For Blasters the secondaries vary a lot in terms of mitigation versus damage.
In general terms Fire and Mental offer the most Damage while Ice and Devices offer the most mitigation (with the note that Devices is relatively slow solo and almost useless on a team). Energy and Electricity are middle of the road with good damage through melee attacks and several mezzes (mostly from those same attacks).
Fire hands down. Most AoE damage I have seen, although you have to stay alive long enough to use all of it 
.
COH 50s
Stone Protector - Stone/Axe tank
Righteous Infernal - Fire/Kin troller
Savior of Paragon - Nrg/Nrg blaster
Lt. Casey - AR/Dev blaster
lil sticker - Spine/Reg scrapper
Cool Fire - Ice/Ice tank
COV 50
Mu Destroyer - Stone/Elec brute
It depends on what you want from your secondary. I don't see Radiation having any particularly magical synergy with one particular Secondary.
I would personally go for Radiation/Energy. The -Defense debuffs that Radiation Blast gives off will help the more powerful ST melee strikes land, and Boost Range is a great way to extend the effective range range of Electron Haze, Cosmic Burst, and Irradiate without using enhancements. I would definitely pick Force Mastery and get Repulsion Field, Repulsion Bomb, and Personal Force Field. I think they're the most fun Blaster epic powers (aside from the LRM Rocket), and great for buying you some time if you get in too deep.
Edit: It could look something like this, if you use Ninja Run as a travel Power.
Archetype: Blaster
Primary: Radiation Blast
Secondary: Energy Manipulation
+----------------------------------------------------------------
01: Neutrino Bolt => Empty(1)
01: Power Thrust => Empty(1)
02: X-Ray Beam => Empty(2)
04: Build Up => Empty(4)
06: Electron Haze => Empty(6)
08: Hurdle => Empty(8)
10: Energy Punch => Empty(10)
12: Bone Smasher => Empty(12)
14: Health => Empty(14)
16: Conserve Power => Empty(16)
18: Cosmic Burst => Empty(18)
20: Stamina => Empty(20)
22: Aim => Empty(22)
24: Irradiate => Empty(24)
26: Neutron Bomb => Empty(26)
28: Power Boost => Empty(28)
30: Hasten => Empty(30)
32: Atomic Blast => Empty(32)
35: Boost Range => Empty(35)
38: Total Focus => Empty(38)
41: Personal Force Field => Empty(41)
44: Temp Invulnerability => Empty(44)
47: Repulsion Bomb => Empty(47)
49: Repulsion Field => Empty(49)
Ok. I'm trying this out now. I'm a newbie to CoX and have been playing for a couple of months. I have a lvl 37 Ill/Storm troller, and absolutly love her. Also playing a Necro/Dark MM. Also a joy. Wnting to try something different. Tried a Fire/Dev blaster but not thrilled. Wanna try out a Rad blaster but there's not alot out there to read about it. I'm the type who has trouble making decisions with AT's and I'm wondering which secondaries might sync best. All suggestions will be appreciated. I'm leaning towards either Ice for the slows because of all the AoE or mabey NRG but not sure if I want to blap. Am I thinking about it to much???? Does it even matter?????