Radioactivity as Sustainable Source of Energy: An Oxymoron or a Nature-Inspired Concept of Resources Management?

Comment: INTRODUCTION     The first alarm about the criticalities  that  could arise out of  an uncontrolled  development  of   technology   has been given   by the Club of Rome , that in 1972 published the famous   Report  “ The Limits to Growth”  [ 1] .This  fundamental study   started to question   the paradigm  that  the humanity  had   undefined  available resources  and could  therefore  perturb  the  ecological equilibrium  without  consequences . The   industrial era  has  been  indeed characterized  by  an antagonism  between  environment  and  technology  ,  this last   designing  and producing  artificial  objects and  anthropomorphic systems  with no  analogs in nature  ,  disrupting  any  balance  between  bio-sphere and   techno-sphere  .We are  now  well aware  of the fact  that   the  limited supply of  non- renewable  energy sources  cannot  meet  future  needs . Moreover,  the  fossil fuels   are not  only “ non-renewable”   , but , what is worse, are the major responsible  of  the  abnormal climate changes,  closely  correlated to  a  CO₂   emission   unparalleled  over the past  .    To respond  to  the  sustainability  challenges  of  the XXI century ,   the  right approach to  manage our  world  is  not only the control of   the impact exerted by  the civilization  on the  natural resources, but  rather   the   change of  strategies to restore a nature-like  self-consistent    cycle of    energy/ matter   inter-change .  THE STATE of  THE  ART   Todays  a lot of scientific work  is going towards  the modeling    of  man-made  activities  on   nature-like   technologies  , and  the inter play  between  the   bio-world  and  the  techno-world  is  opening   new possibilities to  reproduce systems and  processes   inspired  by  nature  . Millions of year of evolution  allowed nature to find  the best  solutions for a series  of problems , such as   the  energy  requirements  of  the  biosphere .For the vegetable kingdom  the solution was  the photosynthesis  process     that  convert  the  solar energy   into  energy  for metabolic reactions .       In the search of innovative  energy  sources   scientists  tried  to  replicate   such a process  by  assembling    photovoltaic  cells based on semiconductors  .In particular  the   dye-sensitized  solar cells  proposed  by  Gratzel in 1991  [ 2 ] mimic   the   methods adopted by nature  that uses  complex bio-organic structures   for  light  absorption and  energy  production .  In  the  “Gratzel –like “  devices   the   role of chlorophyll   to transform   photons  into  chemical energy  is played  by    artificial  dyes    that,   coupled with  semiconducting materials ,   generate   electrical  power .    However the opportunity  to  convert   sunlight  into  available  energy, either metabolic or electrical,   is offered  only  to systems  exposed to the sun radiation  , whereas we  know  that  there  is life also in   regions  isolated from   the photo-sphere .  Under  such conditions  life evolved    developing  alternative mechanisms    for harvesting   energy   from  other  natural resources  [ 3 ] . As disclosed  by  paleo-biological  investigations  , one of  the  other  way  followed  from million  years   by   biosystems    for  life  flourishing    in absence of  solar  light    was  to   exploit  the natural    radioactivity  [ 4 ,5 ] .    When radioactivity come into play, a generalized  anxiety is  generated  , because people connect  such  physical phenomenon to  the big catastrophes that occurred  in the recent years  ( Chernobyl, Fukuiama ) . But  radioactive  decays are  physical  processes   naturally occurring  “on”   and  “around “  the Earth . Mankind   coexisted  ab initio   with radiations   emitted  from   terrestrian rocks  and  landed  meteorites , as well  as  with  cosmic rays  raining down on the  Earth .     The advent of  the nuclear  era  and   the related  escalation  of   civil    and  military  applications   has led   to   a continuous  production of  a series of artificial    radioisotopes ,  either  obtained   as   by-products   of fission reactions   or   purposely generated  for specific applications ,  as  diagnostics and  medical  therapies  [ 6 ].In each case the  safe  disposal of   such  radioisotopes , that can remain radioactive  up to  tens of thousands years ,    is a  problematic   task and  the produced nuclear waste  is  an additive source of   environmental   radioactivity . From the point of view of  safety       the nuclear decays  cause  endless  troubles   but ,  from  a different  perspective ,  radioactivity  can be viewed  as an alternative   source  of   power.      The  idea to  fabricate a nuclear  battery  is not   new  ,  the concept was  firstly  proposed  by  H.G.W.  Moseley  ,who  in 1913  fabricated and tested a device    exploiting   the energy released  in  the α-decay of  Radium  [ 7].  Further research work demonstrated  that  nuclear batteries  could   generate electricity  by  converting   highly energetic  α   or  β   particles  and  γ   radiations  emitted from  a variety of   radioactive isotopes. The   generation of electricity from radioactivity   can be obtained  by means of thermal  and  non-thermal   conversionn mechanisms .  In Fig 1   are schematically  represented the   main   methods  used for such conversion RADIOISOTOPE      EMITTERTHERMAL    CONVERTER  NON-THERMAL CONVERTER  INDIRECT    ENERGY CONVERSION  DIRECT  ENERGY CONVERSION  BETA ALPHAGAMMA        
   Fig 1  Classification of the main mechanism  for conversion  of  radioactivity in  electricity    A complete analysis of  all  the  possible  radiation sources  can be found  in [ 8 ] .   This  article  takes into account   not only  the    α,  β  and  γ   nuclear  decays , but also  the  emissions of neutrons and  fission fragments , and  reports on   the feasibility to  fabricate  nuclear batteries  able to produce  electricity   on the basis of the   various  conversion mechanisms .  A    β-voltaic  solid-state device  that produced electricity  from the     β-induced   ionization  of  intrinsic semiconductors    was patented  in 1953  [ 9].  In such   battery  direct  energy conversion was   achieved   using   a diode  configuration for the converter ,  with   the radioactive source  closely contacting  the   p-n  junction (Fig 2 ) . Fig.2   Scheme of the processes  occurring in  a semiconductor exposed to  β-radiations  and of the electrical  circuit      [From  S.Kumar  .arXiv 17 November 2015 ] From the  ‘70s  to  late ‘80s  ,  β-voltaic  cells   based on the use  of   long  lived  radionuclides (mainly  ¹⁴⁷Pm  and  ²³⁸Pu  )  have been   widely used  to power  pacemakers  [10 ] .   In   the  implanted patients   such devices  showed    an  extraordinary  reliability   combined   with    proper functionality  and  lack of  safety risks .   The  current   replacement  of  the nuclear  cardiac  devices  with  Li-powdered batteries has   being   motivated  by other  concerns  , related more to security  than to safety  [ 11] .The  technology  of   batteries  based on   α-particles emitted from  radioactive nuclides  was developed  in 1954   [ 12] .  Wide band-gap  and  radiation-tolerant  semiconductors are needed   to convert the  α-decay  into  electrical current    , but  the  energy output   of  α-voltaic   device   is   a factor   ≈ 100 greater than  that of   a similar   β-voltaic  power source ( assuming  the same  conversion efficiency )  . The  plot of    specific energy density  (J/Kg)  against  specific power density ( W/Kg)  (Fig.3 )   shows   that  not only the  α-voltaics , but also other nuclear batteries  based on the direct  conversion   mechanism  offer  energy densities  higher  than any other power source [ 13,14 ]. Energy Density (kJ/kg)         Fig. 3  Ragone plot   comparing the  performances of various batteries : capacitors ( grey ) chemical  cells  (blue),  fuel  cells ( green) ,  nuclear  batteries (red) .  The lines indicate  the discharge time for each  technology   [ adapted from Ref.  14  ].     However,  the nuclear  batteries with  long shelf-life  suffer  from some intrinsic limitations,  such as  low  specific power density ,  efficiency   typically  <10%   and    radiation damaging of the converter material [15]. Moreover there are  obstacles  to  decrease the cell size.   The  effective  miniaturization of nuclear  devices  is  presently  a virtually  unattainable   objective , mostly  when   the  highly  penetrating  γ-rays are the radioactive source and a proper shielding  is needed .  The failing  to   down-scale   batteries size   is a critical constraint  that   virtually  hampers    the  integration in  the   ever-smaller and   extremely  compact electronic  devices  used  in several technological fields  . The  whole of   shortcomings  is  nowadays  restricting  the  usage  of  nuclear batteries  only  to  some niche   applications. In particular such   energy sources  are  employed  in  sensors   or  communication  devices/ nodes  located  in remote or harsh environments  ,  which are  required  to last the lifetime of  the  infrastructures   [ 16-17 ]  .    Nevertheless, the  advantages  offered by the  long lifetime  is still  pushing   researchers   to   improve  the performance of  the  radiation-based    solid-state systems , making them appropriate  for  a  wider range of end-uses  . Recent studies   demonstrated  that   a non-conventional  design of  the  converter, together with  the  choice of a proper semiconducting  material   and   the control  of the surface  nanostructure  , can enhance the   electrical  power  output .  The specific  energy  of a   β-battery  based on   diamond   in  a Schottky  diode  configuration   can indeed   reach   values  an order of magnitude higher   than those  of  a  conventional chemical   cell  [18 ] . Up to now  several converter   configurations  have been tested,  producing  devices  with  innovative  quasi-vertical ,  vertical   and also  “corner” architectures.      Even if  there are no doubts  that    optimization of  the  cell  design  and  new  fabrication routes  will  in future   improve   the performances  of  such  batteries  , the   combination of  downsizing    constraints ,  low conversion efficiency  and  limited  power density  is  hindering   a  large scale  commercialization  of  nuclear batteries  for   integration  in  tiny electronic devices .         But   the above  mentioned  drawbacks  are  really   limiting    the possibility  to  use  nuclear  radiations  as  an alternative  energy source  ?   PERSPECTIVES and CHALLENGES     Thinking to  the  mines of  natural  radioactive  elements  as well as  to the  storage facilities  of  spent   nuclear  fuel and  of  radioactive waste , it is clear  that  such  premises   could   be regarded  as  rather  “endless”    sources of  an energy   that wait  only  to be   properly captured .  If the objective is that of  centralized  utility-scale installations   and not  of mobile units,  the  inability  to downsize   the batteries   is no more a concern .  Moreover, one would  no longer  speak  of  a  truly  “battery” , i.e.  of a complete  system    that  supplies   electrical power  by conversion of  energy  from an internal  radioactive     source , but  merely   of  the  transducer,   i.e. the semiconducting  component  that acts  as   power generator  .   Large  plants   could   be fabricated  inside  abandoned  mines of  U, Th and  rare earths , or  storage areas of  artificially produces   isotopes , available and  affordable sources of  radiation  all over the globe  .  When  huge deposits  of  natural or artificial  long-living  isotopes  are  put into play  and  a unexhausted   supply of  high-energy radiation  is  guaranteed  ,   all  the   drawbacks experienced   in harvesting power  by means of nuclear batteries   are expected to fade.      In a  centralized  installation   the issue of reaching   high  power  levels   can   be  easily  tackled  by  connecting  a   number of  converters in   large    modules , overcoming  in  such a way  the  low   efficiency  of   the direct  or un-direct mechanisms   involved  in   the    conversion of    radiations  into   electrical power  .    As regards  the  radioactive sites, , there is  an ever  growing  public concern about  them , however  not all  the  radiation sources are  looked at  with the  same attention . Storage  sites  of  radioactive  waste  produced  in  nuclear   plants face effective  protest   by citizens  and the risk perception  influences  negatively  the public acceptance, even if  such  installations   are object of severe  regulatory issues . Conversely,   little or  no  attention  is  payed   to the mining   sites  of  radioactive minerals  and to the  issues of  safety,  environmental effects  and  also security  related  to  such radioactive  premises .   In this context one  must  figure out  not only   the  operating mines, but  rather  the  disused  ones  . Around   the world  there are indeed  a lot of  old  U, Th and  rare earth mining   areas  that, after the closing  of exploitation,  have been abandoned   by the companies without   any  on- site recovery  ( Fig.4) . Fig.4 . View of a  typical  abandoned   U  mine .      The abandoned sites and  their  surroundings  could represent  a  hazard for  the   populations  exposed  to   high levels of ionizing  radiations .  As   reported  in. [ 19 ]  , in many cases   the   exposure   exceeds   the  reference  values established  for  annual effective  dose limit by the  USCEA   [ 20 ] .The  mitigation of the   effects produced by high-level radiations  needs a deep  rehabilitation of  the  legacy  mines  . However such  topic does not  affect  the  public  perception , therefore  the   protection against  radiations    in former mines  is not seen as a  pressing objective . In this view   the putting in place of recovery strategies  is  very  unlikely,  unless  outstanding  economic interests  were  coming into play   . The  planning of  a centralized  battery bank   based  on multiple   energy converters   should  provide  the   positive  side-effect   to   guarantee    the security of the sites  .  In this context  the  threats  connected  to  the  disposal  of  the large amount  of  nuclear waste   and of   highly radioactive soils    would be  turned  into  an opportunity .        However ,  just speculating  about  the use of  radioactive sites  for power  harvesting  causes  a   widespread  anxiety among  general public  .  The perceiving of  radioactivity as a  frightening   hazard  , no  ifs and  buts ,  could represent a  stumbling obstacle  able  to stop not only any initiative , but also  any   feasibility  study  . What is not perceived  is  that  precautionary  approaches  to the use of radiations,  even of high level ones ,  are currently  feasible .  In  power  installations , how   done   in  damaged reactors or  in nuclear  waste warehouse ,  the  running   of  the  building and  operational  phases  would be   remotely controlled  by mechatronic  systems  and  unmanned  platforms       The  legislative and  regulatory  issues   with  which to comply are  the results of  old    studies  on  the  interferences  of  the radiations  with  the physiology of   living species and on   the  adverse effects  induced in  cellular components . These  studies   led in  the 1950s to  establish the linear no-threshold  (LNT) model  adopted since then by  international advisory  bodies . The LNT  paradigm  assumes that  health risks occur   independently  from  the energy of radiations ,with alterations of  DNA functions and  genetic code and  increase of  cancers   . However,  the  contradictory  concept of radiation hormesis , i.e of the benefits  for health  of low-dose radiations [ 21] is now well   demonstrated  [22-24].   Moreover  in recent  years it  has been discovered  that   for  some   organisms  growing  in radioactive environment  not only  cell survival  is ensured  ,  but  there is evidence  that  the  organisms  utilize radioactivity    as a source of metabolic energy . Such unexpected  behavior   was preliminary noted  at the end of  50’s  in   fungal  colonies   grown  in Nevada nuclear  test sites  [ 25 ]. More recently  it was discovered  a  flourishing  of   single-cell  fungi   in the   highly  radioactive  areas surrounding   the  damaged  Chernobyl  Atomic Energy Station   or in the cooling water  of still  operating   nuclear reactors  [26] .    The  studies on   the  species dominant  in soils contaminated  by   naturally occurring or  anthropogenically originating  radionuclides  , as well as in the high-radiation environment ( i.e. Antarctica  highlands  )  enabled  to   disclose   that  in all cases  such species were  rich in  melanins  [ 25 ,26 ]  . The  broad term   “ melanins  “  indicates    a class of  naturally occurring   conjugated  polymers , based on  C₁₈H₁₀N₂O₄  molecular  sub- units.       Whereas  at the beginning of 2000’s   was well   known the  capability of   melanins    to absorb a wide range of  the electromagnetic  spectrum  , at that time there were only hypothesis about   the  way followed  by  melanins  to  transform   dangerous  α, β  and  γ  radiations  in energy  for   physiological processes .  Evidence  proving  the radiation-induced  increase of metabolic activity   was achieved  from   experiments carried out on   C. neoformans cells  exposed to   high radiation levels  . The   laboratory  studies  highlighted   modifications of  the   melanin electronic structure in  irradiated  cells   and   allowed  to  quantify    the    melanin-mediated  electron transfer  rates , that were  found  up to 4-times   increased   when  compared with  those  of  unexposed cells   [ 27,28] .    Afterthat   other studies  confirmed   the  role of melanins  in  the  growth of  melanized  fungi   exposed to   ionizing  radiations  [ 29-31]. Attempts to understand  the  way  radioactivity  is  transformed  by melanins in  energy available for metabolic processes   evidenced   similarities  with  the mechanisms  adopted  by  chlorophyll    in  turning  energy  from radiations into  bio-energy  . In both cases  what  come into play is the electronic  structure   of the chemical species, but,  whereas  chlorophyll  generates chemical energy   from  non-ionizing radiations  through  the   photo-synthesis  process  ,  melanins   carry  out    radio-synthesis processes ,  converting   the  ionizing  high energy  portion of  the  electromagnetic spectrum as well as  radiations from nuclear decays.    In  living organisms  melanins  exploit   charge transport    following  multi-step  electron  transfer pathways  . This   semiconducting   behavior     accounts for   the   consistent  presence  of melanins   in  some specific locations  of the  human body, as  retina,  inner ear,  midbrain  (substantia nigra ) where  charge transfer phenomena  take place  .    The electronic properties  of  eumelanin,  the most interesting component of the melanin family , had   been   outlined  in  1972  by  J.E. McGinness  [ 32 ]   who   also   tested  the  material   as an amorphous semiconductor  threshold  switch    [ 33 ] . The capability of  the  eumelanin   to  parallel  , or also  to go beyond   the  performances of  inorganic amorphous  semiconductors , led  in recent years to  investigate  the  viability of a  melanin-based  bio- electronics  [ 34]. In Fig.5 is shown the  molecular structure of the eumelanin oligomer .Fig. 5  Structure of the eumelanin oligomer      Even if It is  now   well established  that   the ability  of  some   organisms , as the radiotrophic fungi  ,  to  withstand  high doses of  ionizing radiation is due to their  richness in  melanins  ,  details of  the mechanism   implemented by  melanins to  assure cell survival   safeguarding DNA  are not  yet been clarified  [31 , 35]   . Among  the  hypothesis   proposed  at  a speculation level to  explain the   radiation resistance of  melanized  organisms    , the  most accredited  is that  melanins  succeed  in quenching   the cytotoxic free radicals produced  by radioactivity    [ 35-37] . In this context  studies   on   melanins  are  been mainly  performed by  biologists ,  who   are still  trying  to  reshape  the conventional  schemes  about  electron transfer  in   metabolic  pathways   [ 30 , 35].    The  advances  in the field of organic/ bio-inorganic  electronics and  optoelectronics   yielded   in recent  years   a large number of publications   dealing with   melanins  , and    guidelines  to understand  charge  transport  features  of  such     organic semiconductors  are  now  provided  [ 38 ]   .  However  it is to be noted  that  ,  whereas      a lot of applications    are being proposed for  bioelectronics and  biosensing   [ 39-42 ] ,   the  issue  of energy  harvesting   by melanin-based    devices  is  overlooked  by the scientific community  This is likely  due to the  output  of   several studies  that  evidenced   an intrinsic   low conductivity of  the  material ,   poor  performances  in converting  light into electricity   and  a  scarce stability  of melanins  under long-term  illumination .      However ,  such drawbacks  regard  the  efficiency of  light conversion , in which melanin is certainly  not competitive with inorganic and also  some organic semiconductors. If  ionizing radiations are taken into account as  energy source, the  approach must be completely reversed . It is indeed  well known  that  ionizing radiations    have  a    strong detrimental  impact  on   inorganic semiconductors  , limiting  therefore their use in  highly radioactive environment .   The  low  resistance to radiations  of  conventional semiconductors   is  precisely  one  of  the  arguments  put  forward  against  the  development  of  nuclear batteries . On the contrary ,  due to its properties,  melanins  could  act as a  successful active   material  in   converter devices for  electrical  power   production .     The  consequence of the papers published in  2007 - 2008    [ 27-29 ]  was  a  focus  pulled  on  melanins/ radioactivity  systems for production  of  metabolically useful energy .   Many reports  and commentaries disseminated  information  about  the  melanin’s   performances  in capturing  and  living off  radiations  ,  but  the interest was  exclusively   towards  the  increased  growth of  some  living  organisms   and  the transformation of  radiations  into   “ food”  or  biofuel  .  Some exciting  ideas have been proposed , such as  to use  radiotrophic -fungi to destroy radiations and radioactive particles , or  to feed  astronauts  during  future long  voyages  in space  using   galactic cosmic rays  to grow eatable  organisms  .  The scouring   of the  biomedical  literature allows one  to  find  many  studies and  commentaries   that discuss  the  role of  melanin in  mitigating  the  effects  of  radiation, radioisotopes and  fallout    on living   species . Melanin-rich  fungi  have been  tested  as   radioprotective  food  [ 43,44 ] and  ESA  recently  funded  a   project  to evaluate  melanin-based   materials   for the  human  protection  during  space flight missions    [ 45 ] .    Overall ,in the last decade  the traditional  picture of melanin as   the  pigment  absorbing   harmful    components  of  the  solar spectrum   has been  widely  broadened., to include  the unexpected   features of  radioprotection .   What  are   missing  , conversely ,   are  studies/speculations   about  the  possibility  to capitalize  on the opportunities  that   melanins  can offer  for electrical  power production from    radioactive  sources .   Certainly  not an easy  task  to be pursued  ,  but   a bounded problem with a realistic development timeline.   Considering that  mankind  is  now addressing  much more complex  and challenging task  , as the  colonization of  Mars  ,   the    design of   plants  to convert  radiations   into  electrical power  would be  a  children’s game  . CONCLUDING  REMARKS        Other  are the  issues   that can really   impede   to attain   the objective   to  produce  energy ,with zero-CO_2-emissions ,  from  already existing  radioactive sites  , converting   therefore  a trouble in a  resource  .   The first one is  a   somewhat   compartmentalization  of  researches.  It is to be noted  that  each  topic  briefly  outlined   in this  paper  , namely  radioactivity,   nuclear batteries and  melanins ,  has been  - and still  is -  the object of deep  investigations   carried out   in the frame of   highly specialized   and  sectorial scientific /technological  areas .  As an example , the  idea  of  energy  harvesting  from  radioactivity , launched   inside the restricted community of  researchers  working  in  nuclear   technologies, did not  trigger  up to now  the attention  of  people  addressing   the  topic to  rebalance   energetic cycles  and  processes  for  a sustainable development  .     As a general  rule  the  management  of   the fast changes of  today’s world and the  tackling of  society’s grand challenges   must pass  through the strategic priority  to create  a fertile  ground  for  really  multidisciplinary collaborations  covering  the  whole spectrum of  technologies .    The  second  big issue  is the  negative perception and subsequent  low acceptance of radioactivity , even  of the low-level one ,  by  the  general public . It is  to be noted  that   the concern  about  radioactivity   in our society is greater than the actual  health hazard  .  This depends  mainly  on the   lack of   a correct  scientific information  necessary to  overcome  the  generalized  fear   created   by  the   catastrophic events  of  the last decades . The  matter  to earn  the public’s  trust   should   be addressed  by  launching  initiatives  and  promoting  effective  information  campaigns  about   risk assessment  and  related regulatory issues .  Arguing  about    the  technical  capabilities   to   safely manage  either  natural radioactive  sites  or nuclear  waste deposits  and   debating  of  salient  benefits   could   help in   off-setting   irrational concerns  .   At   this stage  of  global  systems  development   , when “what  to do”  is a challenge  of  the outmost  importance,  the issues of  non-segmental   technologies    and  of  a new  social consciousness   are  strategic priorities  .  As   indicated in the more recent  report of the Club of Rome [ 46] , collective  actions must be planned   through  the cooperation  of  experts on technologies,    governments  and   all sectors of   society. 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