4. Major Accidents and Hypothetical Accidents

1. Postulation of Events

"Major accidents” and “hypothetical accidents” are postulated for the purpose of evaluating the pertinence of reactor site conditions. The “major accidents” and “hypothetical accidents” are selected from the “accidents” discussed in Section 8.3, which have possibility of expanding release of radioactive materials. And, accidents involving release of radioactivity inside the reactor containment vessel and accidents involving release outside the reactor containment vessel are postulated. For “major accidents” release of radioactive materials considered maximum from the technical point of view is postulated, and for "hypothetical accidents” release of a hypothesized larger amount of radioactive materials is postulated.

Specifically, the following two “accident” types are evaluated:

©Accidents involving release inside the containment vessel

• Loss of reactor coolant

©Accidents involving release outside the containment vessel

• Steam generator tube rupture

Although the types of accidents assumed for “major accidents” and “hypothetical accidents” are the same, the assumed amounts of released radioactivity are different. The examples of safety evaluation for 3-loop plants are explained.

2. Loss of Reactor Coolant

To evaluate adequacy of siting of the NPP, assuming a reactor coolant piping break and large release of radioactive materials, the radiological effect on the general public in the vicinity of the plant is evaluated. The evaluation procedure is explained , a taking 3-loop plant as an example, as indicated in Figure 8.4.1 (1) and Figure 8.4.1 (2).

CD Amount of discharge from the reactor core into the reactor containment vessel

-Major accident: 2% of rare gases and 1% of iodine accumulated in reactor core (assuming release from the gap between fuel clad and pellets).

- Hypothetical accident 100% of rare gases and

50% of iodine accumulated in reactor core (assuming fuel melting).

In both the casesT0% of iodine is assumed to be organic iodine.*³

@ Discharge from the reactor containment vessel (the following are the same for both major accidents and hypothetical accidents)

@-150% of the inorganic iodine in the containment vessel is deposited on the containment vessel wall surface, and the rest is reduced at the rate of equivalent half­life of 100s by the internal spray system.

(2>2 97% of the radioactive materials that leak from the containment vessel leak to the annulus part and the remaining 3% leak directly into the atmosphere. The rate of . leakage for 24 hours after the accident is the leak rate corresponding to the rate at the maximum pressure of the containment vessel and after 24 hours the rate of leakage is half that

(2>3 In the annulus part most of the iodine is removed by the cleaning filter, and the rest is discharged into the atmosphere by way of the exhaust stack.

©4 The radioactive materials that dissolve in the recirculation water are 1% of the iodine accumulated in the reactor core immediately after occurrence of the accident (50% in the case of a hypothetical accident).

©-5 The recirculation water containing iodine is assumed to leak to the auxiliary building through valves, etc. 50% of the iodine is deposited on the walls, and the rest leaks into the atmosphere through the iodine filter.

© Dose evaluation

The amount of iodine released into the atmosphere in the processes in (!) and © above is about 1.3 x 10¹² Bq in the case of a major accident and about 6.6 x 10¹³ Bq in the case of a hypothetical accident, compared with the quantity of about 3.3 x 10¹⁸ Bq accumulated in

⁽*³⁾ Since, like rare gases, organic iodine is chemically stable (inactive), deposition on the wall and removal by spray are not expected.

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NSRA, Japan

Unit : Bq (Converted to 0.5MeVy Energy)

To Atmosphere

(Transfer Rate to Atmosphare : 5%)

To Atmosphere

Release by Ixakage out of Containment Leak Rate Zero to One day; 0.15%/day 1 day to 30 days : 0.075%/day

Release by Leakage out of Containment Leak Rate

Zero to One day: 0.15%/day 1 day to 30 days : 0.075%/day

I

Leakage of Recirculation Water

(Leak Rate : 20Min. to 30 Days: 6000cc/h)

(Iodine Release Rale from Core: 1%) (Rate of Iodine Release from Core: 1%)

Rare Gas Inside Containment

Reduction Effect

Inside Containment Ignored Decuy of Radioactivity Accounted

(Rare Gas Release Rate from Core: 2%)

< Iodine >

Figure 8.4.1 (1) Loss of reactor coolant (major accident)-Process of release to atomosphere of iodine and rare gas

NSRA, Japan

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Unit : Bq (Converted to 0.5MeV y Energy)

To Atmosphere

Unit : Bq (1-131 Equivalent)

(Transfer Rale Io Almosphare : 5%

Release by Leakage out of Containment Leak Rate

Zero to One day: 0.15%/day 1 day (o 30 days : 0.075%/day

1

Leakage of Recirculation Waler Ix?ak Rate : 20Min. to 30 Days : 6000cc/h)

Release by Leakage out of Containment Leak Rate Zero to One day: 0.15%/day 1 day Io 30 days : 0.075%/day

Rare Gas Inside Containment

Reduction Effect

Inside Containment Ignored Dccuy of Radioactivity Accounted

(Rare Gas Release Rate from Core : 10096)

(Iodine Release Rate from Core: 1%) (Rate of Iodine Release from Core; 50%)

t f

Amont of Rare Gas Accumulated in Core during Long Term Operation

3.0 x 10” Bq Approx.

Amont of Iodine Accumulated in Core during

Long Term Operation

3.3 x 10^ls Bq Approx.

< Rare Gas >

< Iodine >

Figure 8.4.1 (2) Loss of reactor coolant (hypothetical accident)-Process of release to atomosphere of iodine and rare gas

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NSRA, Japan