Fission product penetration, containment

As far as is reasonably practicable, in the event of a major accident, the release of radioactive fission products from the uranium fuel to the atmosphere is prevented by a series of containment barriers. Should these be penetrated and a release of activity to the environment occur, the Station Emergency Plan is required to be implemented. The immediate response by the operators to this potentially dangerous situation is vitally important if the consequences to station personnel and the local population are to be minimised. 

Unit 421 was encased in concrete and as discussed earlier, a slowly degrading lifetime of 100 years was assumed for this containment barrier. A similar lifetime was assumed for the Furfurol(F) encapsulating the SNF. As the concrete barrier becomes more and more porous, activation products are released from the outside of the RPV. Then the breather hole into the interior of the RPV is corroded open in the year 2005, allowing fuel and interior SS corrosion to begin. The other RPV penetrations and barriers begin to open up in the year 2035, shown by the peak in release rates for the fission products, 370 GBq-a and actinides, 0.2 GBq-a L Coupled with the continuing steel corrosion, the total peak release rate is 370 GBq a. ... 

The primary cooling ciicuit in a PWR is a high-integrity, pressure-resistant system that will contain any fission products released from the fuel in an accident until the internal pressure exceeds the values that would actuate the pressure relief devices. A simple, conqiact primary system will be easier to qualify and inspect and to protect from seismic events and external hazards. The RPV penetrations should be as few as possible and of small diameter. All primary system openings would be kept sealed for die duration of autonomous operation. 

Design basis Fission product containment Containment vessel passive Pressure suppression type with mmimum penetrations... 

If the leak is trough a containment penetration, the penetration should be isolated (e.g. by closing a valve in the leakage path). If this is not sufficient or not possible, the containment spray system should be started (if not yet done) in order to (1) reduce the airborne fission products inventory in the containment, and (2) reduce the driving force for the leak. 

Control had been established with no further hazard likely, although a state of emergency was still considered to exist. Office areas immediately adjacent (40 ft) to the process area were cleared (T-t-1 day) by health physics for occupancy. Contamination was cbi ined to the process area by uranium and fission products with maximum penetrating radiation levels of SO R. There was no evidence that uranium or fission products had been deposited off site. The radioactive materials ivere contained primarily within the plant building and were concentrated in the Immediate vicinity of the reacting vessel (the solvent extraction area). 

In safety analyses it is frequently assumed that the rate of penetration of fission products through the usually very narrow slits would be identical to the leak rate that would be measured using air as a penetrating medium. To be sure, this assumption is true for the fission product noble gases for airborne aerosols, however, narrow slits can act as a trap, so that under otherwise identical conditions aerosol leak rates will usually be lower by some orders of magnitude than the air leak rate. Early observations in this field showed that the presence of steam in a heated vessel atmosphere results in a drastic decrease of the leak rate to values that are well below those measured with dry air at ambient temperature (Witherspoon, 1970). Likewise, quick plugging of natural leak paths was observed when higher steam contents were introduced into the containment atmosphere under such conditions, even artificially made leaks in the vessel effected an appreciable retention of both airborne iodine and particles (Witherspoon and Postma, 1971). 

These results mean that penetration of fission products (except the fission product noble gases) through operational leaks in the containment shell in the course of a severe reactor accident will be very small. The largest fraction of the penetrating fission products will be plated out in the adjacent rooms and compartments with the condensing steam, while the still airborne fraction will be transported to the filters of the annulus air extraction system. Fission product iodine plated out into the water pools formed in the annuli will be subjected to the reactions to be discussed in Section 7.3.4.3. 

The piping which penetrates the containment steel shell (e. g. fresh air and exhaust air ducts) is automatically locked in the very early stages of an accident. Despite these measures, in risk studies it has to be assumed that one of these penetrations will fail to be closed, thus permitting the transport of radionuclides out of the containment. The TMI-2 accident demonstrated that such a situation may occur in reality in this event an open connection to the auxiliary building was the main pathway for the escape of fission product radionuclides from the containment to the environment. 

Concerning the transport of fission products out of the containment, the relative locations of the break in the primary system and of the open penetration in the containment steel shell are of great importance. It can always be assumed that these two positions are in different compartments so that the fission products have to travel a certain distance over which deposition may occur moreover, the time required to cover this distance provides an opportunity for chemical reactions to occur, in particular with respect to iodine. This time delay depends highly on the specific accident sequence, in particular on the rate of pressure increase within the containment and, thus, on the steam mass flow from the primary system to the containment. With a high degree of probability it can be assumed that the open penetration can be locked (e. g. by manual action) within a comparatively short time. 

The potential for containment isolation and containment bypass is lessened by having fewer penetrations to allow fission product release. In addition, normally open and risk important penetrations are fail-closed, thus eliminating the dependence on instrumentation and control (I C) and batteries. 

Cl Containment isolation failure Fission-product release through a failure of the system or valves that close the penetrations between the containment and the environment. Containment failure occurs prior to onset of core damage. Large release 1... 

A containment isolation failure occurs because of the postulated failure of the system or valves that close the penetrations between the containment and the environment. Containment isolation failure occurs before the onset of core damage. For such a failure, fission-product releases fi-om the reactor coolant system can leak directly from the containment to the environment with diminished potential for attenuation. Most isolation failures occur at a penetration that cormects the containment with the auxiliary building. The auxiliary building may provide additional attenuation of aerosol fission-product releases. However, this decontamination is not credited in the containment isolation failure cases. Accident sequences in which the contairunent does not isolate prior to core damage are grouped into release category Cl. 

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