11.5 Radiobiological risk assessment

a. A radiobiological risk assessment shall be performed by comparing the protection and operational quantities calculated according to the definitions in Clause 11.2 with the protection limits defined for the project in accordance with requirement 11.4a.

b. When calculating the protection and operational quantities as specified in requirement 11.5a, the influence of shielding in attenuating the primary particle environment and modification to its spectrum at the location of the astronaut shall be evaluated as follows:

1. Perform initial calculations as specified in Clause 6.2.2 to assess the influence of shielding for worst-case shielding, environment and secondary production.

2. If these indicate that the protection limits are exceeded, perform more detailed calculations using a detailed sector shielding calculation or Monte-Carlo analysis, calculation, as specified in Clauses 6.2.3 and 6.2.4, respectively.

c. The evaluation specified in requirement 11.5b shall include the potential variations in radiation exposure as a function of shielding material and its configuration.

d. Scaling to the equivalent areal mass shall not be performed, unless an analysis is performed that demonstrates that the scaling provides an overestimate of the severity of the environment.

e. The minimum shielding requirements shall be specified for each mission phase.

NOTE The reason is that the shielding issues depend on the mission phase scenario and the associated crew activities within the spacecraft habitats, lunar or planetary habitats, or extra-vehicular activities.

f. The crew exposure shall be assessed for all the following:

1. the nominal environment,

2. energetic solar particle events,

3. radiation belt passages, and

4. conditions where the 30-day radiation environment exceeds the nominal environment by a factor of 5.

NOTE This is to account for anomalous environmental changes that can affect the 30-day dose limits.

g. The linear, no threshold (LNT) hypothesis shall be applied extrapolating high-dose-rate data in order to quantify the risk of radiobiological effects.

NOTE For long-term missions the doses are likely to attain values where extrapolation can be replaced by a look up into epidemiological data.

h. If shielding simulations are performed which include self-shielding, the simulation shall include the variations in a build-up of high LET particles, including the nuclear interactions (“star” events) of these particles.

i. Self-shielding shall be included for simulations where the shielding afforded is less than provided by the self shielding.

NOTE For example, astronauts during an EVA.

j. For simulation of the effects of self-shielding, secondary radiation generated within an organ shall not be included in the calculation of the equivalent dose to that organ.

NOTE 1 The reason is that radiation weighting factors already include secondary particle contribution.

NOTE 2 For extremely densely ionising radiation like HZE (high mass and energy) particles and nuclear disintegration stars the concept of absorbed dose can break down and has therefore become inapplicable, but not having better concepts it is the only one used to calculate effective dose or dose equivalent.