WSRC-MS-99-00956
An Approximate Method to Calculate 12 Rad Zone
A. Blanchard
Westinghouse Savannah River Company
Aiken, SC 29808
D. Biswas and R. Bartholomay
Westinghouse Safety Management Solutions LLC.
Aiken, SC 29801
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At the Savannah River Site (SRS), criticality alarm systems, as discussed in ANS 8.31 and DOE Order2 420.1 are known as Nuclear Incident Monitors (NIMs). The NIM system is provided to cover occupied areas in a facility in which the expected dose exceeds 12 Rad in free air, and it includes criticality accident detection devices and personnel evacuation alarms. At SRS, the area within the 12 Rad zone is defined as the area to be evacuated.
Past evaluations of evacuation zones ranged from simple point kernel to extensive MCNP4A3 models. This paper outlines an improved, yet simple and conservative methodology (based on semi-empirical equations and factors) to estimate the 12 Rad zone boundary. The following sections discuss the different factors considered for this methodology.
a. Fission Yield Estimation
ANS 8.3 requires that a 12 Rad perimeter zone be calculated using the maximum credible fission yield for that facility. A value of fission yield will correspond to the total number of fissions in a maximum credible criticality excursion event and can be evaluated from various published documents4,5,6,7. At SRS, the total fission yield is generally used in determining 12 Rad zone boundaries, regardless of how many bursts are assumed or whether personnel evacuation has been completed prior to second or subsequent bursts, except for cases where the initial fission burst disrupts the system due to thermal expansion or other energy release mechanism.
b. Prompt Primary Gamma and Neutron Dose Estimation
The prompt dose equations in the NRC guideline8 3.34 are used as a basis for the 12 Rad zone analysis. They are modified based on the following consideration:
The modified prompt primary gamma ray and neutron dose equations in Rad become:
These modified equations are used for prompt neutron and gamma ray dose estimate in a criticality excursion accident scenario in SRS facilities.
c. Effect of Concrete Shield
The concrete attenuation factor is defined as unshielded dose divided by shielded dose for a particular thickness of a concrete shield. It includes the dose build-up and must be calculated separately for both photon and neutron dose values. Based on the NRC guideline8 and subsequent modification based on generic MCNP4A and ANISN10 results, a gamma ray attenuation factor (afg ) of 10t is used, where t is the concrete thickness. The neutron attenuation factor (afn) changes depending on the concrete thickness and values of 5.0 for the first foot of concrete and 10.0 for each subsequent foot of concrete appear to be reasonably conservative. These factors yield the prompt gamma ray and neutron dose at locations shielded from the source by concrete barriers.
d. Secondary Gamma Ray Dose Estimation
Secondary gamma ray dose is produced from the (n, g ) reaction in concrete material as well as from inelastic scattering. It can add significantly to the total dose. The secondary gamma ray production depends on material, geometry, and on the source spectrum;
The secondary gamma ray dose is not calculated directly, but is estimated from the neutron dose using secondary gamma to neutron dose ratio (sg df). This ratio was calculated based on generic spectrum and simple geometry models using ANISN and MCNP4A codes, and data11 from ANL-5800. The ratio (sg df) is evaluated to be conservative and can be represented by a linear equation:
e. Room Reflection Effect
The room reflection factors are defined as the constants by which neutron and gamma ray dose will be multiplied to obtain an increased dose due to reflection from the surrounding walls of an enclosed space. This factor is composed mainly of the reflection factors due to the source room and the detector room (dose point), neglecting the contributions from the in-between rooms. This factor will be smaller if the 12 Rad zone falls outside the building, since the contribution is coming only from the source room reflection. Simple MCNP4A models were developed to estimate generic room reflection factors.
The total room reflection is calculated by multiplying the source room reflection factor and the detector room reflection factor. To encompass the variation in geometry and concrete thickness, the following conservative factors are selected for various room reflector factors:
Engineering judgment should be used to select appropriate reflection factors in case the source room or the detector room is not totally enclosed.
f. Multiple Scattering Path
Radiation can reach an evacuation zone point not only from a direct path, but also from a secondary path through single or multiple scattering pathways, such as an elevator shaft, windows, doors, etc. A simplified way to treat the multiple scattered dose contribution is to use the albedo concept using the following equation12:
Based on the available published data12, 13, 14, a nominal bounding value of 0.3 can be used for both the neutron and photon albedo in our analysis and is conservatively valid over any incident or reflected angle and neutron energy.
g. Slant Path through Concrete
The line of sight distance from the criticality site to the dose point may traverse the concrete portion of the shield at different angles and consequently the length of the slant path through the concrete shield may vary depending on the angle of incidence. The slant path angle is defined as the angle between the slant path and the normal at the point of incidence and varies between 0° to 90° . The effective concrete thickness will increase as the slant path increases. The semi-empirical equations might give non-conservative answer if the slant path through the concrete is used as the effective concrete thickness. The basic reason for non-conservative doses at steeper angle is the additional contribution coming through the minimum concrete thickness, which is then scattered in air or in last section of concrete before reaching the dose point.
In order to provide a conservative dose at all slant path angles, it is proposed that the nominal slant path concrete thickness be reduced by a factor, called slant path reduction factor (SPR factor). The following slant path reduction factors are deduced to be reasonably conservative based on MCNP4A calculations. The same factor is applicable for neutron and gamma ray dose calculations.
In summary, the NRC guideline8 is primarily intended for dose calculations outside facilities, where no consideration is given to secondary gamma ray production from concrete shields, room reflection and multiple scattered Radiation doses from different paths. A conservative methodology based on the NRC guideline is developed integrating these additional factors to determine the 12 Rad zone inside buildings. MCNP4A and ANISN models and published data are used to obtain conservative values of these factors. Direct comparison with MCNP results for representative cases show that this method is conservative. This methodology is used in many SRS facilities with significant concrete structures. Table 1 summarizes the equations and factors used in this methodology.
Table 1. Summary of dose factors and dose equations for 12 Rad zone Analysis
References