This is an issue with BigReactors (obviously) and a TCon hammer. The BigReactors dev has not gotten around to fixing it yet. Your best bet is to edit the world with MCEdit.

---- Minecraft Crash Report ----
// I'm sorry, Dave.

Time: 3/29/15 1:59 PM
Description: Exception in server tick loop

java.lang.ClassCastException: erogenousbeef.bigreactors.common.multiblock.tileentity.TileEntityReactorFuelRod cannot be cast to erogenousbeef.bigreactors.common.multiblock.tileentity.TileEntityReactorControlRod
        at erogenousbeef.bigreactors.common.multiblock.MultiblockReactor.updateServer(MultiblockReactor.java:291)
        at erogenousbeef.core.multiblock.MultiblockControllerBase.updateMultiblockEntity(MultiblockControllerBase.java:451)
        at erogenousbeef.core.multiblock.MultiblockWorldRegistry.tickStart(MultiblockWorldRegistry.java:79)
        at erogenousbeef.core.multiblock.MultiblockRegistry.tickStart(MultiblockRegistry.java:25)
        at erogenousbeef.core.multiblock.MultiblockServerTickHandler.onWorldTick(MultiblockServerTickHandler.java:20)
        at cpw.mods.fml.common.eventhandler.ASMEventHandler_462_MultiblockServerTickHandler_onWorldTick_WorldTickEvent.invoke(.dynamic)
        at cpw.mods.fml.common.eventhandler.ASMEventHandler.invoke(ASMEventHandler.java:54)
        at cpw.mods.fml.common.eventhandler.EventBus.post(EventBus.java:138)
        at cpw.mods.fml.common.FMLCommonHandler.onPreWorldTick(FMLCommonHandler.java:273)
        at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:914)
        at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:423)
        at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:798)
        at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:658)
        at java.lang.Thread.run(Thread.java:745)


A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- System Details --
Details:
        Minecraft Version: 1.7.10
        Operating System: Linux (amd64) version 3.16.0-23-generic
        Java Version: 1.8.0_40, Oracle Corporation
        Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
        Memory: 462073312 bytes (440 MB) / 1942769664 bytes (1852 MB) up to 6227755008 bytes (5939 MB)
        JVM Flags: 9 total; -Xms512m -Xmx6144M -XX:PermSize=256m -XX:+UseParNewGC -XX:+CMSIncrementalPacing -XX:+CMSClassUnloadingEnabled -XX:ParallelGCThreads=2 -XX:MinHeapFreeRatio=5 -XX:MaxHeapFreeRatio=10
        AABB Pool Size: 0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
        IntCache: cache: 0, tcache: 4029, allocated: 12, tallocated: 95
        FML: MCP v9.05 FML v7.10.88.1307 git-Cauldron-MCPC-Plus-1.7.10-1.1307.06.218 (MC: 1.7.10) (API v1.7.10-R0.1-SNAPSHOT) Minecraft Forge 10.13.2.1307 156 mods loaded, 156 mods active
        mcp{9.05} [Minecraft Coder Pack] (minecraft.jar) Unloaded->Constructed->Pre-initialized->Initialized->Post-initialized->Available->Available->Available->Available





        Forestry : Warning: You have mods that change the behavior of Minecraft, ForgeModLoader, and/or Minecraft Forge to your client: ^M
Bukkit, Cauldron, or other Bukkit replacement^M
These may have caused this error, and may not be supported. Try reproducing the crash WITHOUT these mods, and report it then.
        AE2 Integration: IC2:ON, RotaryCraft:OFF, RC:ON, BC:ON, MJ6:OFF, MJ5:OFF, RF:ON, RFItem:ON, MFR:ON, DSU:ON, FZ:OFF, FMP:ON, RB:OFF, CLApi:OFF, Waila:ON, Mekanism:OFF, ImmibisMicroblocks:OFF, BetterStorage:OFF
        Profiler Position: N/A (disabled)
        Vec3 Pool Size: 0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
        Player Count: 7 / 20; [EntityPlayerMP['KEEN3R'/175190, l='world', x=-4058.41, y=114.00, z=1591.54](KEEN3R at -4058.4079897095626,114.0,1591.5402495563947), EntityPlayerMP['NEOTYKE'/192252, l='world', x=4043.64, y=64.00, z=-240.05](NEOTYKE at 4043.6407493383376,64.0,-240.05087489347633), EntityPlayerMP['HawaiianAce27'/226911, l='world', x=-4041.76, y=127.79, z=1591.70](HawaiianAce27 at -4041.763043969523,127.78502770378924,1591.699999988079), EntityPlayerMP['ShadowEagle4'/286869, l='world', x=-781.66, y=117.00, z=1772.03](ShadowEagle4 at -781.6607429358003,117.0,1772.0260405617996), EntityPlayerMP['HyperEnergized'/312877, l='DIM-7', x=-724.47, y=35.92, z=1037.45](HyperEnergized at -724.471555002188,35.92159999847412,1037.4459237417734), EntityPlayerMP['HkisAwesome'/318660, l='world', x=-1221.70, y=62.42, z=623.66](HkisAwesome at -1221.699999988079,62.41999998688698,623.6639845083182), EntityPlayerMP['RoboDuckMC'/256743, l='world', x=-712.65, y=72.00, z=1054.79](RoboDuckMC at -712.6499144234765,72.0,1054.7863873909985)]
        Is Modded: Definitely; Server brand changed to 'cauldron,craftbukkit,mcpc,fml,forge'
        Type: Dedicated Server (map_server.txt)