Radioactive release to the atmosphere

For ship accidents that might occur in port or while sailing a coastal route, SNL constructed irradiated nuclear fuel source terms for two hypothetical accidents, a ship collision that fails the seal of a TN-12 irradiated nuclear fuel flask and all of the rods inside of the flask, and a

initiates a severe fire. For the second accident, differential heating of the flask was assumed to cause a buoyant flow through the flask of combustion gases and air which sweeps all of the radioactivity released to the flask interior out into the atmosphere. Thus, the source term for the second accident constitutes a conservative estimate for an upper bound on atmospheric irradiated nuclear fuel source terms for maritime accidents.

7.3.1. Accidents at port (using MACCS calculations)

Table XXII presents consequence estimates for the two hypothetical port accident scenarios assuming that these accidents occur in the port of New York (Port Elizabeth). Both calculations assumed that the irradiated nuclear fuel flask was being carried in a break-bulk freighter that was also carrying other cargo. Both calculations used the irradiated nuclear fuel inventory from a TN-12 flask calculated using the ORIGEN code [24], accident release fractions based on the studies of Wilmot [25], Sprung [26], and Sandoval [27], one year of variable meteorological data recorded at the New York City National Weather Service Station, and a population distribution constructed from census data for 1990 using POPSEC90 [28].

Although no short term emergency response actions (evacuation, sheltering) were assumed to take place, post-accident relocation of population away from and decontamination and/or condemnation of significantly contaminated property was assumed to take place.

Table XXII shows that the normal background radiation doses and normal rates of cancer deaths among the population predicted to be exposed to radiation as a result of the two hypothetical port accident exceed by factors of about 10² to 10⁵ the MACCS predictions of mean population dose and cancer fatalities among the same population that might be caused by these two port accident scenarios.

TABLE XXII. MACCS PREDICTIONS OF THE DOSE TO THE POPULATION OVER A PERIOD OF 50 YEARS AND CANCER FATALITIES ARISING FROM A PORT ACCIDENT

Source Term Probability (per port call)

Population dose (Sv)

Cancer fatalities

Collision only 1.0 × 10^–6 857 37

Collision with fire 4.0 × 10^–12 2.4 × 10⁴ 1.0 × 10³ 50-year background dose >^{1.8 × 106}

50-year cancer fatalities >^{1 × 105}

Exposed population ~^{1 × 106}

Version 1.5 of the MACCS code [17, 18] was used to develop the Cancer Fatality results presented in Table XXII. MACCS 1.5 calculates latent cancer fatalities using organ-specific linear-quadratic models. These models and their parameter values are documented in NUREG/CR-4214, "Health Effects Models for Nuclear Power Plan Accident Consequence Analysis." which was written by an expert panel selected by the U.S. NRC. The chapter on late somatic effects in that report cites 39 references, including references to BEIR I and BEIR III, ICRP 26, NCRP 64, UNSCEAR.

7.3.2. Accidents while sailing a coastal route (using RADTRAN calculations)

SNL used the RADTRAN code [19, 20] to model the consequences that might arise if either of the two hypothetical transportation accidents occurred while irradiated nuclear fuel was being transported in a TN-12 flask from New London, CT around Long Island and then down the east coast of the United States to Charleston, SC at a distance of approximately 40 km from the coast. These calculations used the same inventory and release fractions that were used for the MACCS port accident calculations and three aggregate route segments (one urban, one suburban, and one rural segment). Table XXIII presents the lengths and average population densities of these three aggregate route segments as calculated using the HIGHWAY code [29] and the coastal highway route from New London to Charleston.

TABLE XXIII. AGGREGATE COASTAL ROUTE SEGMENT LENGTHS AND POPULATION DENSITIES

Segment Urban Suburban Rural

Length (km) 133 415 902

Population density (persons per km² ) 2780 386 13.5

The presence of 40 km of open ocean between the ship and the shore was accounted for by subtracting the results of a 40 km RADTRAN calculation from the results of a standard 121 km RADTRAN calculation [30], thereby obtaining an estimate of the consequences that occurred in the 40-to-121 km distance range, which comprises the first 81 km of land next to the shoreline. Table XXIV presents the results of these RADTRAN calculations.

Table XXIV shows that deposition of radioactive material onto the surface of the 40 km wide region of ocean between the sailing route and the shoreline reduces the estimated population dose by a factor of about three. Thus, correcting for the presence of a near-field region that is devoid of population produces a significant reduction in the estimated dose to the population.

Although the 50-year 33 100 Sv dose to the urban population calculated for the collision-followed-by-fire accident scenario seems to be very large, it is in fact about 20 times smaller than the 590 000 Sv background dose that the 3.3 million people in the exposed population would accumulate during the 50 years that follow the hypothetical accident. Thus, even an unusually long epidemiological study of a large portion of that exposed population would not be expected to detect any radiological consequences (e.g. cancer fatalities) attributable to the accident. Finally, not only are the radiological consequences of this extremely severe collision-followed-by-fire accident unlikely to be capable of epidemiological detection, but also, as Table XXV shows, the probability that this accident will occur while sailing near an urbanized shoreline during a voyage from New London to Charleston is so small (4 × 10^–15) that the accident is almost implausible.

TABLE XXIV. FIFTY-YEAR POPULATION DOSES (Sv) CALCULATED USING THE RADTRAN CODE FOR THREE DISTANCE RANGES ON THE NEW LONDON TO CHARLESTON COASTAL SHIPPING ROUTE

Source term Collision only Collision followed by fire Route segment Urban Suburban Rural Urban Suburban Rural

0 to 121 km 1110 255 8.9 106 000 24 400 855 0 to 40 km 795 183 6.4 72 900 16 700 586 40 to 121 km 315 72 2.5 33 100 7700 269

7.4. Accidents at sea