincial Emergency Programme (PEP).

The chance of a contained accident is minimal, approximately one in one million years of reactor operation. CFB Esquimalt has all necessary resources in hand to respond to such an incident.

The Uncontained Accident

The Uncontained accident involves similar initiating events to the contained accident; however, in addition, both the primary containment and the secondary containment systems fail. As a result, radioactive material would be released from the vessel and would be borne by the prevailing winds. This type of an event would be highly unlikely given the robust construction of the containment systems used in these ships.

If this were to occur, the failure of the secondary containment would probably arise as a result of a hatch being opened. Therefore, the duration of such a release from an NPV would be relatively short.

Figure 4. Uncontained release – Failure of Primary Containment and open hatch leading to release to the environment.

In view of the additional factors involved in precipitating an uncontained release, the probability of such an event is consequently even more remote than a contained accident. Estimates place the probability of such an event occurring at 1/10,000,000 years of reactor operation.

Figure 5 – Nuclear material, released from the NPV, will be borne by the prevailing winds

Unlike the contained accident, where all deleterious effects are confined to DND property, the uncontained accident would have an impact on the surrounding communities. In the highly unlikely event of such an occurrence, specially trained teams from the CFB Esquimalt Nuclear Emergency Response Team (NERT) would be dispatched into the local communities to directly measure any radiation consequences of an accident.

Based on the results of these surveys, and in accordance with Health Canada guidelines, DND would provide recommendations for either local sheltering or evacuation to the Provincial Emergency Programme (PEP). PEP, in turn, would liaise with the local municipalities who have the legal responsibility for implementing any mitigating actions.

The Technical Safety Assessment (TSA) was carried out to assess possible radiological consequences of an accident involving an NPV. In this report, even the worst postulated event was found to pose minimal health risks off site. Simple actions such as sheltering would even further reduce the consequences of a release to the environment. The extremely small probability of any such event was reaffirmed during the development of the TSA.

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Last Updated: 5/3/2002		Important Notices