How to get and set column from type "number" by powershell

I am using sp 2010. I have a library with documents. There is a column "CalcultatedWeekNumber" from type "Calculated". I have another column Week from type "Number". I try to get the value of the calculated field and set it on the column week:

$site = new-object Microsoft.SharePoint.SPSite("http://ift.tt/1AThS51") 
$web = $site.openweb()
$list = $web.Lists["BMRSoproTest"] 
$listItems = $list.Items
$listItemsTotal = $listItems.Count
$newName = "2015-06-WeeklyOperationsReport.pdf"
$originalName = "WeeklyOperationsReport.pdf"

"find document with destination name"

$spSourceItems = $list.Items | where {$_['Name'] -eq $newName}

if($spSourceItems -ne $null)
{
    $spSourceItems | ForEach-Object {
        "get original week number: "+$_["WeekNumber"]
        $originalWeekNumber = $_["WeekNumber"]
        "saved original week number: "+$originalWeekNumber
        "delete document with destination name"
        $_.Delete() 
    }
}

"rename original document to destination name"

$spSourceItems = $list.Items | where {$_['Name'] -eq $originalName}

if($spSourceItems -ne $null)
{
    "originalWeekNumber: "+ $originalWeekNumber
    $spSourceItems | ForEach-Object {
        $_["Name"] = $newName
        $_["Week"] = $originalWeekNumber
        $_.Update()
        }
}

$web.Dispose();

The value of variable $originalWeekNumber is:

float;#6.00000000000000

I got this error:

Exception calling "Update" with "0" argument(s): "Invalid number value.

A number field contains invalid data. Please check the value and try again." At C:\Users\jack\Desktop\Untitled2.ps1:36 char:18 + $_.Update <<<< () + CategoryInfo : NotSpecified: (:) [], MethodInvocationException + FullyQualifiedErrorId : DotNetMethodException

This is row 36:

$_["Week"] = $originalWeekNumber

what is the issue? How to set a float value on a number column?