External Dosimetry - doe-std-1136-20040145

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiological Protection in Uranium Facilities

6.0 EXTERNAL DOSIMETRY

The external dosimetry program is an integral part of the external dose control program. DOE G

441.1-4, External Dosimetry Program Guide, provides detailed guidance for implementing an external

dosimetry program that meets the requirements of 10 CFR 835. The reference section of that Guide lists

specific documents applicable to external dosimetry. Because the requirements and recommendations are

explicitly given in these documents, they will not be discussed in any great detail in this chapter. Rather,

the emphasis will be on items that are unique to uranium facilities and the radiological aspects for safe

handling of uranium.

Measuring the external radia tion exposure and the resultant dose is complicated by the many

radiations involved in uranium handling. Chapter 2 of this Technical Standard discusses the radioactive

decay schemes for and radiations emitted by the uranium isotopes and their radioactive daughter products.

Uranium has a wide distribution of beta and gamma energies, with a 2.29-MeV beta as the most significant

of these. The dose rate from photons is relatively low. Uranium also emits alpha particles that may generate

~2 MeV neutrons as a result of interactions with the nuclei of fluorine or other low-Z atoms. The

magnitude of the neutron fluence depends on the enrichment of the uranium and on the interacting

chemical.

The elements of the external dose control program are: detection and characterization of the beta,

gamma, and neutron radiation fields; measurement and quantification of these fields; measurement of

personnel dose; and determination and establishment of dose control practices.

6.1 DOSE LIMITS

10 CFR 835 specifies the applicable limits used for control of external radiations. Table 6-1 lists

the appropriate depths in tissue for measurement of doses to the whole body, lens of the eye, "unlimited

areas of skin," and extremities.

Table 6 -1. Effective Depth of Tissue for Various Organs

Depth of tissue, mg/cm2

Deep (penetrating)

1000

Lens of eye

300

Shallow (skin, extremities)

6.1.1 Limiting Quantities

In 1977, the ICRP introduced a major revision in recommended radiation protection practice with

the introduction of ICRP Public ation 26 (ICRP 1977). The new methodology establishes a "risk-based"

system of dose limitation. The ICRP introduced the terms stochastic and nonstochastic for radiation effects

and set limits for both types of effect. Stochastic effect is defined as one for which the probability of the

effect occurring (as opposed to the degree or severity of effect) is a function of radiation dose.

Nonstochastic effect is defined as one for which the severity of the effect is a function of the dose; a

threshold may exist. Limits were established such that the risk of stochastic effects occurring was

equivalent to about the same risks faced by workers in "safe" industries who were not occupationally

6-1