washington fear nuclear terrorism rise u.s. president barack obama drive lock vulnerable atomic materials worldwide seem lose momentum could slow less 10 months leave office follow one signature foreign policy initiatives obama convene leaders 50 countries washington week fourth final nuclear security summit high-level diplomatic process start end watch boycott russian president vladimir putin apparently unwilling join u.s.-dominated gather time increase tensions washington moscow add doubt meet yield major result recent deadly militant attack brussels fuel concern islamic state could eventually target nuclear plant develop radioactive dirty bomb topic may well uppermost leaders mind meet despite significant progress obama persuade dozens countries rid bomb-making materials reduce safeguard stockpile much world plutonium enrich uranium remain vulnerable theft absence russia one big atomic power could detract decisions reach washington week obama opinion piece washington post say massive cold war nuclear arsenal poorly suit today threats unite state russia together hold 90 percent world nuclear weapons negotiate reduce stockpile further kremlin spokesman dmitry peskov say wednesday russia skip summit shortage mutual cooperation work agenda note moscow continue joint work nuclear security white house spokesman josh earnest say russia go miss opportunity no-show illustrate degree russia isolate russia chafe u.s.-led sanction ukraine conflict efforts make world safe also complicate north korea nuclear weapons advance pakistan move toward small tactical nuclear weapons washington fear may destabilize already volatile region weigh obama agenda prepare host world leaders thursday friday inaugurate event nearly six years ago use landmark speech prague 2009 lay goal rid world nuclear weapons central theme presidency guarantee obama drive force behind initiative leave office january successor keep issue high priority white house preview summit tout list achievements u.s.-led effort tie loose bomb-grade materials arm control advocate commend obama galvanize international response problem however many say progress slow since last summit 2014 countries japan india pakistan prepare activities could increase stockpile nuclear materials nuclear security summit positive effect strategic goal develop effective global nuclear security system remain unachieved nuclear threat initiative anti-proliferation watchdog say report month accord group nuclear security index track safety weapons-usable nuclear materials past two years bring improvement range measure include on-site physical protection security transport ability recover lose radioactive materials report also say many countries nuclear reactors vulnerable online attack seven 24 countries weapons-grade material include china belgium receive low possible score facilities cyber security critics point lack agreed-upon set international standards nuclear security mechanism keep tabs common source radioactive material often find hospitals medical labs however laura holgate obama adviser weapons mass destruction cite commitments 30 countries 2014 summit secure dangerous material international community make hard ever terrorists acquire nuclear weapons make us secure tell reporters summit two brussels suicide bombers secretly film daily routine head belgium nuclear research development program consider attack nuclear site country accord belgian media u.s. experts less concern militants obtain nuclear weapon components thefts ingredients low-tech dirty bomb would use conventional explosives disperse radioactive material sow panic u.s. officials say doubt islamic state control swaths syria iraq interest obtain materials holgate say u.s. authorities explicit indications group try commitments world leaders enhance nuclear security expect summit anti-proliferation group worry without meet high level interest could wane improvements could backslide nuclear terrorism fear loom obama 's final atomic summit