Indoor Air Quality Implications of 222Rn from Lunar Regolith Rutgers Lunar Settlement Symposium

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Indoor Air Quality Implicationsof 222Rn from
Lunar RegolithRutgers Lunar Settlement Symposium

• François Lévy
• John Fardal
• The University of Texas College of Engineering
• Department of Civil, Architectural and
  Environmental Engineering
• contact info_at_francoislevy.com

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• Regolith-Aggregate Concrete
• Future long-term lunar expeditions will rely on
  in-situ resource utilization (ISRU) for the
  construction of bases (Schmitt 2003 and Benaroya
  et al. 2002). Lin et al. (1997) demonstrated that
  lunar soil, or regolith, is a viable concrete
  aggregate using a dry-mix/steam-injection
  technique (DMSI). It is therefore viable as a
  prevalent raw material for lunar bases.

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• 222Rn
• Several researchers have established the lunar
  presence of 222Rn as a natural decay product of
  238U (Lawson et al. 2005, Gorenstein and
  Bjorkholm 1977).
• Yaniv and Heymann (1971), Friesen and Heymann
  (1972), Lambert et al. (1975), and Friesen and
  Adams (1976) all establish the presence of radon
  in lunar regolith, as well as 222Rn levels and
  exhalation rates.

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• Exhalation
• We examine two plausible lunar settlement
  configurations which introduce lunar concrete
  into the habitable space, and use the cited
  exhalation rates in numerical models to make
  preliminary determinations of 222Rn levels.
• This is significant because 222Rn presents
  well-established risks to human health (i.e.
  Nazaroff and Nero, 1988 Krewski et. al 2005)
  222Rn has a 3.8 day half-life, and decays to
  218Po. The National Academy of Science attributes
  15,000 to 22,000 annual lung cancer deaths to
  222Rn in the US.
• We then propose potential remediation and
  directions for future research.

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• Model Parameters
• In order to make first-order estimates of
  regolith concrete quantities within our
  hypothetical habitats, we considered three
  primary functions of a regolith-based structure
• Radiation Protection Silberberg et al. (1985)
  suggest that 2 m of regolith would provide
  adequate shielding from both background galactic
  cosmic radiation (GCR) and uncommon but
  devastating solar proton events (SPE). This
  assumes 80 human occupancy of a base, with the
  balance of time spent outside.
• Micrometeorites Jolly et al. (1994) recommend 3
  to 4 m of regolith as a barrier to primary and
  secondary impacts.
• Thermal Stabilization 1 m of regolith stabilizes
  lunar temperatures to 238K (Heiken et al. 1991)
  or 253K (De Angelis et al. 2002).

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• Bermed, inflated structure
• The first structure is based on a bermed,
  regolith-ballasted inflatable habitat proposed by
  Chow and Lin (1988)

Bermed regolith
Habitable space
Concrete slab (.1 m)
Inflated structure with toroidal support
Compacted regolith subslab fill (2 m) (.1 m)
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• Concrete Habitat
• The second structure consisted of an all-concrete
  enclosure

Habitable space
Concrete slab (.5 m)
Lunar regolith
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Scenario 1 Assumed 222Rn Emission Rate

• 3 atoms cm-2 minute-1 taken from lunar fines
  data, not concrete (Lambert, et al., 1975).
• Concrete surface area of 320 m2 for concrete
  habitat 100 m2 for inflated habitat
• Closed environment in either habitation, and
  neglecting adsorption to surfaces and aerosols.
• Steady state formula (Yaniv and Heymann, 1972)

Where E 222Rn exhalation rate S surface
area of the source V volume by air ? decay
constant of 222Rn (2.06E-6 s-1) (Kovler, et al.
2005, Part 1).
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Scenario 2 Calculated 222Rn Emission Rate

• Determine radon emanation rate (Yaniv and
  Heymann, 1972)
• Assume density of regolith-aggregate from density
  of regolith
• Determine diffusion coefficient for 222Rn, based
  on Rogers et al., 1994 for terrestrial concrete
• Numerical model for flux between layers of
  regolith-aggregate concrete from Ficks First
  LawWhere D diffusion coefficient of
  222Rn dC/dx change in pore space radon
  concentration with respect to a distance
• Flux from the concrete-air interface used as the
  emission rate

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Numerical Modeling

• Regolith-aggregate concrete divided into 50
  layers
• 60 second time intervals
• Time allowed to progress to steady state in
  outermost layer approximately 10 days to reach
  steady state
• Flux calculated from layer at the concrete-air
  interface worst case scenario assumed zero air
  concentration
• Compacted fill in inflatable structure introduced
  a flux of 3 atoms cm-2 minute-1 into lowest
  regolith-aggregate concrete layer

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Numerical Modeling (continued)
Concentration calculated for each layer and time
step using
Ci concentration at the midpoint of the slice of
material (atoms m-3) Cemitted concentration
increase due to emission of radon (atoms
m-3) Cdecayed concentration change due to decay
of radon (atoms m-3) D diffusion coefficient of
radon (m2 s-1) t time interval of the time
steps used in the numerical solution (s) x
thickness of each slice of material (m) n time
step number i number of the layer
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Results
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Radon Discussion

• Noble gas unreactive, low sorption
• No ventilation in habitations
• Once in the habitat airspace, cannot be easily
  removed
• Only way to lower concentration is to lower
  emission rate

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• Acceptable Risk
• The Environmental Protection Agency (EPA) has set
  a standard for 222Rn exposure at 4 pCi L-1 (148
  Bq m-3). OSHA (Occupational Safety Health
  Administration) and the NRC (Nuclear Regulatory
  Commission) have established a standard maximum
  of 100 pCi L-1 (3700 Bq m-3) averaged over a 40
  hour work week for workplaces. Given that
  regolith-concrete lunar base inhabitants would be
  exposed to 222Rn nearly continuously, the EPA
  standard is more appropriate.
• In one of our results we exceed the EPA standard.
  It should be emphasized that according to the EPA
  there is no safe 222Rn level increased
  exposure increases risk of lung cancer.
• Other environmental factors (GCR and SPE
  radiation, meteorite and micrometeorite impacts,
  temperature extremes) present more obvious risks
  to human health, but 222Rn exposure is a
  potential threat that should not be ignored.

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• Control Technologies
• In cases where 222Rn concentration levels exceed
  safety standards, two options exist to reduce
  risk.
• Gao et al. (2002) show that the use of polymer
  additives to a cement plaster reduce 222Rn
  concentration levels by 85 in field tests.
• Daoud and Renken (2001) shows a promising method
  for reducing the diffusion coefficient of radon
  by using flexible thin-film membranes. Such
  membranes reduce diffusion rates by 83.4 to
  96.6.

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• Conclusion
• Using assumed exhalation rates and numerical
  modeling, we have calculated 222Rn rates from
  regolith-aggregate lunar concrete, and these
  concentrations may reach dangerous levels in
  certain types of ISRU lunar habitats. 222Rn
  concentrations are therefore a significant health
  concern which should be considered in the design
  of these types of structures.
• With the lack of ventilation inherent to a lunar
  habitat, it will be of the utmost importance that
  cost-effective means of reducing 222Rn
  concentrations are employed, whether thin film
  membranes or polymer cement plasters.
• Without detailed data on the physical properties
  of regolith-aggregate concrete, it isnt possible
  to accurately determine 222Rn emanation and
  diffusion coefficients from such concrete, and
  thus exhalation rates. More research is required
  to provide this data.