Specificity of radiation risk application for assessment of radiation hazard during a spaceflight

1. Space Weather Effects on Humans:in Space and on EarthInternational ConferenceSpace Research InstituteMoscow, Russia,June 4-8, 2012 Specificity of radiation risk application for assessment of radiation hazard during a spaceflight I.B. Ushacov, V.M. Petrov, A.V. Shafirkin. State scientific center of Russian Federation – Institute for biomedical problems RAS, Moscow

2. LEO Primary sources relevant for space radiation risks (Слайд DLR). SPE radiation GCR: Galactic Cosmic Rays trapped radiation

3. Radiation sources in spaceflight -galactic cosmic rays (GCR): high energy protons and heavy ions – up to uranium with energy up to ~1020eV (HZЕ-particles), isotropic, realize chronic exposure; -Earth radiation belts (ERB): protons with energy up to 1000 MeV, and electrons with energy from 10KeVup to 5 MeV, unisotropic, located in the near Earth space, fractioned exposure; -solar particle events(SPE), generating solar cosmic rays (SCR), consisting mainly of protons with energy from 10 MeV up to 1000 MeV. SCR flux unisotropic at the beginning of event and isotropic at the finish phase – subacfute exposure; -neutrons and gamma - rays: secondary, appearing in interactions of primary radiation with the spacecraft’s matter and biological tissue – combined.

4. “Radiation” peculiarities of deep space mission: • long – term chronic impact of Galactic Cosmic Rays (GCR) with an annual dose of 0.5 – 1.0 Sv on the background of which an acute impact of Solar Cosmic Rays (SCR) with the dose of some Sv depending on the event’s power and shielding thickness; • absence of such a strong protective factor as a geomagnetic field (it can 102 – 104 times decrease a dose); • absence of opportunity to terminate a flight in case of a radiation accident for special protective actions.

5. Characteristics of human’s exposure during spaceflight • Complicity, timely and spatially nonuniform impact of radiation with various Z, E and LET. Energetic and charged spectra of the particles and flux F very in the broad range from Z from 1 up to 92, for flux units up to 104 part/cm2 sec, LET from 1 up to 104 KeV/mcm. LET for gamma – rays is 0.3 – 0.4 KeV/mсm. • Many form of radiation impact: acute, chronic, prolonged, fractioned, total uniform and nonuniform, partial acute with changing dose rate and so on. • Modification of radiobiological effects by the other spaceflight factors can take place during the flight.

6. Dosimetric values for estimation of radiation hazard • Absorbed dose (D) – energy deposition of radiation in material - Gy (Grey) or cGy (1Gy/100); • Equivalent dose (H) – dose used for estimation of radiobiological effect: H = D x QF; Sv (Sivert) or cSv (1Sv/100); • Quality factor (QF) characterizes the radiation quality. QF= 1 for e-, p+; - and X-rays; QF=6-10 for GCR and neutrons • Effective dose (E) , Sv – dose used for cancer risk estimation, E is equal to sum of equivalent doses in given body organs.

7. DOSE DEPTH DISTRIBUTION OF GCR (cSv/year) AND SCR (CSv/event) FOR INTERPLANETARY FLIGHT

8. DOSE CALCULATION MADE FOR SINGLE FLIGHT. PEAKS ON THE GRAPH CORRESPOND TO THE SPE DOSE CONTRIBUTION.

9. GENERAL STATEMENT OF PROBLEM In realizing a space mission (SM), a significant number of competing risks take place. Each of them contributes to the total hazard and, therefore, should be taken into account when assessing the SM safety. One of the major goals of the SM is to realize a flight program with successful return of the crew to the Earth.

10. Clearly, as in any sophisticated system, it is impossible to use some determinant variables in describing the responses of cosmonauts’ body to a mixture of various aggressive factors with wide ranges of variations. Consequently, the safety criteria (standards) have been developed in terms of the probabilistic values of risk R or, in a broader sense, survivability S. If a risk is defined to be a probability for a cosmonaut to die during, or soon after the flight or, more broadly, to be a probability for health disorders to occur, then we shall be able to assess hazard to cosmonaut from any unfavorable in-flight situation using a unified quantitative criterion.

11. Risk definition “ The main peculiarity of radiation consequences after the space radiation impact is their stochastic character. Both deterministic and stochastic radiobiological effects are random quantities having all necessary mathematic attribute. The probability of health damage for exposed individual and his death as a limit case define the radiation risk connected with radiation impact. In the space condition additional peculiarity of a man exposure appears - radiation environment has a stochastic character. The probability of solar particle event (SPE) is possible to give additional exposure to a dose from the background up to lethal level. The problem appears how to use the risk as a quantitative expression of the radiation hazard for current radiation situation and how to use this value for establishing the radiation safety norms for space application.

12. Risk definition Definitions of “radiation risk” Radiation risk (RR) of spacecraft’s crew during a spaceflight is the risk of crew due to radiation impact during the spaceflight. (Russian space radiation standard) The RR is defined as the probability of appearance of some harmful consequences in a man or in his progeny induced by exposure. (Russian “ground” radiation standards NRB – 99) The RR is defined as the probability of a specified effect or response occurring. (US standards on spaceflight radiation safety) In the complete definition RR is defined as Expression of excess risk due to exposure as the arithmetic difference between the risk among those exposed and that obtaining in the absence of exposure. (US standards on spaceflight radiation safety). The United Nations Scientific Committee on the Effects of Atomic Radiation defines RR (quantitatively) as the probability of harmful consequences (events) of the exposure (for example, death) and often expresses this probability in percents.

13. Radiation risk assessment Structure of radiation risk of spaceflight The specificity of cosmonaut’s professional exposure is defined by the two factors: • character of exposure during the flight; • discreteness of the flight fulfillment during the professional activity. In accordance with this specificity it is possible to from two categories of professional radiation risk: - flight radiation risk, determined by the probability of a cosmonauts death both during the flight and after its completion; it consists of two components: -risk connected with acute consequences of the exposure during the flight; -risk connected with delayed consequences of the exposure (life-time risk). -professional risk caused by delayed consequences of the exposure during the carrier.

14. Table 1. List of recommended criteria to be applied to the System of Medical Support for space missions

15. It can be considered that survivabilityS can be expressed through a respective meaning of risk R taken to be a probability of unfavorable outcome (death or health disorders) for crewmember. The relationship between S and R is, then, very simple: S = 1 - R (1) This expression is useful when the S value is chosen to be the severity of hazard from a given factor (or a group of factors), which is usually described as a probability of death of an individual. From this standpoint, and comparing space work with other Earth’s occupations with moderate or high risks, which are thought to be reasonable for the present-day missions (hence, the society establishes strict guaranties for space crews), we may evaluate the criteria shown in Table 1 for a mission duration of up to one year. These data are given in Table 2 [3].

16. Approximate estimates of guaranties of health and life maintenance in long-term space missions

17. Note: Estimations were made for a mission’s duration of up to one year. As seen from the table, the survivability requirements can appear to be very strict and, consequently, the total risk R very small. An integral survivability variable can be calculated as a function of the length of a period, the number and character of missions during the period, and a type of relation of risk function to time R(t) both during and between the missions (post-effect). It is worth saying that the risk function in the later missions must be evaluated with allowance for the post-effects, that is, the delayed effects of the earlier missions.  As a rule, hazard restriction consists in setting the bounding values that limit the impact of an unfavorable factor, for example, dose limits. For the SM the increase of limit hardness for restricting the hazard factor impact and the corresponding increase of the spacecraft’s resource spending can make the mission impossible.

18. Survivability St within time period t is expressed by equation (6) if an astronaut participated earlier in the flights, the number of which is defined as N-1. St = 1 -= 1- Rt (6) where R() is dependence of risk distribution density on time both in the k-th flight (index k) and in intervals between two earlier flights (index l); N is number of flights per carrier (total). Rt is generalized total professional risk per time t.

19. If an astronaut did not fly before, the right-hand addend in brackets expresses the results of his exposure before the flight. In this case the expression of generalized risk for the SM is Rt = (7) In this expression the first integral is a flight radiation risk due to acute radiation effects, the second one is the stochastic radiation risk; the sum is the additional input into the two types of risk connected with preliminary exposure of a crewmember.

20. If the main goal is to implement the mission program and to return the crew to the Earth alive, then the decisive role in emergency situation parameter (ESP) formation will be played by the first integral, which reflects the probability of the established work-capacity level deterioration and the probability of radiation sicknessand death during the flight. In keeping the health within the established limits corresponding to the age norm, the major role belongs to the second integral, naturally with adding the third addend (sum). Safety level is expressed by the probabilistic characteristics (probability for ES to arise from the i-th source of hazard during the j-th stage of the mission) and (probability of getting out from the ES).

21. The fact or forecast of an excess over ESP that separate the allowed and forbidden areas can be considered as an ES. The safety level estimate is expressed by the following probabilistic characteristics: -         PEsij, probability for ES to arise from the i-th source of hazard in j-th stage of mission; -          PExij, probability of getting out from the ES. A risk can be defined as a component of the quantitative measure of hazard within a fixed time interval. The partial risk that characterizes the hazard of i-th unfavorable factor should be taken to be a limiting value of ESP (LVESP).

22. The partial risk should be defined on the time interval during which it can be realized; for example, during a single solar particle event (SPE) throughout a flight, the entire carrier, etc. Adequately, we will obtain different risk values of different amplitudes corresponding to different radiobiological effects. The limiting value of the partial risk should correspond to an analyzed occurrence and must be used to identify and eliminate that given class of events only. THE FEATURES OF REALIZING THE PRESENT APPROACH IN THE SM SAFETY SUPPORT SYSTEM.

23. The SM safety support system must be developed basing on - allowance for the contribution of each partial unfavorable factor to overall safety, -  optimal distribution of total safety components among between the partial factors and the established LVESP corresponding to the factors, - feasibility of controlling the total safety via spending the spacecraft resources for countermeasures on board the spacecraft, -  optimal selection of crewmembers.

24. In terms of the presented approach, the radiation safety should be regarded as a partial case of the total biomedical support of the flight. At same time, the limits used to ensure radiation safety should be of auxiliary, rather than absolute, character, being used as LVESP. From this point of view it is necessary to establish the values of limiting risks for all classes of ES that can occur during the flight. The radiation limits can suitably be expressed in terms of dosimetric values corresponding to adequate values of radiation risks used as LVESP.

25. The following additional criteria should be established to reach a more flexible control over the radiation safety level.  1) The radiation safety norms that classify the adverse outcomes of exposure and prescribe the value characterizing radiation hazard (radiation risk, life shortening, disturbances in ability to work, etc.).  2) The criteria for estimating space radiation environment.  3) Intervention criteria used to timely react to a trend or forecast of changing radiation environment.

26. Regulation of crew radiation exposure during a flight. At present as a rule radiation risk is a value used for radiation hazard estimation. Radiation risk is the quantitative measure of a radiation hazard defined by the increase of individual's death probability during the analyzed interval of time T. This effect caused by a specific consequences of exposure realized on all levels of organism from cellular up to bodies during all analyzed period. For operative control of a radiation hazard level during a flight the dosimetric functionals’ values equal to regulated values of radiation risk can be used as an another version of radiation norms.

27. Radiation risk assessment For the further consideration we shall use the following notations: Ф(t+a) is the probability for a man at the age of “a” to survive up to age of t+a; is the radiation death induced probability density at the moment t. Time t is counted from the moment of exposure. Then lifetime radiation risk is presented as: Rrad = (5) Let’s analyze in detail the structure of RR according to expression (1). Some compromises will be introduced for the mathematical description of the radiation risk: • probability Ф(а+t) for a man at age of a to live to the age of a+t is estimated with the help of demographic life function H(a) by the equation Ф(а+t) = H(t+a)/H(a); the life function H(t) can be presented by Homperz - Machem’s law [6]; • the cosmonaut’s death probability because the earliest consequences of the exposure to dose D during SPE exists in the risk action period TR. Death probability density function during the flight is the function of dose D and equal to zero outside TR interval. It can be assumed that and have normal distributions, the parameters of which are estimated on the basis of the experimental data; • probability of exposure to dose D during SPE is a stochastic value with probability function F*(D); • cosmonaut’s death caused by SPE radiation exposure is an independent stochastic event;

28. If we will take into account all negative consequences of the radiation impact we must rewritten this equation as: Ri(а ,НЕ)=Ф(a+t,HЕ) [hс(t,НЕ) + khз(t,НЕ)]dt where t is a time from the moment of exposure to the individual at age “a”, hсand hзare densities of death or illness probability functions, Ф(а+t)- probability to live to age “a+t” without an exposure. Ф(а+t) is defined as a probability Н(t) to live to given moment of the time and equal to ratio Н(а+t)/Н(а) – two values of Hompertz functions.

29. Risk of crewmember death during the flight caused by constant sources and SCR is given by the equation: • tk -time of SPE appearance; • а- crewmember’s age at the momenttk ; • Dak – acting non repair dose of constant sources at the moment tk , • F* (tk,D) -probability function of dose D for SPE occurred at tk [-tk, D+Dak (tk)]=

30. Approaches to Radiation Limiting in Space In the light of today’s knowledge radiation - induced death probability can be taken as a main factor of multi - dimensional risk conception. If we designate Fi(H) as a cumulative distribution function (CDF) of death probability (caused by i-th somatic effect) on an equivalent dose H, and p(H) as a probability function of an equivalent dose H per flight, then the radiation risk R can be presented as: R = So, for radiation risk estimation it is necessary to determine what radiobiological effects must be taken into account for estimating general death probability, and to know their CDF on an equivalent dose. The time interval, during which radiation death probability is analyzed, is of great importance.

31. Radiation risk assessment Radiation risk caused by stochastic effects For calculation of radiation risk we will use the following relationships: -probability Ф(a+t) is defined by the life-span N(t) as N(t+a)/N(t): , where (6) μ(t) is death probability rate for cohort with age t0, parameters λ0 and μ(0) specified for Russia are 0.062 1/year and 0.0009 1/year. B is coefficient of the mortality rate dependence on generalized dose H0, equal to 0.36 1/Sv; -radiation death probability rate in the moment t is defined by the equation (7) The dose H0 is calculated according to Russian standard [15].

32. Calculation of radiation risk and life-span shortening caused by exposure

33. Radiation risk assessment Radiation risk caused by deterministic effects Probabilistic character of radiation risk of deterministic effects in the spaceflight caused by two factors: stochastic nature of SPE and stochastic regulations of an exposure consequences. The following data are used for the risk calculation: the death probability density for these consequences is describe by the Gaussian with parameters presented in Table 2. This death probability density is given by the equation (8)

34. Radiation risk assessment Table2.Mean value Dand Standard deviation σfor density distribution probability of acute reactions and lethal issue on dose exposure to blood forming organs

35. COMPARISON OF GRAPHS FOR DEATH PROBABILITY CAUSED BY SPE TAKING INTO ACCOUNT THE BLEIR AND GCR MODELS. 10 Al. SHIELDING.

36. COMPARISON OF THE SPE RADIATION RISK DEPENDENCIES ON THE TIME OF FLIGHT TAKING INTO ACCOUNT THE BLEIR AND GCR MODELS FOR VARIOUS SHIELDING THICKNESS.

37. Total crewmember’s radiation risk for all life after the interplanetary mission completion in conditions of the various shielding sickness (X py in g/cm2) of the radiation shelter Risk, relative units Flight duration, months

38. Radiation risk assessmentTable 3.Results of risk calculation (%) for various flight’s duration and shielding thickness Death probability caused by acute and delayed effects of exposure to SCR and GCR for various shielding thickness and flight duration (accounting the repair effects)

39. Total crewmember’s life-span shorteningafter the interplanetary mission completion in conditions of the various shielding sickness (X py in g/cm2) of the radiation shelter life-span shortening, years Flight duration, months

40. Radiation risk assessmentQuantitative classificationof radiation risk during spaceflights

41. RADIATION LIMITS AND DERIVED VALUES • According to modern approaches to radiation safety ensuring (ICRP recommendations No. 60) the individual radiation risk is the value to the best advantage reflecting the detriment induced by radiation • Main radiation limit is carrier limit that consists of effective dose of 1.0 Sv. It is adequate to total radiation risk (it represents all kinds of delayed consequences of exposure) of 10%. The correspond cancer radiation risk as a component of the total risk is ~3% • This limit reflects general tendency to increase of hardness of radiation norms justified in the ICRP recommendations No. 60

42. RADIATION LIMITS AND DERIVED VALUES To manage the dose rate of exposure during the flight the working dose limits are introduced. The main role of these limits consists of ensuring the action of the repair mechanisms for acute and delayed effects of exposure. The following working dose limits were established (equivalent dose for BFO or mean tissue dose for the body): • 250 mSv per 30 days (months limit), • 500 mSv per year (annual limit).

43. General provisions of crew radiation safety concept results from the main goal of the mission’s life support system: ensuring the performance of each crewmember and the safety of the mission as a whole. Some general principles used to ensure radiation safety for occupational exposure to humans are also applicable here: a) principle of exposure limiting – not exceeding the personal dose limits from all the radiation sources;

44. b) principle of usefulness (justification by practice) – the benefit from the crew activities should exceed any possible detriment resulting from exposure to radiation (including the activities required during a radiation disturbances); c) optimization of protection principle (ALARA) – maintaining as low levels of exposure as reasonably achievable during the mission. As applied to for the Mars mission, it is necessary to take into account the number of the peculiarities that make it possible to distinguish the provision for radiation safety in this case from the adequate activity on ground and even during near – Earth space flights.

45. Regulation of crew radiation exposure during a flight. At present as a rule radiation risk is a value used for radiation hazard estimation. For operative control of a radiation hazard level during a flight the dosimetric functionals’ values equal to regulated values of radiation risk can be used as an another version of radiation norms.

46. LOGICAL BASIS OF RUSSIAN SPACE FLIGHT RADIATION SAFETY NORMS • -biological "A' when risk of unfavorable outcome of mission increases for the reason of radiobiological somatic and delayed reactions of the body to in-flight exposure, • -ergonomical "B' when risk growth is associated with radiation-induced impairment of operator's and physical ability of cosmonauts, • -technical "C' when risk to the crew rises in connection with growing probability of failure of different space vehicle systems because of radiation.

47. LOGICAL BASIS OF RUSSIAN SPACE FLIGHT RADIATION SAFETY NORMS • Probability of each of the above types of radiation risk denoted as P(A), P(B), P(C), P(B|A), P(C|B) where • -P(B|A) is the probability of ergonomic failure due to specific radiobiological effects as are, for instance, acute radiation reactions and • -P(C|B) is the probability of technical failure following repair improperly done by radiation-impacted cosmonaut, • total risk P can be determined from equation expressing a full probability of dependent parameters • Р = Р(А) + Р(В) + Р(С) - Р(В|А) - Р(С|В) (1) • Events A and C are assumed to be independent. This is one of the most important differences in estimating radiation hazard in space and on the ground.

48. Functional disturbances of CNS by appearance of asthenia syndrome after longtime radiation impact with small doses (<3-5 cSv per year) -Disturbances of pain and olfactory sensitivity, - Disturbances of a memory, - Reduce of visual and otic functional flexibility after a loading, - Increased fatigability, - Reduce of rate and accuracy of a work, increasing two times of the number of errors.

49. Water consumption by the rates from the water – trough installed outside (1) and inside (2) of the area of radiation impact of gamma – rays with the dose – rate of 1670 mkR/second 1 2 days